Question

Chemical \(A\) is used for water softening to remove temporary hardness. A reacts with \(\mathrm{Na}_{2} \mathrm{CO}_{3}\) to generate caustic soda. When \(\mathrm{CO}_{2}\) is bubbled through \(\mathrm{A}\), it turns cloudy. What is the chemical formula of \(\mathrm{A}\). (1) \(\mathrm{CaCO}_{3}\) (2) \(\mathrm{CaO}\) (3) \(\mathrm{Ca}(\mathrm{OH})_{2}\) (4) \(\mathrm{Ca}\left(\mathrm{HCO}_{3}\right)_{2}\)

Short answer

The chemical formula of \(A\) is \( Ca(\backslash mathrm \ OH \ )_{2} \).

Step-by-step solution

Title - Analyze the reaction with \(\backslash mathrm \ Na_{2}CO_{3} \)

Chemical \(A\) reacts with \(Na_{2}CO_{3}\) to generate caustic soda \(\backslash mathrm \ NaOH \). This suggests that \(A\) could be \(Ca(\backslash mathrm \ OH \ )_{2} \), as it fits the reaction equation: \( Ca(\backslash mathrm \ OH \ )_{2} + Na_{2}CO_{3} --> 2NaOH + CaCO_{3} \).

Title - Analyze the reaction with \( \backslash mathrm \ CO_{2} \)

When \(CO_2\) is bubbled through \(A\), it turns cloudy. This indicates the formation of \(CaCO_{3}\), which is insoluble and causes cloudiness. The reaction \( Ca(\backslash mathrm \ OH \ )_{2} + CO_{2} --> CaCO_{3} + H_{2}O \) supports this.

Title - Confirm the identification

From the reactions and their products, \( Ca(\backslash mathrm \ OH \ )_{2} \) is identified as the substance that fits all given conditions: it reacts with \(Na_{2}CO_{3}\) to form \(NaOH\) (caustic soda) and causes cloudiness when reacting with \(CO_{2}\).

Key concepts

headline of the respective core concept

Understanding how to remove temporary hardness in water is crucial for many industrial and household applications. Temporary hardness is primarily caused by the presence of dissolved bicarbonates of calcium and magnesium. Removal usually involves converting these bicarbonate compounds into insoluble substances. Other treatments might remove or neutralize them with the help of other chemicals.

headline of the respective core concept

Chemical reactions are central to understanding water softening. In the provided problem, Chemical A reacts with \textbackslash \text{mathrm } \text Na_{2}CO_{3} \textbackslash to produce caustic soda (\text{NaOH}). The reaction can be represented as: \( Ca(OH)_{2} + Na_{2}CO_{3} \rightarrow 2NaOH + CaCO_{3} \). Here, calcium hydroxide \( Ca(OH)_{2} \) plays a key role by reacting with sodium carbonate to produce calcium carbonate (\( CaCO_{3} \)), which is insoluble. This calcium carbonate precipitates out of the solution, helping to ‘soften’ the water by removing hardness-causing ions.

headline of the respective core concept

For treatments to remove temporary hardness, it’s essential to use the right chemicals. In this context, calcium hydroxide (\(Ca(OH)_{2}\)) is often used. It reacts with carbonates and bicarbonates to precipitate out calcium carbonate, which can then be filtered out. This reaction helps convert soluble bicarbonates in water into an insoluble form, thus softening the water. This method is efficient and widely used in various water softening processes around the world.