States of Matter Lesson Plan with a Simulation

Updated 2026-10-02

This states of matter lesson plan uses a free particle simulation to answer the question behind every heating and cooling activity: what are the particles actually doing? Students heat a solid until it melts and boils, watch the particles break out of their lattice, compare four substances, and find out why water needs so much more heat than neon. You get learning goals, setup, predictions, a step-by-step sequence with the values students should see, a five-question set for the class link, a heating-curve extension, and ideas for differentiation. Every number below comes from the simulations' own model code.

Lesson at a glance

Level: grades 6–9 (ages 11–15). The interaction potential part also works in grades 10–12.
Time: one 50–60 minute period, plus an optional 15-minute extension.
Prior knowledge: solids, liquids and gases from everyday life; temperature. No equations needed.
Format: pairs on laptops or tablets, or the whole class with one projector.
Simulation: States of matter – solids, liquids, gases and phase changes. Extension: States of matter – particle model on heating and cooling.

Learning goals

By the end of the lesson, students can:

Describe the arrangement and motion of particles in a solid, a liquid and a gas.
Explain melting and boiling as particles gaining enough energy to overcome their attraction.
Link the strength of attraction between particles to how high a substance's boiling point is.
Predict how squeezing a hot gas changes its pressure.
(Extension) Explain why the temperature stays flat on a heating curve while a substance melts or boils.

What the simulation does

Every particle moves under real forces: attraction at a distance, repulsion up close (the Lennard-Jones model). Nothing is pre-animated. You choose neon, argon, oxygen or water; argon is the default.

It has three screens:

States: a sealed container of 80 particles that starts as a solid. Drag the Heat / cool slider toward Hot or Cold, or use the Solid, Liquid and Gas quick-set buttons. +10 particles and −10 particles change the amount, up to 100. The temperature appears in kelvin above the box, and a status line describes the state: "Particles vibrate in place about fixed positions — solid."
Phase changes: 40 particles under a piston. Drag the grey bar to compress or expand, pump in more particles (up to 120), and heat or cool. A gauge shows the pressure in atm, and a small phase diagram marks the current state.
Interaction potential: the energy curves of all four substances on one graph. Drag Distance r from 200 to 900 pm to read the potential energy U between two particles. The status line gives the bottom of the well for the chosen substance.

Three model details are worth knowing before class:

The state label comes from the temperature. Argon is labelled liquid from about 42 K and gas from about 78 K. These cut-offs are scaled from each substance's attraction, so they rank the substances sensibly but don't match real melting and boiling points. Real argon melts at 84 K and boils at 87 K. Tell students to trust the particles more than the label.
The temperature flickers. It comes from the particles' speed, so it jumps by several kelvin even with the heater off, and the label can flicker near a change of state. Treat readings as approximate.
Water is drawn as one round particle. Its stronger attraction stands in for hydrogen bonding. That makes a good "what does the model leave out?" question.

States of matter – solids, liquids, gases and phase changes

Materials and setup before class

Materials: one device per pair (or a projector), the record table below on paper, and a reminder that 0 °C is 273 K.

Setup (10 minutes, once):

Open the simulation. The defaults are the States screen, argon, 80 particles, starting as a solid.
Click Share and create a link for each class, for example "Science · Period 3". The link pins these starting values.
Optional: on the Questions tab, enter the question set below and attach it to the link.
Post the link, or open it in present mode and show the QR code so students join on their own devices.

Lesson sequence

1. Hook and predictions (7 minutes)

Hold up an ice cube and ask: "The water molecules in this ice and in steam are identical. So what is different?" Collect a few answers. Then students commit to two predictions, on paper or on the link:

P1. "When a solid melts, its particles… get bigger / break out of fixed positions and slide past each other, still close together / spread far apart and fill the container / stop moving."
P2. "Neon, argon, oxygen and water all start as solids. Which must be heated to the highest temperature before it becomes a gas?"

Expect "the particles get bigger" from some students in P1. Don't correct anyone yet.

