Gases in the atmosphere

The atmosphere is the blanket of gas held around the Earth by gravity, and you are breathing a sample of it right now. That sample is a mixture: several different gases jumbled together, each keeping its own properties, none of them chemically joined to the others. This chapter takes that mixture apart. You will learn exactly what dry air is made of and in what proportions, then meet the experiments that measure one particular component — oxygen — directly, and finally follow the two gases that matter most for chemical reactions and for the climate: oxygen and carbon dioxide.

The chapter runs on one measuring idea, stated here so everything afterwards has a home: a gas's share of a mixture is quoted as a percentage-by-volume — of every 100 cm³ of dry-air, about 78 cm³ is nitrogen and about 21 cm³ is oxygen. Three different experiments (heated copper, rusting iron, burning phosphorus) all attack the same question from different angles: remove the oxygen from a known volume of air, watch the volume shrink, and do one subtraction and one division. Whichever reagent you use, the arithmetic and the error analysis are the same shape — and that shape is where the exam marks live.

From measurement the chapter moves to reaction. combustion — burning elements in oxygen — produces a family of compounds called oxides, and comparing the oxides of metals with the oxides of non-metals reveals a clean, examinable split in their properties. The same chemistry explains where atmospheric carbon dioxide comes from, and why that particular gas, despite being only about 0.04% of the air, punches far above its concentration: it is a greenhouse-gas, and the final sections of this chapter make you explain — at the level of molecular vibrations — why carbon dioxide absorbs infrared radiation while nitrogen and oxygen, 99% of the air between them, do not.

Every objective in this chapter is core content: it can be examined in both Paper 1C and Paper 2C. By the end you will be able to recall the composition of dry air; describe and calculate the oxygen-percentage experiments using metals or non-metals (the listed practical); describe the combustion of magnesium, hydrogen and sulfur in oxygen, with equations; describe carbon dioxide forming from heated metal carbonates, including copper(II) carbonate; and explain the greenhouse effect in infrared terms.