Properties of Matter — Print · Physical Science: Matter, Forces & Energy, Grade 8

Atomic Structure

Learning goals

• I can identify the three subatomic particles and describe their charge, mass, and location in the atom.
• I can use an element's atomic number and mass number to find its numbers of protons, neutrons, and electrons.
• I can distinguish molecular and extended-structure models.

Readings for this unit

• A container that works once

All ordinary materials, including the air around us, are made of atoms or groups of atoms. An atom is a constituent of an element. Its nucleus, the very small central region, contains positively charged protons and, for most isotopes, electrically neutral neutrons. Negatively charged electrons are described by a surrounding electron cloud. Charge is a property associated with electric interactions; a proton’s positive charge and an electron’s negative charge have equal magnitudes. A neutron has no net electric charge. Hydrogen-1 is a useful exception with one proton and no neutron.

The nucleus contains nearly all the atom’s mass and is much smaller than the atom’s overall extent. A simplified stadium comparison illustrates a size difference but is not a photograph. Electrons are not tiny planets following visible circular tracks, and atoms are not literally hard empty containers. An electron-cloud model describes where an electron may be detected; a classroom dot drawing is mainly a counting aid.

What makes an atom a specific element — carbon, oxygen, gold — is simply the number of protons it holds. That count is the atomic number, and it never changes for a given element: every atom with 6 protons is carbon, and anything with 8 protons is oxygen, no exceptions. Change the number of protons and you literally change what element you have. In a neutral atom, the number of electrons exactly matches the number of protons, so the positive and negative charges cancel out and the atom carries no overall charge.

The mass number counts the protons and neutrons together, since those two supply essentially all of an atom's mass. Once you know the atomic number and the mass number, you can work out all three particle counts for a neutral atom: the protons equal the atomic number, the electrons also equal the atomic number, and the neutrons are simply whatever is left over — mass number − atomic number.

Neutrons are the one particle that can vary without changing the element itself. Every carbon atom has 6 protons, but one might carry 6 neutrons and another 8 — these different versions of an element are called isotopes. Getting comfortable with counting protons, neutrons, and electrons is the foundation for almost everything else in chemistry, from understanding why each element sits where it does on the periodic table to predicting how atoms will bond and react.

Atomic number counts protons. A neutral atom has the same number of electrons as protons, so its charges cancel. If it gains an electron it becomes a negative ion; gaining an electron does not change its element.

Mass number counts protons plus neutrons in one atom. Atoms of the same element can have different neutron counts: these are isotopes. The decimal mass on a periodic table is an average for a mixture of isotopes, not a count of particles in one atom.

A model drawing is useful for counting particles, but its circles are not actual paths followed by electrons. Label the nucleus and electron cloud, and explain what the model leaves out. Use proton count first when identifying an unknown atom.

Build your understanding

Where this fits

This unit helps answer the chapter question: How can the properties of a material help us explain it and choose a useful application? Keep a notebook table with the question, evidence, explanation and one unresolved issue. Revise a previous model only when you can name the new evidence or idea that changes it.

Use three teaching sessions of about 40–45 minutes: first observe the phenomenon, draft a model and study Parts 1–2; next explain Parts 3–4 with the figures and evidence; then work the application, examples, practice, model revision and exit check. Divide any difficult part into smaller steps and revisit after feedback. Investigations, review and quizzes have separate sessions.

Part 1 · Use a model at an invisible scale

An atom has a very small nucleus containing protons and neutrons, surrounded by an electron cloud. The cloud describes where electrons are likely to be found; electrons are not tiny planets following the circles on a classroom drawing. A model helps us discuss something too small to see with an ordinary microscope.

Read the visual model

What useful information can a bead model show, and what does it leave out?

Visual reasoning — after your attempt

It can show particle counts, locations, and charges. It does not accurately show relative sizes, distances, electron behavior, or literal particle colors.

Draw a nucleus and label the three particle types and charges. A large circle around it is a counting aid, not a photograph. Real atoms do not have the colors used in a bead model, and particle sizes and distances are usually not drawn to scale.

Pause and explain

What useful information can a bead model show, and what does it leave out?

Check your reasoning after answering

It can show particle counts, locations, and charges. It does not accurately show relative sizes, distances, electron behavior, or literal particle colors.

Part 2 · Identify an element by its protons

The number of protons determines the element. Every oxygen atom has eight protons, regardless of how many neutrons or electrons it has. Atomic number is this proton count. A neutral atom has equal positive and negative charges because it has as many electrons as protons.

Read the visual model

Read p⁺ as positively charged protons, n as neutral neutrons and e⁻ as negatively charged electrons. Element identity and net charge answer different questions. Changing only electrons cannot turn oxygen into carbon.

A particle has 8 protons, 8 neutrons and 10 electrons. Is it carbon, oxygen or a new element? Explain its charge.

Visual reasoning — after your attempt

It is oxygen, because it still has 8 protons. It has two more electrons than protons and therefore charge −2 in elementary-charge units.

For a neutral atom with six protons, six electrons cancel the six positive charges. Six protons identify carbon. Removing an electron changes its charge, but the nucleus still contains six protons, so the element is still carbon.

Pause and explain

A particle has eight protons and ten electrons. Is it oxygen, and is it neutral?

Check your reasoning after answering

It is oxygen because it has eight protons. It has two excess negative charges, so it is an ion with charge −2, not a neutral atom.

Part 3 · Count neutrons without confusing two masses

Mass number counts protons plus neutrons in one atom. Subtract atomic number from mass number to find neutrons. Isotopes of an element share proton count but differ in neutron count. The decimal atomic mass printed on a periodic table is an average for naturally occurring isotopes, rather than the particle count of one atom.

Read the visual model

How do carbon-12 and carbon-14 remain the same element?

Visual reasoning — after your attempt

Both contain six protons. Carbon-12 has six neutrons and carbon-14 has eight; the different neutron counts make them isotopes of carbon.

