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

States of Matter & Phase Changes

Learning goals

• I can describe how particle arrangement and motion differ in solids, liquids, and gases.
• I can name the phase changes and describe what happens to temperature and energy during each.

Readings for this unit

• Ice melts, but a cold cup gets wetter

Many everyday materials are described using three familiar states — solid, liquid, or gas — and the real difference between them comes down to how the tiny particles inside are arranged and how fast they're moving. In a solid, the particles are packed tightly and locked into fixed positions, so all they can do is vibrate in place; that's why a solid keeps its own shape. In a liquid, the particles are still close together but no longer locked down, so they can slide and tumble past one another — which is why a liquid flows and takes the shape of its container while keeping the same volume. In a gas, the particles have broken free almost entirely, zooming around with lots of empty space between them, so a gas spreads out to fill whatever container it's in.

The state of a substance depends on conditions including temperature and pressure, and on interactions between its particles. Heating can increase average particle kinetic energy and temperature, but during a phase change energy can instead change particle interactions and arrangement. Thermal energy includes particle motion and interactions; it is not simply another name for temperature. Particles do not become bigger when a material expands, and substances such as water can behave differently from a simple always-closer-on-cooling rule.

When enough energy is added or removed, a substance flips from one state to another in a phase change. Adding energy drives the changes that loosen particles: melting turns a solid into a liquid at its melting point, and boiling turns a liquid into a gas at its boiling point (evaporation does the same thing more gently, only at the surface). Removing energy runs these in reverse — freezing turns a liquid back into a solid, and condensation turns a gas back into a liquid, which is the fog you see forming on a cold bathroom mirror. Two less common shortcuts skip the liquid stage entirely: sublimation goes straight from solid to gas (dry ice does this), and deposition goes straight from gas to solid.

For a pure substance at fixed pressure, temperature can remain nearly constant during melting or boiling while energy continues to enter. During melting, solid and liquid may coexist while particle arrangements and interactions change. After the phase change is complete, further heating can raise temperature again. Real samples, mixtures, pressure changes, and unequal heating may produce a less sharply defined plateau, so check the conditions before assuming that every phase change has a perfectly flat temperature record.

This is why one substance, water, can be solid ice, liquid water, or gaseous water vapor without ever becoming a different chemical — it's the very same H₂O molecules the whole time, just arranged and moving differently. Understanding how adding or removing energy shuffles particles between these states is the key to everything from why sweating cools you down to how clouds and rain take shape in the sky.

During a phase change, particles remain the same chemical substance while their arrangement and interactions change. Melting water does not turn water molecules into hydrogen and oxygen. Particle diagrams should change spacing and organization while retaining the same labeled particles.

For a pure substance at fixed pressure, temperature can stay nearly constant during melting or boiling while energy is transferred. The energy changes interactions between particles. Outside a phase change, heating generally increases temperature. Mixtures can melt across a range instead of at one sharp point.

Evaporation happens at a liquid's surface at temperatures below boiling. Faster particles can escape, cooling the remaining liquid. Boiling occurs throughout the liquid when its vapor pressure reaches the surrounding pressure. A change in air pressure can therefore change boiling temperature.

Build your understanding

Where this fits

This unit helps answer the chapter question: How can adding or removing energy change matter? 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 · Represent the same particles in different states

In a solid, particles vibrate around relatively fixed positions. In a liquid, particles remain close but change neighbors. In a gas, particles are much farther apart on average and move through the available space. This model explains shape, flow, and compressibility without turning particles themselves into miniature solid or liquid objects.

Read the visual model

What changes when liquid water freezes, and what remains the same?

Visual reasoning — after your attempt

The molecular arrangement and motion change. The molecules remain water molecules, and the total mass stays the same if none enters or leaves the system.

Draw the same number and kind of water molecules in three equal-area panels. Change their arrangement and spacing, not their identity or drawn size. Real water expands on freezing; the familiar claim that every solid is denser than its liquid has exceptions.

Pause and explain

What changes when liquid water freezes, and what remains the same?

Check your reasoning after answering

The molecular arrangement and motion change. The molecules remain water molecules, and the total mass stays the same if none enters or leaves the system.

Part 2 · Distinguish evaporation from boiling

Evaporation occurs at a liquid surface, including below boiling temperature. Some molecules escape into the gas phase. Boiling occurs throughout the liquid when vapor pressure matches surrounding pressure. Temperature, exposed surface, and air movement can affect evaporation, so a fair comparison changes only the factor being tested.

Read the visual model

Bulk means the material away from only its outer surface. Water vapor is gaseous water and is invisible; visible mist consists mainly of small liquid droplets. The model separates these ideas without asking the learner to boil water.

Which feature distinguishes these two modeled phase changes, and why would a bubble from an unknown mixture need other evidence?

