States of Matter & the Particle Model — Print · Grade 6 Science

Chapter: Matter, Mixtures & Water Properties

States of Matter & the Particle Model

Learning goals

I can describe the arrangement, spacing and motion of particles in each state.
I can read a melting point and a boiling point as evidence about attraction between particles.
I can explain why temperature stops rising while a substance is melting.

Readings for this unit

The Clear Liquid and the Missing Salt

Before this unit

Read the chapter opening and the named observation or diagram. Clarify one word or unit that blocks understanding. Recall the relevant earlier measurement, matter, or energy idea.

Study the unit across three 35–45-minute class sessions. Use the worked examples after an attempt and the supplied records when actual equipment is not available.

Oxygen is a gas in this room, but cool it to minus 183 degrees Celsius and it becomes a liquid, and at minus 218 degrees Celsius it becomes a solid. Iron is a solid here, but heat it past 1538 degrees Celsius and it melts, and past 2862 degrees Celsius it boils. Neither substance changes into anything else along the way.

That tells you the state is not a property of the material but of the material at a particular temperature. Water is familiar in all three states because its melting point of 0 degrees Celsius and boiling point of 100 degrees Celsius happen to sit either side of the temperatures we live in, which is a fact about our planet rather than about water.

The particle model explains all of it with one idea. Particles attract each other, heating makes them move faster, and a state change happens when the movement finally overcomes the attraction. The temperature at which that happens measures how strong the attraction was.

Build your understanding

Where this fits

This chapter opens the study of matter and everything in it rests on the particle model introduced here. The previous chapter ended by insisting that every model comes with a limit, and this one has a famous one you will meet in the fourth section. Properties, chemical change and mixtures all follow in the next three lessons.

Use three flexible 40-45-minute sessions: the three states and what distinguishes them; melting and boiling points as evidence; what happens during a change of state, then the practice set, revision and the exit check. Workshop, review, quiz and exams have separate time.

Part 1 · Three arrangements, one set of particles

In a solid the particles are packed closely in a fixed arrangement and vibrate about their positions without swapping places, which is why a solid keeps both its shape and its volume. In a liquid the particles are still close together, almost as close as in the solid, but they are no longer locked in position and can slide past one another, so a liquid keeps its volume but takes the shape of its container. In a gas the particles are far apart, typically something like ten times further apart than in the liquid, and move freely in all directions, so a gas has neither a fixed shape nor a fixed volume and expands to fill whatever it is in.

The spacing is what you can test most easily. Push the plunger of a sealed syringe full of air and it moves a long way, because there is a great deal of empty space between gas particles for them to be pushed into. Do the same with a syringe full of water and it barely moves at all, because the particles in a liquid are already nearly touching and there is almost nothing to close up. That single comparison is strong evidence for the model: something in the gas is mostly empty, and something in the liquid is not, and you can feel the difference through your thumb.

Nine dots in each panel represent the same modeled amount. The solid dots vibrate near stable positions, liquid dots move past one another, and gas spacing is larger. Dots, sizes, container spacing, and exact positions are simplified, not an actual particle photograph.

Pause and explain

A sealed syringe of air compresses easily and one of water hardly at all. What does this show about the two states?

Check your reasoning after answering

That a gas contains a great deal of empty space between its particles while a liquid does not. Pushing the plunger closes gaps, so a large movement means large gaps were available. In the liquid the particles are already nearly touching, leaving almost nothing to compress, which is why the volume barely changes however hard you push.

Part 2 · Reading a melting point

A melting point is a number you can look up, and it is better understood as a measurement of how strongly the particles of that substance hold on to each other. Oxygen melts at minus 218 degrees Celsius, which tells you its particles barely attract at all, so even the feeble motion left at that temperature is enough to break the solid apart. Water melts at 0 degrees Celsius and iron at 1538 degrees Celsius, so iron's particles must be gripping each other far more tightly, since the particles have to be moving enormously faster before the arrangement gives way.

The same reasoning applies to boiling points, which measure the attraction that remains in the liquid. Here is a comparison worth sitting with: ethanol melts at minus 114 degrees Celsius and boils at 78 degrees Celsius, while water melts at 0 and boils at 100. Ethanol boils at a lower temperature than water even though both are ordinary liquids at room temperature, and that difference is why ethanol evaporates off your hand so much faster. A list of melting and boiling points is not a set of facts to memorise; it is a table of how firmly different substances hold themselves together. One caution about reading such a table: the values assume ordinary air pressure. Water boils at 100 degrees Celsius at sea level, but on the summit of Mount Everest, where the air is far thinner, it boils at about 71 degrees Celsius, which is why food cannot be properly cooked in boiling water up there. A fixed point is fixed only when the conditions are stated.

Pause and explain

Iron melts at 1538 degrees Celsius and oxygen at minus 218. What does the difference tell you?

Check your reasoning after answering

That the attraction between iron particles is vastly stronger. Melting happens when particle motion overcomes the forces holding the fixed arrangement together, so a substance needing an enormous temperature has strong attractions, while one that melts far below room temperature has very weak ones. The melting point is effectively a measurement of that strength.