A carbon atom with mass number 14 has six protons and 14−6=8 neutrons. A neutral carbon-14 atom also has six electrons. Do not round a table average and assume that every atom of that element has the resulting neutron count.

Pause and explain

How do carbon-12 and carbon-14 remain the same element?

Check your reasoning after answering

Both contain six protons. Carbon-12 has six neutrons and carbon-14 has eight; the different neutron counts make them isotopes of carbon.

Part 4 · Keep element identity during ordinary changes

In ordinary chemical reactions, atoms rearrange and electrons can be shared or transferred. The nuclei keep their proton counts. This is why an oxygen atom in water and an oxygen atom in oxygen gas are still oxygen, even though the substances have different properties. Nuclear changes are a different kind of process.

Read the visual model

Use the nucleus count for the element and mass number. Use proton minus electron count for net charge. Keep these accounts separate: a heavier isotope is not necessarily a more positively charged atom.

Starting from the C-12 record, what changes if it gains one electron? What changes if it instead gains two neutrons?

Visual reasoning — after your attempt

One added electron gives 7 electrons and a −1 charge while keeping carbon and mass number 12. Two added neutrons instead give carbon-14 with mass number 14 and unchanged neutral charge.

Use the same color and label for each oxygen atom when drawing a reaction. Changing an oxygen bead into a carbon bead would falsely suggest a changed nucleus. Track atom identity while allowing new groupings.

Pause and explain

Why is changing proton count a poor model of dissolving salt?

Check your reasoning after answering

Dissolving separates and disperses salt ions in water; it does not convert their nuclei into different elements. Proton counts and element identities remain unchanged.

Molecules and extended structures

A formula can name a small separate group or the ratio in a repeating structure. The same drawing rule does not describe both.

An element contains atoms with one proton-defined identity. A compound contains different elements chemically combined in a characteristic composition. A molecule is a distinct bonded group of atoms, such as one water molecule with two hydrogen atoms and one oxygen atom. A chemical bond is an interaction that holds a combination together. Drawing a line represents that connection; it is not a literal stick between colored balls. Subscripts count atoms in one represented group, while a coefficient counts whole groups.

Some solids are extended structures: a connected or repeating arrangement continuing beyond the few particles drawn. Ordinary sodium chloride forms a lattice, a repeating arrangement of positive sodium ions and negative chloride ions. NaCl states a one-to-one ratio, not a separate two-atom molecule floating inside the crystal. A polymer is a material whose molecules include long chains of repeated groups. A repeating-unit drawing can show part of a chain without claiming every chain has identical length.

Composition and arrangement help explain why substances differ in properties. An oxygen atom in water remains oxygen, but water has properties different from oxygen gas because the combinations differ. A molecular or lattice model should name its represented substance, count or ratio, and scale limits. The few drawn particles stand for many actual particles. Ordinary chemical changes rearrange existing atoms; changing an element’s nucleus is a different process, so ordinary chemistry does not explain the creation of all elements in the universe.

Explain and apply

Draw two water molecules and a larger repeating salt patch. Compare total atom counts with a formula ratio, and explain why a two-ball salt molecule is an incomplete crystal model.

• Distinguish molecule from extended arrangement.
• Count water atoms correctly.
• Explain formula ratio and a model limit.

Reasoning and marking guidance — after your attempt

Two water molecules contain four H and two O atoms. A salt patch represents part of an extended arrangement; its bulk Na:Cl ratio is one-to-one rather than a claim that the crystal is a collection of separate NaCl molecules. Retain element/ion labels and note that colors, bonding lines and distances are conventions.

Apply the whole unit · Identify three particles

Illustrative particle records: A has 8 protons, 8 neutrons, 8 electrons; B has 8 protons, 10 neutrons, 8 electrons; C has 8 protons, 8 neutrons, 10 electrons. Oxygen has atomic number 8.

Your task

Identify the element, mass number, and overall charge of each record. Draw a labeled counting model, identify an isotope pair, and explain why the drawings cannot represent true electron paths.

What your response should show

• Use the supplied observations and measurements accurately; include units where needed.
• Explain the science behind your answer, rather than giving only a number or label.
• Show a labeled model, calculation, or evidence comparison as requested.
• State an assumption or limitation and suggest a useful next measurement.

Response guidance — after your attempt

All three are oxygen. A has mass number 16 and charge 0; B has mass number 18 and charge 0; C has mass number 16 and charge −2. A and B are isotopes. A and C differ in electron count. Label the nucleus and electron cloud; show counts without implying scale or circular electron trajectories.

Scientific references and adapted teaching resources

• OpenSciEd 7.1 Chemical Reactions & Matter — public unit overview
• OpenSciEd 6.2 Thermal Energy — public unit overview
• OpenSciEd instructional model — storyline routines
• Creative Commons Attribution 4.0 — middle-school material license

Parent / teacher teaching notes

Begin with counters and a particle-count table. Ask the learner to identify the element before calculating mass number or charge. If the learner changes element when an electron changes, revisit the proton rule with a second example. Adaptation credit: OpenSciEd middle-school teaching routines and public task structures by OpenSciEd, CC BY 4.0. This independent version adds original explanatory text, data and figures, terminology support and a home-study schedule. It does not reproduce the complete teacher editions or their classroom validation. The illustrated study “Molecules and extended structures” is required curriculum content. Teach it before the associated application or distribute it over the assigned unit sessions; it is not an optional enrichment. Its practice/review/quiz response provides evidence of understanding. Model/data work must not be recorded as an actual physical experiment.

Number of Neutrons

neutrons = mass number − atomic number

atomic number = number of protons = number of electrons (in a neutral atom); mass number = protons + neutrons.

Worked examples

Avoid these mistakes

• Changing electrons does not create a new element.

