How does the particle model explain density, internal energy and the pressure of a gas?
Density and its calculation, the particle model of the three states, internal energy and changes of state, specific heat capacity and specific latent heat, and the link between gas temperature and pressure.
A focused answer to the OCR Gateway GCSE Combined Science A topic P1 on matter, covering density and its calculation, the particle model of the three states, internal energy and changes of state, specific heat capacity, and the link between the temperature and pressure of a gas.
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What this topic is asking
OCR wants you to calculate density, use the particle model to describe the three states, explain internal energy and changes of state, use specific heat capacity, and explain the link between the temperature and pressure of a gas.
Density
Density is measured in kilograms per cubic metre (kg/m cubed) or grams per cubic centimetre (g/cm cubed). To find it, measure the mass on a balance and the volume (by measurement for a regular shape, or by displacement of water in a measuring cylinder for an irregular object), then divide. The particle model explains differences in density: a solid has its particles packed closely together, so a lot of mass fits in a small volume and the density is high; a gas has its particles far apart, so the same number of particles takes up a much larger volume and the density is much lower. This is why most gases are far less dense than solids and liquids.
Internal energy and changes of state
The particle model also explains energy in matter. The internal energy of a substance is the total kinetic energy (from the movement of the particles) plus the potential energy (stored in the forces between particles).
During melting or boiling the temperature stays constant even though energy is still supplied, because that energy overcomes the forces holding the particles together rather than speeding them up. Changes of state are physical changes, so the mass is conserved and the change can be reversed.
Gas temperature and pressure
A gas exerts pressure because its particles collide with the walls of the container. Each collision pushes on the wall, and the total effect of many collisions per second is the pressure. When a fixed mass of gas in a sealed container (so the volume is fixed) is heated, the particles gain kinetic energy and move faster, so they collide with the walls more often and with greater force. Both effects increase the pressure, so for a fixed volume the pressure of a gas increases as its temperature increases. This is why a sealed container of gas can burst if it is heated strongly.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20184 marksA block of metal has a mass of 480 g and a volume of 60 cm cubed. Calculate its density in g per cm cubed, and explain how the particle model accounts for a gas having a much lower density than a solid.Show worked answer →
A Physics Paper 5 calculation plus explanation. Method: density g per cm cubed. Markers credit the correct rearrangement and answer with units. For the explanation: in a solid the particles are packed very closely together, so a lot of mass is contained in a small volume, giving a high density; in a gas the particles are far apart with large spaces between them, so the same number of particles occupies a much larger volume, giving a much lower density. Reward the link from particle spacing (close versus far apart) to mass per unit volume.
OCR 20214 marksExplain, using the particle model, why the pressure of a fixed mass of gas in a sealed container increases when it is heated.Show worked answer →
A P1 question on gas pressure. Reward: heating the gas transfers energy to the particles, so they move faster (gain kinetic energy). The faster particles collide with the walls of the container more often and with greater force. Pressure is caused by these collisions with the walls, so more frequent and harder collisions mean a greater pressure. Because the container is sealed (fixed volume and mass), the only change is the increased speed of the particles. Markers credit the particles gaining energy and moving faster, more frequent and harder collisions with the walls, and the link from collisions to pressure.
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