The units you must use

Section 5 expects you to work confidently in these SI units:

• °C degree Celsius and K kelvin — temperature
• kg kilogram — mass; m, m², m³ — length, area, volume
• kg/m³ — density; Pa pascal — pressure; N newton — force
• J joule — energy; m/s and m/s² — speed and acceleration

Paper 2 only (5.2P): for specific heat capacity you also use J/kg °C (joules per kilogram per degree Celsius).

Density: how tightly packed

Density tells you how much mass is squeezed into each cubic metre of a material:

Measuring it (5.4 practical): find the mass on a balance. For a regular solid, measure its sides and calculate the volume; for an irregular solid, use a displacement can or measuring cylinder; for a liquid, pour it into a measuring cylinder. Then divide.

Your turn — density

Why is a litre of steel so heavy?

Same push, so why does only one knife cut?

Pressure: force spread over area

Pressure is the force acting on each square metre of a surface:

The same force over a smaller area gives a bigger pressure — which is why a sharp knife or a drawing pin works.

Acts in all directions (5.6): at any point in a gas or a liquid at rest, the pressure pushes equally in every direction — up, down and sideways — not just downwards.

Your turn — pressure

Same push, so why does only one knife cut?

Pressure grows with depth

Dive deeper and the weight of water above you grows, so the pressure difference increases:

Watch out: pressure in a liquid depends on depth and density, not the shape of the container or how much liquid there is in total. A narrow tube and a wide tank give the same pressure at the same depth.

Your turn — depth pressure

Why does iron warm up more than water?

Three states, three arrangements

The kinetic theory explains the states by how the particles are arranged and moving:

Heating (5.8P–5.9P): heating raises the energy stored in the particles, so the temperature rises or the state changes — a solid melts, then the liquid evaporates/boils to a gas. During a change of state the temperature stays constant while the forces between particles are overcome.

Which state?

Specific heat capacity

The specific heat capacity c is the energy needed to change the temperature of 1 kg by 1 °C:

Water has a high c (4200 J/kg °C), so it heats and cools slowly — useful in radiators and for the climate near the sea.

Practical (5.14P): heat a known mass with an electric heater, measure the energy supplied and the temperature rise, then find c = ΔQ ÷ (m × ΔT). A temperature–time graph (5.11P) shows the flat region while a substance changes state.

Your turn — thermal energy

Why does iron warm up more than water?

Why does squashed air push back harder?

Where gas pressure comes from

Gas molecules have random motion. Each time one hits a wall it bounces off, exerting a tiny force. Huge numbers of collisions every second add up to a steady pressure:

Watch out: gas pressure is not the gas "pressing because it is heavy" — it is the result of countless molecular collisions with the container walls.

What raises the pressure?

For a fixed amount of gas, tap how the molecular collisions explain each change.

Absolute zero and kelvin

Cool a gas and its molecules slow down. At −273 °C they have the least possible energy — you cannot go colder. This is absolute zero, the start of the Kelvin scale:

Absolute zero = 0 K = −273 °C. There is no such thing as a negative kelvin temperature.

Your turn — convert to kelvin

Hotter means faster

Raising the temperature gives molecules more energy, so on average they move faster. In fact:

This is why temperature is measured from absolute zero: at 0 K the average kinetic energy is (as near as possible) zero, so it makes sense to say the energy is proportional to the Kelvin temperature — that statement would fail with the Celsius scale.

Link to pressure: faster molecules hit the walls harder and more often, so for a fixed volume a higher temperature gives a higher pressure.

Temperature and motion

Squeeze it: pressure ∝ 1/volume

For a fixed mass of gas at constant temperature, squeezing it into a smaller volume packs the same molecules into less space, so they hit the walls more often and the pressure rises:

Your turn — Boyle's law

Heat it: pressure ∝ Kelvin temperature

For a fixed mass of gas at constant volume, raising the Kelvin temperature makes the molecules hit the walls harder and more often, so the pressure rises in proportion:

Watch out — use kelvin! The gas laws only work with Kelvin temperatures. Putting °C into p₁/T₁ = p₂/T₂ gives the wrong answer (and dividing by 0 °C is meaningless). Always convert first.

Your turn — the pressure law

The classic trap

Why does squashed air push back harder?

Equation to job

Tap an equation, then tap what it is used for.

The equations to know

You've covered the key ideas in 4PH1 Section 5 — units, density & pressure, change of state, and ideal gas molecules. Press Finish to see your score.