Compression and gas pressure

A gas sample is sealed with no leaks or chemical reaction. What does sealed tell you about its particles?

A sealed boundary prevents material crossing it; it does not automatically stop energy transfer

Compress the sample, not each particle

Volume is the space a sample occupies. Compressing a sample means reducing that volume

Gas particles are far apart compared with their own size. A smaller volume can be made mainly by reducing the gaps, without shrinking or removing particles

Particles in liquids and solids are much closer together, so those materials are much harder to compress under ordinary conditions. Hard to compress does not mean perfectly incompressible

A liquid can flow because its particles move past one another. That movement does not give it the large empty gaps of a gas

Read a model's key before using it as evidence. A selected set of circles can represent spacing without counting the whole real sample or measuring pressure, temperature or speed

Predict whether particle count or size will change, then increase the compression setting and compare with the widest setting. This existing schematic keeps the same twelve selected particles and one snapshot. It shows spacing, not measured pressure, temperature, speed or a complete real gas population

What changes when the model container becomes narrower?

Compare gas with liquid

Collisions act on the walls

Gas particles move in different directions and collide with the container walls. In each collision they exert a force on a wall

Pressure compares the force with the area over which it acts. Over the same wall area, a greater combined collision effect gives greater gas pressure

Gas pressure acts on the sides and top as well as the bottom. A gas is not a pile of particles resting only on the floor

The comparisons here use a simple gas model away from condensation and extreme conditions. It explains useful trends; it does not make the spacing drawing a measuring instrument

Identify the cause of gas pressure

Compare at the same temperature

The same gas sample is sealed behind a movable plunger. After a volume decrease, it is allowed to return to its original temperature. No gas escapes or reacts, and no liquid forms

Predict the pressure change and explain how the controlled conditions support that prediction

The record keeps gas amount and temperature the same but reduces volume

The same moving particles collide with wall areas more frequently, giving higher pressure without requiring smaller or hotter particles

Separate spacing from particle size

State what changes and what stays fixed

At fixed particle number and volume, warmer gas has greater average particle motion energy. More frequent and stronger wall collisions produce higher pressure

At the same temperature and volume, more particles of the same gas give more wall collisions and higher pressure. More particles does not mean each particle is hotter

Rigid means the container volume stays fixed. Sealed means material does not enter or leave. Neither word by itself means the temperature stays fixed

Check all these conditions before comparing pressures. If two relevant conditions change together, the record does not isolate their separate contributions

These are conceptual comparisons, not instructions to heat or pressurise sealed containers. Real experiments need teacher-approved equipment and supervision

Warm the gas at fixed volume

Compare more particles under matched conditions

Read a combined change carefully

Choose both necessary controls

Keep the claim within the drawing

Explain a controlled comparison

Written task, not automatically marked

A teacher supplies two records for the same sealed, non-reacting gas sample. Record A: larger volume, temperature T, lower pressure. Record B: smaller volume, the same temperature T, higher pressure. The gas remains gas in both records; T names the common temperature, not a numerical value

Before reading the model response, draw particle models for A and B with a key. Write an explanation connecting the volume change to particle spacing and wall collisions. Identify the changed, measured and held-fixed quantities, and give one thing your drawing cannot measure

Check the explanation against the givens

Records for the same sealed, non-reacting gas sample show A at larger volume and lower pressure and B at smaller volume and higher pressure, both at temperature T. The sample remains gas. Use the same number of equal-size circles in a wider and a narrower container, labelling each circle as a model particle rather than a visible grain. The narrower model has smaller average gaps, not smaller particles

Volume is changed and pressure is measured. Gas type, particle number, temperature and gas state are held fixed. In the smaller volume, the same particles give more wall collisions per unit area, consistent with the higher pressure

An uncalibrated particle drawing does not measure a pressure value, temperature, particle speed or the real sample's total particle count

Review spacing, collisions and conditions

Compression reduces the available volume mainly by reducing particle gaps. Liquids and solids have much less gap to reduce than gases

Gas pressure comes from particles colliding with walls. Volume, temperature and particle number all matter, so name the held-fixed conditions

Use the complete supplied record to explain a comparison. A model can show an idea without measuring it, and a combined change cannot isolate each cause