Separation techniquesIB MYP Chemistry: Subtopic test

1. 1
   A school laboratory technician has four mixtures to separate. Mixture W is sand and water. Mixture X is salt dissolved in water, and she wants to collect the salt. Mixture Y is seawater, and she wants to collect pure water. Mixture Z is a liquid mixture of ethanol, which boils at 78 °C, and water, which boils at 100 °C, and she wants to collect both liquids.

   (a)

   Which method is best for separating the ethanol and water in mixture Z?

   [1 mark]

   • AFiltration
   • BEvaporation
   • CSimple distillation
   • DFractional distillation

   (b)

   Which method should she use to collect pure water from the seawater in mixture Y?

   [1 mark]

   • ASimple distillation
   • BFiltration
   • CDecanting
   • DFractional distillation

   (c)

   Explain why filtration cannot be used to collect the salt from mixture X.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student wants to obtain pure crystals of copper sulfate from a blue copper sulfate solution. She heats the solution in an evaporating basin until about half of the water has gone. Every so often she dips a clean glass rod in the hot solution and lets the drop cool, until small crystals form on the rod. She then stops heating, leaves the solution to cool, separates the crystals from the liquid and dries them between filter paper.

   (a)

   Why does the student stop heating when crystals form on the glass rod?

   [1 mark]

   • AAll of the water has evaporated
   • BThe copper sulfate has decomposed
   • CThe solution has become saturated
   • DThe solution has reached its boiling point

   (b)

   Which method does she use to separate the crystals from the remaining liquid?

   [1 mark]

   • ASimple distillation
   • BFiltration
   • CEvaporation
   • DFractional distillation

   (c)

   Explain why the solution is left to cool before the crystals are separated, and why they are then dried.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student is given 10.0 g of a mixture of sand, salt and iron filings and must separate and recover all three substances. After she has dried each one, the masses she recovers are: iron 1.0 g, sand 3.6 g and salt 4.9 g.

   (a)

   Outline a method that she could use to separate sand, salt and iron filings from the mixture.

   [3 marks]

   (b)

   Process the results to find the percentage of the mixture that she recovered, and suggest one reason why it is below 100% and how to improve this.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A small company in Mexico makes ethanol for hand sanitiser by fermenting sugar cane juice, which gives a mixture of about 10% ethanol in water. Ethanol boils at 78 °C and water boils at 100 °C. The company separates the two liquids using fractional distillation and burns natural gas to heat the mixture. Local residents are concerned about the fire risk of storing the ethanol, but the company says the sanitiser protects people from infections.

   (a)

   Explain how fractional distillation separates the ethanol from the water.

   [6 marks]

   (b)

   Discuss and evaluate the company's production of ethanol by fermentation and fractional distillation.

   [6 marks]

   Total for question 4: 12 marks

End of questions