Final answer:
Aluminum sulfate is added to municipal drinking water to make it clearer, involving a process known as coagulation and flocculation. Chlorine disinfects water but does not primarily clarify it. Fluoride and sodium hydroxide have other functions and do not serve the purpose of clarification.
Step-by-step explanation:
The substance often added to municipal drinking water to make it clearer is aluminum sulfate. Aluminum sulfate is used in the process of coagulation and flocculation, where it serves as a coagulant to remove dirt and other dissolved particles by forming larger particles, or floc. This treatment effectively clarifies the water, making it visually clearer.
Chlorine is the most common compound added to water to make it safe for humans by disinfecting and removing biological contaminants. However, it is not primarily used to make water clearer, but rather to kill bacteria and other microbes. On the other hand, substances like fluoride are typically added to water to prevent tooth decay and are not involved in the clarification process of drinking water. Likewise, sodium hydroxide is used to adjust pH levels and is not chiefly for water clarity.