Chemistry is a science of substances, their structure, their properties and the reactions, that change them into the other substances.Chemistry is a great and complex subject. Substances may exist in any of the three states. They may exist as solids, liquids and gases. Sulphur, iron, silver are solids at an ordinary temperature, bromine and mercury are liquids in the same conditions and oxygen and hydrogen are gases. But solid substances may become liquids if we heat them to a proper temperature. Liquids turn into gases at very high temperatures.