Words to know

atom

The smallest unit of an element that still has the properties of that element.

proton

A positively charged particle found in the nucleus.

neutron

A particle with no charge (neutral) found in the nucleus, alongside the protons.

electron

A negatively charged particle that moves around the nucleus in an electron cloud.

nucleus

The dense, positively charged center of an atom, containing the protons and neutrons.

atomic number

The number of protons in an atom; it identifies which element the atom is.

mass number

The total number of protons plus neutrons in an atom's nucleus.

isotope

An atom of an element distinguished by neutron count; isotopes share proton count.

ion

An atom or group of atoms with net electric charge because positive and negative charges do not balance.

model

A representation that explains selected features and makes predictions; it leaves other details out.

evidence

Relevant observations or records used to support or challenge a claim.

system boundary

The chosen limit separating the objects being studied from their surroundings.

assumption

A condition taken as true for a model or calculation and needing to be checked against the case.

Why atoms are mostly empty space

If an atom's nucleus were the size of a marble, the nearest electrons would be about two football fields away, with nothing in between. Atoms are roughly 99.9999999% empty space. The reason solid objects still feel solid is not that atoms are packed tight — it's that the electron clouds of neighboring atoms repel each other, so your hand never actually touches the desk; the electrons push back before contact.

Isotopes: same element, different mass

Every atom of an element has the same number of protons, but the number of neutrons can vary. These variations are called isotopes. Carbon-12 and carbon-14 are both carbon (6 protons), but carbon-14 has two extra neutrons. Carbon-14 is slightly radioactive and decays at a known rate — which is exactly how scientists carbon-date fossils and ancient artifacts.

Where the mass really is

Protons and neutrons each have about the same mass, and each is roughly 1,836 times heavier than an electron. So nearly all of an atom's mass sits in the tiny nucleus, while electrons — which take up almost all the atom's volume — contribute almost none of its weight. That's why the atomic mass number counts protons + neutrons and ignores electrons.

In the real world

Doctors use isotopes — atoms of the same element with different numbers of neutrons — in medical imaging and in treating certain cancers.

Exit check

If only electrons rise to 10, what entries change? Explain the relevant mechanism and one model limitation.

Physical vs. Chemical Properties

Learning goals

• I can distinguish a physical property from a chemical property.
• I can classify a change as physical or chemical based on evidence, including calculating density.
• I can plan a mixture separation, evaluate recovery evidence and account for incomplete collection.

Readings for this unit

• A container that works once

Every substance has a set of physical properties — traits you can observe or measure without turning the substance into anything new. Color, hardness, texture, melting point, boiling point, and density (mass ÷ volume) are all physical properties. You can slice a loaf of bread, bend a copper wire, melt an ice cube, or dissolve sugar in tea, and in every one of those cases the material is still fundamentally the same stuff, just in a different form. A change like that — where no new substance appears — is a physical change, and it's often reversible.

Density is worth a closer look, because it's a physical property you'll calculate often. It measures how much mass is packed into a given amount of space (density = mass ÷ volume), and it explains everyday puzzles like why a massive steel ship floats while a small pebble sinks — what matters isn't total weight but how tightly the mass is packed compared with water. A key point: density belongs to the material itself, so a tiny gold ring and a giant gold bar have exactly the same density.

A chemical property, by contrast, describes how a substance behaves when it reacts and turns into something different. Flammability (will it burn?), reactivity with acid (will it fizz or dissolve?), and the tendency to rust are all chemical properties. Here's the catch: you can only truly observe a chemical property by letting the substance change — you can't see that wood is flammable until some of it actually burns.

When a substance genuinely reacts and becomes one or more new substances with new properties, that's a chemical change. Burning wood, rusting iron, and digesting your lunch are all chemical changes: the original material is gone, its atoms rearranged into different substances entirely. And unlike most physical changes, chemical changes are usually very hard to reverse — there's no simple way to un-burn a log or un-cook an egg.

So how do you tell the two apart? Ask a single question: did a genuinely new substance form? Melting butter is physical — it's still butter, just liquid, and it hardens right back if it cools. Burning that same butter is chemical — it becomes soot, smoke, and gases that will never turn back into butter. Watch out for one common trap, though: a dramatic-looking change of state, like water boiling into steam, is still only a physical change, because that steam is nothing but water in gas form.

A physical property can be observed without changing a substance's chemical identity. Measure melting point, density or electrical conductivity under stated conditions. Color alone rarely identifies a substance because different substances can have similar colors.

Chemical properties describe possible reactions, such as burning in oxygen or reacting with an acid. The property exists before the test, but observing it normally involves a chemical change. A chemical reaction rearranges atoms into new substances.

Separate the evidence from the conclusion. Cutting metal changes its size; rusting makes new substances. Dissolving sugar in water usually keeps the sugar molecules intact. Ask whether composition changed, rather than whether the change looks dramatic or is easy to reverse.

Build your understanding

Where this fits

This unit helps answer the chapter question: How can the properties of a material help us explain it and choose a useful application? Keep a notebook table with the question, evidence, explanation and one unresolved issue. Revise a previous model only when you can name the new evidence or idea that changes it.

Use three teaching sessions of about 40–45 minutes: first observe the phenomenon, draft a model and study Parts 1–2; next explain Parts 3–4 with the figures and evidence; then work the application, examples, practice, model revision and exit check. Divide any difficult part into smaller steps and revisit after feedback. Investigations, review and quizzes have separate sessions.

Part 1 · Separate a property from a change

A property describes how a substance behaves under stated conditions. A physical property can be measured without changing chemical identity, such as density or electrical conductivity. A change is something that happens to a sample. Cutting a copper wire changes its length, while conductivity remains a property of the copper.

Read the visual model

Is flammability a change or a property?

Visual reasoning — after your attempt

Flammability is a chemical property describing a tendency to burn. Actual burning is a chemical change producing new substances.

Make two notebook columns: what can be measured, and what happened. Melting point belongs in the first column; a sample melting belongs in the second. Give the temperature or other test conditions when they matter.

