A student dissolves a small amount of sodium hydroxide pellets in water inside an open beaker. After the reaction is complete, the student measures the total mass of the beaker and its contents. She notices the mass is slightly greater than the mass of just the sodium hydroxide pellets and water measured separately before mixing. Explain why the total mass appears to have increased, and describe what the law of conservation of mass tells us about what actually happened to the atoms during this process.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Sodium hydroxide dissolves exothermically in water. The student carried out the experiment on a balance in an open laboratory.
Model answer (4 marks)
The law of conservation of mass states that mass cannot be created or destroyed in a chemical reaction or physical process.
The apparent increase in mass occurs because the student did not account for the mass of water vapour or moisture that was absorbed from the air during the experiment. Sodium hydroxide is hygroscopic and can take up water from the atmosphere, so the beaker and its contents gain mass.
During the dissolving process no atoms are gained or lost; the sodium hydroxide and water molecules simply mix and rearrange.
If the experiment were carried out in a fully closed or sealed system, the total mass before and after would be equal and unchanged.
The apparent increase in mass occurs because the student did not account for the mass of water vapour or moisture that was absorbed from the air during the experiment. Sodium hydroxide is hygroscopic and can take up water from the atmosphere, so the beaker and its contents gain mass.
During the dissolving process no atoms are gained or lost; the sodium hydroxide and water molecules simply mix and rearrange.
If the experiment were carried out in a fully closed or sealed system, the total mass before and after would be equal and unchanged.
Examiner tips
- Use the exact phrase "law of conservation of mass". Explain the hygroscopic nature of NaOH and the role of atmospheric moisture. Mention that no atoms are created or destroyed. State that a closed system would give equal mass.
Common mistakes
- Claiming the mass increased because of a chemical reaction, ignoring the role of moisture. Forgetting to mention that NaOH is hygroscopic. Saying the mass actually increased, rather than explaining it was an apparent increase.
Mark scheme (4 marks)
- The law of conservation of mass states that mass (atoms/matter) cannot be created or destroyed in a chemical reaction or physical process
- The apparent mass increase is because the student did not account for the mass of water vapour / moisture absorbed from the air, OR sodium hydroxide is hygroscopic and absorbs water from the atmosphere
- No atoms are gained or lost during dissolving — the sodium hydroxide and water particles simply mix / rearrange
- If the experiment were carried out in a fully closed / sealed system, the total mass before and after would be equal / unchanged
Key terms in this question
Related
- All Eduqas GCSE Chemistry revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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