Lime is applied to water to adjust ________.

Lime is used in water treatment primarily to adjust the pH or alkalinity of water. This process is crucial because maintaining an appropriate pH level helps to ensure that the water is safe for consumption and effective for treatment processes. When lime (calcium hydroxide) is added to water, it reacts with carbon dioxide and other compounds, leading to an increase in pH and alkalinity.

A higher pH can enhance the efficiency of disinfection processes and can aid in the removal of certain metals and hardness from water. Additionally, adjusting the alkalinity helps to stabilize the pH, making it less susceptible to fluctuations due to other chemicals or environmental factors.

The other options do not relate to the primary application of lime in water treatment. Color is usually controlled through filtration and chemical addition rather than lime; temperature adjustment typically involves physical processes rather than chemical additives; and purity is determined through various other treatment methods, not specifically by the addition of lime. Thus, the adjustment of pH or alkalinity is the key role that lime plays in water treatment processes.