Science (086) — AI-generated practice question

(i) The equation Mg + O₂ → MgO is called a skeletal chemical equation because it only represents the reactants and products using chemical formulae, without indicating the correct number of atoms/molecules of each substance.

(ii) It is unbalanced because the number of atoms of each element is not equal on both sides — there are 2 oxygen atoms on LHS but only 1 on RHS, violating the law of conservation of mass.

Balanced equation:

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

Source: Chapter 1, Section 1.1.1 Writing a Chemical Equation

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• Examiners expect two distinct points: definition of skeletal equation + reason it is unbalanced. Both parts carry marks separately.
• The balanced equation must be shown explicitly — simply stating "add coefficient 2" is not enough.
• Key phrase to use: "law of conservation of mass" — it signals you understand why balancing is necessary.
• Do not change the formula of any compound (e.g., do not write MgO₂); only add coefficients.