Sample evaluation
Consider the skeletal equation Mg + O₂ → MgO. Is it balanced? Explain what a skeletal equation is, and write the balanced equation for burning magnesium in air.
Student’s answer: Mg + O₂ → MgO is not balanced because there are 2 oxygen on the left and 1 on the right. Balanced: 2Mg + O₂ → 2MgO.
Marks: 3 / 5
What earned marks
- Spotted that oxygen atoms do not match on both sides.
- Wrote the correct balanced equation.
Where marks were lost
- Did not explain what a skeletal equation is.
- Did not mention the law of conservation of mass.
Fix next: Define the term first, then show the atom count on each side before balancing.
