Science — AI-generated practice question

Law of Conservation of Mass: Mass can neither be created nor destroyed in a chemical reaction. The total mass of reactants equals the total mass of products.

Why a skeletal equation must be balanced: A skeletal equation only shows the formulae of reactants and products but does not confirm equal numbers of atoms on both sides. Since the number of atoms of each element must remain the same before and after a reaction, the equation must be balanced.

Example: Mg + O₂ → MgO is a skeletal equation.

• LHS: 1 Mg, 2 O atoms
• RHS: 1 Mg, 1 O atom

The atoms are unequal, so it is unbalanced. The balanced equation is:
$$2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}$$
Now both sides have 2 Mg and 2 O atoms.

Source: Chapter 1, Sections 1.1.1 and 1.1.2

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• 1 mark for stating the law correctly (mass neither created nor destroyed; total mass of reactants = total mass of products).
• 1 mark for explaining why balancing is needed (number of atoms of each element must be equal on both sides to satisfy the law).
• 1 mark for correctly illustrating with Mg + O₂ → MgO — showing the imbalance in the skeletal equation and writing the balanced form 2Mg + O₂ → 2MgO.

Key reminder: Never change the formula of a compound to balance (e.g., do not write MgO₂); only use coefficients in front of the formula.