Chlorine demand increases when __________ increases.

The correct answer is that chlorine demand increases when organic matter increases. Organic matter in water supplies can come from various sources such as decaying plants and animal matter, as well as pollutants from human activity. When organic matter is present, it requires more chlorine to achieve effective disinfection. Chlorine reacts with organic compounds, and this reaction not only consumes chlorine but can also produce disinfection by-products, which may pose health risks.

In contrast, while factors such as alkalinity, phosphate concentration, and pH can influence water quality and chemical reactions, they do not directly correlate to an increase in chlorine demand like organic matter does. For instance, increased alkalinity might stabilize pH and assist in the effective functioning of chlorine but does not inherently increase the demand for chlorine. Similarly, while a higher phosphate concentration can contribute to algal growth, leading to potential chlorine demand increases indirectly, it is the organic matter which directly leads to a greater requirement for chlorine for disinfection purposes. Lastly, pH can affect the form of chlorine present in water (such as hypochlorous acid vs. hypochlorite ion) but does not directly correlate with an increase in chlorine demand compared to the presence of organic matter.

Thus, understanding the relationship between chlorine