Question

The reagent(s) used for softening the temporary hardness of water is(are) A) \(\mathrm{Ca}_{3}\left(\mathrm{PO}_{4}\right)_{2}\) B) \(\mathrm{Ca}(\mathrm{OH})_{2}\) C) \(\mathrm{Na}_{2} \mathrm{CO}_{3}\) D) \(\mathrm{NaOCl}\)

Short answer

The reagents used for softening temporary hardness of water are \(\mathrm{Ca(OH)}_{2}\) and \(\mathrm{Na}_{2} \mathrm{CO}_{3}\), corresponding to Options B and C.

Step-by-step solution

Understanding the Concept of Temporary Hardness

Temporary hardness in water is due to the presence of dissolved bicarbonate minerals (calcium and magnesium bicarbonates). It can be removed through a process called water softening, often by converting bicarbonates into insoluble carbonates, which can then be filtered out.

Identifying the Reagent for Removing Temporary Hardness

To soften water with temporary hardness, a reagent that can react with the bicarbonate ions to produce insoluble carbonates must be used. Calcium hydroxide (\(\mathrm{Ca(OH)}_{2}\)) and sodium carbonate (\(\mathrm{Na}_{2}\mathrm{CO}_{3}\)) are commonly used for this purpose. They react with calcium and magnesium bicarbonates to precipitate calcium carbonate and magnesium carbonate, respectively.

Selecting the Correct Answer

Among the given options, both Calcium hydroxide (Option B) and Sodium carbonate (Option C) can be used to remove temporary hardness from water. Thus, the correct answer includes both Option B and Option C.

Key concepts

Water Softening Process

The water softening process is a treatment method aimed at reducing and removing the minerals that cause water hardness, specifically temporary hardness. This type of hardness is typically characterized by the presence of dissolved bicarbonate minerals and can be treated to make the water more suitable for various uses, including drinking, cooking, and cleaning.

During softening, chemical reactions are induced to change the soluble bicarbonates into insoluble carbonates, which can then be easily filtered from the water. This removal of calcium and magnesium, the ions responsible for water hardness, not only improves the taste and quality of the water but also prevents scale formation, which can damage plumbing and appliances.

Dissolved Bicarbonate Minerals

Dissolved bicarbonate minerals primarily refer to calcium bicarbonate and magnesium bicarbonate. These are soluble compounds that can make water 'temporarily' hard. The reason it's called 'temporary' hardness is because it can be removed quite easily by heating the water or by adding certain chemicals that cause these bicarbonate minerals to precipitate or settle out of the water.

This is significant because the presence of these minerals can interfere with the action of soap, forming a scum, and can lead to the build-up of limescale in water systems, which is detrimental to heating elements and surfaces where water evaporates.

Calcium Hydroxide

Calcium hydroxide, often known as slaked lime, is a chemical compound with the formula \(\mathrm{Ca(OH)}_{2}\). It's a mainstay reagent in the softening of temporary water hardness. When introduced to water containing dissolved bicarbonate ions, calcium hydroxide triggers a chemical reaction that produces calcium carbonate, a white insoluble solid that can be filtered from the water:

\(\mathrm{Ca(HCO3)_2} + \mathrm{Ca(OH)_2} \rightarrow 2\mathrm{CaCO_3}\downarrow + 2\mathrm{H_2O}\)

This reaction illustrates how calcium hydroxide effectively reduces hardness by precipitating dissolved bicarbonates, resulting in softened water.

Sodium Carbonate

Sodium carbonate, also called soda ash, is another key agent in removing temporary hardness from water. Its chemical formula is \(\mathrm{Na}_{2}\mathrm{CO}_{3}\). This compound adds to the softening process by converting soluble calcium and magnesium bicarbonates into insoluble carbonates, which then precipitate. The reactions can be summarized as follows:

\(\mathrm{Ca(HCO3)_2} + \mathrm{Na2CO3} \rightarrow \mathrm{CaCO3}\downarrow + \mathrm{2NaHCO3}\)
\(\mathrm{Mg(HCO3)_2} + \mathrm{Na2CO3} \rightarrow \mathrm{MgCO3}\downarrow + \mathrm{2NaHCO3}\)

This demonstrates that sodium carbonate is effective in reducing water hardness by facilitating the removal of calcium and magnesium through precipitation.