Separation Techniques

Which of the following is most likely a pure compound?

• Black substance that produces three fractions when distilled.

• Blue crystals that melt at exactly 58 ºC.

• Ice that melts over a range from -10 ºC to -5 ºC.

• Liquid that has particles settling to the bottom when left to stand.

Answer explanation

Test for pure substances:

1) Fixed melting and boiling point

2) Only one spot on chromatogram

The chromatogram of sample X is shown below. Is sample X pure? Explain your answer.

Sample X is ​ (a)   because it contains ​ (b)   spots on the chromatogram.

• impure
• two
• pure
• three
• one

Which of the following equipment is NOT required for filtration?

• Filter funnel

• Conical flask

• Filter paper

• Measuring cylinder

Answer explanation

How would you obtain pure sand from a mixture of salt and sand?

• Dissolve in water --> Filter --> Wash and dry

• Dissolve in water --> Evaporate --> Crystallisation

• Filter --> Wash and dry

• Evaporate --> Crystallisation

Answer explanation

First step is always to dissolve in a solvent. One will b able to dissolve, the other solid cannot dissolve.

Then filter.

Copper(II) sulfate decomposes easily under heat. Copper(II) oxide is insoluble in water. How would you obtain pure and dry copper(II) sulfate crystals from a mixture of copper(II) sulfate and copper(II) oxide?

• Dissolve in water --> Filter --> Wash and dry the residue

• Dissolve in water --> Filter --> Crystallise the filtrate

• Dissolve in water --> Filter --> Evaporate the filtrate

• Filter --> Evaporate the water

Which of the following is true about evaporation to dryness?

• Evaporation to dryness does not require all the solvent to be evaporated.

• Evaporation to dryness is suitable for compounds that decompose under heat.

• Evaporation to dryness uses the evaporating dish, wire gauze and tripod stand.

• Evaporation to dryness is used to obtain insoluble solids from a mixture.

Salt X that decomposes on strong heating is dissolved in water.

Which separation technique is best used to recover the solid salt?

• Crystallisation

• Evaporation to dryness

• Filtration

• Simple distillation

Answer explanation

Crystallisation --> used for solids that decompose upon heating.

What component is present in fractional distillation but not in simple distillation?

How to obtain pure water from sea water?

• Fractional distillaiton

• Simple distillation

• Paper chromatography

• Evaporation to dryness

Answer explanation

Simple distillation is used to obtain pure solvents (liquids) from solutions (mixture of liquid and soluble solids).

Evaporation to dryness is used to obtain the soluble solid from solution.

In the simple distillation of seawater, what is the temperature recorded on the thermometer when the water is coming out of the condenser?

• 0^oC

• 78^oC

• 100^oC

• 120^oC

Answer explanation

Boiling point of the distillate (which is water).