What are the two types of hardness present if alkalinity is less than total hardness?

When the total hardness of water is greater than its alkalinity, it indicates the presence of two distinct types of hardness: carbonate hardness and non-carbonate hardness.

Carbonate hardness is associated with dissolved alkaline earth metals like calcium and magnesium, specifically when they are in the form of carbonate ions (CO₃²⁻) and bicarbonate ions (HCO₃⁻). This type of hardness can potentially be neutralized by the addition of acids and is typically responsible for the temporary hardness of water, which can change depending on conditions such as pH and the presence of CO₂.

Non-carbonate hardness, on the other hand, comprises hardness caused by other soluble salts of calcium and magnesium, such as sulfates and chlorides. This type cannot be neutralized by acids and contributes to the permanent hardness of water.

When total hardness exceeds alkalinity, it means that there are excess calcium and magnesium ions that do not have sufficient carbonate or bicarbonate ions to balance them out, resulting in both types of hardness being present. Therefore, the correct understanding involves recognizing that the situation provides both carbonate and non-carbonate hardness in the water.