A state of matter is a form that matter takes that has certain properties. These properties are the same regardless of the material used to make them, which is why they are good groups. The traditional states of matter are solid, liquid, and gas. Sometimes other states can be found, such as colloids, plasma, and superfluids, but those are more complicated and will not be discussed here. Matter can change state from one form to another with the gain or loss of energy. This energy usually exists in the form of heat, but occasionally can be found in other forms.

Solids

A solid is the most basic form of matter and the easiest to understand. We use solids for almost everything. A solid can be hard or soft, but it cannot go through itself without being broken. A solid has a definite shape and a definite volume, which means that without exerting a force on it, neither the shape nor volume can change. You can change the shape of a solid by affecting it via bending or moving, but you must bend or move the object itself to move it: it can't move on it's own. In a solid, the atoms or molecules that make it up are held together very strongly by intermolecular forces: forces or bonds that act between molecules. The only motion that atoms or molecules in a solid can do are vibrations. The faster the particles vibrate, the more the temperature increases. Solids are very useful in that they occupy space and will push against other matter with a force strong enough to prevent you from moving through them. This is good, otherwise we'd fall through the ground!

Liquids

A liquid is the normal form of things like water and milk. It is a form of matter that has a definite volume but not a definite shape. This means that the shape of a liquid is determined by the container holding it, but the amount of liquid is always the same, even if it is spilled into a puddle. Liquids can move on their own, but they have to follow gravity, and once they are in the lowest spot available without going up, they will stop. Unlike a solid, things can move through a liquid without breaking it. In fact, the idea of breaking a liquid doesn't make a lot of sense to most people. Like solids, liquids can push back against other bits of matter, but the push is much weaker, so that you will sink into the liquid before it will hold you up. Notice that boats sink into the water a bit, even though we say they float. Not all things float in liquids, though; you must compare the density of a solid to the density of a liquid to see if it will float or not. If it is too dense, a liquid will nto be able to push back hard enough to keep the object floating. The atoms or molecules in a liquid have more energy than in a solid, but less energy than a gas. They are able to vibrate, rotate, and slide past each other but they are not able to move wherever they want. The intermolecular foces are strong enough to hold them together, but not strong enough to keep them from moving. An important property of liquids is that they are always evaporating and will eventually become gases and will "dry up."

Gases

A gas is the normal form of things such as air and carbon dioxide. It is a form of matter that is like a liquid in that it has no definite shape, but unlike a liquid a gas will expand to fill the entire volume of the container it is in. The amount of gas can never change but the volume, or the space that it takes up, can change. The atoms or molecules in a gas are usually moving about very rapidly and the intermolecular forces are too weak to hold them still. Because of this, it is very easy to move around in a gas and very easy to make gases move. This property means that gases can't push against other forms of matter very easily. It takes very large amounts of gases to create significant pressure (note that the same amount of a solid or liquid would create a much larger pressure). Despite this, air pressure, the force applied by air pushing on things, is responsible for many things in life and is strong enough to hold things up in certain situations. Another important behavior of gases is that they will move from areas of high pressure to areas of low pressure, even if that means they move against gravity.