Pause and explain

Is flammability a change or a property?

Check your reasoning after answering

Flammability is a chemical property describing a tendency to burn. Actual burning is a chemical change producing new substances.

Part 2 · Ask whether chemical identity changed

A physical change can alter size, shape, arrangement, or state while retaining the substance. A chemical change produces substances with different chemical identities by rearranging atoms. The decision concerns composition, not whether the event looks dramatic or whether it is easy to reverse.

Read the visual model

Why is reversibility alone an unreliable rule?

Visual reasoning — after your attempt

Some physical changes are hard to undo, such as breaking glass, and some chemical reactions can be reversed under appropriate conditions. Chemical identity is the deciding idea.

Boiling water makes water vapor, so it is a physical change. Rusting iron produces iron-containing compounds with oxygen, so it is chemical. Breaking a glass is difficult to undo but does not, by itself, create a new substance.

Pause and explain

Why is reversibility alone an unreliable rule?

Check your reasoning after answering

Some physical changes are hard to undo, such as breaking glass, and some chemical reactions can be reversed under appropriate conditions. Chemical identity is the deciding idea.

Part 3 · Build a comparison from several measurements

One property rarely establishes identity. Different substances may share a color or approximate density. Collect a pattern of evidence using comparable conditions: appearance, density, melting behavior, or a suitable approved test. Treat a proposed identity as a claim that should agree with all relevant evidence.

Read the visual model

The slope of this mass-versus-volume graph is density, not speed. Both axes name measured properties. Use several properties, with conditions stated, before deciding whether a substance’s identity has changed.

For a 12 cm³ sample following this pattern, predict mass. Can that result alone identify the substance?

Visual reasoning — after your attempt

Predicted mass is 12×3 = 36 g. Density alone is insufficient because different substances can share similar values; compare additional characteristic properties.

Two white powders may look identical but behave differently in water. Describe the observations without tasting or handling unknown chemicals. A label, a known safe reference sample, and a controlled test are stronger evidence than appearance alone.

Pause and explain

Do two equal densities prove two samples are the same substance?

Check your reasoning after answering

No. Different substances can have similar densities, and measurements have uncertainty. Additional independent properties and known comparison samples are needed.

Part 4 · Describe dissolving at the particle level

When sugar dissolves, sugar molecules disperse among water molecules. The mixture looks uniform at the everyday scale, but the sugar has not simply vanished. For this familiar example, dissolving keeps the sugar molecules intact. Some substances can also react when mixed with water, so dissolving should not be treated as one universal chemical rule.

Read the visual model

How can a clear liquid contain more than one substance?

Visual reasoning — after your attempt

Particles can be mixed too finely to distinguish by eye. A clear sugar solution contains both water molecules and dispersed sugar molecules.

A particle drawing of sugar water should show both labeled water and sugar particles. Evaporation can leave sugar behind, provided heating does not cause decomposition. Explain the chosen example rather than claiming every clear solution contains only water.

Pause and explain

How can a clear liquid contain more than one substance?

Check your reasoning after answering

Particles can be mixed too finely to distinguish by eye. A clear sugar solution contains both water molecules and dispersed sugar molecules.

Mixtures and physical separation

Sand and salt can be combined without automatically forming a new compound. A physical combination needs investigation, not a reaction claim based only on mixing.

A mixture contains substances physically combined in variable proportions. A solution is a mixture that is uniform at the visible scale because one substance is distributed among particles of another. A solute is the dissolved substance and a solvent is the dissolving medium. Salt dissolved in water remains matter even when it is not visible. A compound has a chemical composition; changing the mixture proportions is different from changing a compound’s formula.

Separation can use differences in physical properties. A filter traps sufficiently large suspended particles but ordinarily does not remove dissolved salt. Decanting means carefully pouring off a liquid while leaving settled material. Evaporation can leave dissolved salt behind while water enters the surroundings; it does not destroy water or create new salt atoms. Separation behavior, recovered characteristic properties and mass accounting supply evidence for a physical combination.

Plan before observing: specify known substances, their amounts, the property used for separation, the recovery method and how records will be kept. Test the components and their combination with matched water amounts and stirring/settling time. A sample’s clear appearance does not establish purity or drinking safety. Incomplete recovery may reflect transfer loss or dissolved material left behind, rather than a chemical reaction. Report all recovered and unrecovered matter within the chosen boundary.

Evidence to interpret

Original components	Recovered sand (g)	Recovered salt (g)
5.0 g sand + 3.0 g salt	4.8	2.9

Use this method safely

Adult support for material selection and spills. Use only clean known nonhazardous samples, avoid dust inhalation and keep small items from younger children. Do not taste, drink, heat or mix unknown chemicals. Use provided records when balance, filters or safe samples are unavailable.

Investigate or evaluate the supplied records

1. Use clean approved sand and table salt, or the supplied records. Record the known components and masses.
2. Compare each component and the mixture in equal room-temperature water volumes; record dissolving/settling observations.
3. Filter through a suitable paper filter into a clean collection vessel and record which component remains.
4. Leave a small labeled salt-solution sample in a safe tray to evaporate at room temperature over several days; no stove or boiling is needed.
5. Compare recovered solids with original component records and account for incomplete recovery.
6. Record actual measurements separately from supplied values and propose a repeated improved method.

Explain and apply

Plan and, if safe materials are available, conduct the sand/salt comparison. Use the supplied recovery record to argue whether it is consistent with a physical mixture, and explain why a mass shortfall is not automatically a reaction.

• Specify components and a property-based method.
• Use recovered identity and mass evidence.
• Explain a limitation or controlled improvement.

Reasoning and marking guidance — after your attempt

The separate-component tests and recovery of sand and salt are consistent with a physical combination. The illustrative recovered masses are 4.8+2.9=7.7 g versus 8.0 g originally added, leaving 0.3 g unaccounted for in that collection. Transfer residues, remaining dissolved salt or reading uncertainty can explain a shortfall. Better rinsing, complete collection and repeated matched tests can improve the account; the data do not establish a new compound.

