One thing that makes the classification scheme of all matter into elements, compounds, and mixtures, is that it does not directly account for what the natural phenomena of phase changes. The uniformity of phase changes are used to establish the purity of a substance and cannot then be regarded as a chemical change. It is a physical change.
In general, chemists regard physical changes as changes in which no chemical bonds are broken or formed. This means that when a substance changes phase, goes from a solid to a liquid by melting, or from a liquid to a gas by evaporating, the same types of compounds or elements that were there at the beginning of the change are there at the end of the change. Because the ending materials are the same as the beginning materials, the properties (such as color, boiling point, etc) will also be the same. Physical changes involve changing the spatial arrangement of the basic units that make up a substance, without changing their identity.
Under normal conditions we distinguish three phases – solid, liquid, and gas.
When we heat a solid the parts vibrate more vigorously. This increases the volume of the solid and makes it less dense. At some point, the energy of the vibrating parts is great enough to overcome the attractive forces that keep them in place. They loose their orderly ranking and turn into a liquid with parts sliding past each other. The solid melts. If we continue to heat the substance after it is all liquid, the parts gain more energy and usually increase their overall volume – they become less dense. At some point, the energy of all the parts is great enough to overcome the attractive forces that keep them in a cluster. The parts separate from each other – they have vaporized and turned into a gas.
For example, grains of sand are much larger than water molecules. To separate a mixture of water and sand we simply pour the mixture through a filter. The sand does not pass through but the water does.
Chromatography works in a similar way. A mixture of substances is passed through a stationary phase (similar to the filter paper) which interacts differently with the various substances. Those that interact more take longer to reach the bottom of the column than those that interact less, allowing for separation.
Distillation works by heating the mixture. The more volatile substances, the one with the lower boiling point, will evaporate first. If the vapor is cooled by passing it through a condenser, the more volatile substance will be separated from the less volatile. For example, distillation can separate ethanol from water. The ethanol will boil off first leaving the water behind. Another example would be purifying salt water. In this case, the water will boil off first leaving the salt behind.