5 Solids, liquids and gases

Use the units degree Celsius (°C), kelvin (K), joule (J), kilogram (kg), kilogram per cubic metre (kg/m³), metre (m), square metre (m²), cubic metre (m³), metre per second (m/s), metre per second squared (m/s²), newton (N) and pascal (Pa).

Use the unit joule per kilogram degree Celsius, J/(kg °C).

Know and use density = mass ÷ volume, ρ = m/V.

Practical: investigate density using direct measurements of mass and volume

Know and use pressure = force ÷ area, p = F/A.

Understand how the pressure at a point in a gas or liquid at rest acts equally in all directions

Know and use pressure difference = height × density × gravitational field strength, p = hρg.

Explain why heating a system will change the energy stored within the system and raise its temperature or produce changes of state

Describe the changes that occur when a solid melts to form a liquid, and when a liquid evaporates or boils to form a gas

Describe the arrangement and motion of particles in solids, liquids and gases

Practical: obtain a temperature–time graph to show the constant temperature during a change of state

Know that specific heat capacity is the energy required to change the temperature of an object by one degree Celsius per kilogram of mass (J/kg °C)

Use change in thermal energy = mass × specific heat capacity × change in temperature, ΔQ = mcΔT.

Practical: investigate the specific heat capacity of materials including water and some solids

Explain how molecules in a gas have random motion and that they exert a force, and hence a pressure, on the walls of a container

Understand why there is an absolute zero of temperature, which is –273 °C

Describe the Kelvin scale of temperature and be able to convert between the Kelvin and Celsius scales

Understand why an increase in temperature results in an increase in the average speed of gas molecules

Know that the Kelvin temperature of a gas is proportional to the average kinetic energy of its molecules

Explain, for a fixed amount of gas, the qualitative relationship between:

• pressure and volume at constant temperature
• pressure and Kelvin temperature at constant volume

Use p₁/T₁ = p₂/T₂ for the pressure and kelvin temperature of a fixed mass of gas at constant volume.

Use p₁V₁ = p₂V₂ for the pressure and volume of a fixed mass of gas at constant temperature.