What Air Is Made Of空气的组成

Big Idea · Scale, Proportion, and Quantity (SPQ)AP Science Practice 2 · Question and MethodAP link · Unit 3~25 min

Learning goals

  • State the main components of air and their percentages by volume
  • Explain how burning phosphorus measures the oxygen in air, and analyze sources of error
  • Describe uses of nitrogen and the noble gases, and the main air pollutants and how to reduce them

Before you start

  • Pure substances and mixtures (1.2.5)
  • Physical and chemical properties (1.2.4)

What’s in the air?

In the 1770s, the French chemist Antoine Lavoisier heated a small amount of mercury in a sealed container for 12 days. The volume of air in the container shrank by about one-fifth, and a red powder (mercury(II) oxide) formed on the mercury. The gas left over couldn’t keep a mouse alive or a flame burning. Lavoisier concluded that air is mainly a mixture of oxygen and nitrogen, with oxygen making up about one-fifth.

The composition of air

By volume, dry air is made of:

ComponentPercent by volume
Nitrogen, NX2\ce{N2}78%
Oxygen, OX2\ce{O2}21%
Argon and other noble gasesabout 0.93%
Carbon dioxide, COX2\ce{CO2}about 0.04%
Other gasestraces

Air is a mixture of gases, and each component keeps its own properties. (Many older textbooks, including Chinese ones, list carbon dioxide at 0.03%. Burning fossil fuels has pushed it up to about 0.04% today.)

Measuring the oxygen in air

A classic demonstration uses burning red phosphorus to measure how much of the air is oxygen:

  1. Put a little water in a gas jar and mark the space above the water into 5 equal parts.
  2. Light some red phosphorus in a combustion spoon, quickly lower it into the jar, and seal the stopper.
  3. The phosphorus burns, giving off thick white smoke and heat.
  4. Once the jar has cooled to room temperature, open the clamp. Water flows from the beaker through the tube into the jar and fills about 1/5 of the air space.
4 P(s)+5 OX2(g)→2 PX2OX5(s)\ce{4P(s) + 5O2(g) -> 2P2O5(s)}

How it works: the burning phosphorus uses up the oxygen in the jar and forms solid phosphorus pentoxide, which takes up almost no space. The pressure inside drops. When the clamp is opened, the pressure of the air outside pushes water into the jar, and the volume of water that enters equals the volume of oxygen used up.

The water stops rising at about 1/5, which tells you the gas left behind (mostly nitrogen) doesn’t dissolve much in water and neither burns nor supports burning.

Choose a procedure and click Run experiment to see whether too much or too little water enters:

Try it: measure the oxygen in air with burning phosphorus, and see how mistakes change the result
12345ClampWaterRed phosphorus
Procedure:
  1. Light the phosphorus, lower it into the jar, and seal the stopper
  2. The phosphorus burns, giving off thick white smoke
  3. Let the jar cool to room temperature
  4. Open the clamp: water flows from the beaker into the jar

Nitrogen and the noble gases

  • Nitrogen is a colorless, odorless, unreactive gas. It’s used as a protective gas (for example, in food packaging and in welding), as a raw material for nitric acid and fertilizers, and, as liquid nitrogen, for freezing.
  • The noble gases (helium, neon, argon, krypton, xenon, and radon) are very unreactive, so they’re also used as protective gases. They glow in different colors when electricity passes through them, which is how neon signs work. Helium is much less dense than air, so it’s used to fill weather balloons.

Keeping the air clean

Main pollutants: harmful gases such as sulfur dioxide, carbon monoxide, nitrogen dioxide, and ozone, plus particulate matter such as PM2.5 and PM10. In the US, the Air Quality Index (AQI) reports these pollutants; the higher the AQI, the worse the air.

Ways to reduce pollution: switch to cleaner energy and burn less fossil fuel, treat factory exhaust before releasing it, plant trees, and control vehicle emissions.

Example 1: Why phosphorus and not charcoal?

Step-by-step solution
  1. Look at the products. Burning phosphorus forms phosphorus pentoxide, a solid. Burning charcoal forms carbon dioxide, a gas.
  2. Think about pressure. With charcoal, the carbon dioxide produced takes the place of the oxygen used up, so the pressure barely changes and almost no water enters.
  3. Conclusion. The substance you burn must burn in air, react only with oxygen, and not produce a gas. That’s why red phosphorus works and charcoal or sulfur doesn’t.

Example 2: Analyzing an error

A student finds that the water fills only about 1/7 of the air space. What could have caused this?

Step-by-step solution
  1. The result is too low. 1/7 is less than 1/5, so too little water entered.
  2. Oxygen wasn’t used up. There wasn’t enough phosphorus.
  3. Gas got back in. The apparatus leaked, letting outside air into the jar.
  4. The pressure was too high. The clamp was opened before the jar had cooled to room temperature.
Practice: does each mistake make the measured oxygen percentage too high, too low, or have no effect?
  1. Not enough red phosphorus
  2. The apparatus leaks
  3. The clamp is opened before the jar cools
  4. The clamp wasn't closed during burning
  5. The burning phosphorus is lowered into the jar too slowly
  6. Using more phosphorus than needed
  7. Using charcoal instead of phosphorus
  8. The tube wasn't filled with water beforehand

Common mistakes

Mistake 1: “Air is 21% oxygen by mass.” It’s 21% by volume. 100 L of air contains about 21 L of oxygen, but 100 g of air does not contain 21 g of oxygen.

Mistake 2: “Any fuel would work instead of phosphorus.” Charcoal and sulfur produce gases, iron wire won’t burn in air, and magnesium also reacts with nitrogen, which makes the result too high.

Mistake 3: “Noble gases never react with anything.” They’re very unreactive, but some can react under special conditions. That’s why chemists no longer call them “inert gases.”

Quick check

Which gas makes up about 78% of air by volume?

Which of these is NOT one of the pollutants reported in the Air Quality Index?

Summary

  • By volume, air is about 78% nitrogen, 21% oxygen, 0.93% argon and other noble gases, and 0.04% carbon dioxide. Air is a mixture.
  • Burning phosphorus uses up the oxygen and forms a solid; after cooling, water fills about 1/5 of the jar, showing that oxygen is about 1/5 of the air.
  • Too little phosphorus, a leak, or opening the clamp too early makes the result too low; an open clamp or slow insertion makes it too high.
  • Nitrogen and the noble gases are unreactive and serve as protective gases; noble gases glow in signs. Cutting harmful gases and particulates keeps the air clean.
On to AP (for later)

For gases, percent by volume equals mole fraction. Dalton’s law of partial pressures (AP topic 3.4) then says that in air at 1 atm, the partial pressure of oxygen is about 0.21 atm. High in the mountains, oxygen is still 21% of the air, but the total pressure is lower, so its partial pressure drops, and that’s why people feel short of breath at altitude.

Practice this chapter: Module 7 · Air, Water, and Solutions · 10 questions

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