Separating Mixtures (Extended Reponse)

Question 1

Substance: nitrogen | oxygen | water vapour | CO[2]
Melting
point °C: -210 | -219 | 0 | -78.5
Boiling
point °C: -196 | -183 | 100 | -78.5

Nitrogen and oxygen are useful substances in air.
They can be separated from air by:
-cooling air until water vapour and carbon dioxide solidify
-filtering the chilled air
-further cooling the filtered air to -200 degrees C
-carrying out fractional distillation

The table gives information about four substances found in air. Use the information above, and your knowledge of separation methods, to explain how nitrogen and oxygen may be separated in this way.

Answer 1

(Use data from the table to suggest the range of temperatures needed to solidify water and carbon dioxide. Use your knowledge and understanding of separation techniques to suggest how they could be separated from oxygen and nitrogen.)

If the air is cooled to below -78.5°C but above -183°C, the water vapour and carbon dioxide will become solid. The nitrogen and oxygen will stay as gases, so the solid water and carbon dioxide could then be filtered to remove them. This would stop them contaminating the oxygen and nitrogen later in the process.

(Use data from the table to describe what happens to the oxygen and nitrogen at -200°C. Use your knowledge and understanding of fractional distillation to describe the temperature gradient needed in the column.)

When the filtered air is cooled to -200°C, the oxygen and nitrogen will condense to form a mixture of liquids. This mixture can then be piped into a fractionating column. The column should be warmer at the bottom and colder at the top.

(Use data from the table to explain why oxygen and nitrogen can be separated by fractional distillation. Use your knowledge and understanding to describe where each substance will leave the fractionating column.)

In the fractionating column, the nitrogen will boil more easily than the oxygen, because it has the lower boiling point. Nitrogen gas can leave from the top of the column and oxygen from the bottom of the column. Depending on the temperature at the bottom, oxygen could leave as a liquid or a gas.

(You need to show comprehensive knowledge and understanding. Be prepared to apply what you know to a new context. Use the data given to you in a structured way with clear lines of reasoning.)
Location

Question 2

Substance: sodium chloride | calcium carbonate
Solubility: 307 | 0.013
at 25°C
(g dm[-3] water)

Sodium carbonate and calcium carbonate are both white powders at room temperature. The table shows some information about them. A student accidentally makes a mixture of the two powders.
Use the information in the table, and your knowledge and understanding of separation methods, to explain how the student could produce separate, dry sample of sodium carbonate and calcium carbonate from this mixture.

Answer 2

• Add water to the mixture
• Stir the mixture
• Sodium carbonate should dissolve but calcium carbonate should not
• This is because their solubility is different/calcium carbonate is (almost) insoluble
• Filter the mixture
• Calcium carbonate stays behind as a residue.
• Wash the residue in the filter paper (to remove any sodium carbonate)
• Leave washed residue in a warm place to dry
• Filtrate is sodium carbonate solution
• Heat to evaporate
• Leave solution to cool/crystallise sodium carbonate
• Filter/decant to separate crystals
• Leave crystals in a warm place to dry.