As rain begins to fall from the clouds, it is very pure. However, as it falls through the air it dissolves some of the gases in the air. It also collects bits of dust and microbes   that are floating in the air. When the rain reaches the ground it begins to pick up more dust and dirt. Many kinds of mineral substances   dissolve in the water. Sand and mud cannot dissolve in water.    Since water is so important to us, it is very important that the water we use should be clean. Otherwise, we might get some of the diseases that are carried by water. Many microbes are harmful. They are so small that we cannot see them. These microbes must be removed or killed before the water is suitable for drinking. For example, microbes of diseases like cholera live well in water. If the water supply is not clean, people who drink this water may get these diseases. Thus, to provide people with clean water, the government purifies   the water.   Purification is a long process and includes three stages: storing, purifying and distributing . The first stage is storing. Man-made lakes are used for this purpose. Water from the rains, other lakes, streams and rivers is collected in the lakes. This water is usually dirty and muddy.    In the second stage, water from the man-made lakes is filtered   in a special pond. Stones, sand and other unwanted tiny things are caught in it. After that, tiny microbes are still present in the water. These microbes are removed by "airing" the water and by adding a chemical substance called chlorine   to the water. This kills all the microbes in the water. After this, the water can be safely used for drinking. The third stage is the distribution of the purified water to homes, hospitals, offices, factories and other places. This is done through a network of pipes of various sizes. These pipes must be made of a suitable material. What is the purpose of adding chlorine to the water?
to air the water to catch tiny things to kill tiny microbes to remove harmful gases