Visual reasoning — after your attempt

Surface-only escape versus vapor-bubble formation within the liquid distinguishes these cases. Unknown bubbles could arise from other processes, so gas identity and conditions must be established.

A wet towel dries at room temperature without boiling. Spread-out water can evaporate faster than the same amount in a narrow container under otherwise comparable conditions. The water molecules move into the air; they have not been destroyed.

Pause and explain

How can water disappear from an open dish without reaching 100°C?

Check your reasoning after answering

Surface molecules can escape by evaporation below boiling temperature. They enter the surrounding air as water vapor.

Part 3 · Interpret a temperature plateau

For a pure substance at fixed pressure, melting or boiling can occur at nearly constant temperature while energy is transferred. The transferred energy changes interactions and arrangement rather than simply increasing average particle kinetic energy. Real samples and mixtures may not show a perfectly flat plateau.

Read the visual model

Why can heating continue while temperature stays nearly constant?

Visual reasoning — after your attempt

During a phase change, energy can change particle interactions and arrangement while average kinetic energy, and therefore temperature, changes little.

On a temperature-time heating graph, a rising section indicates warming; a nearly flat melting section can indicate solid and liquid coexisting. A flat line does not mean the heater necessarily stopped supplying energy. Check what happened to the sample as well as the temperature.

Pause and explain

Why can heating continue while temperature stays nearly constant?

Check your reasoning after answering

During a phase change, energy can change particle interactions and arrangement while average kinetic energy, and therefore temperature, changes little.

Part 4 · Trace energy and matter through condensation

Water vapor is invisible. Visible mist contains tiny liquid droplets formed when vapor condenses. On a cold glass, water from surrounding air can condense on the outside; it need not have leaked through the glass. Condensation transfers energy from the condensing water to its surroundings.

Read the visual model

Condensation is a gas-to-liquid phase change. Keep chemical identity, water origin and energy transfer as separate questions. The container wall can be dry initially yet gain outside water without leaking from its interior.

Where do the droplets’ water and the transferred energy come from in this model?

Visual reasoning — after your attempt

The water comes from vapor in surrounding air. During condensation, energy transfers from the vapor/surrounding system toward the cooler wall; no new water is created.

Use a labeled before-and-after drawing with vapor outside the cup and droplets on the cold surface. A sealed cup of colored water can help distinguish a leak from outside condensation, but several observations are stronger than one picture.

Pause and explain

Are the visible white clouds above hot water water vapor itself?

Check your reasoning after answering

The visible cloud is mostly tiny condensed liquid droplets. Gaseous water vapor itself is invisible; the droplets form as warm moist air cools.

Apply the whole unit · Read a melting record

Illustrative record for a pure solid at fixed pressure: temperature rises from −5°C to 0°C, stays near 0°C while solid and liquid coexist, then rises to 8°C after the solid is gone. A heater supplies energy throughout.

Your task

Sketch and label the temperature-time pattern. Draw particle models for three stages, identify where phase change occurs, and explain energy transfer during the flat section. State one condition needed for this interpretation.

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

The graph rises, has a near-flat melting section, then rises again. During melting, added energy changes particle interactions while temperature changes little. Retain the same particle identities in all sketches. The interpretation assumes a pure substance, controlled pressure, and reliable temperature measurements; a mixture may show a broader melting range.

Scientific references and adapted teaching resources

• 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 a graph and particle drawing together. Ask what the graph shows directly and what the model explains. Correct any drawing that enlarges molecules during heating or turns water into different elements. 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.

Worked examples

Avoid these mistakes

• Evaporation does not require the liquid to reach its boiling point.

Words to know

solid

A state of matter with a fixed shape and volume; particles vibrate in place, tightly packed.

liquid

A state of matter with a fixed volume but no fixed shape; particles are close but can flow past each other.

gas

A state of matter with no fixed shape or volume; particles are far apart and move freely and quickly.

melting point

The temperature at which a solid turns into a liquid.

boiling point

The temperature at which a liquid turns into a gas throughout the whole liquid.

condensation

The phase change from gas to liquid.

thermal energy

Internal energy associated with random particle motion and particle interactions; temperature alone does not give its total amount.

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 hidden 'plateau' when ice melts

If you heat a beaker of ice and graph temperature over time, you'll see something strange: the temperature stops rising at 0°C even though you keep adding heat. That flat plateau is the ice melting. All the incoming energy is going into breaking the bonds holding the solid together, not raising the temperature. Only once every bit of ice is liquid does the temperature start climbing again. The same plateau appears at 100°C during boiling. This 'hidden' energy is called latent heat.

Why sweating cools you down

Evaporation is secretly a cooling machine. When the fastest-moving water molecules escape from your skin as vapor, they carry away energy, leaving the slower (cooler) molecules behind. That's why you feel cold stepping out of a pool, and why sweating works: your body uses evaporative cooling to dump heat. It's also why rubbing alcohol feels icy — it evaporates even faster than water.

