Nature of matter

Matter? What is matter?

Matter is quite simply everything around us that has a physical presence. It is anything we can touch. Your glass of water, the computer, the air, your own body, all are composed of matter.

3 Physical states of matter

Matter can be found in three physical states - solid, liquid and gas. (There are actually more than this, but that is a topic for another time). These are called physical states because in each case the substance is still chemically the same, but how it occupies space, is different:

Solid

A solid has several important characteristics:

It has a fixed shape.
It has a fixed volume.
The molecules are close together and cannot move around.
The molecules vibrate in place.

The first two points should be easy to see. If we pick up a glass and look at it, we can see it has a definite shape. The shape of the glass stays the same (unless we break it), and the space it occupies remains constant. The last two points maybe harder to understand. Between each molecule there is some attraction which keeps the molecules from just flying apart. If there were no such attractions then the universe would instead be replaced by some weird soup of atoms and molecules. Another important thing to realise is that all molecules have a certain amount of energy. The atoms and molecules are not static in space but instead are in constant motion. However, in a solid the attraction between the molecules is so strong that the molecules can only move by vibrating in place. If we heat a solid above a certain point, its melting point, then the molecules gain more energy and vibrate more. Eventually, when the amount of energy they gain is enough to beat the attraction between the molecules, then the solid turns into a liquid.

Liquid

The characteristics of a liquid are:

The molecules are still close to each other, but can move past each other.
The molecules are not held together as tightly as in a solid.
A liquid can flow.
A liquid will take the shape of the container it is in.
A liquid has a fixed volume.

The molecules in a liquid are able to move about much more freely than in a solid, but they still remain close to each other. If we think of water, we can this effect on how a liquid behaves. We are unable to stand water on the table, like you can with a glass. Instead it drops onto the table and spreads out. We are able to pour it from the tap into a cup. When it goes in to the cup it settles and fills the space available, until it reaches a certain level and no more. The volume does not change. If I measure the amount of water in one cup, it will be exactly the same in another. We can apply more heat to the liquid and it will mostly stay the same until we get to the boiling point. At this point the individual molecules now have a lot of energy and can easily pull apart from one another. We now have a gas.

Gas

The characteristics of a gas are:

The molecules are now far apart from each other.
The molecules can move freely.
A gas will fill the shape of the container it is in.
A gas has a changeable volume.

When we get to the gaseous stage, the molecules posses a lot of kinetic energy and can move around as far apart as they want. A gas will spread out to fill any space it is held in and has no fixed shape. Think about air, or steam. The molecules are so far apart that you can push your hand through the air with little resistance. Do not forget, there are still molecules there, just not as closely packed as in a liquid or a solid.

We can also reverse the process by removing energy, otherwise known as cooling. As we cool a gas, the molecules lose their energy and become closer and closer to each other, until eventually the attraction between the molecules becomes greater than their energy to move away. At this point the gas turns into a liquid (known as the condensation point). If we keep cooling the molecules in the liquid, they possess less and less energy. The forces of attraction between the molecules eventually becomes much larger than their kinetic energy and the molecules can no longer move passed each other. At this point the liquid turns into a solid (also known as the freezing point).

These changes are physical, not chemical. The reason is that the chemical properties of the molecules have not changed. The difference is in the physical state.