Before it comes to your house, water is usually treated to ensure that you do not get diseases from the water. After you use it, the water that leaves the drains in your house must be treated again before it reenters the environment. Once back in the environment, it may evaporate, getting into the air where it will condense and rain will again fall. Thus, it should not contain organisms or chemicals that are harmful to us. The water is NOT pure.
Pure water has no other substances in it at all.
It is very expensive to purify water. Read the materials listed above. Work in groups of three students. Discuss the water cycle and the treatment of water which you use. Draw a diagram of what you think your water system looks like.