Chemical compounds can be broadly divided into three categories: acids, bases and salts. Acids are sour in taste, corrosive in nature, and change blue litmus to red. Bases are generally slippery in nature, bitter in taste, and change red litmus to blue. The reaction of acids and bases with other substances helps us to understand their properties and their uses in our lives.

Non-metallic oxides such as oxides of nitrogen, sulphur and carbon, dissolve in rainwater, making rainwater slightly acidic. This leads to acid rain. Acid rain has many implications on the environment.