Apply the whole unit · Classify with evidence

Illustrative observations: ice melts into liquid water; a glass breaks into pieces; iron develops a new reddish coating after exposure to moist air; sugar dissolves and is recovered after gentle evaporation.

Your task

Classify each event and explain the chemical-identity evidence. Identify a property used in an observation and explain why a dramatic appearance or difficult reversal is not enough to classify a change.

What your response should show

• Use the supplied observations and measurements accurately; include units where needed.
• Explain the science behind your answer, rather than giving only a number or label.
• Show a labeled model, calculation, or evidence comparison as requested.
• State an assumption or limitation and suggest a useful next measurement.

Response guidance — after your attempt

Melting, breaking, and the stated sugar dissolution are physical changes; rusting is chemical because new iron compounds form. State identity evidence rather than relying on size or reversibility. Color is an observed physical property, but color change alone would not establish a chemical reaction without more evidence.

Scientific references and adapted teaching resources

• OpenSciEd 7.1 Chemical Reactions & Matter — public unit overview
• OpenSciEd 6.2 Thermal Energy — public unit overview
• OpenSciEd instructional model — storyline routines
• Creative Commons Attribution 4.0 — middle-school material license

Parent / teacher teaching notes

Use familiar pictured events and ask for because-statements. Keep properties, observations, and conclusions in separate columns. Do not ask learners to taste samples or test unknown household chemicals. Adaptation credit: OpenSciEd middle-school teaching routines and public task structures by OpenSciEd, CC BY 4.0. This independent version adds original explanatory text, data and figures, terminology support and a home-study schedule. It does not reproduce the complete teacher editions or their classroom validation. The illustrated study “Mixtures and physical separation” is required curriculum content. Teach it before the associated application or distribute it over the assigned unit sessions; it is not an optional enrichment. Its practice/review/quiz response provides evidence of understanding. Model/data work must not be recorded as an actual physical experiment.

Density

density = mass ÷ volume

mass in grams (g); volume in cm³; density in g/cm³.

Worked examples

Avoid these mistakes

• An irreversible-looking change is not automatically a chemical reaction.

Words to know

physical property

A characteristic you can observe or measure without changing what the substance is, like color, density, or melting point.

chemical property

A characteristic that describes how a substance reacts or changes into a different substance, like flammability or reactivity with acid.

physical change

A change in form or appearance where no new substance is created.

chemical change

A change where one or more new substances with new properties are formed.

density

A physical property found by dividing an object's mass by its volume (density = mass ÷ volume).

model

A representation that explains selected features and makes predictions; it leaves other details out.

evidence

Relevant observations or records used to support or challenge a claim.

system boundary

The chosen limit separating the objects being studied from their surroundings.

assumption

A condition taken as true for a model or calculation and needing to be checked against the case.

The one-question test for a chemical change

Students mix these up constantly, so here's the key question: Is a new substance with new properties formed? If yes, it's a chemical change; if no, it's physical. Crushing chalk into powder is physical — it's still chalk. But chalk fizzing in vinegar makes a new gas (carbon dioxide) — that's chemical. Tearing paper is physical; burning it into ash and smoke is chemical.

The clues that a chemical reaction happened

You can't always see bonds breaking, but chemical changes leave fingerprints: a color change (a nail rusting), a gas (bubbles that aren't from boiling), a precipitate (a solid forming when two liquids mix), light or heat released or absorbed, and — the big one — it's hard to reverse. Any one clue is a hint; several together are strong evidence.

Why 'hard to reverse' is the giveaway

Physical changes are usually easy to undo: melt ice, then refreeze it; dissolve salt, then evaporate the water to get it back. Chemical changes are stubborn because new bonds have formed. You can't un-bake a cake or un-burn a log by cooling them, because the atoms have been rearranged into entirely new molecules. That difficulty of reversal is often the clearest sign a chemical change occurred.

The sneaky in-between: dissolving

Is dissolving sugar in water physical or chemical? It's a classic trick question — the answer is physical. The sugar molecules are still sugar; they've just spread out among the water molecules, and you can get the sugar back by evaporating the water. No new substance formed. This trips people up because it looks dramatic, but 'looks dramatic' isn't the test — 'new substance' is.

In the real world

Firefighters need to know which materials are flammable (a chemical property) so they can fight fires safely and predict how they'll spread.

Exit check

Which case changes substance identity? Explain the relevant mechanism and one model limitation.

Density & Floating

Learning goals

• I can calculate density from mass and volume.
• I can use average density to predict whether an object floats in a fluid.
• I can argue from density evidence without claiming a unique identity from an overlapping value.

Readings for this unit

• A container that works once

Mass tells how much matter a sample contains. Volume tells how much space it occupies. Density compares them: density = mass ÷ volume. A small metal sample can be denser than a large wooden block even though the wooden block has more total mass. Include units, such as grams per cubic centimeter (g/cm³).

Samples of the same uniform substance have approximately the same density at the same temperature and pressure. If one sample has twice the volume, it also has twice the mass. For an irregular solid that does not dissolve or absorb water, volume can be measured by the rise in water level when it is submerged: 1 mL is the same volume as 1 cm³.

An object with average density below that of its surrounding fluid can float; one with greater average density tends to sink. Average density includes enclosed air. A hollow steel ship can float even though solid steel is denser than water. The upward buoyant force balances the ship’s weight once it floats steadily.

Density compares equal volumes: a cubic centimeter of a denser material has more mass. For a regular box, find volume by multiplying length, width and height. For an irregular solid that does not dissolve, measure the rise in water level after complete immersion.

Floating depends on an object's average density compared with the fluid. A hollow steel boat can float because its enclosed air greatly increases volume without much added mass. The material's density and the whole object's average density are different quantities.