The fourth (and most common) state of matter

Solid, liquid, gas — and plasma. If you heat a gas hot enough, electrons rip away from atoms, creating a glowing, electrically charged soup. It sounds exotic, but plasma is actually the most common state of matter in the universe: every star, including the Sun, is plasma. On Earth you see it in lightning, neon signs, and the flame of a candle's hottest region.

Sublimation: skipping the liquid entirely

Some substances jump straight from solid to gas without ever becoming a liquid — that's sublimation. Dry ice (frozen carbon dioxide) does this, which is why it produces that spooky fog with no puddle. It happens because at normal air pressure, CO₂ has no stable liquid form. The reverse, gas straight to solid, is deposition — it's how frost forms on a cold window overnight.

In the real world

Meteorologists track evaporation and condensation in the atmosphere to help predict clouds, fog, and rain.

Exit check

What changes without changing particle identity? Explain the relevant mechanism and one model limitation.

Temperature & Thermal Energy

Learning goals

• I can distinguish temperature from the total thermal energy of a sample.
• I can predict the direction of energy transfer between objects at different temperatures.
• I can plan separate material and mass temperature-change comparisons.

Readings for this unit

• Ice melts, but a cold cup gets wetter

Particles of matter are always moving. In a simple particle model, temperature relates to their average kinetic energy. A higher temperature means faster average particle motion for the same substance in the same phase. Temperature is an average; not every particle has the same speed.

Thermal energy depends on the amount of matter, its temperature, and the material. Two cups of water at 40°C have the same temperature as one cup at 40°C, but together they contain more thermal energy relative to the same reference state. You cannot rank the thermal energy of arbitrary objects using temperature alone.

When objects at different temperatures interact, energy transfers from the warmer object to the cooler one overall. This transfer is called heat. In an insulated system, transfer continues until the objects reach thermal equilibrium: the same temperature. Heat is energy being transferred, not a substance stored inside an object.

Temperature describes average particle kinetic energy in a simple model. Thermal energy depends on how many particles there are as well as their motion and interactions. A large warm bath can store more thermal energy than a tiny hotter droplet, even though its temperature is lower.

Heat is energy transferred because of a temperature difference. For objects in contact without an external energy supply, the warmer object transfers energy to the cooler one until thermal equilibrium is reached. The thermometer measures temperature; it does not directly measure all the stored energy.

Compare equal amounts of materials when investigating temperature change. Control starting temperature, heating time and energy input. Different materials can change temperature by different amounts under the same heating, so temperature change alone does not tell you that the energy inputs differed.

Build your understanding

Where this fits

This unit helps answer the chapter question: How can adding or removing energy change matter? 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 · Compare averages with totals

Temperature relates to average particle kinetic energy in a simple introductory model. Thermal energy also depends on amount of matter, material, and particle interactions. Objects with the same temperature need not contain the same amount of thermal energy relative to a specified reference state.

Read the visual model

Why does twice as much water at 30°C not have a temperature of 60°C?

Visual reasoning — after your attempt

Temperature describes an average rather than a sum. Adding an equal amount at the same temperature increases the amount of matter, not its average particle kinetic energy.

Two identical cups of water at 30°C have the same temperature as one cup at 30°C. Together, the two cups contain more thermal energy relative to the same reference. Doubling water amount does not double its Celsius temperature.

Pause and explain

Why does twice as much water at 30°C not have a temperature of 60°C?

Check your reasoning after answering

Temperature describes an average rather than a sum. Adding an equal amount at the same temperature increases the amount of matter, not its average particle kinetic energy.

Part 2 · Separate stored energy from heat transfer

Heat is energy transferred because of a temperature difference. In ordinary passive contact, net transfer is from warmer to cooler matter. Calling heat a substance inside an object hides the distinction between an energy store and a transfer. Insulation slows transfer; it does not manufacture energy.

Read the visual model

Heat names energy transferred because of a temperature difference. Thermal energy names an energy contribution within an account. When temperatures match, the simple model has no net heat transfer between these two objects, though microscopic exchanges can still occur.

Would simply adding the initial temperatures give the equilibrium temperature? Explain what is held comparable here.

Visual reasoning — after your attempt

No. For this ideal equal-mass, same-substance case the result is the midpoint 50 °C, not 100 °C. Amount, substance, phase and outside transfers affect the prediction.

A warm cup cools while nearby cooler surroundings gain energy. Draw an arrow from the cup to the surroundings and label it energy transfer by heating. Do not draw cold flowing into the cup as though cold were a material.

Pause and explain

What changes when a warm cup is wrapped in insulation?

Check your reasoning after answering

Insulation reduces the rate of energy transfer to cooler surroundings. It does not necessarily change the starting temperature or create thermal energy.

