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The Earth’s atmosphere contains several gases - Edexcel - GCSE Chemistry - Question 1 - 2018 - Paper 1
Question 1
The Earth’s atmosphere contains several gases. Figure 1 shows the relative amounts of gases thought to be in the Earth’s early atmosphere. | gas | r... show full transcript
Worked Solution & Example Answer:The Earth’s atmosphere contains several gases - Edexcel - GCSE Chemistry - Question 1 - 2018 - Paper 1
Step 1
Explain why the amount of water vapour in the atmosphere has decreased.
Answer
The amount of water vapour in the Earth's atmosphere has decreased due to several factors. Initially, the Earth cooled over time, allowing water vapour to condense and form oceans. Additionally, changes in climate and weather patterns may have led to decreased evaporation rates. Lastly, the formation of land and vegetation could have reduced the water that remains as vapour in the atmosphere.
Step 2
Calculate the percentage of oxygen in this sample of dry air.
Answer
The initial volume of air in the apparatus is 50 cm³, and the final volume of gas is 40 cm³. To find the volume of oxygen removed:
Volume of oxygen = Initial volume - Final volume = 50 cm³ - 40 cm³ = 10 cm³.
To calculate the percentage of oxygen in the dry air:
Percentage of oxygen = ( \frac{10}{50} \times 100 = 20% )
Step 3
At the end of the experiment, the apparatus and its contents must be allowed to cool because
Answer
The correct option is C: the gas has expanded when it is hot. Allowing the apparatus to cool ensures that the gas contracts to its actual volume, providing a more accurate measurement of the gas present.
Step 4
Explain what happened to allow this change to occur.
Answer
As the Earth evolved, oxygen levels in the atmosphere began to rise due to processes like photosynthesis. This increased oxygen reacted with iron sulfide in early rocks, oxidizing it to form iron oxide. As a result, the rocks transitioned from containing iron sulfide to containing both iron sulfide and iron oxide, reflecting the changing composition of the atmosphere.