0620 · 1.1Solids, liquids and gases

Particle model of the states

0:00An ice cube keeps a fixed form, liquid water takes the shape of its container, and water vapour spreads throughout available space. These are three states of the same substance. The differences come from the arrangement and movement of particles. By the end of this video, you will understand changes of state, temperature graphs, and the behaviour of gases. Our starting point is the particle model. All substances consist of tiny particles, which may be atoms, molecules or ions.

0:25For this topic, we can represent them using simple circles. The circles show positions and movement, not the actual appearance of particles. Forces of attraction between particles help explain why substances behave differently. A solid has a fixed shape and a fixed volume. Its particles are very close together in a regular arrangement. They vibrate about fixed positions rather than moving freely from place to place. Attractions hold them in those positions, so the solid keeps its shape.

0:51A solid does not flow. The particles in a solid are not completely motionless. Vibration means moving backwards and forwards around a position without travelling through the substance. A solid is difficult to compress because its particles are already closely packed. Compression means reducing volume by applying pressure. There is very little space available for the particles to move closer together. A liquid has a fixed volume but no fixed shape.

1:15Its particles are close together, but their arrangement is irregular. They move around and slide past one another. This allows a liquid to flow and take the shape of its container. Like a solid, a liquid is difficult to compress because its particles are already close together. A gas has neither a fixed shape nor a fixed volume. Its particles are far apart and arranged randomly. They move rapidly in random directions and spread throughout the available space.

1:38A gas therefore fills its container and can flow. Unlike solids and liquids, gases are easily compressed because there are large spaces between their particles. Imagine transferring a liquid between containers without spilling any. Its shape changes, but the volume remains the same. A gas behaves differently because it spreads throughout a larger container. The particles themselves do not become larger. Instead, the average separation between them increases, so the gas occupies more space.

Changes of state and energy

2:06Melting is the change from solid to liquid, and freezing is the change from liquid to solid. Boiling and evaporating both change a liquid into a gas. Condensing changes a gas into a liquid. During these changes, the particles remain those of the same substance. Their arrangement, separation and movement change instead. Evaporation happens at the surface of a liquid and can occur below its boiling point. A puddle can therefore disappear without boiling.

2:30Boiling happens throughout a liquid at its boiling point for a particular pressure. Bubbles of the substance in its gaseous state form within the liquid. These bubbles rise and escape at the surface. Forgetting that surface distinction is a common examination mistake. Evaporation does not require the whole liquid to reach its boiling point. Boiling produces vapour bubbles throughout the liquid, not just particles escaping from its surface. Also, bubbles in boiling water contain water vapour, not air produced by the heating process.

2:59This next part is for the Extended paper. Kinetic particle theory explains these changes using particle movement and energy. Kinetic energy means the energy a particle has because it is moving. Temperature is related to the average kinetic energy of particles. When temperature rises, their average kinetic energy increases. Heating a solid initially makes its particles vibrate more vigorously about their fixed positions. At the melting point, supplied energy overcomes enough attraction for particles to leave those positions.

3:27They can then move past one another, so the substance becomes liquid. The particles remain close together, which explains why melting does not produce a gas. Heating a liquid increases the average kinetic energy of its particles. At the boiling point, supplied energy overcomes the attractions holding particles close together. The particles separate widely, forming a gas. They then move freely in random directions. During boiling, the energy supplied changes the state rather than increasing the temperature.

3:53Particles in a liquid do not all have the same kinetic energy. During evaporation, some particles at the surface have enough energy to overcome attractions and escape. This can happen even below the boiling point. The escaping particles carry energy away. The remaining liquid can therefore cool, which explains the cooling effect of evaporation. Cooling a gas reduces the average kinetic energy of its particles. As they move more slowly, attractions can bring them close together, producing a liquid.

4:20This is condensation, and energy is released to the surroundings. During freezing, particles become held in fixed positions as attractions establish an ordered arrangement. They still vibrate, but can no longer slide past one another.

Heating and cooling curves

4:32A heating curve shows how temperature changes while a substance receives energy. Time is on the horizontal axis, and temperature is on the vertical axis. For a pure substance at constant pressure, changes of state produce horizontal sections. Follow the curve from left to right. Each sloping section represents warming within one state. The first rising section shows the solid warming, with particles vibrating more vigorously. The first flat section shows melting, with solid and liquid present together.

