O-LevelChemistryChemistry of the Environment5070/21 · Oct/Nov 2021 · Q210 Marks

Dry air contains nitrogen, oxygen, noble gases and carbon dioxide.

(a)State the percentage of oxygen present in dry air.[1]
(b)Carbon dioxide is removed from a sample of air by passing the air through aqueous sodium hydroxide. Explain why aqueous sodium hydroxide removes carbon dioxide from air.[2]
(c)Describe how oxygen, nitrogen and the noble gases are separated from each other after carbon dioxide has been removed.[2]
(d)Describe the test for oxygen. test observation[2]
(e)(i)Ozone, O₃, is formed in the atmosphere by the reaction of nitrogen dioxide with oxygen in the presence of ultraviolet light. State the type of chemical reaction that takes place when ozone is formed in this way.[1]
(e)(ii)Nitrogen dioxide is formed in internal combustion engines. State one other source of nitrogen dioxide in the atmosphere.[1]
(f)A layer of ozone is present high in the atmosphere. State one problem for humans that can arise if the ozone layer is depleted by CFCs.[1]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2021). The mark or mark type is shown in brackets.

[1] 2(a) 21(%)
[1] 2(b) carbon dioxide is an acidic oxide
[1] 2(b) reacts with base / reacts with alkali / neutralised by sodium hydroxide
[1] 2(c) air liquefied
[1] 2(c) fractional distillation
[1] 2(d) glowing splint
[1] 2(d) relights
[1] 2(e)(i) photochemical / redox
[1] 2(e)(ii) lightning
[1] 2(f) (more) skin cancer / (more) sunburn / (more) harm to eyes

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