Problem 139 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following processes will produce hard water? (a) saturation of water with \(\mathrm{CaCO}_{3}\) (b) addition of \(\mathrm{Na}_{2} \mathrm{SO}_{4}\) to water (c) saturation of water with \(\mathrm{MgCO}_{3}\) (d) saturation of water with \(\mathrm{CaSO}_{4}\)

Step-by-Step Solution

Answer

Option (d) saturation of water with \(\mathrm{CaSO}_{4}\) will produce hard water.

Step 1: Hard Water Definition

Hard water is water that has a high concentration of calcium (Ca²⁺) and magnesium (Mg²⁺) ions. These ions make it difficult for soap to lather and can cause scale formation in kettles and boilers.

Step 2: Analyze Option (a)

When water is saturated with \({CaCO}_{3}\), it is unlikely to produce hard water because \({CaCO}_{3}\) is not very soluble in water and contributes little \({Ca}^{2+}\) ions compared to other compounds like \({CaSO}_{4}\).

Step 3: Analyze Option (b)

Adding \({Na}_{2} {SO}_{4}\) to water will introduce Na⁺ ions and SO₄²⁻ ions but won't add Mg²⁺ or Ca²⁺ ions, hence it won't make the water hard.

Step 4: Analyze Option (c)

Saturating water with \({MgCO}_{3}\) can potentially produce hard water because \({MgCO}_{3}\) can dissolve to release Mg²⁺ ions, thereby contributing to water hardness.

Step 5: Analyze Option (d)

Saturation of water with \({CaSO}_{4}\) leads to hard water as it dissolves to release \({Ca}^{2+}\) ions, which contribute to water hardness.

Step 6: Determine the Correct Answer

Both options (c) and (d) can produce hard water. However, given the option list and typical scenarios, \({CaSO}_{4}\) (option (d)) is more soluble than \({MgCO}_{3}\), hence it more commonly contributes to the hardness of water.

Key Concepts

Solubility of Calcium Compounds

Calcium compounds vary in their solubility in water. A compound like calcium carbonate ( CaCO₃ ) is considered poorly soluble. This means that it doesn't easily dissolve in water to release calcium ions ( Ca^{2+} ). When water is saturated with CaCO₃ , only a small amount of Ca^{2+} ions are released, which are not enough to significantly contribute to the water’s hardness. This is why CaCO₃ alone is less likely to cause hard water.
In contrast, calcium sulfate ( CaSO₄ ) is more soluble than CaCO₃ . This higher solubility means that when CaSO₄ is present, more Ca^{2+} ions are introduced into the water. These Ca^{2+} ions are what mainly contribute to water hardness, making CaSO₄ a common culprit in hard water formation. Understanding the solubility of these compounds is essential in determining which substances will impact the hardness in water.

Magnesium Ions in Water

Magnesium ions ( Mg^{2+} ) play a crucial role in the hardness of water. Just like calcium ions, Mg^{2+} ions react with soap and form insoluble precipitates, which hampers soap’s ability to lather effectively and clean properly. This is a key characteristic of hard water.
When compounds like magnesium carbonate ( MgCO₃ ) are present in water, they can dissolve and release Mg^{2+} ions. Although the solubility of MgCO₃ is not very high, any contribution of Mg^{2+} ions can still make the water hard. It's important to note that even small amounts of Mg^{2+} in water can lead to the formation of scale, which can be particularly problematic in pipes and water heaters.

Water Hardness

Water hardness is primarily due to the presence of calcium ( Ca^{2+} ) and magnesium ( Mg^{2+} ) ions. When these ions are plentiful in the water supply, the water is labeled as 'hard'. The issues with hard water range from cosmetic ones, like soap not lathering, to practical ones, such as scale buildup in plumbing.
The degree of hardness depends on the concentration of these ions. In general, "temporary hardness" is usually associated with carbonate minerals that can be removed by boiling. On the other hand, "permanent hardness" usually involves sulfate or chloride compounds, which do not precipitate out easily and thus cannot be removed by boiling.
Hard water is not just a nuisance in households; it can also cause greater maintenance costs and efficiency losses in industrial settings. Understanding water hardness is essential in deciding if water treatment, such as softening processes, is necessary to mitigate these effects.