Part 3 · Predict thermal equilibrium with assumptions

Objects in thermal contact can approach the same temperature. A final temperature depends on masses, materials, phase changes, and energy exchange with surroundings. An arithmetic mean works only for special cases, such as equal masses of the same liquid with negligible losses and no phase change.

Read the visual model

Would 30°C necessarily result from mixing any two objects at 20°C and 40°C?

Visual reasoning — after your attempt

No. Different amounts, heat capacities, phase changes, or energy exchange with the surroundings can produce a different final temperature.

Equal masses of water at 20°C and 40°C mix in an ideally insulated container to approach 30°C. More cold water would pull the final temperature closer to 20°C. The result is a prediction from a model, not a rule for all mixtures.

Pause and explain

Would 30°C necessarily result from mixing any two objects at 20°C and 40°C?

Check your reasoning after answering

No. Different amounts, heat capacities, phase changes, or energy exchange with the surroundings can produce a different final temperature.

Part 4 · Use a thermometer rather than touch alone

Touch senses energy transfer through skin and is not a reliable thermometer. Two surfaces at the same room temperature may feel different because they transfer energy at different rates. A fair measurement uses the same thermometer procedure and enough time for the sensor to respond.

Read the visual model

A thermometer’s reading is a measurement with a method and an uncertainty. Allow the sensor to settle, read the scale correctly and note conditions. Skin sensation alone cannot supply an exact temperature or safe handling decision.

Could two objects at the same room temperature feel different to the same hand? How would you test their temperatures?

Visual reasoning — after your attempt

Yes, their energy-transfer rates with skin can differ. Compare safe, suitable thermometer readings under comparable conditions rather than relying on how they feel.

Room-temperature metal may feel cooler than wood because it transfers energy away from warmer skin more rapidly. Check actual temperature before claiming the metal started colder. Compare readings, not just sensations.

Pause and explain

Why can equally warm wood and metal feel different?

Check your reasoning after answering

Their thermal transfer properties differ. Metal typically conducts energy away from the skin faster, so sensation does not directly establish a temperature difference.

Compare heating by mass and material

Equal energy inputs do not require equal temperature changes when the samples differ in amount or material.

Temperature describes a quantity related to average particle motion; thermal/internal energy also depends on amount, phase and interactions. Heat capacity describes how much energy input is needed for a stated temperature rise under specified conditions. We compare supplied temperature records here rather than calculate every sample’s total thermal energy. A smaller temperature rise can occur because energy is distributed through more material or because the material responds differently.

To isolate mass, use the same material at different masses, matched starting temperature, phase, container conditions and energy input. To isolate material type, match mass and the other conditions while comparing materials. A source that operates for equal time may not deliver equal energy if its power or contact changes. The environment and container can also receive energy, so an apparatus account should specify the measurement boundary.

Plan and interpret the observations: record starting/final temperatures and times with a suitable thermometer, identify sample amounts and material, and keep all trials. An investigation can compare warming or cooling under an approved safe method. At home, use room-temperature/cool-water comparisons or supplied data without heaters; some quantitative comparisons require controlled equipment. State whether an unexpected change could reflect a changed input, sample or exchange with surroundings.

Evidence to interpret

Sample	Mass (g)	Starting temperature (°C)	Final temperature (°C)	Input
Water	50	20	24	Same modeled amount
Water	100	20	22	Same modeled amount
Model solid	50	20	28	Same modeled amount

Investigate or evaluate the supplied records

1. State the mass question or material question, not both together.
2. Define starting temperature, phase, measuring interval and energy-input comparison.
3. Use the supplied quantitative records or an approved supervised method; collect real readings only when the input can be justified.
4. Compare temperature changes and graph the intended factor.
5. Repeat, account for environment/container effects and evaluate a changed case.

Explain and apply

Design separate mass and material comparisons using the table. Explain the two trends and specify how a real investigation would verify matched energy input and limit environmental transfers.

• Choose separately controlled pairs.
• Explain amount/material response without equating heat with temperature.
• Specify input, tools, records and uncertainties.

Reasoning and marking guidance — after your attempt

50 versus 100 g water isolates mass in the supplied model and gives rises 4 versus 2 °C. The 50 g water versus 50 g model solid comparison isolates material and gives rises 4 versus 8 °C. Equal input does not impose equal temperature change. Verify input method, starting state, container and surroundings, repeat readings, and label the supplied values rather than claim actual collection.

Apply the whole unit · Explain a mixing prediction

Illustrative case: equal masses of liquid water at 20°C and 40°C are mixed. The prediction model assumes negligible heat loss, the same heat capacity, and no phase change. A measured final temperature is 28°C.

