Which addition is needed to precipitate non-carbonate magnesium hardness out of water?

• A. Lime
• B. Soda ash
• C. Chlorine
• D. Carbon dioxide

Answer: (A)

To precipitate non-carbonate magnesium hardness from water, the addition of lime is necessary. Lime, which consists of calcium hydroxide (Ca(OH)₂), reacts with the magnesium ions present in the water. In this reaction, magnesium ions combine with hydroxide ions provided by lime to form magnesium hydroxide (Mg(OH)₂), which is an insoluble compound. As a result, this precipitation process effectively removes magnesium hardness from the water.

The role of the hydroxide ions is critical, as they help increase the pH of the water, facilitating the formation of the precipitate. Therefore, the introduction of lime is key to managing non-carbonate hardness, as it specifically targets the magnesium ions that contribute to water hardness beyond the capacity of carbonates.

Other substances listed do not precipitate non-carbonate magnesium hardness effectively. Soda ash, while useful for carbonate hardness, does not address non-carbonate magnesium. Chlorine is primarily a disinfectant and does not participate in the precipitation of hardness. Carbon dioxide, on the other hand, can increase acidity and may assist in dissolving some forms of hardness rather than precipitating them.