This is a good question! In pools, or any water systems that are used by people or animals for drinking, washing, living (such as a fish tank), or recreation, it is important to keep the people and animals healthy and safe. "Chlorine," or more formally either a chloride ion (Cl-) or a hypochlorite ion (ClO-) in its most active form, is commonly used in water systems to kill bacteria and algae. There are many forms of chlorine used in pools including gaseous chlorine, which is very strong and typically found in older public pools. liquid chlorine, which is like a more concentrated form of liquid bleach, and solid chlorine tablets or granules of either "tri-chlor," "di-chlor," or "calcium chloride."

Chlorine works in two ways: 1) Disrupting the membranes that hold bacteria cells together, causing them to "pop," quite like a balloon, and 2) causing the proteins used by bacteria and algae cells to carry out various cell functions to unravel and break down. The level of chlorine necessary for a given water system depends on what types of plants or animals will be using the water, but it is usually in a "parts per million" concentration, or 1/1,000,000 kilograms or pounds of chlorine per 1 kilogram or pound of water.

Chlorine, in most of the forms above, actually does not contribute to the color of pool water. Pure water scatters blue light and absorbs red light (which are both present in white light from the sun), which causes the water to appear blue/greenish-blue if there is enough water all in one spot, like a pool. However, two of the forms of chlorine above, tri-chlor and di-chlor, come with a stabilizer called cyanuric acid which protects chlorine from breaking down in bright sunlight, but also scatters more blue light than water alone, deepening the shade of blue in a pool.

I hope all this helps! More info can be found at: this site if you're interested. Keep the questions coming!