seegongsik
Grade 6 (age 11-12)

grade 6 Various Gases

Air is invisible, but it is a mixture of several gases, including nitrogen, oxygen and carbon dioxide. Nitrogen is the largest share at about 78 percent, and oxygen, which we need to breathe, makes up about 21 percent. A gas is made of tiny particles: warm it and the particles speed up and the gas expands; squeeze it into a smaller space and it pushes back harder on the walls. On this page you change the temperature and the volume, and you also see how a burning incense stick and limewater tell the three gases apart.

Warm gas, faster particles

50
💨 Warmer gas, faster particles, more volume
①You cannot see gas particles, but they never stop moving in every direction and bumping into the walls
②When the temperature rises, the particles move faster and hit the walls harder and more often. That is why the lid in the picture is pushed up and the gas takes up more space
③A dented table-tennis ball pops back out in hot water, and a balloon stretched over a flask swells when the flask is warmed, for the same reason
④The slider numbers are a relative scale, not °C

Three gases in the air

🌬️ Telling gases apart with incense and limewater
①Air is a mixture: about 78% nitrogen, about 21% oxygen, and about 1% of other gases such as argon and carbon dioxide
②Oxygen helps other things burn, so a burning incense stick flares up in it
③Carbon dioxide puts out flames, so the incense goes out, and it turns limewater milky
④Nitrogen hardly reacts with anything, so filling a crisp packet with it slows the crisps from going stale or spoiling

Squeeze it: less volume, more pressure

80
💡 Volume and pressure
①Seal the tip of a syringe and push the plunger, and the air inside takes up less space
②The number of particles stays the same, but in the smaller space they hit the walls more often, so the pressure rises and the air pushes back on the plunger harder
③Halve the volume and the pressure roughly doubles
④A syringe full of water hardly shrinks however hard you push. Only a gas can be squeezed like this

Summary

What air is made of
Air = nitrogen (about 78%) + oxygen (about 21%) + others (about 1%)
Carbon dioxide is a tiny part of that 1% (about 0.04%)
Temperature, pressure and the volume of a gas
Temperature ↑ → volume ↑ / squeezing (pressure) ↑ → volume ↓
A gas changes volume with temperature and pressure far more than a liquid or a solid does
🎯 Key points
①Air is mostly nitrogen (about 78%) and oxygen (about 21%)
②Oxygen makes incense burn more brightly; carbon dioxide puts it out and turns limewater milky
③Nitrogen hardly reacts, so it is used in food packaging
④When the temperature rises, the particles speed up and the gas expands
⑤Squeezing a gas into a smaller volume raises its pressure

Examples and Unit Test

Example 1
Three gas jars hold oxygen, carbon dioxide and nitrogen, one each. How can you tell them apart with a burning incense stick and limewater?
1
The jar where the incense flares up and burns more brightly holds oxygen.
2
In the other two jars the incense goes out. Pour limewater into both and shake: the one that turns milky is carbon dioxide, and the one that stays clear is nitrogen.
▸ Incense burns brightly → oxygen; incense goes out and limewater turns milky → carbon dioxide; incense goes out and limewater stays clear → nitrogen
The incense alone cannot separate carbon dioxide from nitrogen. You need the limewater as well to tell all three apart.
Example 2
You cover the tip of a syringe with your finger and push the plunger, so the air inside takes up less space. What happens to the pressure, and why?
1
The number of particles stays the same, but the air is crowded into a smaller space.
2
In the smaller space the particles hit the walls more often, so the pressure rises. The harder you push, the harder the plunger pushes back against your finger.
▸ The pressure rises, because the same number of particles hit the walls more often in a smaller space.
Halving the volume roughly doubles the pressure. In the picture above, setting the volume to 50 gives a pressure of 2.0 × the starting value, which is the same idea.
Unit-test style
Which statement about gases is correct?
①Oxygen is the most plentiful gas in air.
②Squeezing a gas into a smaller volume raises its pressure.
③Carbon dioxide makes limewater clearer.
④Gas particles slow down when the temperature rises.
⑤A burning incense stick flares up in a jar of nitrogen.
▸ ② Squeezing a gas into a smaller volume raises its pressure.
1
When the volume shrinks, the same number of particles hit the walls more often in the smaller space, so the pressure rises.
2
The most plentiful gas in air is nitrogen (①), and carbon dioxide turns limewater milky (③). Particles speed up as the temperature rises (④), and it is oxygen, not nitrogen, that makes the incense flare up (⑤).
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