Animated Solution for Chemistry - s and p-Block Elements: Which one of the following processes will produce hard water?

The concept of water hardness is one of those beautiful intersections where everyday life meets rigorous chemistry. We all have heard of "hard water" leaving white scales on our taps or making it difficult to form a lather with soap. But what exactly is happening at the molecular level? Let's dive into this problem and decode the chemistry of hard water.

When we say water is "hard," we are not talking about its physical state. We are talking about the presence of specific dissolved metal cations—primarily calcium ($C{a}^{2+}$) and magnesium ($M{g}^{2+}$).

For water to be hard, these ions must be freely floating in the solution. If a calcium or magnesium compound is insoluble, it will simply sit at the bottom of the container like sand, completely ignoring the water molecules. It won't interfere with your soap, and it won't cause scaling. Therefore, solubility is the absolute key to this question.

Let's look at the first two options: saturating water with calcium carbonate ($CaC{O}_3$) or magnesium carbonate ($MgC{O}_3$).

If you have ever seen marble or chalk, you have seen calcium carbonate. Does chalk dissolve easily in pure water? No. Both $CaC{O}_3$ and $MgC{O}_3$ are practically insoluble in pure water. Because they do not dissolve, they cannot release those crucial $C{a}^{2+}$ or $M{g}^{2+}$ ions into the water.

Thus, simply adding these carbonates to pure water will not make it hard.

Next, we have option (d), which suggests adding sodium sulfate ($N{a}_{2}S{O}_4$).

Sodium salts are famous for being highly soluble. When you drop sodium sulfate into water, it immediately dissociates:

The water is now full of ions. But does it make the water hard? Absolutely not. The definition of hardness is strictly tied to multivalent cations like calcium and magnesium. Alkali metal ions like sodium ($N{a}^{+}$) or potassium ($K^{+}$) do not form insoluble scums with soap. So, despite being completely soluble, sodium sulfate is a decoy.

Finally, we arrive at option (c): calcium sulfate ($CaS{O}_4$).

Unlike the carbonate, calcium sulfate is soluble enough in water to undergo dissociation:

As it dissolves, it releases free $C{a}^{2+}$ ions directly into the aqueous environment. The moment these calcium ions are present, the water officially becomes hard.

Furthermore, because the anion is a sulfate ($S{O}_4^{2-}$), this creates permanent hardness. Unlike temporary hardness (caused by bicarbonates), permanent hardness cannot be removed by simply boiling the water. You would need chemical treatments, like adding washing soda ($N{a}_{2}C{O}_3$), to precipitate the calcium out.

By systematically evaluating the solubility and the specific ions released, we can confidently conclude that saturating water with calcium sulfate is the process that will produce hard water.