Problem 4 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The temporary hardness of a water sample is due to compound \(\mathrm{X}\). Boiling this sample converts \(\mathrm{X}\) to compound Y. \(\mathrm{X}\) and \(\mathrm{Y}\), respectively, are : [Main April 12, 2019 (II)] (a) \(\mathrm{Mg}\left(\mathrm{HCO}_{3}\right)_{2}\) and \(\mathrm{Mg}(\mathrm{OH})_{2}\) (b) \(\mathrm{Ca}\left(\mathrm{HCO}_{3}\right)_{2}\) and \(\mathrm{Ca}(\mathrm{OH})_{2}\) (c) \(\mathrm{Mg}\left(\mathrm{HCO}_{3}\right)_{2}\) and \(\mathrm{MgCO}_{3}\) (d) \(\mathrm{Ca}\left(\mathrm{HCO}_{3}\right)\), and \(\mathrm{CaO}\)

Step-by-Step Solution

Answer

(c) \(\mathrm{Mg(HCO}_3\mathrm{)}_2\) and \(\mathrm{MgCO}_3\).

Step 1: Understanding Temporary Hardness

Temporary hardness in water is primarily caused by the presence of bicarbonate compounds of calcium and magnesium, such as \(\text{Ca(HCO}_3\text{)}_2\) and \(\text{Mg(HCO}_3\text{)}_2\). These bicarbonates decompose upon heating, leading to other compounds.

Step 2: Identifying the Result of Boiling

When a water sample containing temporary hardness is boiled, the bicarbonate compounds decompose to form their respective carbonates. For example, \(\text{Ca(HCO}_3\text{)}_2\) decomposes to \(\text{CaCO}_3\) and \(\text{Mg(HCO}_3\text{)}_2\) decomposes to \(\text{MgCO}_3\). \(\text{Ca(OH)}_2\) and \(\text{Mg(OH)}_2\) are further products formed in oxidative environments. Compare this with the options listed.

Step 3: Reviewing the Given Options

- Option (a): \(\text{Mg(HCO}_3\text{)}_2\) converts to \(\text{Mg(OH)}_2\). - Option (b): \(\text{Ca(HCO}_3\text{)}_2\) converting to \(\text{Ca(OH)}_2\). - Option (c): \(\text{Mg(HCO}_3\text{)}_2\) converting to \(\text{MgCO}_3\). - Option (d): \(\text{Ca(HCO}_3\text{)}\) converting to \(\text{CaO}\).Upon boiling, the bicarbonates form carbonates rather than hydroxides or oxides directly.

Step 4: Validating the Correct Answer

Since \(\text{Mg(HCO}_3\text{)}_2\) upon boiling forms \(\text{MgCO}_3\) and \(\text{Ca(HCO}_3\text{)}_2\) forms \(\text{CaCO}_3\), option (c) \(\text{Mg(HCO}_3\text{)}_2\) and \(\text{MgCO}_3\) matches the expected chemical behavior.

Key Concepts

Bicarbonates

Bicarbonates play a key role in determining the temporary hardness of water. Temporary hardness is caused due to the presence of bicarbonate salts of calcium and magnesium—that's compounds like \( \text{Ca(HCO}_3\text{)}_2 \) and \( \text{Mg(HCO}_3\text{)}_2 \). These bicarbonates dissolve readily in water, contributing to hardness.

• Easily Dissolvable: Bicarbonates dissolve in water creating ions that contribute to the hardness.
• Sources: These compounds often originate from the natural leaching of mineral deposits, common in areas with limestone or chalk.
• Reversible Effect: Unlike permanent hardness, the effects of bicarbonates can be reversed by boiling, which precipitates them out of the water.

When water with such bicarbonates is boiled, it leads to the decomposition of these compounds, forming more stable carbonates. This is why bicarbonates are considered to cause temporary, rather than permanent, water hardness.

Calcium Carbonate

Calcium carbonate is the product of decomposed calcium bicarbonate \( \text{Ca(HCO}_3\text{)}_2 \) when water is boiled. It becomes well-known because it often forms scales inside kettles and hot water pipes.

• Formation: When calcium bicarbonate is heated, it breaks down into calcium carbonate \( \text{CaCO}_3 \), water, and carbon dioxide.
• Deposition: This insoluble compound precipitates out of the water, leading to deposits or scales, which can clog plumbing systems.
• Impact on Hardness: Since it precipitates out, calcium carbonate does not contribute to water hardness after boiling.

This transformation is a perfect natural demonstration of chemical reactions involved in temporary hardness.

Magnesium Carbonate

Magnesium carbonate forms from magnesium bicarbonate \( \text{Mg(HCO}_3\text{)}_2 \) when the latter is heated in water. Like calcium carbonate, magnesium carbonate is a solid, insoluble byproduct.

• Decomposition: Upon boiling, the magnesium bicarbonate decomposes into magnesium carbonate \( \text{MgCO}_3 \), releasing water and carbon dioxide.
• Characteristics: Unlike the bicarbonates, magnesium carbonate does not dissolve in water, thus forming particles or deposits that remain after the water is boiled.
• Role in Hardness: This helps to reduce the hardness of water since these particles are effectively removed from the water solution once formed.

Magnesium carbonate showcases how temporary hardness can be effectively managed by simply heating the water.