Students should be able to: describe a simple model of the Earth’s…

The idea

A complete description of the atmospheric model has four parts: air consists of particles in constant random motion; gravity holds them in a thin shell around the Earth; this shell is densest at the bottom due to compression by the air above; and pressure arises from particles colliding with surfaces. The variation in pressure follows directly from this model. For example, as a balloon rises, the column of air above it becomes shorter and less dense, reducing the weight of air pressing down. Simultaneously, the local air density decreases, meaning fewer particles strike each square metre per second. Both mechanisms explain the pressure drop. Examiners look for these logical links, so always write the full chain of reasoning.

What earns the marks

• Describe the atmosphere as a thin layer of particles in random motion held by gravity.
• State that the atmospheric layer is denser near the ground due to compression.
• Explain that atmospheric pressure is produced by particle collisions with a surface.
• Link greater height to fewer molecules above and fewer collisions, resulting in lower pressure.

Common mistakes and how to put them right

• Describing the particle model but failing to use it to explain the variation in pressure.

• Stating 'because it is higher up' as an explanation, without providing the physical mechanism.

• Claiming there is less gravity at altitude, rather than explaining the reduced weight of air.

Exam tips

• Structure your answer in ordered steps: particles, gravity, density, and collisions.
• Use linking words like 'so' or 'therefore' to show the logical progression of your explanation.