Gases in the Atmosphere
5 marks · EasyThis question is about gases in air and combustion in oxygen.
State the approximate percentage by volume of oxygen and nitrogen in dry air.
Describe the complete combustion of magnesium in oxygen. State the chemical formula of the product formed.
Which property of oxygen explains why elements burn more brightly in oxygen than in air?
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(a) oxygen: about 21%, nitrogen: about 78%
(b) Magnesium burns in oxygen with a bright white flame, forming magnesium oxide. Chemical formula: · must show mentions bright/white light, identifies product as magnesium oxide, gives formula for magnesium oxide or balanced equation
(c) Oxygen has a higher concentration in pure oxygen than in air; more oxygen available for combustion. · must show reference to higher concentration of oxygen or more oxygen for burning
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · States correct approximate percentage of oxygen in dry air (about 21%).
B2 (a) · 1 mark · States correct approximate percentage of nitrogen in dry air (about 78%).
B3 (b) · 1 mark · Describes the burning of magnesium in oxygen (bright/white flame).
B4 (b) · 1 mark · Identifies product as magnesium oxide and gives either the name or the correct chemical formula () or balanced equation.
B5 (c) · 1 mark · Mentions that oxygen concentration is higher in pure oxygen (more oxygen available for combustion).
Worked solution
a) The approximate percentage by volume of oxygen in dry air is 21%. The approximate percentage by volume of nitrogen in dry air is 78%.
b) Magnesium burns in oxygen with a bright white flame. It reacts to form magnesium oxide. The chemical formula for this reaction is:
c) Oxygen supports combustion because it is more concentrated in pure oxygen than in air, so there is more oxygen available for elements to burn more brightly.