Your task

Predict the ideal final temperature, draw the direction of net energy transfer, and give two possible reasons for the measured difference. Explain why a larger amount of cold water would change the prediction.

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

The equal-mass ideal prediction is 30°C, with net transfer from warmer water to cooler water. A cooler container or surrounding heat loss could lower the measured value; thermometer error or unequal masses could also matter. More cold water increases its contribution and shifts the final temperature toward 20°C. Identify a measurement to distinguish these explanations.

Scientific references and adapted teaching resources

• 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 colored mass counters rather than a formula for heat capacity at first. Ask the learner to explain why the arithmetic mean is conditional. Use supplied data or comfortably warm water; avoid hot-water handling. 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 “Compare heating by mass and material” 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

• Heat and temperature are related but are not the same quantity.

Words to know

temperature

A measure related to average particle kinetic energy.

heat

Energy transferred because of a temperature difference.

thermal equilibrium

A state in which interacting objects have the same temperature.

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

Does four times the mass imply four times the temperature? Explain the relevant mechanism and one model limitation.

Gas Particles, Diffusion & Pressure

Learning goals

• I can explain diffusion and gas pressure with particle motion and collisions.
• I can test a fixed-amount, matched-temperature pressure–volume model and identify its limits.

Readings for this unit

• Ice melts, but a cold cup gets wetter

A closed bag can contain matter even when its contents cannot be seen. Air has mass and occupies space.

A gas consists of widely separated particles moving in many directions. A particle is a small constituent of matter; in air, many particles are molecules, which are groups of atoms held together.

The particles do not need a fan to begin moving. Random means their individual directions vary, not that their motion has no consequences.

Their collisions with walls produce a measurable push. We can use this model to explain spreading, squeezing and warming a gas without pretending to photograph its individual molecules.

An observation is what a person or instrument records. An inference is a proposed interpretation of those records. Keep the two separate: a graph can show a pattern while more than one mechanism might explain it. State the chosen system, or part of the world being studied, and its boundary before accounting for material or energy transfers.

Use each figure as a reasoning tool. Read its caption and labels, describe the pattern, then explain a cause. If the picture is schematic, use the stated numbers rather than measuring its printed size. Check whether its simplified conditions match the problem before applying a formula.

Build your understanding

Where this fits

This unit helps answer the chapter question: How can adding or removing energy change matter? 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 · Explain spreading without a conveyor belt

Diffusion is the spreading and mixing produced by random particle motion. If particles are initially more concentrated in one region, more tend to leave that region than enter it until the concentration differences become smaller. Concentration here means the number of particles in a given volume.

Read the visual model

Diffusion is the spread of particles through their motion. The snapshots show distributions, not each particle’s exact path. Red is a tracking label for this model, not a literal color or proof of a chemical change.

Does more even spreading mean particles were created, or that every particle moved only to the right?

Visual reasoning — after your attempt

Neither. The four marked particles remain four. Individual motion can occur in many directions while the overall distribution becomes more even.

A drop of food coloring spreads through still room-temperature water. This is a liquid example of particle motion. Stirring also mixes water, but it adds large-scale fluid movement; a still-water observation should not automatically be explained as stirring.

Pause and explain

Does diffusion require every particle to move in the same direction?

Check your reasoning after answering

No. Individual particles move in varying directions. The overall distribution changes because more particles start in one region than another.

Part 2 · Connect collisions with pressure

Gas pressure is force per unit wall area resulting from many particle impacts. We cannot see an individual impact with our eyes, but a flexible wall can move as the collective push changes. Pressure is not a substance inside the bag.

Read the visual model

Pressure is force per area. Individual particles do not need to grow or stick to a wall to explain it. More frequent or stronger impacts can change the total force delivered per area over time.

Why can gas push on every wall rather than only the wall in the highlighted example?

Visual reasoning — after your attempt

Particles move in many directions and strike all walls. The highlighted collision explains a mechanism, not the only place where gas pressure acts.

Use an empty needle-free plastic syringe, if available, and cover its tip with a fingertip while pushing gently. The trapped air resists compression. Do not force the plunger or aim the tip at anyone. Supplied data give the same reasoning opportunity without this equipment.

Pause and explain

Why does trapped air resist being squeezed?

Check your reasoning after answering

The particles have less space and collide with the walls more frequently under comparable temperature conditions, increasing the pressure.

Part 3 · Change volume while keeping the comparison fair

Volume is the space occupied by the gas. In an approximately constant-temperature model with a fixed particle number, reducing volume increases pressure. The particles become closer together on average; each particle does not become smaller. A leak changes particle number and invalidates this particular comparison.

Read the visual model

A curve means the change is not a constant subtraction for each extra volume unit. Gas particles remain the same size; less volume changes collision opportunities. Use a supplied record instead of heating a sealed container.

Predict pressure at volume 3. Why would heating while compressing prevent a clean test of volume alone?

