Pools are in danger from getting contaminated from rain, debris, dust, animals and swimmers. Chlorine is used to sanitize pool water. It is used to kill bacteria and prevent the growth of viruses, algae and other contaminants to keep it safe to swim.
How is chlorine created? Chlorine is a product made through the electrolysis of salt water. The way that works is the use of electricity passing through Salt (2NaCl) and water (2H2O). The electricity causes the atoms to separate into Chlorine (Cl2), Sodium Hydroxide (2NaOH) and Hydrogen (H2). From there the chlorine is isolated into a gaseous form and use to create different forms of chlorine compounds to sanitize pool water.
Chlorine basically kills off bacteria, viruses, microorganisms and contaminants through a chemical reaction when added to water. The chemical reaction after chlorine is added to water leaves the pool water with hypochlorous acid (HOCl) and hypochlorite ion (ClO-). Both kill off microorganisms and bacteria at different rates. HOCl breaks down the microorganisms and bacteria within seconds and ClO- can take up to at least 30 minutes.
The chemical reaction when chlorine is added to water kills off the microorganisms bacteria, viruses and algae by attacking the lipids in the cell walls and destroying their inner enzymes/structures inside the cell. This makes the molecule change shape not allowing them to function properly. This leaves them oxidized. Basically the cells are deactivated and harmless.
Free Chlorine is a measurement in the amount of chlorine left in the water to kill off new bacteria, viruses and microorganisms entering the pool water. Once the chlorine has completed sanitizing and killing off the containments, the hypochlorous acid and hypochlorite ions combine with other chemicals or break down into atoms. Once the break downs is complete it is no longer working and you will need more chlorine. Direct sunlight and heat speed up the breakdown process and dissipates chlorine quickly.