2. Solid, liquid, gas (12 minutes)

On the States screen, pairs set the heater to about 30% heating and watch argon warm up from around 25 K. They pause with Pause whenever the status line changes and write one sentence about the motion:

Argon	Temperature (about)	What the particles do
Solid	below 42 K	vibrate about fixed places in a regular pattern
Liquid	42–78 K	slide past each other but stay together at the bottom
Gas	above 78 K	fly freely and hit every wall

Keep heating. Above about 239 K the lid opens by itself and particles escape; the readout counts them. The heater stops at about 359 K. Then cool the box back down and watch the gas condense and the solid re-form.

Ask: "Did any particle get bigger?" No. Only the motion and the spacing changed. Check P1 now.

3. Same heater, different substances (10 minutes)

Choosing a substance restarts the box as a solid. Pairs heat each substance from solid and note the temperature at which the status line first says gas:

Substance	Labelled liquid from	Labelled gas from	Depth of potential well
Neon	about 13 K	about 23 K	3.10 meV
Oxygen	about 39 K	about 73 K	9.70 meV
Argon	about 42 K	about 78 K	10.30 meV
Water	about 138 K	about 256 K	34.00 meV

Leave the last column blank on the student copy; it comes from step 4. Neon goes first, water last, by far. Check P2.

Note: real oxygen boils at 90 K, just above argon's 87 K, so the model swaps those two. Strong students enjoy spotting that.

4. Interaction potential: why boiling points differ (10 minutes)

Open Interaction potential. All four curves share one graph. The readout starts at "argon · r = 380 pm · U = −10.28 meV". Students drag Distance r to find the lowest point of each curve:

Substance	Bottom of the well	U at the bottom
Neon	308 pm	−3.10 meV
Argon	383 pm	−10.30 meV
Oxygen	388 pm	−9.70 meV
Water	354 pm	−34.00 meV

Questions to ask:

"What happens to U when you pull the particles further apart than the bottom of the well?" It rises toward zero: they attract, and you must do work to separate them.
"And when you push them closer?" U shoots up: they repel.
"How many times deeper is water's well than neon's?" 34.00 ÷ 3.10 ≈ 11. A deeper well means more energy is needed to pull particles apart, so the boiling point is higher.

Tick Mark the mean distance from the States screen while the States screen holds a solid. The marker lands close to the bottom of the well: in a solid, particles sit about where the attraction and repulsion balance.

5. Phase changes: squeeze the gas (8 minutes)

Open Phase changes. It has its own 40 particles, starting as a solid, so heat them until they fill the box. Pairs drag the piston down slowly and watch the gauge. The pressure rises as the particles hit the walls more often. Then they heat while the piston is low. Above about 59 atm the container bursts with "💥 Burst — overpressure!", and Reset screen brings a new one. Give two minutes of free play, then ask: "Why does a hot gas burst the container sooner than a cold one?"

6. Exit check (5 minutes)

Use questions 3–5 of the set below. Then open View answers and show the Prediction and After columns for P1 and P2 side by side. The formative assessment guide shows how to read those columns across classes.

Question set for this lesson

Enter these on the simulation's Questions tab. Suggested Instructions for students: "Start on the States screen. Choosing a substance restarts the box as a solid. Temperatures flicker, so read them as approximate."

1. Multiple choice · Before, as a prediction · Ask again after the simulation

Question: "When a solid melts, what happens to its particles?"
Options: They get bigger / They break out of fixed positions and slide past each other, still close together (correct) / They spread far apart and fill the container / They stop moving
Explanation: "The particles stay the same size. Heating makes them move faster until they break out of the regular pattern. In a liquid they still touch and attract each other, but they can slide past one another. Spreading out to fill the container is what happens when the liquid boils."

2. Multiple choice · Before, as a prediction · Ask again after the simulation

Question: "Neon, argon, oxygen and water all start as solids. Which one must be heated to the highest temperature before it becomes a gas?"
Options: neon / argon / oxygen / water (correct)
Explanation: "Water particles attract each other far more strongly than the others. Its potential well is 34.00 meV deep against 3.10 meV for neon. More energy is needed to pull the particles apart, so water turns into a gas at a much higher temperature."

