Anything which occupies space, has mass and can offer resistance (opposition) is known as matter. Matter is made up of small component particles called molecules.
A molecule is the smallest particle of a substance which can exist in a free state, and has the properties of that substance. The diameter of an average molecule is
An atom is the smallest particle of an element which may or may not exist in a free state and which can take part in a chemical reaction. We say an atom may or may not have a free existence because mostly, atoms in a molecule are bound together by forces of attraction.
Matter exists in three physical states, depending on temperature and pressure of the surroundings.
Solids are those substances having a definite mass and volume. For instance - iron, gold. Each piece of gold or iron will have a definite shape, mass and volume. Of course, you can give it any desired shape, but it will not take the shape of the container which it is put into.
Liquid is that form of matter which has a definite mass and volume but no definite shape. For example, if you take one litre of water in a glass, it will assume the shape of a glass. Subsequently, if you pour it into a fry pan, it will assume the shape of the fry pan. Of course, the volume remains the same in this case.
Are those substances which have neither a fixed volume nor a fixed shape. For example, Hydrogen, Oxygen etc. You can change their shape and volume by increasing / decreasing the pressure applied on them. Thus they lack a definite shape or volume.
Recently, another state of matter known as plasma has been discovered by nuclear scientists. A plasma is produced during a fission or fusion reaction when the atoms get ionized due to tremendous heat and high temperature. Such a state is known as plasma, because it has distinctive properties of its own. And most recently, some scientists in the US were able to detect a fifth state of matter under very extraordinary conditions. Such a state was termed the Bose-Einstein Condensate and its exists for a very small fraction of a second.
In fact, dubbing any substance as solid or liquid is partly a matter of perception. Any state of matter can be converted into the other by suitably changing the conditions of temperature and pressure. For example, water which exists as a liquid normally can be easily converted into a gas i.e. steam, by heating it to 1000 Celsius, whereas the same liquid water can be made a solid by cooling it to 00 Celsius. So whenever we say that a particular substance is a solid, in reality, we imply that the particular substance exists as a solid at normal conditions of temperature and pressure.
There is a particular temperature and pressure at which all the three states of matter exist together. For example, water exists as a solid, liquid and gas at 00 Celsius and 1 Atmosphere Pressure. Such a point is known as Triple Point and any substance can be made to exist in all these three states by applying suitable conditions of temperature and pressure.