Free available chlorine refers to the forms of chlorine that are effective in disinfection and are readily available to kill pathogens in water. The following are the different forms of free available chlorine:

1. Hypochlorous acid (HOCl): Hypochlorous acid is the most effective form of chlorine for disinfection. It is a weak acid that dissociates into hydrogen ions (H+) and hypochlorite ions (OCl-) in water. The hypochlorous acid molecule (HOCl) is the active form of chlorine that can penetrate the cell walls of microorganisms and disrupt their metabolic processes, leading to their destruction.

2. Hypochlorite ions (OCl-): Hypochlorite ions are formed when hypochlorous acid dissociates in water. They are less effective than hypochlorous acid but can still provide disinfection. Hypochlorite ions work by oxidizing the cellular components of microorganisms, leading to their inactivation.

3. Molecular chlorine (Cl2): Molecular chlorine is a gas that can dissolve in water to form hypochlorous acid and hypochlorite ions. It is a powerful disinfectant and can effectively kill pathogens. Molecular chlorine can penetrate the cell walls of microorganisms and disrupt their enzymes and proteins, causing their destruction.

Explanation:
- Hypochlorous acid, hypochlorite ions, and molecular chlorine are all forms of free available chlorine.
- Hypochlorous acid is the most effective form of chlorine for disinfection, while hypochlorite ions and molecular chlorine are less effective but still provide disinfection.
- Hypochlorous acid and hypochlorite ions are formed when chlorine is added to water, while molecular chlorine is a gas that can dissolve in water.
- All three forms of free available chlorine work by oxidizing and disrupting the cellular components of microorganisms, leading to their destruction.

In conclusion, free available chlorine includes hypochlorous acid, hypochlorite ions, and molecular chlorine. These forms of chlorine are effective in disinfection and can kill pathogens in water.