Three states of matter

The kinetic theory model pictures all matter as tiny particles in constant motion. Edexcel wants you to describe each state by the arrangement, movement and energy of its particles:

• Solid — particles packed in a regular, fixed pattern, touching, vibrating about fixed positions. Lowest energy. Fixed shape and volume.
• Liquid — particles still touching but randomly arranged, able to slide past each other. More energy. Fixed volume, takes the shape of its container.
• Gas — particles far apart, random, moving quickly in all directions. Most energy. Fills any container; easily compressed.

Which state?

Density

Density tells you how much mass is packed into a given volume. It depends on the mass of the particles and how closely packed they are:

Because particles are packed tightly in a solid and a liquid but spread far apart in a gas, the same substance is usually densest as a solid and least dense as a gas.

Watch your units. The SI unit is kg/m³, but lab data often comes in g/cm³. Use the same units throughout, and remember 1 g/cm³ = 1000 kg/m³ (water is 1.0 g/cm³ = 1000 kg/m³).

Your turn — density

Measuring density

Edexcel's Core Practical is to investigate the densities of solid and liquids. You always need a mass (balance) and a volume.

• Liquid: place a measuring cylinder on a balance, zero it, pour the liquid in. Read the mass and the volume straight off.
• Regular solid: weigh it, then measure its sides and calculate the volume.
• Irregular solid: weigh it, then find its volume by displacement — lower it into water and measure the volume of water pushed out (e.g. with a eureka/displacement can).

Why is a litre of steel so heavy?

Where does the energy go when water boils?

Changes of state are physical

Melting, freezing, boiling, evaporating, condensing and subliming are all changes of state. They are physical changes, not chemical:

• Mass is conserved — no particles are created or destroyed, so the mass stays exactly the same.
• They are reversible — the material recovers its original properties if the change is reversed (unlike many chemical changes, which make new substances).

Misconception: ice melting to water is not a chemical reaction. It's still H₂O — cool it down and the water freezes straight back to ice with its original properties. Mass in = mass out.

Physical, not chemical

Heating changes stored energy

The internal energy of a substance is the total energy stored in its particles (their movement and their positions). When you heat a system, that stored energy increases, and one of two things happens:

• the particles speed up, so the temperature rises; or
• at a melting or boiling point, the energy goes into breaking bonds between particles — a change of state — and the temperature stays constant while it does.

Misconception: the temperature does not keep climbing while something melts or boils. During a change of state the temperature pauses even though you keep supplying energy.

Specific heat capacity

The specific heat capacity, c, is the energy needed to raise the temperature of 1 kg of a substance by 1 °C. To warm a mass m by a temperature change Δθ:

Water has a high specific heat capacity (4200 J/kg°C), so it takes a lot of energy to warm up and is slow to cool — which is why it makes a good coolant and hot-water bottle.

Your turn — heat capacity

Investigating water

The second Core Practical is to investigate the properties of water: determine the specific heat capacity of water and obtain a temperature–time graph for melting ice.

• Heat a known mass of water with an electric immersion heater, using a joulemeter (or power × time) for the energy ΔQ supplied and a thermometer for the temperature change Δθ.
• Rearrange ΔQ = m c Δθ to get c = ΔQ ÷ (m Δθ).
• For melting ice, plot temperature against time: the line is flat at 0 °C while the ice melts, then rises again once it is all liquid.

Why your value comes out a bit high: some energy escapes to the surroundings instead of going into the water, so the measured c is usually a little larger than the true 4200 J/kg°C. Insulating the beaker reduces this — the same idea as thermal insulation (14.10): a lid and a layer of a poor conductor such as foam or trapped air cut unwanted energy transfer, just as loft and cavity-wall insulation do in a house.

Specific latent heat

To melt or boil a substance you must keep supplying energy, yet its temperature does not change. The specific latent heat, L, is the energy needed to change the state of 1 kg of a substance without changing its temperature:

This energy goes into separating the particles (breaking bonds), not into making them move faster — so the thermometer stays still during melting or boiling.

Analogy: latent heat is like paying to unlock the particles from one another. While you are paying the "unlock fee", the temperature pauses; only once they're free does heating start to raise the temperature again.

Your turn — latent heat

Where does the energy go when water boils?

Why does squashed air push back harder?

Where gas pressure comes from

Gas particles fly about randomly and constantly collide with the walls of their container. Each collision pushes on the wall; the combined effect of billions of collisions per second is the gas pressure.

Analogy: imagine hundreds of bouncy balls drumming on the inside of a box. You don't feel the individual hits — you feel a steady outward push. That steady push is gas pressure.

Heat a gas, raise its pressure

The temperature of a gas is a measure of the average kinetic energy of its molecules. Heat a fixed mass of gas in a sealed, rigid container (constant volume) and:

• the particles gain kinetic energy and move faster;
• they hit the walls harder and more often;
• so the pressure rises.

This is a qualitative link for Edexcel: higher temperature → faster particles → more frequent, harder collisions → higher pressure (volume kept constant).

Reading the collisions

Absolute zero & the kelvin scale

Absolute zero is −273 °C — the lowest possible temperature, where particles have the least possible movement (their kinetic energy is as low as it can be).

The kelvin temperature scale starts there: 0 K = −273 °C. To convert between the scales:

Your turn — converting temperatures

Squeezing a gas (separate Physics)

A gas can be compressed or expanded by changing the pressure, and its pressure produces a net force at right angles to any surface. For a fixed mass of gas at constant temperature:

• Squeeze the gas into a smaller volume → particles hit the walls more often (less distance to travel) → higher pressure.
• Let it expand → collisions become less frequent → lower pressure.

Your turn — pressure & volume

Why does squashed air push back harder?

Doing work on a gas warms it

Pushing in a piston (or pumping up a tyre) does work on the gas. That work is transferred to the particles, increasing their kinetic energy — so the gas gets hotter, even with no flame in sight.

This is why a bicycle pump feels warm after you've used it: as you compress the air, you do work on it and its temperature rises.

Spot the state

Tap the state of matter that each description fits best.

Which equation?

Tap a situation, then tap the equation you'd use.

Squeezing at constant temperature

The equations to know

You've covered the key ideas in Edexcel Topic 14 — density and states of matter, changes of state, internal energy, specific heat capacity and latent heat, and gas pressure including the Higher Tier work-on-a-gas idea. Press Finish to see your score.