What type of ions cause hardness in water?

Hardness in water is primarily caused by the presence of multi-positive ions, specifically divalent cations such as calcium (Ca²⁺) and magnesium (Mg²⁺). These ions contribute to the ionic composition of water, leading to the formation of insoluble salts when they react with bicarbonates or sulfates that are also present. The more of these divalent ions present in the water, the greater the hardness, which can affect water quality, soap effectiveness, and scale formation in plumbing systems.

In contrast, single positive ions, neutral ions, and negative ions do not significantly contribute to water hardness. Single positive ions, like sodium (Na⁺) or potassium (K⁺), do not have the same dual charge and hence do not precipitate in the same manner as the multi-positive ions do. Neutral ions lack any charge and consequently do not participate in the reactions that determine water hardness. Negative ions, such as chloride (Cl⁻) or sulfate (SO₄²⁻), can interact with cations in water but do not themselves contribute to hardness; rather, they can influence solubility aspects and the overall ionic balance of the water.

Thus, the correct choice highlights the significance of multi-positive ions