What small amount of substance can be used to ensure drinking water is safe by killing bacteria?

Chlorine is commonly used as a disinfectant in water treatment processes because it effectively kills bacteria and other pathogens that could pose health risks. When introduced to water, chlorine reacts with organic matter and microorganisms, rendering them inactive and making the water safe for consumption.

In contrast, while bleach also contains chlorine as its active component, it is typically not recommended for direct use in treating drinking water due to its potential impurities and the presence of additional chemicals that could be harmful. Alcohol is not an effective disinfectant for water treatment in the same way that chlorine is; it is a solvent and can even be harmful if ingested in large quantities. Hydrogen peroxide can act as a disinfectant but is not the standard choice for water treatment, as it breaks down into water and oxygen and may not provide continuous protection against recontamination.

Therefore, chlorine is the best option among these substances for ensuring drinking water safety.