4:59Energy is still being supplied, but it overcomes forces between particles instead of increasing their average kinetic energy. The temperature therefore stays constant until all the solid has melted. The next rising section shows the liquid warming until it reaches its boiling point. The second flat section represents boiling, with liquid and gas present together. Supplied energy overcomes attractions so particles can separate widely, while temperature remains constant. Once all the liquid has become gas, further heating increases the gas temperature.

5:29Reading across from the first flat section gives the melting point, and the second gives the boiling point. A cooling curve follows the reverse sequence. The gas cools and then condenses into a liquid. The temperature of the liquid falls until it freezes, and then the solid cools further. During condensation and freezing, energy is released as forces of attraction pull particles together, so the temperature stays constant. Condensation occurs at the boiling point, and freezing occurs at the melting point under the same pressure.

5:57Let us work through a heating curve as an examination example. A pure substance starts as a solid and is heated until it becomes a gas. The question asks you to identify the first flat section and explain its constant temperature. Begin by finding its position in the sequence. Because it follows the solid warming, it must represent melting. Next, identify what happens to the supplied energy. It overcomes forces of attraction that hold particles in fixed positions.

6:23It does not increase the average kinetic energy during the flat section. The temperature therefore remains constant while the substance melts. This explanation connects the graph shape to the particle behaviour, rather than simply naming a change. A flat section does not mean that heating has stopped or that the particles have stopped moving. Energy transfer continues throughout the change of state. Also, the particles themselves do not melt or expand.

6:47The substance changes state because the arrangement and movement of its particles change. Keep particle identity separate from the behaviour of the whole substance.

Gas volume, temperature and pressure

6:56The volume of a gas depends on both temperature and pressure. For a fixed amount of gas at constant pressure, increasing temperature increases volume. Cooling the gas decreases its volume. The constant pressure condition matters because the gas must be able to expand or contract. A sealed rigid container cannot change its volume when heated. For a fixed amount of gas at constant temperature, increasing pressure decreases its volume. Reducing pressure allows its volume to increase.

7:22A sealed syringe provides a simple example. Pushing the plunger compresses the trapped gas into a smaller space. The gas contains the same number of particles, but their average separation decreases. This next part is for the Extended paper.

Gas pressure and volume changes

7:36Gas pressure results from particles colliding with the walls of their container. Each collision exerts a force on the wall. Many collisions together produce pressure. We can explain changes in gas volume by connecting particle speed, collisions and the movement of a container boundary. When a gas is heated, its particles gain average kinetic energy and move faster. Before expansion, they strike the walls more frequently and with greater force.

8:00In a cylinder with a movable piston, the increased gas pressure pushes the piston outwards. The gas expands until its pressure balances the external pressure again. Its final volume is larger. Cooling produces the opposite effect because the particles move more slowly. Their collisions initially produce less pressure inside the cylinder. The external pressure pushes the piston inwards, reducing the gas volume. As the space decreases, wall collisions become more frequent.

8:26The piston settles when the internal and external pressures balance again. Increasing external pressure pushes the piston inwards and reduces the space available to the gas. At constant temperature, the particles retain the same average kinetic energy. They reach the walls more frequently because the distances across the container are shorter. This raises the internal pressure until it balances the increased external pressure. The result is a smaller volume. A common mistake is to say that compression makes the particles smaller.

8:53Their size does not change; the spaces between them decrease. Another mistake is to say that particles must move faster when pressure increases. In this comparison, temperature is constant, so their average kinetic energy remains unchanged. More frequent wall collisions explain the increased pressure. An examination question might ask you to explain why heating a gas increases its volume at constant pressure. Start with the temperature change, then describe particle motion and collisions.

9:20Finally, connect the collisions to expansion. Avoid writing only that particles spread out, because that describes the result without explaining its cause. The particles gain average kinetic energy. They move faster. Before expansion, they collide with the walls more frequently and with greater force. The gas expands until its pressure equals the constant external pressure. Its volume is therefore larger. The central idea is that particle arrangement and movement explain the properties of each state.

9:47Solids have closely packed particles vibrating in fixed positions. Liquid particles remain close but move past one another. Gas particles are widely separated and move rapidly in random directions. These differences explain shape, volume, flow and compressibility. Changes of state involve changes in particle arrangement and the attractions between particles. Flat sections on heating and cooling curves show energy transfer without a temperature change. A gas expands when heated at constant pressure and contracts when pressure increases at constant temperature.

10:17Always connect your explanation to particle movement, separation and collisions, rather than treating the particles themselves as changing size.