The Wonders of Air
FoundationAir is about 78% nitrogen and 21% oxygen; it has weight and takes up space. Atmospheric pressure lets us suck drinks through a straw, and rising warm air makes wind.
- Air: about 78% nitrogen, 21% oxygen, 1% others.
- Air has weight and occupies space.
- Straws and suction hooks both work by atmospheric pressure.
文體小知識：說明文把看不見的空氣講得有聲有色。這一篇用實驗和例子，證明空氣有成分、有重量、有力量。
§Text Passage
1Hold a cup of bubble tea, put in a straw and suck gently — the drink climbs obediently up the tube. Who pushed it up? The invisible air. Air wraps around the whole Earth and is everywhere; it has power and performs many tricks, waiting for us to discover them.
2Air is actually a mixture of several gases. Scientists found that nitrogen makes up about 78%, oxygen about 21%, and the rest about 1% is carbon dioxide and other rare gases. Nitrogen is the biggest share; though oxygen is only about one fifth, our breathing and every fire depend on it.
3Can we see air? No, but it truly exists. Blow up a balloon and it bulges, taking up space; put an inflated balloon on a balance and the inflated side sinks, proving air has weight. Stick a dry paper ball inside a cup and push the cup upside down into water — the paper stays dry, because air fills the cup and keeps the water out.
4Air also has astonishing power. When we suck a drink through a straw, our mouth removes the air inside the tube; the pressure inside falls, and the atmospheric pressure outside pushes the drink up the straw. When a suction hook is pressed hard onto a tile, the air is squeezed out, and the atmospheric pressure outside presses it firmly against the wall, so it can hold a towel.
5What happens when air is heated? Warm air becomes lighter and rises, while cooler air flows in to fill its place — this moving air is wind. When a sky lantern is lit, the air inside grows hot and lifts the lantern slowly into the night sky; by the sea on a sunny day the land heats quickly, warm air rises, and cooler air from the sea blows in as a refreshing sea breeze.
6Air cannot be seen or touched, yet it has composition, weight, space, pressure, and it moves as wind. Keep the air clean: walk more and ride private cars less, never burn rubbish in the open, and let this invisible friend stay fresh — then both we and the Earth will be healthier.
1Summary 重點摘要
Air is about 78% nitrogen and 21% oxygen; it has weight and takes up space. Atmospheric pressure lets us suck drinks through a straw, and rising warm air makes wind.
2Vocabulary cards Vocabulary
The gas making up about 78% of air, its main ingredient.
The gas making up about 21% of air, used for breathing and burning.
How much a part is of the whole, such as oxygen being about 21% of air.
Air fills the space inside containers, keeping water out of an upside-down cup.
The push of the air all around us; it presses a drink up a straw.
A slender tube that, with atmospheric pressure, lifts drinks to our mouth.
A hook that grips smooth walls by atmospheric pressure after the air is squeezed out.
Heated air that becomes lighter and rises, lifting hot-air balloons.
Air flowing from place to place as warm air rises and cool air fills in.
Moving air that can turn wind turbines and carry seeds.
3Step by step Steps
- Step 1: Know the composition and proportions of airAir is a mixture of gases with two leading players: nitrogen makes up about 78%, roughly four fifths; oxygen about 21%, roughly one fifth; and the remaining 1% or so is carbon dioxide and rare gases like argon. As a metaphor: if air were 100 marbles, 78 would be nitrogen, 21 oxygen and 1 other gases. Remember the numbers 78 and 21, and you will know most of every breath is nitrogen, while oxygen is the part our bodies truly use.
- Step 2: Prove that air has weightThough invisible, air truly has weight. Place an empty balloon on one side of a balance and an inflated balloon on the other: the inflated side sinks, because the air inside weighs something. Scientists calculate that the air in a classroom can weigh dozens of kilograms — we just cannot feel it around us. Since air has weight, it presses on us and everything on the ground, and that is exactly where atmospheric pressure comes from.
- Step 3: Prove that air occupies spaceStick a dry tissue ball inside the bottom of a cup, push the cup straight down into water with the mouth facing down, then lift it out — the tissue is still dry. The reason: the cup was already full of air, and the air, occupying the space inside, keeps the water out. Tilt the cup, let the bubbles escape, and water rushes in. This little experiment proves that air, though invisible, genuinely takes up space — that is why an inflated ring can float a person.
- Step 4: Atmospheric pressure, straws and suction hooksThe weight of the air presses on the ground and everything there, forming atmospheric pressure. When we suck a drink through a straw, we first remove the air inside the tube; the pressure inside falls, and the atmospheric pressure outside pushes the drink up the straw — so drinking through a straw is really the air helping us. A suction hook works the same way: press it hard onto a tile, the air inside is squeezed out, and the atmospheric pressure outside presses the cup firmly to the wall, holding a towel without falling.
- Step 5: Warm air risesWhen air is heated, its particles move faster, the volume expands and the density falls, so it grows lighter and rises — like a block of wood floating up in water. Sky lanterns and hot-air balloons use exactly this principle: burning fuel heats the air inside, and the warm air carries the lantern slowly upward. In an air-conditioned room, cool air sinks and warm air rises, forming convection that cools the whole room. Remember this and you will see why air moves briskly at high places.
- Step 6: How wind formsWind is simply moving air. The sun heats the ground unevenly: where the air grows hot it rises, and cooler air flows in to take its place — moving air is wind. By the sea on a sunny day the land heats fast, and wind blows from sea to land as a cool sea breeze; at night the land cools quickly and the wind blows back out to sea. Wind can turn turbines to make electricity and carry pollen and seeds, but typhoons also wreck homes, so we must learn to live with the wind.
5Worked example Example
Example: Ming tries to drink bubble tea through a straw but fails — the straw has a tiny hole on its side. With a good straw he succeeds at once. He also presses a suction hook onto a smooth tile and it holds firmly. Explain both with atmospheric pressure.
- How drinking works: the mouth removes the air inside the straw, the pressure inside falls, and atmospheric pressure outside pushes the drink up the tube.
- With a hole in the straw, outside air keeps leaking in, so the pressure inside stays the same as outside, and atmospheric pressure cannot push the drink up — hence failure.
- A good straw lets the mouth remove the air, creating a pressure difference; atmospheric pressure immediately pushes the tea up — hence success.
- When the suction hook is pressed onto the tile, the air inside the cup is squeezed out, leaving almost no air and very little pressure inside.
- The atmospheric pressure outside then presses the cup tightly to the wall, so it can bear the weight of a towel and hold without falling.
- Answer: Drinking through a straw relies on atmospheric pressure: the mouth removes the air in the tube, the inside pressure drops, and the outside pressure pushes the drink up. A hole lets air leak in, so the pressures stay equal and the drink cannot rise; a good straw creates the pressure difference and works at once. A suction hook holds because, after its air is squeezed out, atmospheric pressure presses it firmly against the tile.
6Common mistakes Mistakes
7Quick check Quick check
Q1Which gas makes up about 78% of air?
Q2Which gas makes up about 21% of air?
Q3What pushes a drink up a straw?
Q4What does air do when heated?
Q5How does wind form?
8Recap Recap
- Air: about 78% nitrogen, 21% oxygen, 1% others.
- Air has weight and occupies space.
- Straws and suction hooks both work by atmospheric pressure.
- Warm air grows lighter and rises.
- Wind is moving air from temperature differences.