AI-generated practice question — model-generated for extra practice, not a previous-year CBSE board question.
The equation Mg + O₂ → MgO is called a skeletal chemical equation because it only shows the reactants and products using chemical formulae, but the number of atoms of each element is not equal on both sides. On the LHS, there are 2 oxygen atoms (O₂), but only 1 on the RHS (MgO). This makes it unbalanced.
To make it scientifically valid, it must be balanced by adding appropriate coefficients:
$$2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}$$
This step is necessary because of the Law of Conservation of Mass, which states that mass can neither be created nor destroyed in a chemical reaction. Therefore, the number of atoms of each element must be equal on both sides.
Source: Chapter 1, Sections 1.1.1 and 1.1.2
Examiners look for three things here (1 mark each): (1) defining "skeletal equation" and identifying the imbalance, (2) writing the correctly balanced equation, and (3) naming the Law of Conservation of Mass and stating it. Don't just say "atoms are not equal" — explain which atom is unequal as evidence. Always write the balanced equation explicitly; it earns a dedicated mark.