Solids, Liquids and Gases
One of the commonest substances on earth is water and we are well aware of its presence all around us in liquid form. Yet it is very easy to change the liquid state of water into a gas by boiling it in a kettle until it becomes steam, or to change it into a solid as ice in a refrigerator. Why do these three states of the same substance have such very different physical properties? The explanation appears to lie in the fact that particles of matter attract each other.
This in itself is hard to believe, for in the ordinary way we do not sense that all the particles around us, making up the matter in our world, have any forces acting between them.
While magnets will attract each other if they are brought together in the right way - and really strong ones can be made to jump at each other along a table - it would be most surprising if the cups, plates, knives and forks on the dinner table jumped at each other in this way. The forces of attraction between particles of matter are there, however, but are much smaller than those between magnets.
The forces are so small that their presence is not 'felt' until the particles are quite close together.
How does one particle 'know' that another particle is nearby? How do magnets 'know' when other magnets are attracting or repelling them? Scientists have been trying to answer these questions for many years and have not yet found a complete answer.
It could be that some message is being sent from one particle to another. Is this message always being transmitted, like the flashing light on a beacon? For want of a better scientific model, scientists invented message lines called 'fields of force', and imagined these fields to be radiating outwards in all directions from particles of matter.
The particles have gravitational fields, but we will see later that they may also have electrical fields and magnetic fields as well. All these fields have one thing in common - like light signals from a beacon, they grow weaker as the distance from them increases.
In solids, however, where the particles are very close together, the forces become quite strong so that the particles cannot easily escape one another's clutches. Some solids need saws, knives or even sledgehammers to separate them. Furthermore, in a number of solids the particles arrange themselves in orderly patterns to form crystals, and these crystal structures can become very strong indeed. One crystal pattern can be seen when ice forms on a window pane, and other patterns are revealed by taking photographs of the pictures formed in modern electron microscopes.
There is another way of making the particles move apart without using a saw or a hammer, though this method; too, requires a certain amount of energetic work. The energy can be supplied in the form of heat which, we say, 'melts the solid'.
When solids melt, the distances between the particles increase, while the forces between them grow weaker. Now the particles have more freedom, allowing the liquid to flow and be poured from one container to another.
Some particles can escape from the surface of a liquid and shoot off into space like little spacecraft: very energetic particles can even escape from a solid, as anyone will sense in the presence of moth balls. The liquid then evaporates into the atmosphere.
Given even more heat the liquid reaches its boiling point and particles begin to form vapor bubbles deep down inside the body of the liquid. The liquid boils and turns into vapor.
Vapor particles in the atmosphere are now much farther apart, and the forces between particles are so small that the particles dash about in space hardly noticing each other unless they happen to have a head-on collision.