Part 3 · The flat part of the graph

Heat a beaker of crushed ice steadily and record the temperature every half minute, and the graph does something that surprises most people. The temperature climbs from minus 10 degrees Celsius up to 0, and then it stops climbing and stays at 0 for several minutes while the ice turns to water, even though the heater is still running. Only once all the ice has melted does the temperature start rising again, towards 100 degrees Celsius, where it flattens out a second time while the water boils.

The energy has not gone missing. Temperature measures how fast the particles are moving on average, and during melting the energy arriving from the heater is going entirely into pulling particles out of their fixed arrangement rather than into speeding them up. The attractions have to be broken before the particles can move faster, and breaking them takes a considerable amount of energy: melting 1 kg of ice takes about 334,000 J, which is roughly the same as heating that water from 0 to 80 degrees Celsius once it has melted. The same thing happens, on a larger scale, at the boiling point. Turning 1 kg of water at 100 degrees Celsius into steam at 100 degrees Celsius takes about 2,260,000 J, nearly seven times the energy needed to melt the same mass of ice. That is why steam causes far worse burns than boiling water: it releases all of that energy again on your skin as it condenses.

Pause and explain

Why does the temperature stay at 0 degrees Celsius while ice is melting, even with the heater running?

Check your reasoning after answering

Because the incoming energy is being used to break attractions rather than to speed particles up. Temperature reports average particle speed, so while energy goes into pulling particles out of the fixed arrangement the reading does not change. Only when all the ice has melted does further energy start increasing the speed again.

Part 4 · What the simple model leaves out

The model as described predicts that any solid should be denser than its own liquid, because the particles are packed more closely. For almost every substance that is exactly right: solid wax sinks in melted wax, and solid iron sinks in molten iron. Water is the famous exception. Liquid water has a density of about 1.00 g/cm3 and ice about 0.92 g/cm3, so ice floats, and the simple model has made a clear prediction that the world contradicts.

The correction is to add the detail the simple picture left out. As water freezes, its molecules lock into an open six-sided arrangement that holds them further apart than they sit when sliding freely as a liquid, so the solid takes up more room than the same mass of liquid. This is worth more than a footnote: because ice floats, a lake freezes downward from the surface and keeps liquid water beneath it, which is why fish survive winters that would otherwise freeze a pond solid from the bottom up. Freezing water also expands with enough force to split things open. Water sitting in a crack in a rock expands by about 9 per cent as it freezes and levers the crack wider, a process that breaks mountains apart over many winters and is the reason burst pipes are a feature of cold weather. You will meet this again in chapter 5 under the name of weathering.

Pause and explain

Why does water freezing to ice contradict the simple particle model, and why does it matter beyond the classroom?

Check your reasoning after answering

The simple picture says solids pack more tightly and so should be denser, yet ice at 0.92 g/cm3 is less dense than liquid water at 1.00 and floats. The missing detail is the open six-sided structure water forms on freezing. It matters because floating ice makes lakes freeze from the top down, leaving liquid water below where fish can survive.

Apply the whole unit · Predict the state

A substance has a melting point of minus 39 degrees Celsius and a boiling point of 357 degrees Celsius. A second substance melts at 801 degrees Celsius and boils at 1465 degrees Celsius. Room temperature is about 20 degrees Celsius.

Your task

State what each substance is at room temperature and compare the attractions between their particles.

What your response should show

Places room temperature correctly against both fixed points for each substance
Identifies the state of each substance from that comparison
Ranks the attractions using the melting points as evidence
Names the boiling point each substance must pass and notes the temperature plateau

Response guidance — after your attempt

For each substance, compare room temperature with its two fixed points and name the state.
Say which substance has the stronger attractions between particles, with your reason.
Describe what you would have to do to each substance to turn it into a gas.

For the first substance, 20 degrees Celsius sits above its melting point of minus 39 and well below its boiling point of 357, so at room temperature it is a liquid; this is mercury. For the second, 20 degrees Celsius is far below its melting point of 801, so it is a solid; this is common salt. The second substance has much stronger attractions between its particles, because it takes a temperature of 801 degrees Celsius to shake them out of their fixed arrangement while the first gives way at minus 39, a difference of 840 degrees Celsius in the energy needed to break the solid apart. To turn the first into a gas you would heat it past 357 degrees Celsius; to turn the second into a gas you would have to reach 1465 degrees Celsius, and in both cases the temperature would hold steady at the boiling point while the change of state was taking place.

Scientific references for this unit

Parent / teacher teaching notes

Begin with the literal observation, quantity, or diagram relationship. Ask the learner to explain why the evidence supports the claim, check a relevant unit or boundary, then discuss an alternative. The supplied numbers are illustrative, not real observations. A parent can read aloud and allow a spoken rehearsal before writing. Judge science and evidence rather than political, faith, or family agreement. Use the complete data option whenever an actual procedure is unavailable or unsuitable.

The key idea

Solid, liquid and gas are not three different substances but three arrangements of the same particles, differing in how closely they are packed, how strongly they hold each other and how fast they move. Every melting point and boiling point in this lesson is a measurement of how strong that holding is.

