A student reacts zinc with dilute hydrochloric acid in an open conical flask placed on a balance. The student notices that the reading on the balance decreases as the reaction proceeds. Explain why the mass appears to decrease, even though mass is conserved during the reaction.

AQA GCSE Chemistry (8462) — 4.3.1 Chemical measurements, conservation of mass and the quantitative interpretation of chemical equations · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

1. The reaction between zinc and dilute hydrochloric acid produces hydrogen gas.
2. Because the flask is open, the hydrogen gas escapes into the atmosphere.
3. The escaping gas is no longer on the balance, so the measured mass decreases.
4. The atoms are still conserved; the loss of mass from the balance is only due to the gas leaving the system, not to a violation of the law of conservation of mass.

Examiner tips

  • Use the word "produces" to show the reaction product. Mention the open flask to explain escape of gas. State that the gas is not on the balance. Conclude that conservation of mass still holds.

Common mistakes

  • Forgetting to mention that the gas escapes. Saying the mass actually disappears instead of explaining the gas leaving the system. Using vague terms like "something is lost" without specifying hydrogen gas.

Mark scheme (4 marks)

  1. Hydrogen gas is produced as a product of the reaction
  2. The gas escapes / is released into the atmosphere (from the open flask)
  3. The mass of the escaping gas is not measured / not included in the balance reading
  4. The law of conservation of mass is still obeyed because no atoms are lost or made during the reaction

Key terms in this question

balance

Related

More Chemical measurements, conservation of mass and the quantitative interpretation of chemical equations questions

▶ Try answering this question with AI marking (free) →