Visual reasoning — after your attempt

The model gives 120/3 = 40 kPa. Heating changes another influential variable, so the result would no longer isolate volume at fixed temperature.

Illustrative model values are 100 kPa at 40 mL and 200 kPa at 20 mL. The pressure is absolute pressure, measured relative to a vacuum, rather than only the extra pressure above room air. Use the stated numbers as a model, not a forecast for every syringe.

Pause and explain

What stayed fixed in this volume comparison?

Check your reasoning after answering

Particle number and temperature are assumed fixed. Volume changes, while particle size does not. A leaking syringe would not meet the fixed-particle assumption.

Part 4 · Distinguish a particle model from direct evidence

Warming a sealed rigid gas container increases average particle motion and can increase pressure. A diagram represents a proposed mechanism; measurements test the predicted relationship. The model needs a fixed amount of gas and a container that does not expand substantially.

Read the visual model

An observation is a record from a person or instrument. An inference interprets it. A model gains support when predictions fit several appropriate tests; a single gauge change does not reveal every microscopic detail.

Which entries are observations, and which are inferences? Suggest a useful next test.

Visual reasoning — after your attempt

Pressure readings and the measured smaller volume are observations. More frequent impacts and unchanged particle count are model claims supported by controlled conditions. Restore volume at fixed temperature and predict the pressure response.

Do not heat closed household containers. Instead compare supplied records of the same rigid vessel at two temperatures. Keep the vessel size and particle number explicit, and ask whether a different-size vessel would be a fair temperature test.

Pause and explain

Why not compare a warm large vessel with a cool small vessel to isolate temperature?

Check your reasoning after answering

Both temperature and volume change. Either can influence pressure, so the difference would not isolate temperature.

Apply the whole unit · A compressed gas

Illustrative case for analysis, not measurements collected by the learner. Illustrative constant-temperature records give 80 kPa at 60 mL and 120 kPa at 40 mL for one fixed gas sample.

Your task

Explain the trend and test whether pressure × volume stays constant. Use the supplied conditions, show intermediate reasoning and distinguish the supported conclusion from a broader claim.

What your response should show

• Use the specified quantities and units.
• Explain an intermediate step or comparison.
• Connect the pattern to the physical mechanism.
• State an assumption or limitation.

Response guidance — after your attempt

1. Compute the first pressure–volume product: 80×60=4,800 kPa·mL.

2. Compute the second: 120×40=4,800 kPa·mL, matching the model product.

3. Compare volume and pressure: decreasing volume at fixed temperature and particle number increases wall impacts; it does not shrink particles.

4. State that a leak or changing temperature would invalidate the intended comparison.

80×60=4,800 and 120×40=4,800 kPa·mL. The smaller volume gives greater pressure while the model product stays constant. The gas particles do not shrink; more frequent wall impacts explain the increased pressure. This evidence is conditional on fixed temperature and particle number.

Scientific references and adapted teaching resources

• 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

Ask the learner to describe the phenomenon before naming a formula. Discuss each checkpoint after an attempt. Use the supplied records when equipment is unavailable and label them illustrative rather than actual measurements. The final explanation should identify a mechanism and a limit, with different defensible wording accepted. 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.

Worked examples

Avoid these mistakes

• The space between gas particles decreases; the individual molecules retain their sizes in this model.
• Do not treat a schematic drawing as a scale measurement or a universal guarantee.
• Keep the stated comparison conditions and measurement boundary explicit.
• A correct numerical result needs units and an explanation of its meaning.

Words to know

diffusion

Spreading caused by random particle motion.

concentration

Amount or number of particles per stated volume.

gas pressure

Force per unit area from gas impacts on a surface.

compression

Reduction of the space occupied by a material.

absolute pressure

Pressure measured relative to zero pressure, rather than only above local air pressure.

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

Illustrative constant-temperature records give 80 kPa at 60 mL and 120 kPa at 40 mL for one fixed gas sample.

Exit check

What changes without changing particle identity? Explain the relevant mechanism and one model limitation.

Evaporation, Condensation & a Wet Cold Cup

Learning goals

• I can account for water during evaporation and condensation using a boundary and a particle model.
• I can use a supplied dew point and plan a comparison that separates influential conditions.

Readings for this unit

• Ice melts, but a cold cup gets wetter

A dry cold cup can acquire droplets on its outside while a puddle elsewhere dries. Water has not been created in one place and destroyed in the other.

Evaporation means liquid water changes to gas at its surface; condensation means gaseous water changes to liquid. Water vapor is the gas form and is normally invisible.

Visible fog is mainly tiny liquid droplets suspended in air. Begin by drawing separate boundaries around the cup contents, the cup surface and the surrounding air.

This prevents the common mistake of assuming that outside droplets must have leaked through the cup wall.

