How can we explain the differences in physical properties between the three states of matter?
The answer is in terms of particles! Remember, everything is made up of particles, whether a solid, liquid or a gas.
Solids have a regular shape, the particles are packed in a regular pattern.
Solids cannot be compressed, the particles are as close together as they can possibly get.
Solids are dense, there is very little space between the particles.
Liquids take up the shape of the bottom of a container in which you place them, the particles are not in any fixed position.
Liquids are very difficult to compress, as the particles cannot get much closer.
Liquids are quite dense, there is not much empty space between the particles.
Gases have no shape, they fill up whatever space is available as theparticles move rapidly around in all directions.
Gases are easily compressed, the particles have large spaces between them, so it is easy to push them closer together.
Gases have very low densities, there are huge amounts of space between particles.
Compressing a gas
Due to the large spaces between particles in a gas you can force the particles closer together using a plunger.
Remember liquids and solids cannot be compressed as their particles are already close together.
The diffusion of gases
The lighter the particles of gas, the faster the gas will diffuse.
Example: A particle of ammonia gas (the pink particle) has about half the mass of a particle of hydrogen chloride (the grey particle), so it will diffuse faster. This is shown in the experiment below. The cloud formed when the two gases meet is nearer to the hydrogen chloride than the ammonia.
How gas pressure changes with temperature
When you raise the temperature of a gas you increase the number of collisions that the particles have with one another and the container. They hit the container wall with more force and more often. So gas pressure increases as temperature increases for a fixed mass of gas.
How gas pressure changes with volume
If you reduce the size of the container i.e. the volume of the gas, the number of collisions between the gas particles and the container wall increases since they have less space to move within. Hence, as you decrease volume of a gas you increase its pressure, for a fixed mass of gas.
How gas volume changes with temperature
When a gas is heated the particles gain more energy and therefore move around quicker bouncing off each other and the container wall with more energy. This means that they take up a greater volume as they bounce further from one another. This shows that increasing the temperature of a gas increases its volume, for a fixed mass of gas.
When a solid is heated, its particles gain more energy and vibrate more. Due to the increase in vibrations, the solid expands. At melting point, the particles vibrate so much that they break away from their positions. It is at this point that a solid becomes a liquid.
When a liquid is supplied with heat, its particles gain more energy and therefore move around quicker. This increase in movement causes the liquid to expand. At boiling point the liquid particles gain enough energy to overcome the forces holding them together - these particles break away from one another and the liquid now becomes a gas.
Not all particles in a liquid contain the same amount of energy - some have more! This is why evaporation of a liquid can take place below its boiling point. This is called evaporation and explains why puddles dry up on sunny days.
When you cool a gas, the particles lose energy. This loss in energy causes the particles to move more slowly as they move closer together. When the particles no longer have sufficient energy to move away as they bump into one another the gas becomes a liquid.
When a liquid is cooled, the particles once more lose energy, they vibrate less as they slow down. Eventually they stop moving, except for vibrations in fixed positions. A solid has now formed.