Which of the following is NOT an example of a hazardous substance?

In the context of hazardous substances, the distinguishing factor often revolves around the potential risks associated with exposure or the dangerous properties of the substance. Water is generally classified as a non-hazardous substance because, under normal circumstances, it is safe for human use and consumption, and does not pose significant health risks.

On the other hand, ammonia, mercury, and chlorine are all considered hazardous substances due to their toxic properties and potential for causing environmental and health issues. Ammonia can lead to respiratory irritation and other toxicity when inhaled or ingested. Mercury is highly toxic, causing severe health issues when exposure occurs, particularly through ingestion or inhalation. Chlorine, while widely used as a disinfectant, is a corrosive gas that can cause respiratory distress and other harmful effects upon exposure.

Thus, water stands out as the option that is not associated with potential hazards in ordinary use, making it the correct answer in this scenario.