Assertion (A): Following is a balanced chemical equation for the action of steam on iron: $3\text{Fe} + 4\text{H}_2\text{O} \longrightarrow \text{Fe}_3\text{O}_4 + 4\text{H}_2$ .

Reason (R): The law of conservation of mass holds good for a chemical equation.

Chemical Reactions and Equations

Balanced Chemical Equations

Options

(a)Both (A) and (R) are true and (R) is the correct explanation of the assertion (A).

(b)Both (A) and (R) are true, but (R) is not the correct explanation of the assertion (A).

(c)(A) is true, but (R) is false.

(d)(A) is false, but (R) is true.

Official Answer

Both (A) and (R) are true and (R) is the correct explanation of the assertion (A).

balanced chemical equationlaw of conservation of massequal number of atomscorrect explanation

Marking Scheme

11 mark for identifying that both Assertion and Reason are true and Reason is the correct explanation.

Hint

Count the number of atoms of each element on both sides of the equation to check if it is balanced, and recall why we balance chemical equations.

Quick Oral Answer

We balance a chemical equation to satisfy the Law of Conservation of Mass, which states that mass can neither be created nor destroyed in a chemical reaction. Hence, the total number of atoms of each element must be equal on both sides of the equation.

Analysis & Explanation

The assertion states that $3\text{Fe} + 4\text{H}_2\text{O} \longrightarrow \text{Fe}_3\text{O}_4 + 4\text{H}_2$ is a balanced chemical equation for the action of steam on iron. Counting the atoms on both sides: LHS has $3\text{ Fe}$ , $8\text{ H}$ , and $4\text{ O}$ atoms; RHS has $3\text{ Fe}$ , $8\text{ H}$ , and $4\text{ O}$ atoms. Thus, the equation is balanced, making the assertion true. The reason states that the law of conservation of mass holds good for a chemical equation, which is also true because mass can neither be created nor destroyed in a chemical reaction. Since the law of conservation of mass is the fundamental reason why we must balance a skeletal chemical equation (to ensure the number of atoms of each element remains the same before and after the reaction), the reason correctly explains why the given equation is balanced. Therefore, option (a) is correct.

Common Mistakes

1Students often fail to recognize that water in the form of steam is represented as $\text{H}_2\text{O(g)}$ .
2Some students might think the reason is not the correct explanation, confusing the definition of balancing with the law that necessitates it.

Interesting Facts

Iron does not react with cold or hot water, but it reacts vigorously with steam to form magnetic iron oxide (ferrosoferric oxide, $\text{Fe}_3\text{O}_4$ ) and hydrogen gas.

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Frequently Asked Questions

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

This question carries 1 mark 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 "Balanced Chemical Equations" from CBSE Class 10 Science.

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

This is a MCQ question from Section A in the CBSE Class 10 Science 2020 paper.