Air — Class 5 Science (Samacheer Kalvi)
TN State Board (Samacheer Kalvi) Class 5 Science. What air is made of, what it can do, and how we are changing it.
1. What air is made of
Air is not a single gas but a mixture.
| Gas | Share | Role |
|---|---|---|
| Nitrogen | about 78% | Dilutes the oxygen; plants need it for proteins |
| Oxygen | about 21% | Needed for breathing and burning |
| Argon | about 0.9% | An unreactive gas |
| Carbon dioxide | about 0.04% | Used by plants to make food |
| Water vapour and other gases | about 0.06% | Includes neon, helium and methane |
Oxygen is the gas we cannot live without, but nitrogen is the most abundant. That is fortunate. Nitrogen dilutes the oxygen, so fires stay controllable.
2. Air occupies space
Invert an empty glass and push it straight down into water. The water does not rise inside, because air already occupies that space.
Blow into a balloon and it swells for the same reason.
3. Air has weight
Weigh a deflated balloon on a sensitive balance, then inflate it and weigh it again. The inflated one is heavier. Air is not nothing — a cubic metre of it weighs roughly 1.2 kg at sea level.
4. Air exerts pressure
Because air has weight, the column of air above us presses down. This is atmospheric pressure, and it acts in every direction.
The crushing can experiment shows it dramatically. Boil a little water in a tin can so steam drives the air out, then seal it and cool it suddenly. The steam condenses into a few drops of water, leaving almost a vacuum inside. With nothing pushing back, the atmosphere outside crushes the can flat.
Nothing was added or removed from outside. The air pressure had been there all along; the experiment simply took away the pressure that had been balancing it.
5. Warm air rises: convection
Heat air and its particles move faster and spread apart. The air becomes less dense, and so it rises. Cooler, denser air rushes in to take its place.
That movement of air is wind.
You can see it with a paper pinwheel held above a candle. The rising warm air spins it, though nobody is blowing.
The same idea explains why hot air balloons float, why smoke goes up a chimney, and why the upstairs of a house is warmer than the ground floor.
6. Weather and climate are not the same
Weather is what the air is doing now or over a few days — sunny, rainy, windy.
Climate is the average pattern of weather over many years in a place.
So a cold day in summer is weather, not evidence against a warming climate. One day proves nothing about a long-term trend.
7. The greenhouse effect and global warming
Sunlight passes through the atmosphere and warms the Earth. The warmed Earth then radiates heat back out.
Certain gases — carbon dioxide, methane, water vapour — trap part of that outgoing heat instead of letting it escape. These are greenhouse gases, and the trapping is the greenhouse effect.
Some greenhouse effect is essential; without it the Earth would be frozen. The problem is that burning fuels has added far more of these gases than before, so more heat is trapped and the average temperature rises. This is global warming.
Global warming is not the same as the ozone hole. They are two different problems. The ozone hole is the thinning of the ozone layer, which shields us from harmful ultraviolet rays. Global warming is the trapping of heat. Do not mix them up.
8. Worked examples
Example 1. Which gas is most abundant in air?
Solution: Nitrogen, at about 78%.
Example 2. How would you show that air occupies space?
Solution: Invert an empty glass into water. The water cannot enter, because air fills the glass.
Example 3. Why does the can collapse in the crushing can experiment?
Solution: The steam condenses, leaving near vacuum inside. With no pressure pushing outward, atmospheric pressure outside crushes the can.
Example 4. Why does warm air rise?
Solution: Heating makes its particles spread apart, so the air becomes less dense than the cooler air around it.
Example 5. What causes wind?
Solution: Warm air rising and cooler, denser air rushing in to take its place.
Example 6. Is a cold day in summer evidence against global warming?
Solution: No. That is weather, a short-term change. Global warming is a long-term change in climate.
9. Practice
- Name the four main gases in air with their approximate percentages.
- Why is it fortunate that air is mostly nitrogen rather than oxygen?
- Describe an experiment showing that air has weight.
- What is atmospheric pressure?
- Explain the difference between weather and climate.
- Name three greenhouse gases.
- What is the greenhouse effect?
- How does the ozone hole differ from global warming?
10. Answers
- Nitrogen about 78%, oxygen about 21%, argon about 0.9%, carbon dioxide about 0.04%.
- Oxygen supports burning, so an atmosphere of mostly oxygen would let fires blaze uncontrollably.
- Weigh a deflated balloon, then inflate it and weigh it again; the inflated balloon is heavier.
- The pressure exerted by the weight of the column of air above us, acting in all directions.
- Weather is the short-term state of the air; climate is the average pattern over many years.
- Carbon dioxide, methane and water vapour.
- Greenhouse gases trapping part of the heat the Earth radiates back out, keeping the planet warmer.
- The ozone hole is thinning of the ozone layer that blocks ultraviolet rays; global warming is heat being trapped by greenhouse gases.
11. Summary
- Air is a mixture: about 78% nitrogen, 21% oxygen, 0.9% argon and 0.04% carbon dioxide.
- Nitrogen is the most abundant gas and dilutes the oxygen so fires stay controllable.
- Air occupies space, has weight, and exerts pressure in all directions.
- The crushing can experiment shows atmospheric pressure by removing the pressure inside.
- Heated air becomes less dense and rises; cooler air rushing in makes wind.
- Weather is short term; climate is the long-term average.
- Greenhouse gases trap outgoing heat, and burning fuels has increased them, causing global warming.
- The ozone hole and global warming are two different problems.
