A sealed bag of crisps is packed at sea level and then taken to the top of a high mountain. Explain why the bag appears more inflated at the top of the mountain than it did at sea level.

OCR GCSE Physics A: Gateway Science (J249) — P1.1 The particle model · Explain · 4 marks · View as Markdown

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

A hiker notices that a sealed bag of crisps, which felt firm at sea level, feels puffy and expanded when she reaches the summit of a high mountain.

Model answer (4 marks)

At sea level the atmospheric pressure is higher than at the summit. The weight of the air column above the hiker is larger, so the external pressure on the bag is greater. The bag is sealed, so the gas inside cannot escape; its pressure stays the same. With lower external pressure, the pressure inside the bag is now greater than the pressure outside, so the gas pushes the bag’s walls outward. The bag therefore expands and looks more inflated at the top of the mountain.

Examiner tips

  • Use the word ‘pressure’ and ‘external’/‘internal’ clearly. Show the cause–effect chain: lower external pressure → higher internal‑external pressure difference → expansion. Keep the answer concise and use the exact terms from the mark scheme.
  • Mention that the bag is sealed so the gas cannot escape – this explains why the internal pressure remains unchanged.
  • Use UK spelling: ‘pressure’, ‘seals’, ‘inflated’.

Common mistakes

  • Confusing the direction of pressure change (saying the bag expands because external pressure increases).
  • Forgetting to state that the bag is sealed and the internal pressure remains constant.

Mark scheme (4 marks)

  1. Atmospheric pressure decreases with increasing altitude / at the top of the mountain atmospheric pressure is lower than at sea level
  2. There are fewer air molecules above the hiker at greater altitude, so the weight of air above is smaller
  3. The pressure of the gas inside the bag remains the same / the gas inside the bag exerts a greater force outward than the air outside pushes inward
  4. The greater pressure inside than outside causes the bag to expand / inflate

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