Density = mass packed into a volume

Eduqas opens this topic by asking you to define density and explain why the three states differ. Density is the mass per unit volume:

Because a gas has its particles spread far apart, the same mass fills a far bigger volume — so gases have a much lower density than solids or liquids of the same substance.

Units tip: Eduqas expects you to work in both kg/m³ and g/cm³ (water ≈ 1000 kg/m³ = 1 g/cm³), and you may need to convert between them: 1 g/cm³ = 1000 kg/m³.

Why are gases least dense?

Measuring density

To find a density you need a mass (from a balance) and a volume. How you get the volume depends on the shape:

• Regular solid (e.g. a cube): measure the sides with a ruler and calculate volume = length × width × height.
• Irregular solid (e.g. a stone): lower it into a measuring cylinder of water and read the rise in volume — the water it displaces equals the object's volume.
• Liquid: mass an empty measuring cylinder, add a known volume, re-mass, and subtract to find the liquid's mass.

Your turn — find a density

Why is a litre of steel so heavy?

Where does the energy go when water boils?

Melting, boiling — and mass is conserved

When a substance melts, freezes, evaporates, condenses or sublimates, it changes state. These are physical changes, not chemical ones:

• No new substance is made — the particles are simply rearranged.
• Mass is conserved: 1 kg of ice melts into exactly 1 kg of water.
• They are reversible — cool the steam and you get the same water back (ice → water → steam → water → ice).

Watch out: a chemical change (like burning) makes a new substance and isn't easily reversed — a change of state always lets you recover the original substance.

Does the mass change?

The energy stored inside matter

Heating a substance raises its internal energy — the total energy stored by its particles. It comes in two parts:

• the kinetic energy of the particles (how fast they vibrate or move) — this sets the temperature;
• the potential energy from the bonds and spacing between particles — this changes during a change of state.

Spec note: on the Eduqas course the terms and equations for internal energy, specific heat capacity and specific latent heat are formally introduced in Topic 1 (Energy). They sit naturally alongside changes of state, so we revisit them here — and the next three screens are flagged as the Topic 1 link.

Specific heat capacity

While a substance stays in one state, adding energy raises its temperature. How much energy depends on the mass and the material's specific heat capacity, c:

The specific heat capacity is the energy needed to raise the temperature of 1 kg of a substance by 1 °C. Water's is high (4200 J/kg°C), so it is slow to heat and slow to cool.

Your turn — specific heat capacity

Specific latent heat & the heating curve

During melting or boiling the temperature stops rising, even though energy is still flowing in. That energy is breaking bonds and pulling particles apart. The energy needed per kilogram is the specific latent heat:

Don't mix them up: use Q = mcΔθ on the slopes (temperature changing, one state) and Q = mL on the plateaus (state changing, temperature constant).

Your turn — specific latent heat

Which equation, and why?

Tap an item on the left, then its correct match on the right.

Where does the energy go when water boils?

Why does squashed air push back harder?

Pressure comes from particle collisions

In a gas the particles fly around at high speed in random directions. Each time one hits a wall it pushes on it. Billions of these tiny pushes per second add up to gas pressure:

Raising the temperature gives the particles more kinetic energy, so they move faster. They hit the walls harder and more often, so at constant volume the pressure rises as the temperature rises.

Heating a sealed can

Pressure acts at right angles to a surface

Gases can be compressed or expanded when the pressure changes. The pressure produces a net force at right angles to any surface it touches:

Key idea: the same pressure pushes outwards on every wall of a container, always perpendicular to the surface — never along it.

Your turn — force from pressure

Squeeze a gas and the pressure rises

Keep the temperature constant and change the volume of a fixed mass of gas. Squeeze it into a smaller space and the same particles hit the walls more often, so the pressure goes up. Pressure is inversely proportional to volume:

Your turn — pV = constant

Why does squashed air push back harder?

Pump it and it warms up

Pumping up a bicycle tyre, the bottom of the pump gets noticeably warm. Pushing the piston in does work on the gas: that energy is transferred to the internal energy of the gas particles, so their temperature rises.

Why is the pump warm?

The equations to know

You've covered the key ideas in Eduqas Topic 2 — density and its practical, states & changes of state, gas pressure and the gas laws — plus the internal-energy ideas it shares with Topic 1. Press Finish to see your score.