A compound 'A' is used in the manufacture of cement. When dissolved in water, it evolves a large amount of heat and forms compound 'B'. (i) Identify A and B. (ii) Write chemical equation for the reaction of A with water. (iii) List two types of reaction in which this reaction may be classified.

Q: A compound 'A' is used in the manufacture of cement. When dissolved in water, it evolves a large amount of heat and forms compound 'B'. (i) Identify A and B. (ii) Write chemical equation for the reaction of A with water. (iii) List two types of reaction in which this reaction may be classified.

(i) Compound 'A' is **Calcium oxide** (Quick lime, $\text{CaO}$) and Compound 'B' is **Calcium hydroxide** (Slaked lime, $\text{Ca(OH)}_2$). (ii) The balanced chemical equation for the reaction of 'A' with water is: $$\text{CaO(s)} + \text{H}_2\text{O(l)} \rightarrow \text{Ca(OH)}_2\text{(aq)} + \text{Heat}$$ (iii) This reaction can be classified into the following two types: 1. **Combination Reaction**: Two reactants (calcium oxide and water) combine to form a single product (calcium hydroxide). 2. **Exothermic Reaction**: A large amount of heat is released during the reaction, making the reaction mixture warm.

Chemical Reactions and Equations

Combination Reaction

Official Answer

(i) Compound 'A' is Calcium oxide (Quick lime, $\text{CaO}$ ) and Compound 'B' is Calcium hydroxide (Slaked lime, $\text{Ca(OH)}_2$ ).

(ii) The balanced chemical equation for the reaction of 'A' with water is:

$\text{CaO(s)} + \text{H}_2\text{O(l)} \rightarrow \text{Ca(OH)}_2\text{(aq)} + \text{Heat}$

(iii) This reaction can be classified into the following two types:

Combination Reaction: Two reactants (calcium oxide and water) combine to form a single product (calcium hydroxide).
Exothermic Reaction: A large amount of heat is released during the reaction, making the reaction mixture warm.

Calcium oxideCaOCalcium hydroxideCa(OH)2Combination reactionExothermic reactionHeat

Marking Scheme

11 mark for identifying Compound A as Calcium oxide (CaO) and Compound B as Calcium hydroxide (Ca(OH)2).
21 mark for writing the correct balanced chemical equation with state symbols: $\text{CaO(s)} + \text{H}_2\text{O(l)} \rightarrow \text{Ca(OH)}_2\text{(aq)} + \text{Heat}$ .
31 mark for listing the two types of reactions: Combination reaction and Exothermic reaction (0.5 marks each).

Hint

Think about the compound used in cement manufacturing that reacts vigorously with water to release heat.

Analysis & Explanation

According to the NCERT text, Calcium oxide ( $\text{CaO}$ ), commonly known as quick lime, has a major industrial application in manufacturing cement. When water is added to it, a highly vigorous reaction occurs, releasing a significant amount of heat (exothermic nature) and forming calcium hydroxide ( $\text{Ca(OH)}_2$ ), commonly known as slaked lime. Because two reactants combine to form a single product, it is classified as a combination reaction. Because heat is evolved, it is also classified as an exothermic reaction.

Common Mistakes

1Writing incorrect chemical formulae like $\text{CaOH}$ instead of $\text{Ca(OH)}_2$ .
2Forgetting to mention 'Heat' in the chemical equation or failing to classify it as an exothermic reaction.

Interesting Facts

A solution of slaked lime is used for whitewashing walls. It reacts slowly with carbon dioxide in the air to form a thin, shiny layer of calcium carbonate ( $\text{CaCO}_3$ ) after 2 to 3 days.

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Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2020?

This question carries 3 marks in the CBSE Class 10 Science 2020 examination.

Which chapter does this question come from in Science?

This question is from the chapter "Chemical Reactions and Equations" in the CBSE Class 10 Science syllabus.

What topic does this question cover in Science?

This question covers the topic "Combination Reaction" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2020 paper?

This is a Short Answer question from Section B in the CBSE Class 10 Science 2020 paper.