Solids are substances in which the atoms or molecules are attached tightly to each other. The attachment can either be in a regular geometric network, as is the case with crystals or with metals, or in an amorphous state, such as wood. A good example of a solid that can be converted into both a liquid and a gas is ice. Ice is a solid that occurs naturally when water is cooled below zero degrees Celsius. It can appear as hail, icicles, glaciers and snowflakes, which are crystals of ice.
A liquid is a form of matter in which the atoms or molecules are not attached tightly to each other, but are free to move around each other. However, they do not have enough energy to be completely free of one another. A liquid takes the shape of the vessel that contains it, and settles into that shape with the help of gravity. When ice is heated, the bonds between the water molecules are broken and the ice melts to form liquid water. Metals melt at a much higher temperature than ice. This is because the bonds holding the atoms together are much stronger, so more energy is needed to split them.
Gases are a form of matter in which the atoms or molecules are completely free of each other. It has no definite shape and takes up the entire shape of the vessel that contains it. This is because heating of a liquid gives the molecules so much energy that they are no longer bonded together at all. Further heating of a gas initially causes the molecules to have still more energy and to move around faster, increasing the pressure of the gas. Water vapor is the gaseous state of water. Water vapor at 100 degrees Celsius is called steam.
Nuclear and quantum physics have shown that there are, in fact, a number of other forms of matter. The most common of these is plasma. Plasma occurs when a gas is heated to such a temperature that the electrons are given so much energy that they depart from their parent atoms within the gas. Plasma is the most common form of matter in the universe; all of the stars are made up from it. Artificially-produced plasma can be seen in plasma television displays, fluorescent lamps and in the arc welder or plasma torch. Natural plasma can be seen in lightning and ball lightning.