States of Matter
Matter occupies space and has mass. Chemistry deals with the composition, properties and behavior of matter. There are three states of matter: solid, liquid and gaseous. (There are other exotic states of matter but these are not normally encountered.) Solids are rigid and have a definite volume and shape. Liquids and gases are "fluid"and do not have a particular form or shape. They will take the shape of a container to some extent.
Some substances can be transformed from one state to another by heating or cooling and/or by increasing or decreasing the pressure. This is particularly true of simple materials such as elements or simple compounds. We are all familiar with water in the form of solid ice, liquid water, and water vapor (often experienced as the humidity of the air or observed as steam when heating water in the kitchen). On a damp and humid day there is a lot of water vapor in the air. The dry air of the desert has a distinct feel. At high altitudes the pressure is lower and water boils at a lower temperature. Thus a gas can be converted to a liquid by decreasing the temperature and/orincreasing the pressure.
Solids and liquids are much more dense than gases. In most cases a substance in the liquid state is slightly less dense than the solid state. Water is an exception. Ice floats on water because it is lighter than the liquid water. Because of this a lake can freeze over while the fish are still swimming around under the ice. Polar bears can walkabout on the polar ice and catch seals that have to come up for air from the water. The world would be very different if ice were heavier than water.
Solids and liquids are more obviously effected by gravity than are gases. In a glass, water assumes the lowest position it can under the influence of gravity. Since it is fluid and can flow it will arrange itself so that all parts are as low as possible. Thus the water assumes the shape of the glass in the lowest position possible. The upper surface in contact with the air is completely flat when it is not disturbed. Every point on the flat upper surface will be the same distance from the floor (or the center of the earth). Liquids with a "free" upper surface will form a level plane. That is why a carpenter's level can be made with a liquid with a small gas bubble under a curvedsurface. The liquid is used to indicate the direction of gravity, or the plane perpendicular to gravity.
The air is gaseous. We breath air. If you think of the room you're in as a container, you know that the air fills the entire room. The air doesn't flop down onto the floor but fills the whole room. Of course the air is not quite completely the same throughout theroom. If the odor of burning toast is emanating from the kitchen you notice that theodor is stronger in some places than in others. Eventually the odor will permeatemost of the room. So you gather that things can move about in the air even if theair seems still. How is it that air is so light and can distribute itself ratherevenly about the room? The particles of the air (molecules) are far apart and movingvery fast, so fast that they overcome the force of gravity on them to a largeextent. If undisturbed, that is not mixed, the heavier gases will tend toconcentrate more toward the floor. I'm sure you have noticed that the air in a roomis warmer toward the ceiling and that the floor is the coolest part of the room. Theparticles in the warm air are moving faster and the warm air is less dense than the colder air.
Air molecules are very far apart compared to liquids and solids. In a liquid the molecules are very near each other but, unlike those in a solid, they circulate around and are not attached to each other in a rigid structure. In a solid the molecules (or particles) are jiggling around but not so much as to break free from each other. They remain in rather fixed positions with respect to their neighbors. Because gas molecules are so far apart a gas can be compressed. A large volume can be forced into a smaller volume. This is very difficult to impossible with a liquid or solid under ordinary conditions.
We can get a pretty good idea of what a liquid a solid and a gas are. On the other hand, many things we are familiar with seem not to fall into these simple categories. Is ice-cream a solid? It doesn't exactly keep its shape when it hits the floor. Vaseline or warm candle wax - solid or liquid? What about a plastic container in a microwave? It may not have kept its shape when heated. Well, the heat melted it. Does wood melt when heated? No. What about mayonnaise? Is it a liquid? Well it won't take on the shape of a container, nor can you pour it out of a bottle. Yet it doesn't have a permanent shape. You can squish it around. The kitchen is full of materials that have properties of both liquids and solids. Peanut butter, butter, Jell-O, whipped cream, catsup, a sponge, rubber, the list goes on and on. These things are generally rather complex on a microscopic level and often consist of solids suspended in liquids or liquids contained in solid films or air contained in solid compartments or air pockets in liquids, etc. Such materials usually fall in the category of "colloids". Glasses and many polymers get progressively softer as they are heated. These are non-crystalline solids.