Make a fair density test with a balance, a graduated container and recorded units. Read water level at eye level, remove trapped bubbles and repeat the measurement. If densities are very close, uncertainty can prevent a confident prediction; collect better measurements instead of forcing a conclusion.

Worked examples

Avoid these mistakes

• Large objects do not necessarily have a greater density than small objects.

Words to know

density

Mass divided by volume.

displacement

The fluid volume moved aside by a submerged object.

average density

Total mass divided by the total volume enclosed by an object.

model

A representation that explains selected features and makes predictions; it leaves other details out.

evidence

Relevant observations or records used to support or challenge a claim.

system boundary

The chosen limit separating the objects being studied from their surroundings.

assumption

A condition taken as true for a model or calculation and needing to be checked against the case.

In the real world

Exit check

Why does B not have twice A’s density? Explain the relevant mechanism and one model limitation.

The Periodic Table

Learning goals

• I can describe how the periodic table is organized by atomic number, groups, and periods.
• I can identify metals, nonmetals, and metalloids, and describe basic periodic trends.

Readings for this unit

• A container that works once

The periodic table can look like an intimidating wall of boxes, but it's really one of the most brilliant organizing tools in all of science — a map that sorts every known element by its structure and behavior. The elements are laid out left to right and top to bottom in order of increasing atomic number (that's the number of protons), so each box holds exactly one more proton than the box before it. What makes the table 'periodic' is that similar properties keep showing up again and again at regular intervals, like a repeating pattern.

Each horizontal row is a period, and each vertical column is a group (also called a family). These aren't just labels. Elements in the same period share the same number of electron shells, and — more importantly — elements in the same group behave alike chemically. Sodium and potassium sit in the same column and are both soft, silvery metals that react vigorously with water, even though one is considerably heavier than the other.

For main-group elements, similar outer-electron arrangements often explain similar chemical behavior within a group. The outer electrons participate in bonding. Lithium, sodium, and potassium each have one outer electron in the introductory shell model, but they are different elements with different measured properties. Hydrogen occupies a special position and should not be assumed to behave exactly like an alkali metal. Transition-element patterns require more detailed models than this simple shortcut.

The table also sorts elements into three big regions. Metals — most of the table, filling the left and center — are typically shiny, bendable (malleable), and good conductors of heat and electricity. Nonmetals, clustered in the upper right, are usually dull, brittle when solid, and poor conductors. Squeezed along the zigzag staircase between them are the metalloids, in-betweeners like silicon that share a mix of metal and nonmetal traits — which is exactly what makes them so valuable in computer chips.

Finally, smooth patterns called periodic trends run across the table. Atomic size, for example, generally shrinks as you move left to right across a period, because each added proton tugs the electron cloud in a little tighter. The real payoff of all this organization is prediction: once you can read an element's position, you can make a confident guess about how it will behave — its reactivity, its bonding, even roughly its size — without ever running an experiment.

A period is a horizontal row; a group is a vertical column. Main-group elements in one group often have similar chemical behavior because their outer electron arrangements are similar. This pattern helps make predictions, but it does not make every member identical.

Metals generally conduct electricity and can be shaped; nonmetals show a wider range of properties. A material's actual use also depends on strength, density and reaction with its surroundings. Read the key supplied with a periodic table because colors and symbols vary between tables.

Use a table in a sequence: find the symbol, read atomic number, then look at group and period. Do not confuse atomic number with atomic mass. At this level, use recurring patterns as evidence for a prediction and check actual data before calling the prediction certain.

Worked examples

Avoid these mistakes

• An element symbol's letter count does not tell you its number of atoms.

Words to know

periodic table

A chart that organizes all known elements by increasing atomic number and by shared properties.

period

A horizontal row ordered by atomic number; in the introductory shell model, the period relates to the highest occupied principal shell.

group (family)

A vertical column showing related property patterns. Main-group elements often share an outer-electron count, with qualifications for helium and transition elements.

valence electron

An electron in an atom's outermost shell; it mostly determines how the atom bonds with others.

metalloid

An element with properties between metals and nonmetals, found along the zigzag line on the periodic table.

model

A representation that explains selected features and makes predictions; it leaves other details out.

evidence

Relevant observations or records used to support or challenge a claim.

system boundary

The chosen limit separating the objects being studied from their surroundings.

assumption

A condition taken as true for a model or calculation and needing to be checked against the case.

How Mendeleev predicted elements that didn't exist yet

When Dmitri Mendeleev arranged the known elements by properties in 1869, he did something bold: he left empty gaps where the pattern demanded an element that hadn't been discovered. He even predicted the properties of these missing elements. When gallium and germanium were later found and matched his predictions almost exactly, the periodic table went from a clever chart to one of science's most powerful predictive tools.

Reading the table: periods and groups

The table isn't just a list — its shape is the information. Each row (period) tells you how many electron shells an atom has: period 3 elements have 3 shells. Each column (group) shares the same number of valence (outer) electrons, which is why they behave alike. Group 1 metals all have 1 valence electron and all react violently with water; Group 18 (noble gases) all have full outer shells, which is why they barely react at all.

The families you should know

Alkali metals (Group 1) are so reactive they're stored in oil so they don't react with air. Halogens (Group 17) are one electron short of full, so they're 'hungry' and grab electrons — great at forming salts (sodium + chlorine → table salt). Noble gases (Group 18) are the loners with full shells: stable, unreactive, and used where you need something that won't react, like argon in lightbulbs. Transition metals in the middle (iron, copper, gold) are the everyday metals — good conductors, strong, and often colorful in compounds.

Why the table predicts reactivity

Atoms 'want' a full outer shell (usually 8 electrons — the octet rule). An element's spot on the table tells you how it will get there. Sodium (1 valence electron) easily loses that electron; chlorine (7 valence electrons) easily gains one. Put them together and sodium hands its electron to chlorine — that transfer is a chemical reaction, and it's completely predictable from their positions. This is the bridge from the periodic table into the next unit on chemical reactions.

