Which one of the following is used to disinfect water? 

Chlorine 

The correct answer is option 1.Chlorine.

Out of the options listed, chlorine is the primary disinfectant used in water treatment.

Chlorine is a highly effective disinfectant that kills harmful bacteria, viruses, and other microorganisms in water. It is added to water in a controlled amount and remains as a residual disinfectant, meaning it continues to kill pathogens even after the initial treatment process.

Here's a brief overview of the other options:

Washing soda (sodium carbonate): While it can increase the pH of water, making it slightly alkaline, it is not an effective disinfectant and cannot kill harmful microorganisms in water.

Silica: This is a common mineral found in sand and rock and is not used for disinfection purposes.

Coal: Coal is a fossil fuel and is not used in water treatment for disinfection.