--- title: "Air, a Mixture of Gases" book: "School Chemistry — Grades 1 to 12" subject: chemistry language: en chapter: 4 exercises: 11 source: https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases license: CC-BY-NC-SA-4.0 credit: "One Chemistry Book, One Course (one-course.com)" --- # Chapter 4 — Air, a Mixture of Gases On a windy day a kite tugs at its string, and on the lake a sail fills and pushes a boat along. Something is pushing them, yet there is nothing to see. That something is [air](#def-g4-air-a-mixture-of-gases-air). We cannot see it, but it is all around us, it fills every empty bottle and every room, and we take it in with every breath. What is [air](#def-g4-air-a-mixture-of-gases-air) made of? **You already know.** To [mix](https://one-course.com/books/chemistry/1/en/chapter/2-mixing-and-dissolving#def-g2-mixing-and-dissolving-mix) is to put different things together; what we get is a [mix](https://one-course.com/books/chemistry/1/en/chapter/2-mixing-and-dissolving#def-g2-mixing-and-dissolving-mix) of those things, and nothing is lost when we [mix](https://one-course.com/books/chemistry/1/en/chapter/2-mixing-and-dissolving#def-g2-mixing-and-dissolving-mix) them ([Chapter 2](https://one-course.com/books/chemistry/1/en/chapter/2-mixing-and-dissolving#ch-g2-mixing-and-dissolving)). ![A kite and a sail: air cannot be seen, but it pushes.](https://one-course.com/images/onecourse/chapters/chemistry-1/g4-air-a-mixture-of-gases/img-1870ce84784f.jpg) *A kite and a sail: [air](#def-g4-air-a-mixture-of-gases-air) cannot be seen, but it pushes.* ## 4.1 Air is all around us **Example 4.1 (An upturned glass).** A dry paper tissue is pushed to the bottom of an empty glass. The glass is turned upside down and pushed straight down into a bowl of water. When it is lifted out again, the tissue is still dry. The glass was not empty: it was full of [air](#def-g4-air-a-mixture-of-gases-air), and the [air](#def-g4-air-a-mixture-of-gases-air) kept the water out. ![The glass is pushed upside down into the water. The air trapped inside keeps the water out, and the tissue stays dry.](https://one-course.com/images/onecourse/chapters/chemistry-1/g4-air-a-mixture-of-gases/fig-efc41ed8fac7.svg) *The glass is pushed upside down into the water. The [air](#def-g4-air-a-mixture-of-gases-air) trapped inside keeps the water out, and the tissue stays dry.* **Remark 4.2 (Air is a gas).** [Air](#def-g4-air-a-mixture-of-gases-air) is a gas: it has no shape of its own and spreads out to fill any container. It is matter all the same, and a balloon full of [air](#def-g4-air-a-mixture-of-gases-air) weighs a little more than the same balloon empty. What gases are, and why they behave like this, is told in the physics book. ## 4.2 Air is a mixture **Definition 4.3 (Air).** *Air* is the mixture of gases that surrounds the Earth. It is not one single gas: several gases are mixed in it, and they can be separated from one another. **Proposition 4.4 (What air is made of).** Leaving out its water vapour, [air](#def-g4-air-a-mixture-of-gases-air) is made of: - **nitrogen** , about four fifths of the [air](#def-g4-air-a-mixture-of-gases-air) ; - **oxygen** , about one fifth of the [air](#def-g4-air-a-mixture-of-gases-air) ; - a little **argon** , about 1 part in 100; - a very little **carbon dioxide** , about 4 parts in 10 000, and tiny amounts of other gases. [Air](#def-g4-air-a-mixture-of-gases-air) also holds **water vapour**, invisible: very little on a dry cold day, up to about 4 parts in 100 on a hot damp day. **Example 4.5 (One hundred bottles of air).