3. Number · After the simulation

Question: "On the Interaction potential screen, find the bottom of the well for water and for neon. How many times deeper is water's well?"
Answer: 11, tolerance ± 0.5
Explanation: "The bottom of water's well is at U = −34.00 meV and neon's at U = −3.10 meV. 34.00 ÷ 3.10 ≈ 11. The attraction between water particles is about eleven times stronger in this model."

4. Multiple choice · After the simulation

Question: "On the Phase changes screen, you push the piston down on a hot gas. What happens to the pressure?"
Options: It falls / It stays the same / It rises, and the container can burst (correct) / It drops to zero
Explanation: "The same particles now have less room, so they hit the walls more often and the pressure rises. A hot gas pushes harder because its particles move faster. Above about 59 atm the container in the simulation bursts."

5. Short answer · After the simulation

Question: "Use the particle model to explain why water boils at a higher temperature than neon."
Accepted answers (optional): leave empty and read the answers yourself.
Model answer: "Boiling means the particles move fast enough to escape their neighbours' attraction. Water particles attract each other much more strongly than neon particles (a deeper potential well), so they must move faster, which means a higher temperature, before they break free."

Questions 1 and 2 are the predictions, asked again after the simulation. For more on writing questions like these, see writing good questions for virtual labs.

Extension: the heating curve

The second simulation shows what a thermometer records during a change of state. It is simpler, uses °C and suits grades 6–7.

States of matter – particle model on heating and cooling

It opens with water at 20 °C and starts heating at once, at 15 °C per second. Students watch the particles and the temperature–time graph together:

After about 5.3 seconds the water reaches 100 °C.
The graph stays flat at 100 °C for 2.5 seconds while the particles switch to gas one by one.
Then the temperature climbs again.

Press Cool to run it backward: condensing at 100 °C, then freezing at 0 °C, each with its own flat section. Press Hold on the flat section and ask: "The heater was on the whole time. Where did the energy go?" It went into pulling the particles apart, not into speeding them up.

Then use the T: slider to compare the four substances at the same temperature:

Substance	Melts	Boils	State at −50 °C	State at −200 °C
Water	0 °C	100 °C	solid	solid
Ethanol	−114 °C	78 °C	liquid	solid
Mercury	−39 °C	357 °C	solid	solid
Oxygen	−219 °C	−183 °C	gas	liquid

One limitation to discuss: both flat sections last 2.5 seconds for every substance. Real water needs about seven times more energy to boil than to melt, so its boiling plateau should be much longer.

Differentiation

Support:

Use only argon and water on the States screen, and the Solid, Liquid and Gas buttons instead of the slider.
Give sentence starters: "In the solid, the particles… When I heated it, they…"
Start with the heating-curve simulation, which uses °C and labels each change of state.

Stretch:

Explain why the temperature flickers even with the heater off.
Compare the model's labels with real boiling points (neon 27 K, argon 87 K, oxygen 90 K, water 373 K). What does the model get right, and what doesn't it?

English learners: the simulation is available in six languages. Create a second link in the student's language so the labels are familiar while the discussion stays in English.

Standards alignment

This lesson matches NGSS MS-PS1-4: develop a model that predicts and describes changes in particle motion, temperature and state of a pure substance when thermal energy is added or removed. It fits the particle model topic of GCSE Combined Science and introductory high school chemistry.

For more chemistry activities, see interactive chemistry lesson ideas. For routines such as pair work and projector demos, see how to use interactive simulations in the classroom. To run steps 1–2 as a full Predict–Observe–Explain cycle, see Predict–Observe–Explain with simulations.

FAQ

Why does the simulation use kelvin?

Particle motion depends on absolute temperature, and the substances here change state far below 0 °C. Give students the rule "kelvin = °C + 273". The heating-curve extension uses °C if your class isn't ready.

Why do the particles sit at the bottom of the box?

The simulation adds a little gravity, so a liquid settles like a real one. The gas still fills the whole container.

Can students use phones?

Yes. Students open the link without an account, and the sliders, buttons and piston work on touch screens. Water has the most work to compute, so on an older phone it may run a little slower.