How does the particle model explain density, changes of state and pressure?
The particle model and density of materials, the changes of state and internal energy, specific heat capacity and specific latent heat, and the effect of temperature on the pressure of a gas.
A focused answer to the AQA GCSE Combined Science: Trilogy Particle model of matter topic, covering the particle model and density, changes of state and internal energy, specific heat capacity and specific latent heat, and gas pressure and temperature.
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What this topic is asking
AQA wants you to use the particle model to explain density and states of matter, describe changes of state and internal energy, use specific heat capacity and specific latent heat, and explain how temperature affects gas pressure.
The particle model and density
In a solid, particles are held in a regular arrangement by strong forces and only vibrate in place, so a solid has a fixed shape and volume and is hard to compress. In a liquid the particles are still close together but can move past each other, so a liquid flows and takes the shape of its container while keeping a fixed volume. In a gas the particles are far apart with negligible forces between them and move quickly in random directions, so a gas has no fixed shape or volume and can be compressed. Density usually decreases from solid to liquid to gas because the particles get further apart.
Changes of state and internal energy
Heating a substance has one of two effects. It can raise the temperature, which increases the kinetic energy of the particles (they move or vibrate faster). Or, during a change of state, it can increase the potential energy of the particles (overcoming the forces between them) with no change in temperature, which is why a graph of temperature against time has flat sections during melting and boiling.
Specific heat capacity and latent heat
Gas pressure and temperature
A gas exerts a pressure on the walls of its container because its particles are constantly colliding with the walls, and each collision exerts a small force. For a fixed mass of gas at constant volume, increasing the temperature increases the pressure: the particles gain kinetic energy and move faster, so they hit the walls more often and with greater force, increasing the total force per unit area. (Doing work on a gas by compressing it quickly also raises its temperature, which is why a bicycle pump warms up.)
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20193 marksCalculate the energy needed to raise the temperature of 2 kg of water by 30 degrees Celsius. The specific heat capacity of water is 4200 J/kg degrees Celsius.Show worked answer →
A Physics Paper 1 specific-heat-capacity calculation. Method: use , where is the mass, the specific heat capacity and the temperature change. Substituting, J (252 kJ). Markers award the correct equation, the substitution, and the answer with the unit. Watch that the temperature change (30) is used, not a final temperature, and that the large value is sensible because water has a high specific heat capacity, which is why it is used in heating systems.
AQA 20214 marksExplain, using the particle model, why heating a fixed mass of gas in a sealed rigid container increases the pressure of the gas.Show worked answer →
A Physics Paper 1 explanation. Reward: the particles of the gas are in constant random motion and exert a force, and therefore a pressure, on the container walls when they collide with them. Heating the gas transfers energy to the particles, increasing their average kinetic energy, so they move faster. Because the container is rigid (fixed volume), the faster particles collide with the walls both more frequently and with greater force, so the total force per unit area (the pressure) increases. Markers credit the link from heating to faster particles, to more frequent and harder collisions, to greater pressure, and reward referring to the constant volume.
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