Metals, nonmetals, and the staircase

A zig-zag 'staircase' line on the right side of the table separates metals (left, most of the table — shiny, conductive, bendable) from nonmetals (right — dull, brittle, poor conductors). The elements hugging that line, like silicon and germanium, are metalloids: they act in-between, which is exactly what makes them perfect semiconductors for the computer chips in every phone and laptop.

In the real world

Chemists use the periodic table's patterns to predict how newly created or rarely-seen elements will behave, even before testing them.

Exit check

What fixes the order shown? Explain the relevant mechanism and one model limitation.

Materials: Properties, Uses & Impacts

Learning goals

• I can connect a material’s measured properties to a particular use.
• I can explain how a synthetic material comes from natural resources and evaluate a trade-off.
• I can compare synthetic-material resources, properties and impacts using distinct sources.

Readings for this unit

• A container that works once

A material is useful because of its properties. A raincoat needs to resist water and bend; a cooking pan should transfer thermal energy and keep its shape. There is no single “best” material. A choice depends on the task, the evidence, and limits such as cost, mass, or durability.

People make synthetic materials by processing resources and, often, using chemical reactions to create substances with different properties. Many plastics start with chemicals obtained from petroleum or natural gas. Some are made from plant-derived resources. “Synthetic” describes how a material is made; it does not by itself tell us whether the material is harmful or helpful.

Evaluate the whole use: where the resources come from, energy used in production, useful lifetime, and what happens afterward. A reusable container may reduce waste if it is used many times. A material that lasts a long time can be useful in a building yet create a disposal problem when discarded. Support comparisons with evidence and identify information you still need.

Start a material choice by naming the job and constraints. A raincoat must keep water out and allow movement; a bridge cable needs strength under tension. Rank the important properties before comparing materials, because one useful property may come with an unwanted trade-off.

Use measurements rather than labels such as natural or synthetic as a substitute for evidence. Record how many times a material can be reused, how much mass is needed and how it behaves in the intended environment. Production methods can change the properties of materials made from similar resources.

A life-cycle comparison includes resource extraction, manufacture, use and disposal. Compare equivalent jobs, such as carrying the same groceries for a year. A durable product used only once may lose the benefit of durability. State missing evidence and explain how it could change the decision.

Build your understanding

Where this fits

This unit helps answer the chapter question: How can the properties of a material help us explain it and choose a useful application? Keep a notebook table with the question, evidence, explanation and one unresolved issue. Revise a previous model only when you can name the new evidence or idea that changes it.

Use three teaching sessions of about 40–45 minutes: first observe the phenomenon, draft a model and study Parts 1–2; next explain Parts 3–4 with the figures and evidence; then work the application, examples, practice, model revision and exit check. Divide any difficult part into smaller steps and revisit after feedback. Investigations, review and quizzes have separate sessions.

Part 1 · Define the job before choosing a material

A material is useful when its properties match a job. Begin with criteria such as strength, flexibility, insulation, or water resistance. Constraints are limits such as cost or total mass. A material that succeeds in one job can fail in another, so the word best needs a stated purpose.

Read the visual model

What is the difference between a criterion and a constraint?

Visual reasoning — after your attempt

A criterion is a desired performance, such as preventing leaks. A constraint is a limit, such as a maximum mass or budget, that a proposed solution must meet.

A food container may need to resist leaks and repeated washing. A window must transmit visible light. List must-have requirements separately from desirable features before reading a comparison table; otherwise a convenient feature can hide a failure of the main job.

Pause and explain

What is the difference between a criterion and a constraint?

Check your reasoning after answering

A criterion is a desired performance, such as preventing leaks. A constraint is a limit, such as a maximum mass or budget, that a proposed solution must meet.

Part 2 · Link structure and processing to properties

Materials may come directly from resources or be processed into new forms. Synthetic describes manufacture, not an automatic judgment about safety or usefulness. Processing, composition, and structure can affect flexibility, strength, or durability, so materials from a similar resource need not behave the same way.

Read the visual model

Structure describes how materials are arranged; a property describes their measured response under conditions. Processing and assembly can change the arrangement. A useful design explanation connects the feature to a particular mechanism and a measurable outcome.

Why is “two walls means no heat can enter” not supported? Name a remaining pathway and a relevant test.

Visual reasoning — after your attempt

Radiation or conduction through contacts can remain, and moving air may contribute. Compare matched temperature-versus-time records and other design criteria instead of claiming all energy transfer stops.

A rigid plastic container and a flexible plastic film can serve different jobs. The label plastic alone does not identify a single property set. Compare measurements for the actual product and its intended temperature and use conditions.

Pause and explain

Why is natural versus synthetic insufficient for choosing a container?

Check your reasoning after answering

Those labels do not supply evidence of leakage, durability, safety in intended use, or disposal impacts. The actual material properties and use conditions matter.

Part 3 · Compare an equivalent amount of service

A life-cycle comparison considers extraction, manufacture, transport, use, and disposal. Choose a common service before comparing totals, such as carrying the same load for 100 trips. A reusable product needs enough reuse to realize its advantages; repair, cleaning, and replacement can affect the comparison.

Read the visual model

Why compare 50 trips rather than one container of each kind?

Visual reasoning — after your attempt

One container of each may deliver different amounts of service. Comparing the same number of trips makes the performance and resource comparison meaningful.

If one container lasts 50 trips and another lasts 5, compare the number needed for the same 50 trips. Their masses and production demands may differ, and mass discarded is only one environmental measure, not a complete impact score.

Pause and explain

Why compare 50 trips rather than one container of each kind?

Check your reasoning after answering

One container of each may deliver different amounts of service. Comparing the same number of trips makes the performance and resource comparison meaningful.

