Bleaching powder, also known as calcium hypochlorite, is commonly added to water used for drinking purposes. The main role of bleaching powder in this case is as a disinfectant (option 1).

Option 2 suggests that bleaching powder is a bleaching agent. While bleaching powder does have bleaching properties, in the context of drinking water, its primary function is to disinfect rather than to bleach.

Option 3 states that bleaching powder is used to remove impurities. While bleaching powder may help to remove some impurities in water, its primary purpose is to kill or neutralize harmful microorganisms.

Option 4 suggests that bleaching powder decreases the pH of water. However, this is not its primary role in the case of drinking water. The primary role of bleaching powder is to act as a disinfectant by killing bacteria, viruses, and other microorganisms that may be present in the water.

In summary, the correct answer is option 1, as bleaching powder is added to water for drinking purposes primarily as a disinfectant to kill harmful microorganisms.