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 \(\mathrm{CaSO}_{4}\)

Short answer

Saturation of water with MgCO₃ will produce hard water.

Step-by-step solution

Define Hard Water

Hard water contains high concentrations of divalent cations, specifically calcium (Ca²⁺) and magnesium (Mg²⁺) ions. It is typically caused by the dissolution of minerals such as calcium carbonate (CaCO₃), magnesium carbonate (MgCO₃), calcium sulfate (CaSO₄), and magnesium sulfate (MgSO₄) into the water.

Evaluate Option (a)

Saturation of water with \(\mathrm{CaCO}_3\) will introduce calcium ions (Ca²⁺) into the solution as it dissolves, which can contribute to water hardness.

Evaluate Option (b)

Adding \(\mathrm{Na}_2 \mathrm{SO}_4\) to water does not contribute to hardness, as sodium ions (Na⁺) do not cause hard water. Hard water is mainly caused by calcium and magnesium ions, not sodium ions.

Evaluate Option (c)

Saturation of water with \(\mathrm{MgCO}_3\) will introduce magnesium ions (Mg²⁺) into the water. Since magnesium ions contribute to water hardness, this process increases the water's hardness.

Evaluate Option (d)

Saturation of water with \(\mathrm{CaSO}_4\) will dissolve calcium ions (Ca²⁺) into the water. Calcium ions contribute to water hardness, so this will result in hard water.

Conclusion

From the given options, the processes that produce hard water are the ones where calcium ions (Ca²⁺) or magnesium ions (Mg²⁺) are added to the water: Options (a), (c), and (d). However, since the question asks for a singular process that causes hard water, and considering commonly used context in such questions implying exclusivity, we assume only one process is intended as the main hardening factor. Both types of ions found in options (a), (c), and (d) will contribute to hardness. But each option's contribution will vary in natural sources depending on factors like solubility.

Key concepts

Water Hardness

Water hardness is an essential concept in understanding water quality, especially when considering its effects on appliances, plumbing, and daily use. Hard water is primarily characterized by high concentrations of divalent cations, particularly calcium ( Ca^{2+} ) and magnesium ( Mg^{2+} ) ions.
It occurs naturally when water flows over rocks and through soil, dissolving minerals that contain these ions. The more minerals dissolved, the harder the water becomes.
Hard water can lead to several practical issues such as:

• Scaling, which affects plumbing and reduces the efficiency of heating systems
• Soap's reduced ability to lather, impacting cleaning processes
• Potential long-term effects on skin and hair due to soap scum

Thus, understanding and managing water hardness is vital for maintaining both household systems and personal care practices.

Calcium Ions

Calcium ions ( Ca^{2+} ) are one of the key contributors to water hardness. They originate from minerals like calcium carbonate (CaCO₃) or calcium sulfate (CaSO₄) dissolved in water.
The presence of calcium ions is often responsible for the formation of limescale in pipes and appliances, which can lead to reduced efficiency and higher maintenance costs.
So how do these ions get into the water? As water percolates through the earth, it dissolves calcium-containing minerals, thus increasing the Ca^{2+} content.
Some common sources of calcium in natural water include:

• Limestone deposits, which consist largely of calcium carbonate (CaCO₃)
• Gypsum, which is rich in calcium sulfate (CaSO₄)

This process of mineral dissolution leads to increased water hardness and requires attention to drinkability and appliance longevity.

Magnesium Ions

Magnesium ions ( Mg^{2+} ) are also major players in the development of water hardness. These ions often come from minerals such as magnesium carbonate (MgCO₃) and magnesium sulfate (MgSO₄) that dissolve in the water.
Similar to calcium ions, magnesium ions contribute to the buildup of scale in pipes and heating systems, which can reduce their efficiency and increase energy usage.
Magnesium ions are dissolved into water when it comes into contact with magnesium-rich rocks and soils. This natural interaction increases water's mineral content.
Some sources of magnesium in water include:

• Dolomite, which contains calcium and magnesium carbonate
• Magnesite, primarily composed of magnesium carbonate (MgCO₃)

Striking a balance in magnesium levels in water is crucial to preventing scale formation while maintaining nutrient adequacy.