States of matter: solids, liquids and gases explained with particles

Guide · Updated 2026-11-10

How the particle model explains solids, liquids and gases, why temperature stops rising while a substance melts or boils, how kelvin works, and why gases mix faster than liquids.

Open the States of matter Free, in your browser, nothing uploaded.

Ice, water and steam are the same stuff. What changes between them is how the tiny particles inside behave. The states of matter tool shows this with a box of particles that you can heat and cool, so you can watch them shake in place, slide around or fly apart, and a second view where two colours of particles spread into each other. It suits Class 8 and Class 9 students. Afterwards, the heating curve shows the temperature graph for the same changes, kinetic theory explains how fast gas particles create pressure, and random walk looks at the maths of particles wandering.

What it is

The particle model says all matter is made of very small particles (atoms or molecules) that never stop moving. Two things decide the state:

• how strongly the particles pull on each other, and
• how much energy of movement they have, which rises with temperature.

In a solid the pull wins. Particles sit close together in fixed places and can only vibrate, so a solid keeps its shape. In a liquid the particles are still close, but they have enough energy to slip past each other: the volume stays fixed, but the shape follows the container. In a gas the particles have broken free. They are far apart, move fast in straight lines and fill any space they are given.

At the melting point and boiling point something surprising happens. You keep heating, but the temperature holds steady until the change is complete. The heat is being used to pull particles away from each other, not to speed them up.

Temperature on the kelvin scale starts at absolute zero, the coldest anything can be:

T (K) = t (°C) + 273.15

Schools often round this to 273. The average energy of movement of one particle is

E = (3/2) × k × T

where k is the Boltzmann constant, 1.380649 × 10⁻²³ J/K, and T is in kelvin. Double the kelvin temperature and the average energy doubles.

Diffusion is the spreading of one substance through another because its particles keep moving. It is faster in gases than in liquids, and faster when hot.

How to use it

1. Stay on Heat and cool and choose a Substance: water, ethanol, oxygen or iron. Its melting and boiling points appear under the controls.
2. Press Heat (+10 °C) or Cool (−10 °C), or drag the Temperature slider. The buttons pause exactly on the melting and boiling points.
3. Read the headline (such as “Liquid at 20 °C”), the sentence under it, and the facts for state, spacing and particle motion.
4. Switch the level from Class 8 to Class 9 to add the temperature in kelvin and the average energy of one particle.
5. Choose Diffusion, set the two colours to Gases or Liquids and the temperature to Cold, Warm or Hot, then press Lift the divider. The tool shows how mixed the box is and the time to reach 80%. Start again puts the divider back.
6. Use Pause to freeze the particles, and try the five Check yourself questions.

A worked example

Choose ethanol and switch to Class 9. The tool starts at 20 °C, where ethanol is a liquid. The kelvin line reads 293 K (293.15 K with the exact value), and the average particle energy is

E = 1.5 × 1.380649 × 10⁻²³ J/K × 293.15 K = 6.07 × 10⁻²¹ J

Heat it. Five presses take it to 70 °C. The sixth would go to 80 °C, but the button stops at 78 °C, the boiling point. Here T = 351.15 K and E = 7.27 × 10⁻²¹ J. One more press gives 88 °C, and the box is full of gas.

Cool it instead. From 20 °C, thirteen presses reach −110 °C. The fourteenth stops at −114 °C, the melting point, where T = 159.15 K and E = 3.3 × 10⁻²¹ J (3.296 × 10⁻²¹ J unrounded). Below that, ethanol is a solid.

Compare the two changes of state. The ratio of the unrounded energies is 7.272 / 3.296 = 2.21, the same as the ratio of kelvin temperatures, 351.15 / 159.15 = 2.21. Celsius values give nonsense here. That is why energy calculations always use kelvin.

Where it is used

• Cooking at altitude. Water boils below 100 °C high up, because lower air pressure lets particles escape more easily. Pressure cookers help there.
• Medical gases. Oxygen in hospitals is stored as a very cold liquid, because a liquid takes up far less space than the same amount of gas.
• Casting metal. Iron must be heated past 1,538 °C to melt before it can be poured into moulds.
• Smells and gas leaks. A smell crosses a room by diffusion helped by moving air. Gas companies add a smelly chemical to natural gas so leaks are noticed.

Common mistakes

1. Thinking the temperature keeps rising while ice melts. It stays at the melting point until all the ice has gone.
2. Forgetting to change to kelvin before using (3/2)kT. The formula needs absolute temperature. Celsius values give wrong or even negative energies.
3. Saying gas particles are bigger. The particles are the same size in every state. Only the spaces between them and their movement change.
4. Mixing up evaporation and boiling. Evaporation happens at the surface at any temperature. Boiling happens all through the liquid, and only at the boiling point.
5. Treating the picture as real. The box has a few dozen large particles and a simplified motion.

Questions people ask

Why does a gas fill its container but a liquid does not? Gas particles have enough energy to escape the pull of their neighbours completely, so they spread until the walls stop them. Liquid particles still hold together.

Why is oxygen a gas in the room but iron a solid? The pull between oxygen molecules is weak, so they break free far below room temperature. Iron atoms are held together very strongly, so they need a huge amount of heat to separate.