Chemists use chemical reactions to make plastics, medicines, dyes, and many other materials that are important in everyday life. They also study what substances are made of and how they can be combined to make new materials. Chemicals are the raw materials used by a chemist. More than 4 million different chemicals have been made by chemists; there are about 35,000 chemicals in common use. These chemicals can be made by combining simple substances, called elements, into more complicated substances called compounds. Early chemists considered four elements-fire, water, air, and earth. Today, we know there are 92 that occur in nature, and a few others that can be made in laboratories. The most common element in the universe is hydrogen, which is the main component of stars.
Chemistry of water:
When different substances combine together to form new materials, a chemical reaction occurs. Some reactions need heat to start them off; others produces heat as the reaction proceeds.
Chemists use a shorthand to describe chemicals. H2O is the symbol for water and shows that each water molecule contains two hydrogen atoms (H) and one oxygen atom(O).