** Imagine 100 bottles of dry [air](#def-g4-air-a-mixture-of-gases-air), and the gases of the [air](#def-g4-air-a-mixture-of-gases-air) sorted into separate bottles. About 78 bottles would hold nitrogen, 21 bottles oxygen, and the last bottle argon, with a few drops’ worth of carbon dioxide and other gases. ![One hundred squares of dry air. The square at the bottom right holds the argon, and the carbon dioxide too small to draw on its own.](https://one-course.com/images/onecourse/chapters/chemistry-1/g4-air-a-mixture-of-gases/fig-70453815da87.svg) *One hundred squares of dry [air](#def-g4-air-a-mixture-of-gases-air). The square at the bottom right holds the argon, and the carbon dioxide too small to draw on its own.* **Remark 4.6 (Names of gases).** Nitrogen, oxygen, argon and carbon dioxide are the names of gases. They are all colourless and have no smell. A later chapter shows what each of them is made of, down to the tiny pieces that make up all matter. ## 4.3 What we breathe in and out **Proposition 4.7 (Breathed-out air).** The [air](#def-g4-air-a-mixture-of-gases-air) we breathe out is not the [air](#def-g4-air-a-mixture-of-gases-air) we breathed in. Our body takes some of the oxygen and gives out carbon dioxide and water vapour: - about 21 parts in 100 of the [air](#def-g4-air-a-mixture-of-gases-air) breathed in are oxygen, but only about 16 parts in 100 of the [air](#def-g4-air-a-mixture-of-gases-air) breathed out; - carbon dioxide goes from about 4 parts in 10 000 to about 4 parts in 100: about a hundred times more; - the nitrogen is breathed out just as it was breathed in. ![Breathed in and breathed out, in parts in 100 (water vapour left out, numbers rounded). The carbon dioxide of the air breathed in, 4 parts in 10 000, is too small to show as a bar.](https://one-course.com/images/onecourse/chapters/chemistry-1/g4-air-a-mixture-of-gases/fig-524697b89f47.svg) *Breathed in and breathed out, in parts in 100 (water vapour left out, numbers rounded). The carbon dioxide of the [air](#def-g4-air-a-mixture-of-gases-air) breathed in, 4 parts in 10 000, is too small to show as a bar.* **Example 4.8 (A crowded room).** In a small classroom with the windows shut, thirty people breathe out carbon dioxide all morning. Little by little the [air](#def-g4-air-a-mixture-of-gases-air) of the room holds less oxygen and more carbon dioxide, and people feel tired. Opening the windows brings fresh [air](#def-g4-air-a-mixture-of-gases-air) back in. ![A diver breathes air from the tank on the back; the bubbles are the air breathed out.](https://one-course.com/images/onecourse/chapters/chemistry-1/g4-air-a-mixture-of-gases/img-f21b21b5af1a.jpg) *A diver breathes [air](#def-g4-air-a-mixture-of-gases-air) from the tank on the back; the bubbles are the [air](#def-g4-air-a-mixture-of-gases-air) breathed out.* **Remark 4.9 (Why divers breathe air).** A diver’s tank is filled with [air](#def-g4-air-a-mixture-of-gases-air) squeezed into a small space, not with pure oxygen: breathing pure oxygen deep under water is dangerous. The nitrogen of the [air](#def-g4-air-a-mixture-of-gases-air) is there to dilute the oxygen. ## 4.4 Oxygen for life and for fire **Proposition 4.10 (Oxygen is used up by living things and by flames).** Of all the gases of the [air](#def-g4-air-a-mixture-of-gases-air), oxygen is the one living things and flames need. Animals and people take it in when they breathe; a flame takes it from the [air](#def-g4-air-a-mixture-of-gases-air) around it. Green plants, in daylight, give oxygen back to the [air](#def-g4-air-a-mixture-of-gases-air). **In the lab — A candle under a jar.