Gases in the Atmosphere
5 marks · EasyNitrogen and oxygen are the main gases in air. Burning elements in oxygen produces different products and different observations.
State the percentage by volume of nitrogen and oxygen in dry air.
Describe an experiment using copper to determine the percentage of oxygen in air, including what you would observe during the experiment.
Hydrogen burns in oxygen to form a product. Name the product and describe what you would see when hydrogen burns in oxygen.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) Nitrogen: 78%; Oxygen: 21% · must show nitrogen 78%, oxygen 21%
(b) — · must show copper heated, air passed over copper using gas syringes, copper turns black/copper(II) oxide forms, volume of air decreases as oxygen is used up
(c) Water; a colourless liquid is formed and a pale blue/colourless flame is seen · must show water, blue/colourless/pale flame
Show mark scheme and worked solution
Marking points
B1a1 (a) · 1 mark · States nitrogen makes up about 78% of dry air.
B1a2 (a) · 1 mark · States oxygen makes up about 21% of dry air.
B1b1 (b) · 1 mark · Describes warm copper in a tube with air repeatedly passed over it, or mentions gas syringes measuring air volume.
B1b2 (b) · 1 mark · Mentions observation: copper turns black/air volume goes from about 100 to about 79 cm³ or decrease due to oxygen reacting.
B1c (c) · 1 mark · Correctly names product (water), and describes observation (pale blue or colourless flame, or water formed as drops).
Worked solution
a) Nitrogen: about 78%, oxygen: about 21% (by volume).
b) Pass air repeatedly over heated copper in a tube connected to gas syringes. Copper turns black as it reacts with oxygen to form copper(II) oxide. The volume of air in the syringes decreases as oxygen is removed.
c) When hydrogen burns in oxygen, it forms water. A pale blue/colourless flame is seen, and drops of water appear on the container walls.