Worked examples

Example 1 · A changing state and an open boundary

Explain both records using particles and system boundaries.

Identify which sample exchanges water with the surroundings.
Distinguish melting from evaporation.
Explain what remains unmeasured.

Answer: Melting in the closed sample changes arrangement while retaining its 50 g of water. The open dish loses 6 g from the measured liquid; evaporation is a possible pathway into the air. The water has not been destroyed. Without further records, the mass loss alone does not exclude a spill or specify the evaporation rate over a particular time.

Example 2 · Transfer to a different case

A learner says a gas has no mass because it cannot be seen. Explain the error.

Identify the distinction and conditions in this new case.
Use the relevant mechanism or quantity; preserve units and the chosen boundary.
Explain why the answer fits and what further evidence would be needed.

Answer: Visibility is not the definition of matter. Gas particles have mass and occupy space; a suitable comparison or measurement can show these properties.

Example 3 · Connect properties with particle arrangement · a focused example

What mainly changes during gas compression?

State the literal condition or observation in the example.
Apply the distinction explained in this teaching part.
Check whether the claim stays within the given evidence.

Answer: Particle spacing decreases; the model does not require individual particles to shrink.

Avoid these mistakes

Misleading claim: Solid particles are completely motionless.
Misleading claim: A gas has no mass because it is invisible.
Omitting units, measurement timing, or a model boundary makes the evidence hard to interpret.
Treating illustrative classroom data as real field measurements or copying a model is not an independent explanation.

Check your understanding

Does a lower water level prove less water?

Discuss the answer

No. Container shape affects height; compare volume with an appropriate method before claiming loss.

What mainly changes during gas compression?

Discuss the answer

Particle spacing decreases; the model does not require individual particles to shrink.

Must water boil before it can evaporate?

Discuss the answer

No. Surface evaporation occurs below boiling; its rate depends on conditions such as temperature, airflow, and exposed area.

Why is the mass unchanged in this model?

Discuss the answer

The same water remains within the boundary; state and arrangement change, not the amount of matter.

Sustained scientific explanation and revision

These activities are scheduled in the parent plan. Save a first response, a correction, and the reason for the change in the science notebook.

Activity 1 · States of Matter & the Particle Model · Model and explain

Assigned reading and preparation

Read The Clear Liquid and the Missing Salt and the linked unit teaching; use its complete supplied case and diagrams.

Your task

For States of Matter & the Particle Model, study Parts 3–4 and attempt the worked case: Explain both records using particles and system boundaries. Explain the steps before opening the model.

Success criteria

An accurate scientific claim answering the actual question.
Relevant evidence or a labeled model with quantities and units when applicable.
Reasoning that connects evidence with the mechanism rather than only repeats it.
A specific method, boundary, scale, or alternative-explanation limit.

Hint

Begin with the actual observation, quantity, or diagram relationship. Explain how it bears on the question before making a broad claim.

Response guidance — discuss after your attempt

Melting in the closed sample changes arrangement while retaining its 50 g of water. The open dish loses 6 g from the measured liquid; evaporation is a possible pathway into the air. The water has not been destroyed. Without further records, the mass loss alone does not exclude a spill or specify the evaporation rate over a particular time.

Activity 2 · States of Matter & the Particle Model · Practice, revise and exit

Assigned reading and preparation

Read The Clear Liquid and the Missing Salt and the linked unit teaching; use its complete supplied case and diagrams.

Your task

For States of Matter & the Particle Model, complete practice questions 1–6 and both choice checks, then answer independently: A learner says a gas has no mass because it cannot be seen. Explain the error. Save a correction explaining one changed claim or method.

Success criteria

An accurate scientific claim answering the actual question.
Relevant evidence or a labeled model with quantities and units when applicable.
Reasoning that connects evidence with the mechanism rather than only repeats it.
A specific method, boundary, scale, or alternative-explanation limit.

Hint

Begin with the actual observation, quantity, or diagram relationship. Explain how it bears on the question before making a broad claim.

Response guidance — discuss after your attempt

Visibility is not the definition of matter. Gas particles have mass and occupy space; a suitable comparison or measurement can show these properties.

Words to know

particle model: A simplified explanation of matter using tiny moving constituents and their arrangement.
evaporation: A change from liquid to gas at a surface, possible below boiling.
evidence: A relevant observation, measurement, or record used to evaluate a claim.
system boundary: The chosen limit specifying what is included and which exchanges cross it.
uncertainty: A stated limit in measurement or knowledge, distinct from a careless mistake.

In the real world

The particle model explains the whole water cycle: the Sun's heat evaporates water from oceans and lakes into vapor, which then cools and condenses high in the sky to form the clouds that bring us rain and snow.

Exit check

A learner says a gas has no mass because it cannot be seen. Explain the error.

Check your explanation after answering

Visibility is not the definition of matter. Gas particles have mass and occupy space; a suitable comparison or measurement can show these properties.

Explain your reasoning without notes. If a key idea is missing, revisit the lesson and try again.