** The teacher lights a candle on a tile and slowly lowers a large glass jar over it. The flame shrinks, flickers and goes out after a few seconds: around the flame, the [air](#def-g4-air-a-mixture-of-gases-air) holds too little oxygen to keep it burning. Under a jar twice as large, the candle burns about twice as long, because there is about twice as much [air](#def-g4-air-a-mixture-of-gases-air), and so twice as much oxygen, to use. ![A candle under a glass jar goes out when too little oxygen is left around its flame.](https://one-course.com/images/onecourse/chapters/chemistry-1/g4-air-a-mixture-of-gases/img-cab6b262b20d.jpg) *A candle under a glass jar goes out when too little oxygen is left around its flame.* **Remark 4.11 (Nitrogen does not burn).** Nitrogen is the largest part of the [air](#def-g4-air-a-mixture-of-gases-air), yet it does not keep a flame burning and it is not used by our body. If [air](#def-g4-air-a-mixture-of-gases-air) were all nitrogen, we could not breathe and nothing could burn. ## 4.5 Exercises **Exercise 4.1 ★.** Is [air](#def-g4-air-a-mixture-of-gases-air) one single gas or a mixture? Name the two main gases of the [air](#def-g4-air-a-mixture-of-gases-air). **Solution of Exercise 4.1.** A mixture. Its two main gases are nitrogen and oxygen. **Exercise 4.2 ★.** About what fraction of the [air](#def-g4-air-a-mixture-of-gases-air) is nitrogen? About what fraction is oxygen? **Solution of Exercise 4.2.** About four fifths of the [air](#def-g4-air-a-mixture-of-gases-air) is nitrogen, about one fifth is oxygen. **Exercise 4.3 ★.** In 100 litres of dry [air](#def-g4-air-a-mixture-of-gases-air), about how many litres are nitrogen? About how many litres are oxygen? **Solution of Exercise 4.3.** About 78 litres of nitrogen and about 21 litres of oxygen. **Exercise 4.4 ★.** An empty bottle is put upside down into a basin of water. Why does the water not fill the bottle? **Solution of Exercise 4.4.** The bottle is not empty: it is full of [air](#def-g4-air-a-mixture-of-gases-air), and the trapped [air](#def-g4-air-a-mixture-of-gases-air) keeps the water out. **Exercise 4.5 ★.** Look at the bar chart. Which gas is there less of in the [air](#def-g4-air-a-mixture-of-gases-air) breathed out? Which gas is there more of? **Solution of Exercise 4.5.** There is less oxygen (16 parts in 100 instead of 21) and more carbon dioxide (4 parts in 100 instead of almost none). **Exercise 4.6 ★.** Which gas of the [air](#def-g4-air-a-mixture-of-gases-air) does a flame need? Which gas of the [air](#def-g4-air-a-mixture-of-gases-air) do we need when we breathe? **Solution of Exercise 4.6.** Oxygen in both cases: a flame and our body both use the oxygen of the [air](#def-g4-air-a-mixture-of-gases-air). **Exercise 4.7 ★.** Look at the figure with 100 squares. How many squares are nitrogen? How many are not nitrogen? **Solution of Exercise 4.7.** 78 squares are nitrogen; $100 - 78 = 22$ squares are not. **Exercise 4.8 ★.** Why does a candle under a jar go out after a while? **Solution of Exercise 4.8.** The flame uses the oxygen of the [air](#def-g4-air-a-mixture-of-gases-air) in the jar. When too little oxygen is left around the flame, it goes out. **Exercise 4.9 ★.** In 500 litres of [air](#def-g4-air-a-mixture-of-gases-air), about how many litres are oxygen? (One fifth of the [air](#def-g4-air-a-mixture-of-gases-air) is oxygen.) **Solution of Exercise 4.9.** One fifth of 500 litres: $500 \div 5 = 100$ litres of oxygen. **Exercise 4.10 ★★.** Why is it a good idea to open the windows of a classroom between lessons? **Solution of Exercise 4.10.** The people in the room breathe in oxygen and breathe out carbon dioxide. Opening the windows brings back fresh [air](#def-g4-air-a-mixture-of-gases-air), with more oxygen and less carbon dioxide. **Exercise 4.11 ★★.** A candle burns for 10 seconds under a small jar. About how long would it burn under a jar holding three times as much [air](#def-g4-air-a-mixture-of-gases-air)? **Solution of Exercise 4.11.** Three times as much [air](#def-g4-air-a-mixture-of-gases-air) holds three times as much oxygen: about $3 \times 10 = 30$ seconds.