Science (086) — AI-generated practice question

Meaning of a Balanced Equation:
A chemical equation is said to be balanced when the number of atoms of each element is the same on both the reactant side (LHS) and the product side (RHS). This is required by the law of conservation of mass, which states that mass can neither be created nor destroyed in a chemical reaction.

Hit-and-Trial Method (Example):

Consider the skeletal equation:
$$\text{Fe} + \text{H}_2\text{O} \rightarrow \text{Fe}_3\text{O}_4 + \text{H}_2$$

Step 1: Draw boxes around each formula; do not change any formula while balancing.

Step 2: Count atoms on each side:

| Element | LHS | RHS |
|---------|-----|-----|
| Fe | 1 | 3 |
| O | 1 | 4 |
| H | 2 | 2 |

Step 3: Start with the compound having the most atoms — Fe₃O₄. Balance O first: put coefficient 4 before H₂O → 4 O on LHS.

Step 4: Now H = 8 on LHS; put coefficient 4 before H₂ → 8 H on RHS.

Step 5: Balance Fe: put coefficient 3 before Fe on LHS.

Balanced equation:
$$3\text{Fe} + 4\text{H}_2\text{O} \rightarrow \text{Fe}_3\text{O}_4 + 4\text{H}_2$$

Verify: Fe = 3, H = 8, O = 4 on both sides. ✓

This method of using smallest whole-number coefficients by trial is called the hit-and-trial method.

Source: Chapter 1, Section 1.1.2 — Balanced Chemical Equations

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• Key concept to state first: link balancing to the law of conservation of mass — examiners look for this explicitly.
• The example: The textbook uses Fe + H₂O → Fe₃O₄ + H₂ as the standard worked example; use it unless you choose the Mg + O₂ example, but the Fe one better illustrates the steps.
• Steps matter: CBSE expects you to show the step-by-step logic (identify unbalanced element → add coefficients → verify). A bare answer without steps loses method marks.
• Never change formulae — only add coefficients. This is a common mistake examiners watch for.
• Verification at the end is expected for full marks on a 5-mark question.