Part 4 · Revise a design from measured evidence

Test relevant properties using a repeatable method and a safe sample. Keep load, exposure, and test time comparable. Record failures as well as successes. Select a design that satisfies required criteria, then explain the trade-off and a missing measurement that could change the choice.

Read the visual model

A design criterion is a requirement for success; a constraint is a limit such as cost or size. Use the graph to evaluate the stated leak criterion, then test whether the same revision still meets the other requirements.

At three minutes, how much less water has B leaked? What further check is needed before claiming the improvement is reliable?

Visual reasoning — after your attempt

A leaked 6 mL and B 1.5 mL, a difference of 4.5 mL. Repeat matched tests and consider variation; a single modeled comparison does not establish all design criteria.

For a leak test, use the same water volume and observation interval. For repeated-use data, record how many trials each sample survives. One successful test supports only that tested condition; it does not prove unlimited durability.

Pause and explain

How could an unequal test load make a comparison misleading?

Check your reasoning after answering

A weaker load may make one material appear more durable. Comparable loads and methods are needed to attribute a difference to the material or design.

Natural resources and different plastics

A flexible film and a rigid plastic container can both be called plastic yet serve very different purposes.

A natural resource is material or energy obtained from the environment, such as petroleum, natural gas, minerals or plant matter. Synthetic means made through human processing, often with chemical changes. The distinction describes origin and manufacture, not an automatic judgment that natural is safe or synthetic is harmful. A processed natural resource can become a new material with properties suited to a task.

Many plastics are polymers, materials containing long molecular chains. Chain composition, arrangement, branching, interactions and additives can change strength, flexibility and response to warming. An additive is an ingredient included to alter performance or processing. We do not need to memorize every plastic code to understand why all plastics are not equivalent. Some soften enough to reshape on warming, while others have networks that resist such reshaping; neither group should be heated or burned in a home identification test.

Evaluate a material using more than one source and the same useful service. A manufacturer’s durability claim, a measured reuse record and a disposal guide answer different questions. Ask who produced each source, which conditions it tested, what evidence it provides and what it omits. Include benefits such as clean-water delivery or durable equipment as well as concerns about resource use, production, waste and exposure. A balanced explanation assesses evidence and affected people, not whether the learner prefers a brand.

Evidence to interpret

Illustrative source	Record	Limit
Performance trial	Film: 1 service; rigid container: 30 services	Matched task; production energy not measured
Disposal information card	Different products need different collection/processing	Local acceptance must be verified; not a durability test

Explain and apply

Use the supplied two source records to explain one benefit, one concern and a needed follow-up for a film/container choice. Do not treat a resource label as a full safety or impact rating.

• Connect a synthetic material to a resource and processing.
• Use two different source records.
• Assess benefits, concerns and missing evidence fairly.

Reasoning and marking guidance — after your attempt

The reuse record supports longer tested service for the rigid container, while the disposal record warns that acceptance depends on product and local processing. Resource origin alone does not establish total impacts. Compare the same task, include cleaning and replacement, and obtain specific production/disposal information. Describe chains and processing as a plausible property explanation rather than proof from the label plastic.

Apply the whole unit · Choose a container for repeated use

Illustrative product data: A has mass 20 g, survives 5 trips, and passes the leak test; B has mass 60 g, survives 30 trips, and passes the same test; C has mass 35 g, survives 30 trips, but leaks. The job requires 30 trips without leaks.

Your task

Choose a qualifying design, calculate the material mass needed to provide 30 trips for A and B, and discuss why this calculation alone cannot establish the environmentally best option. Propose a useful follow-up test.

What your response should show

• Use the supplied observations and measurements accurately; include units where needed.
• Explain the science behind your answer, rather than giving only a number or label.
• Show a labeled model, calculation, or evidence comparison as requested.
• State an assumption or limitation and suggest a useful next measurement.

Response guidance — after your attempt

C fails the required leak criterion. A needs 6 containers totaling 120 g; B needs 1 totaling 60 g under the stated lifetimes. B uses less supplied product mass for this service. Manufacturing energy, cleaning, transport, disposal, and variation in lifetime are missing, so this is not a complete environmental ranking. Repeat leak and durability tests under the intended conditions.

Scientific references and adapted teaching resources

• OpenSciEd 7.1 Chemical Reactions & Matter — public unit overview
• OpenSciEd 6.2 Thermal Energy — public unit overview
• OpenSciEd instructional model — storyline routines
• Creative Commons Attribution 4.0 — middle-school material license

Parent / teacher teaching notes

Have the learner mark disqualifying criteria first, then calculate equivalent service. Separate a supported choice for this job from a claim about all environmental impacts. The application uses supplied data; physical testing is optional and should use safe containers. Adaptation credit: OpenSciEd middle-school teaching routines and public task structures by OpenSciEd, CC BY 4.0. This independent version adds original explanatory text, data and figures, terminology support and a home-study schedule. It does not reproduce the complete teacher editions or their classroom validation. The illustrated study “Natural resources and different plastics” is required curriculum content. Teach it before the associated application or distribute it over the assigned unit sessions; it is not an optional enrichment. Its practice/review/quiz response provides evidence of understanding. Model/data work must not be recorded as an actual physical experiment.

Worked examples

Avoid these mistakes

• Natural does not automatically mean harmless, and synthetic does not automatically mean harmful.

Words to know

synthetic material

A material made through human-directed processing, often involving chemical reactions.

constraint

A limit that a design must satisfy.

trade-off

A benefit gained at the cost of another desirable feature.

model

A representation that explains selected features and makes predictions; it leaves other details out.

evidence

Relevant observations or records used to support or challenge a claim.

system boundary

The chosen limit separating the objects being studied from their surroundings.

assumption

A condition taken as true for a model or calculation and needing to be checked against the case.

In the real world

Exit check

Which stage is measured by a one-minute leak test? Explain the relevant mechanism and one model limitation.