Methods of Separating & Purifying Substances Practice Questions

Explain why a chemist would not describe this apple juice as pure.

State what is meant by a pure substance and by a mixture in chemistry.

Identify the two samples that are pure substances.

Explain how the data show that sample X is a mixture.

A small amount of a soluble impurity is added to a pure sample of benzoic acid. State the effect this has on the melting point of the sample.

Which process is used to kill microorganisms in the water?

A sedimentation

B filtration

C chlorination

D crystallisation

State what happens to the water during sedimentation.

Explain why the water is filtered after sedimentation.

The treated water is described as potable. State what is meant by potable water.

Mixture 1 is solid calcium carbonate stirred into water. Calcium carbonate is insoluble in water.

Name the technique used to separate the calcium carbonate from the water, and give the name used for the solid that is left behind on the filter paper.

Mixture 2 is copper sulfate solution. Name the technique used to obtain pure water from this solution.

Mixture 3 is the ink from a black felt-tip pen. Name the technique used to separate the coloured dyes in the ink.

Name the pieces of apparatus labelled A and B.

Explain how this apparatus separates the water from the ink.

Describe how the student could test the colourless liquid collected to show that it is pure water.

State why this mixture cannot be separated by filtration.

Describe how fractional distillation is used to obtain a sample of methanol from this mixture.

Give the temperature shown on the thermometer while the methanol is distilling over.

Describe how the student can obtain a sample of pure, dry potassium nitrate crystals from this solution.

Explain why the solution is allowed to cool slowly.

Explain why the line on the chromatography paper is drawn in pencil.

Explain why the level of the solvent in the beaker must be below the pencil line.

Calculate the $R_f$ value of dye X.

State what the chromatogram shows about the green ink.

Explain, in terms of the mobile phase and the stationary phase, why the colourings separate on the paper.

Use the table to identify the colourings present in the sweet. Justify your answer.

Calculate the $R_f$ value of E133.

A builder washes sand and the washings collect in a bucket. The liquid in the bucket is a mixture of solid sand, water and dissolved sodium chloride.

A student is asked to separate this mixture to obtain three products:

• dry sand
• pure water
• solid sodium chloride

Devise a method the student could use. In your answer, name the techniques used and explain what happens to each component of the mixture.

Calculate the distance, in cm, moved by dye X in the ethanol experiment.

In the water experiment, dye X had an $R_f$ value of 0.32. Explain why the same dye can have two different $R_f$ values.

In the water experiment, one of the dyes stayed on the pencil line. State what this shows about that dye.