### Correct Option: B. Adding excess calcium hydroxide #### Explanation of the Correct Answer: To understand why adding excess calcium hydroxide (Ca(OH)₂) is the correct method for removing hardness from water containing calcium sulfate (CaSO₄) and calcium hydrogen carbonate (Ca(HCO₃)₂), we need to look at the nature of hardness in water and the chemical reactions involved. 1. **Types of Hardness**: - **Temporary Hardness**: Caused by the presence of calcium hydrogen carbonate. This type of hardness can be removed by boiling the water, which converts the bicarbonate ions (HCO₃⁻) into insoluble calcium carbonate (CaCO₃). - **Permanent Hardness**: Caused by calcium sulfate and other soluble salts. This type of hardness cannot be removed by boiling. 2. **Role of Calcium Hydroxide**: - When excess calcium hydroxide is added to hard water, it reacts with both types of hardness: - For **calcium hydrogen carbonate**: \[ \text{Ca(HCO}_3\text{)}_2 + \text{Ca(OH)}_2 \rightarrow 2\text{CaCO}_3 \downarrow + 2\text{H}_2\text{O} \] Here, calcium carbonate precipitates out of the solution, effectively removing the hardness. - For **calcium sulfate**, while it does not react directly with calcium hydroxide, the presence of excess calcium ions can help in the precipitation of calcium sulfate under certain conditions, but primarily, the focus is on the removal of temporary hardness. 3. **Why Other Options Are Incorrect**: - **Option A: Boiling the water**: - Boiling is effective for removing temporary hardness (calcium hydrogen carbonate) but does not affect permanent hardness (calcium sulfate). Therefore, this option is not suitable for a sample containing both types of hardness. - **Option C: Adding a calculated amount of calcium hydroxide**: - While adding a calculated amount of calcium hydroxide can help in softening water, it may not be sufficient to ensure complete removal of hardness, especially if the water contains a high concentration of hardness ions. Excess calcium hydroxide ensures that all the bicarbonate ions are converted to insoluble calcium carbonate, making this option less effective than option B. - **Option D: Adding sodium carbonate**: - Sodium carbonate (Na₂CO₃) can precipitate calcium ions as calcium carbonate, but it does not address the calcium sulfate directly. Additionally, it may lead to the formation of soluble sodium sulfate, which does not help in softening the water. Thus, it is not as effective as adding excess calcium hydroxide. #### Summary of Key Points: - **Temporary hardness** can be removed by boiling, while **permanent hardness** requires different methods. - **Calcium hydroxide** reacts with calcium hydrogen carbonate to form insoluble calcium carbonate, effectively removing hardness. - **Excess calcium hydroxide** ensures complete precipitation of hardness-causing ions. - Other options either do not address all types of hardness or are less effective. ### Revision Summary: - Hard water contains calcium compounds that cause hardness (temporary and permanent). - Adding excess calcium hydroxide precipitates calcium carbonate, removing hardness. - Boiling only removes temporary hardness; sodium carbonate does not effectively address permanent hardness. - Always consider the type of hardness present when choosing a method for water softening.