Chapter 1 · Question 4

State the law of conservation of mass. Why must a chemical equation be balanced? Give an example of an unbalanced (skeletal) equation and show how it is balanced.

Answer Revealed

Direct Answer:

The law of conservation of mass states that mass can neither be created nor destroyed in a chemical reaction — the total mass of reactants equals the total mass of products. A chemical equation must be balanced so that the number of atoms of each element is the same on both sides. Skeletal equation:

\text{Mg} + \text{O}_2 \rightarrow \text{MgO}

. To balance:

2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}

. Now atoms of Mg (2 on both sides) and O (2 on both sides) are equal.

Simple Explanation

The law of conservation of mass says you cannot create or destroy matter in a chemical reaction — whatever goes in must come out, just rearranged. So we balance equations: we count atoms on each side and add coefficients to make them equal. Start with

\text{Mg} + \text{O}_2 \rightarrow \text{MgO}

— there is one Mg on the left but only one on the right (OK), but two O atoms on the left and only one on the right (not OK). Putting a 2 before MgO gives

\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}

. Now O is balanced but Mg is 1 on left vs 2 on right. Put a 2 before Mg:

2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}

. Done.

Exam-Ready Structure

The law of conservation of mass is a fundamental principle that governs all chemical equations: 1. Statement: In a chemical reaction, matter (mass) can neither be created nor destroyed. The total mass of the reactants before the reaction is exactly equal to the total mass of the products after the reaction. 2. Implication for chemical equations: Since atoms are neither created nor destroyed during a chemical reaction (they are only rearranged), the number of atoms of each element must be exactly the same on the reactant side (LHS) and the product side (RHS) of a chemical equation. 3. When an equation does not have an equal number of atoms for each element on both sides, it is called a skeletal chemical equation or an unbalanced equation. For example:

\text{Mg} + \text{O}_2 \rightarrow \text{MgO}

— one Mg on LHS, one on RHS (balanced for Mg), but two O on LHS and only one on RHS (unbalanced for O). 4. Balancing is done by placing coefficients (integer numbers) before the chemical formulae — the subscripts within the formulae must never be changed because that would change the identity of the compound. The hit-and-trial method involves comparing atom counts and adjusting coefficients until equality is achieved. For the example: - Step I:

2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}

. Check: Mg = 2 on both sides, O = 2 on both sides. Balanced. 5. A balanced equation is essential because it reflects the actual quantitative relationship in which substances react and ensures all calculations based on the equation are correct.

Key Points

Law of conservation of mass: mass is neither created nor destroyed in a chemical reaction
In a balanced chemical equation, the number of atoms of each element is equal on LHS and RHS
A skeletal/unbalanced equation has unequal atom counts (e.g., $\text{Mg} + \text{O}_2 \rightarrow \text{MgO}$ )
Balancing uses coefficients before formulae; subscripts within formulae must never be changed
The hit-and-trial method compares element-wise atom counts and adjusts coefficients until balanced

Common Mistakes

Changing subscripts inside a formula to balance the equation (e.g., writing MgO₂ instead of 2MgO) — this changes the compound
Forgetting that O₂ represents two oxygen atoms, not one

State the law of conservation of mass. Why must a chemical equation be balanced? Give an example of an unbalanced (skeletal) equation and show how it is balanced.