Problem 9 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The temporary hardness of water is due to: (a) \(\mathrm{Na}_{2} \mathrm{SO}_{4}\) (b) \(\mathrm{NaCl}\) (c) \(\mathrm{Ca}\left(\mathrm{HCO}_{3}\right)_{2}\) (d) \(\mathrm{CaCl}_{2}\)

Step-by-Step Solution

Answer

Option (c) \(\mathrm{Ca}\left(\mathrm{HCO}_{3}\right)_{2}\) causes temporary hardness.

Step 1: Understanding Water Hardness

Water hardness is determined by the presence of dissolved minerals, primarily calcium and magnesium compounds. Temporary hardness is specifically due to bicarbonates of calcium and magnesium, which can be removed by boiling the water, causing them to precipitate as carbonate salts.

Step 2: Analyzing the Options

Let's analyze each given option to identify if it contributes to temporary hardness:(a) \(\mathrm{Na}_{2} \mathrm{SO}_{4}\): Sodium sulfate is a soluble salt but does not contribute to hardness as it lacks calcium or magnesium.(b) \(\mathrm{NaCl}\): Sodium chloride is common table salt and also does not contribute to water hardness since it does not contain calcium or magnesium.(c) \(\mathrm{Ca}\left(\mathrm{HCO}_{3}\right)_{2}\): Calcium bicarbonate is a bicarbonate salt. Its presence in water causes temporary hardness, which can be removed via boiling.(d) \(\mathrm{CaCl}_{2}\): Calcium chloride contributes to permanent hardness, as it cannot be easily removed by boiling.

Step 3: Identifying the Correct Answer

From the analysis, \(\mathrm{Ca}\left(\mathrm{HCO}_{3}\right)_{2}\) is responsible for temporary hardness in water because it is a bicarbonate and can be removed by heating (boiling).

Key Concepts

Temporary Hardness

Temporary hardness in water is primarily caused by the presence of calcium bicarbonate and magnesium bicarbonate. This type of hardness can be identified by its ability to be removed through boiling. When water containing these bicarbonates is heated, a chemical reaction takes place where the bicarbonates decompose into carbonate salts, releasing carbon dioxide in the process. This reaction forms an insoluble precipitate that can be filtered out:\[ \text{Ca(HCO}_3\text{)}_2 \rightarrow \text{CaCO}_3 + \text{CO}_2 + \text{H}_2\text{O} \]Here, calcium bicarbonate decomposes into calcium carbonate, a white solid, making the water softer.

• Temporary hardness is so named because it can be removed temporarily by boiling.
• This type of hardness doesn't require chemical treatment to be resolved, contrasting with permanent hardness which does require such methods.

Boiling is a common method used in households to soften drinking water temporarily.

Calcium Bicarbonate

Calcium bicarbonate, denoted as \( \text{Ca(HCO}_3\text{)}_2 \), is a soluble compound found naturally in water systems, especially those that flow over limestone. It forms when carbon dioxide in the air dissolves in water, creating carbonic acid, which then reacts with calcium carbonate to form calcium bicarbonate:\[ \text{CaCO}_3 + \text{H}_2\text{O} + \text{CO}_2 \rightarrow \text{Ca(HCO}_3\text{)}_2 \]Calcium bicarbonate itself is not directly extractable, and it typically exists dissolved in natural water.

• This compound is responsible for the temporary hardness of water.
• When it decomposes, it yields calcium carbonate which forms scale, often seen in kettles and boilers.

Understanding the role of calcium bicarbonate helps in identifying causes of water hardness and informs methods to treat or test water quality.

Dissolved Minerals in Water

Dissolved minerals are naturally occurring substances that are carried by water as it flows through rocks and soil. These minerals include calcium, magnesium, sodium, potassium, and others. The presence of these dissolved minerals contributes to the water's properties, including its hardness.

• Calcium and magnesium are the primary contributors to water hardness.
• Sodium and potassium do not contribute to hardness but can affect the water's taste and nutritional balance.

The presence of these minerals makes water an essential nutrient source. However, excessive mineral content can lead to hardness, impacting water's usability for industrial, domestic, and personal purposes. Thus, understanding the type and amount of dissolved minerals is vital for determining water treatment needs and ensuring water quality.