Solid calcium oxide reacts vigorously with water to form calcium hydroxide accompanied by the liberation of heat, this process is called slaking of lime. Calcium hydroxide dissolves in water form its solution called lime water. Which among the following is (are) true about slaking of lime and solution formed?

(i) It is an endothermic reaction.

(ii) It is an exothermic reaction.

(iii) The pH of the resulting solution will be more than seven.

(iv) The pH of the resulting solution will be less than seven.

A. (i) and (ii)

B. (ii) and (iii)

C. (i) and (iv)

D. (iii) and (iv)

To tackle the question about the slaking of lime and the properties of the resulting solution, let's break down the process and analyze each statement provided. The reaction between solid calcium oxide (CaO) and water (H2O) is indeed a significant chemical reaction in various applications, particularly in construction and environmental science.

Understanding the Reaction

When calcium oxide reacts with water, it forms calcium hydroxide (Ca(OH)2). This reaction can be represented as:

• CaO (s) + H2O (l) → Ca(OH)2 (aq) + heat

This reaction is characterized by the release of heat, indicating that it is exothermic. The term "slaking of lime" refers to this process, where the solid lime (calcium oxide) is transformed into a more usable form (calcium hydroxide) while generating heat.

Evaluating the Statements

Now, let's assess each of the statements provided in the question:

• (i) It is an endothermic reaction. - This statement is false. Since the reaction releases heat, it is not endothermic.
• (ii) It is an exothermic reaction. - This statement is true. The release of heat during the reaction confirms that it is exothermic.
• (iii) The pH of the resulting solution will be more than seven. - This statement is also true. Calcium hydroxide is a strong base, and when it dissolves in water, it increases the pH of the solution above seven.
• (iv) The pH of the resulting solution will be less than seven. - This statement is false. As mentioned, the solution formed is basic, leading to a pH greater than seven.

Conclusion on the Statements

From our analysis, we find that statements (ii) and (iii) are correct. Therefore, the correct answer to the question is:

B. (ii) and (iii)

Why This Matters

Understanding the slaking of lime and the properties of calcium hydroxide solution is crucial in various fields, including construction, where lime is used for mortar and plaster, and in environmental science for water treatment. The basic nature of lime water makes it useful for neutralizing acidic conditions in soil and water.