Problem 16 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The temporary hardness of water due to calcium bicarbonate can be removed by adding - (a) \(\mathrm{CaCO}_{3}\) (b) \(\mathrm{Ca}(\mathrm{OH})_{2}\) (c) \(\mathrm{CaCl}_{2}\) (d) \(\mathrm{HCl}\)

Step-by-Step Solution

Answer

(b) \(\mathrm{Ca(OH)_2}\) is used to remove temporary hardness by forming \(\mathrm{CaCO}_3\).

Step 1: Understanding Temporary Hardness

Temporary hardness in water is usually caused by the presence of dissolved bicarbonate minerals, such as calcium bicarbonate \((\mathrm{Ca(HCO_3)_2})\). This kind of hardness can be removed by boiling or by chemical treatment.

Step 2: Identifying Chemical Reactions

To remove temporary hardness by chemical treatment, we need a substance that can react with calcium bicarbonate to form an insoluble compound, such as calcium carbonate \((\mathrm{CaCO_3})\), which precipitates out of the solution.

Step 3: Selecting the Appropriate Chemical

When calcium hydroxide \((\mathrm{Ca(OH)_2})\) is added to water containing calcium bicarbonate, it reacts to form calcium carbonate \(\mathrm{CaCO_3}\), an insoluble substance. The reaction is as follows: \[\mathrm{Ca(HCO_3)_2 + Ca(OH)_2 \rightarrow 2CaCO_3 + 2H_2O}.\] This means that option (b) \(\mathrm{Ca(OH)_2}\) can effectively remove temporary hardness.

Key Concepts

Calcium Bicarbonate

Calcium bicarbonate, represented chemically as \(\mathrm{Ca(HCO_3)_2}\), is a compound found naturally in water. It's one of the main culprits behind what's known as temporary hardness in water.
Calcium bicarbonate forms when carbon dioxide dissolves in water, which then reacts with calcium carbonate rocks, commonly found in the Earth's crust. This reaction creates a clear bicarbonate solution, but it adds undesired hardness to water. The term "temporary" is used because this hardness can be removed by boiling the water—this process causes calcium bicarbonate to decompose into calcium carbonate, water, and carbon dioxide, where calcium carbonate is insoluble and precipitates out.

• Formation: Dissolved carbon dioxide reacts with calcium carbonate.
• Removability: Boiling causes it to precipitate as calcium carbonate.
• Nature: It creates temporary hardness in water due to its solubility and breakdown properties.

Chemical Treatment of Water

Chemical treatment is a method used to soften water, especially when dealing with temporary hardness caused by calcium bicarbonate. This approach often involves adding a chemical that reacts with the calcium bicarbonate to form an insoluble precipitate.
In the case of water containing calcium bicarbonate, adding calcium hydroxide \((\mathrm{Ca(OH)_2})\) is an effective solution. Calcium hydroxide reacts with dissolved calcium bicarbonate to produce calcium carbonate \((\mathrm{CaCO_3})\), which is insoluble in water, and water itself.

• Method: Use chemicals to induce reactions that precipitate hardness-causing compounds.
• Effectiveness: Helps remove temporary water hardness by forming insoluble compounds.
• Example: Adding \(\mathrm{Ca(OH)_2}\) to water with \(\mathrm{Ca(HCO_3)_2}\) results in softer water.

Calcium Carbonate Precipitation

Calcium carbonate \(\mathrm{(CaCO_3)}\) forms when calcium ions react with carbonate ions in water, often as a result of chemical treatment of water hardness. This forms a solid, or a precipitate, that can be easily removed from the water system. This is how water softness is achieved from untenable hardness.
In the process of chemical treatment, calcium hydroxide \(\mathrm{(Ca(OH)_2)}\) is added. It reacts with calcium bicarbonate, as seen in the reaction formula \[\mathrm{Ca(HCO_3)_2 + Ca(OH)_2 \rightarrow 2CaCO_3 + 2H_2O}.\] The result—calcium carbonate precipitates out because unlike calcium bicarbonate, it is not soluble in water.

• Formation: Results from treatment reactions in water.
• Benefit: Allows for easy removal of hardness from water.
• Outcome: Turns hard water into soft by precipitating as solid.