Which chemical is used for purification (purity) of water?

Which chemical is used for purification (purity) of water?

پانی کو صاف کرنے کے لیے کون سا کیمیکل استعمال ہوتا ہے؟
Explanation

Chlorine is a strong oxidant that rapidly kills many harmful microorganisms.

Chlorine is a toxic gas.