Clean air and photosynthesis
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The teaching part (1 min) is free to watch. A free account opens the whole lecture: 3 worked examples, 5 practice questions, 2 real past-paper questions with the answers explained, the mistakes that lose marks, the full-marks tip, a one-card summary and a quick quiz.
What this lecture covers
- The air you are breathing
- Clean, dry air
- Measuring the oxygen
Read the transcript of the teaching part
Last time
Before we start: last time, in Testing for water and treating domestic water, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
The air you are breathing
Take a deep breath. Only about one fifth of it is the oxygen you need. Nearly four fifths is nitrogen that you breathe in and straight out again. Where does the oxygen come from? From plants, which have been refilling the air for hundreds of millions of years.
Clean, dry air
Clean, dry air is about 78 percent nitrogen and 21 percent oxygen. The remaining 1 percent is mostly argon, with small amounts of other noble gases. There is also a tiny amount of carbon dioxide, about 0.04 percent. Water vapour varies, so we say dry air.
Measuring the oxygen
Put 100 cm cubed of air in a syringe, with heated copper. The copper reacts with oxygen: 2Cu plus O 2 gives 2CuO. Push the gas back and forth over the copper, and then let it cool to room temperature. The volume falls to about 79 cm cubed. The oxygen has been used up, so 100 minus 79 is 21 cm cubed.
How often it comes up
How often Cambridge has asked each part, counted by ExamYard from every past paper in its question bank: Composition of clean air · Photosynthesis.
The whole chapter: Chemistry of the Environment, Chemistry of the Environment.
Chapter 10: Chemistry of the environment
- Clean air and photosynthesis 6 min
- Air pollutants, catalytic converters and global warming 5 min
- Water tests and domestic water treatment 5 min
Also for IGCSE Chemistry 0620
Which topics come up most in IGCSE Chemistry 0620 past papers, counted from every paper since 2004.