An observation is what a person or instrument records. An inference is a proposed interpretation of those records. Keep the two separate: a graph can show a pattern while more than one mechanism might explain it. State the chosen system, or part of the world being studied, and its boundary before accounting for material or energy transfers.

Use each figure as a reasoning tool. Read its caption and labels, describe the pattern, then explain a cause. If the picture is schematic, use the stated numbers rather than measuring its printed size. Check whether its simplified conditions match the problem before applying a formula.

Build your understanding

Where this fits

This unit helps answer the chapter question: How can adding or removing energy change matter? 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 boundary to ask where water came from

A system is the part of the world chosen for analysis; its boundary separates it from the surroundings. A cup containing colored water can help distinguish inside water from water already present as vapor in the surrounding air. The color is evidence about origin, not a complete molecular photograph.

Read the visual model

Why compare with a room-temperature cup?

Visual reasoning — after your attempt

It helps test the role of cooling while keeping the cup construction similar. Clear outside water on the cold cup supports an air-water source rather than a colored leak.

Illustrative observations show blue water inside a sound cup but clear droplets outside after cooling. A room-temperature identical cup stays dry. These observations favor condensation from the air over a colored-water leak, though checking the cup for cracks remains sensible.

Pause and explain

Why compare with a room-temperature cup?

Check your reasoning after answering

It helps test the role of cooling while keeping the cup construction similar. Clear outside water on the cold cup supports an air-water source rather than a colored leak.

Part 2 · Explain evaporation below boiling

Liquid particles have a range of motion energies. Some at the surface can escape into the gas phase even while the liquid is well below its boiling temperature. Boiling produces vapor bubbles throughout liquid under suitable pressure; evaporation occurs at the surface and does not require visible bubbles.

Read the visual model

A surface is the boundary between liquid and its surroundings. Molecules in a liquid have a range of motions; an escaping molecule retains its chemical identity. Matter leaving also carries energy, linking evaporation to cooling.

Why can a puddle dry at room temperature without the whole puddle boiling?

Visual reasoning — after your attempt

Some surface particles can escape into the gas phase under those conditions. Boiling is a different bulk process involving vapor bubbles throughout the liquid.

A shallow wet tray can lose mass at room temperature. Covering it limits exchange with room air and permits vapor to return. The escaped water remains matter in the surroundings; a reduced reading for the tray is not destruction of water.

Pause and explain

Does a drying cloth need to reach 100 °C?

Check your reasoning after answering

No. Surface evaporation occurs below boiling. Water transfers from the cloth into the air, rather than vanish.

Part 3 · Explain condensation through energy transfer

When vapor contacts a sufficiently cool surface, energy transfer can favor liquid droplets. Humidity describes the water vapor in air; relative humidity compares the amount present with the saturation amount at that temperature. The dew point is the temperature at which air reaches saturation on cooling under the specified conditions.

Read the visual model

Condensation is a gas-to-liquid phase change. Keep chemical identity, water origin and energy transfer as separate questions. The container wall can be dry initially yet gain outside water without leaking from its interior.

Where do the droplets’ water and the transferred energy come from in this model?

Visual reasoning — after your attempt

The water comes from vapor in surrounding air. During condensation, energy transfers from the vapor/surrounding system toward the cooler wall; no new water is created.

If a supplied room dew point is 12 °C, a cup surface at 8 °C can collect condensation while one at 18 °C may remain dry. You do not need to calculate dew point here. Use the supplied value and distinguish surface temperature from the water temperature inside.

Evaporation and condensation can both occur at the same room temperature: particles leave a liquid surface while vapor particles can return. The net direction depends on the surroundings, vapor content and exchange conditions. There is no single universal evaporation temperature equivalent to a pure substance’s ordinary-pressure boiling point. Melting/freezing comparisons also need matching pressure and composition. This resolves the district map’s shared-temperature question without teaching that every evaporation and condensation event occurs at one fixed temperature.

Pause and explain

Which supplied cup is more likely to condense water?

Check your reasoning after answering

The 8 °C surface is below the supplied 12 °C dew point; the 18 °C surface is above it. The prediction depends on those air conditions.

Part 4 · Plan a comparison and account for energy

Air movement, exposed area and humidity can all affect evaporation. To investigate area, use equal water masses at the same starting temperature in two clean shallow vessels with different exposed areas, kept in comparable locations. Record mass changes without tasting any experimental water.

Read the visual model

Separate the outcomes: temperature measures thermal conditions and a balance measures matter remaining. A lid can affect several pathways, so use the results to evaluate the proposed mechanism rather than calling every difference proof.

At 30 minutes, how much cooler is the uncovered sample? What additional measurement tests evaporation directly?

Visual reasoning — after your attempt

It is 19.2−15 = 4.2 °C cooler. Measuring comparable mass loss tests whether more matter left the uncovered sample, while still considering other energy pathways.

