Which statement about lime in water treatment is true?

• A. Lime lowers pH
• B. Lime adds chlorine
• C. Lime reduces turbidity
• D. Lime raises pH and hardness, neutralizes acidity, and precipitates metals

Answer: (D)

Lime in water treatment is used to modify the water chemistry by raising pH and alkalinity, increasing hardness, and helping to remove dissolved metals. When lime (calcium hydroxide) dissolves, it supplies hydroxide ions that neutralize acidity, so the water becomes less acidic and its buffering capacity increases. It also adds calcium, which increases hardness. As the pH rises, many dissolved metals form insoluble metal hydroxides and precipitate out, making it easier to remove them from the water.

This is why the statement that lime raises pH and hardness, neutralizes acidity, and precipitates metals is the best description. The other ideas don’t fit as well: lime does not lower pH (it raises it), it does not add chlorine, and while it can help reduce turbidity indirectly through precipitation, turbidity reduction is not the primary or defining effect of lime in treatment.