Chlorine is used in water treatment primarily for what purpose?

Chlorine is primarily used in water treatment to kill bacteria and other pathogens. This is essential for ensuring that drinking water is safe and free from harmful microorganisms that can cause diseases. When chlorine is added to water, it reacts with organic matter and microorganisms, effectively disinfecting the water. This process is crucial for public health because it helps to prevent waterborne diseases that can arise in untreated or improperly treated water supplies.

Using chlorine in water treatment does not significantly affect the pH of the water, produce oxygen gas, or balance chemical reactions, as these are not the primary purposes of chlorine's application in this context. Its main function remains as a disinfectant, making option B the correct answer.