Physics and chemistry Year 8: Particles, states of matter and gases
Physics and chemistry, Year 8 (Portugal): All matter is made of moving particles; how free they are and how close they sit sets solid, liquid or gas, and explains how a gas's temperature, volume and pressure are linked.
Matter is made of particles
Everything that is matter is made of tiny particles that are always moving, with empty space between them. That is why a drop of dye spreads through a glass of water without anyone stirring it. In a solid the particles are very close and only vibrate in place; in a liquid they are close but slide past one another; in a gas they are far apart and move freely in every direction.
How do we know particles exist?
Nobody sees the particles, but we see what they cause. Dye spreads by itself in water. Perfume reaches the other side of a room. Air can be squeezed inside a closed syringe, yet water hardly can: a gas has a lot of empty space between its particles, a liquid almost none. Each observation is evidence for the particle model, and that is how a model earns its place in science: it explains what we observe.
The closed syringe: volume, pressure, temperature
Close the tip of a syringe holding 60 mL of air and push the plunger to 30 mL. The particles are the same, but the volume is half: there are twice as many per mL (60 ÷ 30 = 2) and more collisions with the walls each second, so the pressure rises and the plunger pushes back. Now fit a balloon on a bottle and stand it in hot water: the particles move faster and hit harder, and the balloon inflates. Qualitatively: at constant temperature, less volume means more pressure; more temperature raises pressure or volume.
Heating does not make particles bigger
Many people think that when a gas is heated its particles swell up. They do not: each particle keeps the same size. What changes is how agitated they are, that is, how fast they move. In a warmer gas the particles travel farther and hit harder, so the gas takes more volume or pushes with more pressure. When you think «the balloon inflated», think «faster and farther apart», never «bigger».
Pumps, tyres and spray cans
When you pump up a bicycle tyre you squeeze more air into the same volume: more particles, more pressure. After a long drive the pressure in a car tyre is a little higher because the air has warmed up. That is why aerosol cans warn you not to leave them in the sun or near a flame: the gas inside heats up and the pressure rises. A sealed crisp packet puffs up on a mountain, where the pressure outside is lower.
Test what you learned
Which sentence describes the particles of a gas?
- a) They are very close together and only vibrate in place
- b) They are close together but slide past one another
- c) They are far apart and move freely in every direction
Correct answer: c) They are far apart and move freely in every direction — Right: large distances and free movement are what make a gas easy to compress and to spread out.
A syringe with its tip closed holds 60 mL of air. The plunger is pushed to 30 mL, temperature constant. What happens?
- a) The pressure falls, because there is less room
- b) There are twice as many particles per mL and more collisions with the walls, so the pressure rises
- c) The particles shrink to fit the smaller volume
- d) Nothing changes, because the number of particles is the same
Correct answer: b) There are twice as many particles per mL and more collisions with the walls, so the pressure rises — Right: 60 ÷ 30 = 2, so the same particles crowd into half the space and hit the walls more often.
A balloon fitted on a bottle inflates when the bottle is placed in hot water. What explains it?
- a) The air particles move faster and hit the walls harder; their size does not change
- b) Each air particle swells when it is heated
- c) Heat creates extra air particles inside the bottle
Correct answer: a) The air particles move faster and hit the walls harder; their size does not change — Right: heat makes the particles more agitated, not bigger, and the gas pushes the balloon out.
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