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 or a liquid, use a displacement can or measuring cylinder. Then divide.

Your turn — density

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

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

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 bonds are broken.

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

Where gas pressure comes from

Gas molecules have random motion. Each time one hits a wall it bounces off, exerting a tiny force. Billions 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

Equation to job

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

The equations to know

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