Table 1 shows the composition of dry air measured beside a busy road and at a coastal weather station. Fig. 1 is a simplified diagram of a plant that separa...
Question 1 Report
Table 1 shows the composition of dry air measured beside a busy road and at a coastal weather station. Fig. 1 is a simplified diagram of a plant that separates gases from air after the air has been cooled until it becomes liquid. Nitrogen is collected for making ammonia, and oxygen is supplied to hospitals. The small amounts of carbon dioxide and water vapour are removed before cooling because they could freeze and block the apparatus.
| gas | busy road / % | coastal station / % |
|---|---|---|
| nitrogen | 78.0 | 78.1 |
| oxygen | 20.8 | 20.9 |
| carbon dioxide | 0.08 | 0.04 |
(a) Name the method used in the column to separate liquid air. [1]
(b) State why air is a mixture rather than a compound. [2]
(c) Which location has the greater percentage of carbon dioxide? [1]
(d) Give one possible reason for this difference. [1]
(e) Explain why carbon dioxide is removed before liquid air enters the column. [2]
(f) Calculate the percentage of gases other than nitrogen, oxygen and carbon dioxide at the coastal station. [2]
Answer Details
(a) Liquid air is separated by fractional distillation. [1]
(b) Air is a mixture because it contains different gases which are not chemically combined. Its composition can also vary slightly, as the table shows. [2]
(c) The busy road has the greater percentage of carbon dioxide: 0.08% compared with 0.04%. [1]
(d) More fuel combustion from vehicles in busy traffic can produce more carbon dioxide. [1]
(e) Carbon dioxide is removed because it freezes at the very low temperatures used to liquefy air. Solid carbon dioxide could block the apparatus or fractionating column. [2]
(f)
\[78.1+20.9+0.04=99.04\%\]
\[100.00-99.04=0.96\%\]
The percentage of other gases is 0.96%. [2]
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