Higher: C8 - Chemical analysis

Question 1

How would you tell whether a substance is pure?

Answer 1

• Testing the melting/boiling point. If not what expected, it is a mixture.
• For an ink/dye, chromatography can be used

Question 2

What are formulations?

Answer 2

Mixtures, designed to be useful products, made by mixing the components in exact quantities.

E.g. fuels, cleaning agents, paints, medicines, alloys, fertilisers and foods.

Question 3

What is chromatography used for?

Answer 3

Can be used to separate mixtures (inks/dyes) and can give information to help identify substances.

Question 4

What are the mobile and stationary phases in chromatography?

Answer 4

• Mobile phase = solvent (bc it moves up the paper).
• Stationary phase = pencil line (it doesn’t move).

Question 5

Practical

Explain how paper chromatography can be used to separate mixtures.

Answer 5

• Draw a line near the bottom of a sheet of filter paper in pencil (since it’s insoluble).
• Add a spot of ink to this line.
• Place the sheet in a beaker of solvent (depends on what’s being tested), making sure the ink doesn’t touch the solvent.
• Put a lid on the container to stop the solvent evaporating.
• The mobile phase (solvent) seeps up the paper, carring the ink with it.
• Each dye within the ink moves up the paper at a different rate; the dyes separate out into spots.
• If any of the dyes are insoluble, they stay on the stationary phase (pencil line).
• When the solvent has nearly reached the top, the paper is taken out and allowed to dry; the result is a chromatogram.

Question 6

How would you calculate R_f value in chromatography?

Answer 6

distance moved by substance / distance moved by solvent

Question 7

How would you test for hydrogen?

Answer 7

• Hold a burning splint over a test tube of the gas
• Positive result: burns rapidly and makes a “pop” sound

Question 8

How would you test for oxygen?

Answer 8

• Insert a glowing splint into a test tube of the gas
• Positive result: the splint relights

Question 9

How would you test for carbon dioxide?

Answer 9

• Use an aqueous solution of calcuim hydroxide (limewater)
• Bubble the gas through the limewater
• Positive result: limewater goes cloudy/milky

Question 10

How would you test for chlorine?

Answer 10

• Place damp litmus paper into the gas
• Positive result: litmus paper turns white (is bleached)