Magnesium reacts with hydrochloric acid to form magnesium chloride and hydrogen gas. The balanced symbol equation for this reaction is: Mg + 2HCl → MgCl₂ + H₂. Describe what the balanced symbol equation shows about the atoms and formula units involved in this reaction, and explain why the equation must be balanced.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Magnesium is a reactive metal that fizzes vigorously when added to dilute hydrochloric acid. The balanced symbol equation for this reaction is: Mg + 2HCl → MgCl₂ + H₂.
Model answer (4 marks)
One atom of magnesium reacts with two molecules of hydrochloric acid to give one formula unit of magnesium chloride and one molecule of hydrogen gas.
The equation shows that the numbers of each type of atom are the same on both sides – one Mg, two Cl and two H – so the atoms are conserved.
Because atoms cannot be created or destroyed, the equation must be balanced to reflect the conservation of mass.
The equation shows that the numbers of each type of atom are the same on both sides – one Mg, two Cl and two H – so the atoms are conserved.
Because atoms cannot be created or destroyed, the equation must be balanced to reflect the conservation of mass.
Examiner tips
- State the stoichiometry of each species clearly; use the word "atom" for Mg and "molecule" for HCl and H₂.
- Explain that the same number of each atom appears on both sides – this shows conservation of mass.
- Mention the law of conservation of mass explicitly.
- Use correct symbols and units (e.g. Mg, HCl, MgCl₂, H₂).
Common mistakes
- Using the wrong unit for HCl (e.g. molecule vs formula unit).
- Failing to mention the conservation of mass or the law of conservation of mass.
- Leaving out the fact that one Mg atom reacts with two HCl molecules.
Mark scheme (4 marks)
- One atom/formula unit of magnesium reacts with two molecules/formula units of hydrochloric acid (HCl)
- One formula unit of magnesium chloride (MgCl₂) and one molecule of hydrogen (H₂) are produced
- The total number of each type of atom is the same on both sides of the equation / atoms are conserved
- Atoms cannot be created or destroyed in a chemical reaction, so the equation must be balanced to reflect this / law of conservation of mass
Key terms in this question
balanced symbol equation · atom · formula unit
Related
- All Pearson Edexcel International GCSE Chemistry (4CH1) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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