What happens when we apply pressure to the particles of matter: gases compress, then liquefy, and why solids and liquids barely change, explained for Class 9 Matter in Our Surroundings with diagrams

What happens when we apply pressure to the particles of matter? is a question from Class 9 CBSE chapter 1, Matter in Our Surroundings, and it appears in almost the same words in Year 8 and Year 9 science in the UK, Australian and IB middle-school courses. The one-line answer is that the particles are pushed closer together. The full answer explains why that compresses a gas, why it can turn a gas into a liquid, and why it does almost nothing to a solid or a liquid, and that full answer is built from the four characteristics of particles that the chapter starts with.

The four characteristics of the particles of matter

1. Particles are very small. A drop of water has about 10²¹ molecules; a single crystal of potassium permanganate colours a whole beaker.
2. Particles have space between them. Sugar dissolves in a full glass of water without the water overflowing: the sugar particles fit into the spaces between the water particles.
3. Particles are continuously moving. They have kinetic energy; the higher the temperature, the faster they move. The smell of an incense stick reaches the far corner of a room (diffusion) because gas particles move and spread.
4. Particles attract each other. The force is strong in solids (a block of iron cannot be pulled apart), weaker in liquids (water can be cut with a hand and flows back), and very weak in gases.

Pressure works on characteristic 2 (the space) and, at the end, characteristic 4 (the attraction).

What pressure does to a gas

A gas has its particles separated by distances many times their own size, so most of the volume of a gas is empty space. When the gas is squeezed into a smaller volume (a bicycle pump, a cylinder), the particles are pushed closer together and the empty space is reduced: the gas is compressed. The same number of particles now hit the walls of a smaller container more often, which is why the pressure inside rises.

Keep increasing the pressure and the particles come close enough for the force of attraction between them to become important. If the temperature is low enough that the particles are not moving too fast to be held, the attraction wins, the particles settle into contact with one another and the gas becomes a liquid. This is liquefaction, and it is how:

• LPG (liquefied petroleum gas: butane and propane) is stored as a liquid in the cooking cylinder at room temperature under about 5 to 8 atmospheres. Open the valve and the liquid boils back into gas.
• CNG (compressed natural gas, mostly methane) in vehicles is compressed to about 200 atmospheres but stays a gas, because methane cannot be liquefied at room temperature by pressure alone; it needs cooling to −162 °C to become LNG.
• Oxygen cylinders in hospitals hold compressed gas; liquid oxygen for large supplies is made by cooling to −183 °C.
• Solid carbon dioxide (dry ice) is made by compressing CO₂ gas to a liquid and then letting it expand quickly so that it cools and freezes. It goes straight back to gas at atmospheric pressure (sublimation), skipping the liquid state, because liquid CO₂ only exists above about 5 atmospheres.

The chapter's sentence, worth writing exactly: gases can be liquefied by applying pressure and reducing temperature. Pressure brings the particles close; cooling slows them so the attraction can hold them.

What pressure does to a liquid and a solid

Almost nothing. Their particles are already touching, so there is no empty space to remove; pushing harder pushes the electron clouds of neighbouring particles into each other, which they resist strongly. Water compresses by about 0.005 % per atmosphere; a block of steel by far less. This is why a hydraulic brake works: the brake fluid transmits the pressure instead of shrinking, and the force reaches the wheels. It is also why a liquid-filled bottle cracks if it freezes: the ice needs more room and the liquid, already incompressible, cannot give it any.

The effect of temperature, which the same question often adds

Heating a substance gives its particles more kinetic energy: they move faster and, in a gas, hit the walls harder and more often, so the pressure of a gas in a closed container rises with temperature (which is why a sealed can must not be heated). Heating a solid makes the particles vibrate more until they break out of their fixed positions: melting. Heating a liquid makes the particles move fast enough to escape the attraction of their neighbours: boiling, or, from the surface only and at any temperature, evaporation. Pressure and temperature pull in opposite directions on a gas: pressure pushes the particles together, temperature pushes them apart. Liquefaction needs the first to win.

Temperature in this chapter is also measured in kelvin: K = °C + 273. 0 °C is 273 K; 100 °C is 373 K; 25 °C is 298 K. The kelvin scale starts at absolute zero, the temperature at which the particles would have their minimum possible energy.

The five questions the chapter asks, with the answers

How we teach Class 9 science

A 1-on-1 online class where the child draws the particle diagram before and after pressure, writes the four characteristics from memory, and then answers the chapter's questions in full sentences that are marked against the CBSE scheme. Chapter 1 takes two lessons; chapter 2 (Is Matter Around Us Pure, solutions and mixtures) takes three. The first 30-minute class is free: send the topic and the last test on WhatsApp or the contact page. How to draw the diagrams is in particle diagrams, and the solutions chapter is in solution, solute and solvent; the subject page is Class 9 and 10 science tutoring.

Questions parents ask

See how we teach this, 1-on-1 online →