It’s A Gas…

There are three common states of matter; solid, liquid and gas. This statement ignores other less common states of matter like plasma (very hot) or Bose-Einstein condensate (very cold). Exam questions often ask you to explain events in terms of the behaviour of the particles involved. This is easier if you can picture how particles are arranged in solids, liquids and gases. Matter is any substance that has mass and takes up space.

Solid

Solids have particles arranged in a regular pattern. They are close together, in fixed positions and vibrating. The higher the temperature, the greater are the vibrations. If the material becomes hot enough, the vibrations become enough that the particles break their bonds and it melts. Solids have a fixed shape and volume.

Liquid

Liquids have particles that are randomly arranged. They are also close together but moving freely. The higher the temperature, the faster the particles can move. Some of the particles will have enough energy to escape, this is why water will evaporate slowly even at low temperatures. At boiling point, the particles have enough energy to escape in large numbers. Liquids assume the shape of their containers but have a fixed volume.

Gas

Gases have particles that are randomly arranged. They are far apart and whizzing in all directions. The higher the temperature, the faster the particles move and the more space that they occupy. This is why gas pressure increases with temperature. Gases take the shape of their containers but can be compressed into a smaller volume.

This is a somewhat better animation of the behaviour of particles in a gas. Pump some light (as in not heavy) particles in and watch them bounce around. You can increase and decrease the temperature and see how they change.

powered by Advanced iFrame

Here is a video demonstration of the effects of air pressure on a can. It’s from Twig (so is a bit annoying) but it shows the principle clearly. Perhaps you remember seeing this demonstrated in class?

When you are asked to “describe in terms of particles”, you need to write about how the behaviour of the particles changes. So, when describing evaporation you could say that the particles go from being close together and sliding over one another to being far apart and moving very rapidly. Then, as the gas is cooled back into a liquid, you could describe the particles getting closer together again as they lose kinetic energy and the particles moving less quickly.

Don’t make the mistake of saying that the particles themselves expand or contract. The particles don’t change except that they increase or decrease in kinetic energy. The particles in a hot material have more kinetic energy than those in a cold one.

Questions…

1. What word describes…
   1. a solid turning into a liquid?
   2. a liquid turning into a gas?
   3. a gas turning into a liquid?
   4. a liquid turning into a solid?
   5. a solid changing into a gas without first becoming a liquid?
2. Name a substance that behaves in the way described in (e) above.

Embarrassing video alert…