Evaporation can cool the remaining liquid because escaping particles remove energy. Water and energy accounts therefore need separate arrows: a water arrow indicates where matter moves; an energy arrow shows a transfer, not a stream of a material called heat.

Pause and explain

Why is greater loss from a sunny wide tray not proof of an area effect alone?

Check your reasoning after answering

Area, illumination and possibly temperature differ. A comparison changing only intended area is needed to isolate its effect.

Apply the whole unit · Droplets and missing water

Illustrative case for analysis, not measurements collected by the learner. Illustrative records show a cold intact cup with clear outside droplets. An open tray loses 6 g while a covered matched tray loses 1 g in an hour.

Your task

Explain both observations and identify a limit of the mass comparison. Use the supplied conditions, show intermediate reasoning and distinguish the supported conclusion from a broader claim.

What your response should show

• Use the specified quantities and units.
• Explain an intermediate step or comparison.
• Connect the pattern to the physical mechanism.
• State an assumption or limitation.

Response guidance — after your attempt

1. Identify two matter boundaries: inside/outside the cold cup, and the water tray/room air.

2. Clear outside droplets on the intact cup can come from air vapor, while the tray can lose water by surface evaporation.

3. Subtract the tray losses: 6−1=5 g greater loss from the open tray in the stated hour.

4. Covering changes exchange and may change other conditions, so the comparison does not isolate every mechanism or imply water creation.

The droplets can be condensed water from surrounding air; the tray loss can be water evaporating into that air. The open tray loses 5 g more under the stated comparison. Covering changes exchange and can also change temperature or airflow, so the result does not isolate every mechanism. Neither observation requires water creation or destruction.

Scientific references and adapted teaching resources

• 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

Ask the learner to describe the phenomenon before naming a formula. Discuss each checkpoint after an attempt. Use the supplied records when equipment is unavailable and label them illustrative rather than actual measurements. The final explanation should identify a mechanism and a limit, with different defensible wording accepted. 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.

Worked examples

Avoid these mistakes

• Existing water vapor from the air can condense on the cold surface; identify its source and energy transfer.
• Do not treat a schematic drawing as a scale measurement or a universal guarantee.
• Keep the stated comparison conditions and measurement boundary explicit.
• A correct numerical result needs units and an explanation of its meaning.

Words to know

evaporation

Liquid changing to gas at its surface.

condensation

Gas changing to liquid.

water vapor

Water in the normally invisible gas phase.

humidity

Water vapor content of air, expressed with a specified measure.

dew point

Temperature at which cooling air reaches saturation under the specified conditions.

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

Illustrative records show a cold intact cup with clear outside droplets. An open tray loses 6 g while a covered matched tray loses 1 g in an hour.

Exit check

What energy pathway should a fuller model add? Explain the relevant mechanism and one model limitation.

Heating Curves & Energy During Phase Change

Learning goals

• I can interpret a heating graph’s axes, rising intervals and phase-change plateau.
• I can account for energy input during melting and distinguish temperature from a complete energy account.

Readings for this unit

• Ice melts, but a cold cup gets wetter

A thermometer can stop rising while ice melts even though energy continues to enter. Temperature describes a quantity related to average particle motion, whereas an energy transfer can also change the arrangement and interactions of particles.

A heating curve is a graph of temperature during energy input. A phase is a physical state such as solid or liquid.

A plateau is a nearly level section of a graph. These words name patterns we can observe; they do not replace the explanation.

Use supplied data for melting rather than an unsupervised boiling experiment.

An observation is what a person or instrument records. An inference is a proposed interpretation of those records. Keep the two separate: a graph can show a pattern while more than one mechanism might explain it. State the chosen system, or part of the world being studied, and its boundary before accounting for material or energy transfers.

Use each figure as a reasoning tool. Read its caption and labels, describe the pattern, then explain a cause. If the picture is schematic, use the stated numbers rather than measuring its printed size. Check whether its simplified conditions match the problem before applying a formula.

Worked examples

Avoid these mistakes

• Continued input can change phase fraction and particle interactions while temperature remains nearly steady.
• Do not treat a schematic drawing as a scale measurement or a universal guarantee.
• Keep the stated comparison conditions and measurement boundary explicit.
• A correct numerical result needs units and an explanation of its meaning.

Words to know

heating curve

Graph of temperature during energy input.

phase

A physical state or distinct material region, such as solid or liquid.

plateau

Approximately level portion of a graph.

latent heat

Energy per amount of substance associated with a phase change at nearly steady temperature.

coexistence

Presence of two phases together under the stated conditions.

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

Illustrative ice–water data remain near 0 °C for four minutes while a heater supplies 100 J each minute.

Exit check

What changes during the level interval? Explain the relevant mechanism and one model limitation.