Chemical analysis

1. What distinguishes a pure substance from a mixture, in terms of melting point?

1. Pure substances melt over a range of temperatures
2. Pure substances melt at one specific temperature
3. Mixtures melt at one specific temperature
4. There's no difference

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B: Pure substances melt at one specific temperature. Impurities usually lower and broaden the melting point range.

2. What is a formulation?

1. A pure element
2. A mixture designed as a useful product with precise proportions
3. A type of atom
4. A gas test

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B: A mixture designed as a useful product with precise proportions. Formulations are made to a specific 'recipe' for a purpose.

3. In paper chromatography, what are the two phases?

1. Solid and gas
2. Stationary phase (paper) and mobile phase (solvent)
3. Acid and alkali
4. Metal and non-metal

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B: Stationary phase (paper) and mobile phase (solvent). Components separate based on how they distribute between these phases.

4. How is an Rf value calculated?

1. Distance moved by solvent ÷ distance moved by substance
2. Distance moved by substance ÷ distance moved by solvent
3. The speed of a reaction
4. The mass of the sample

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B: Distance moved by substance ÷ distance moved by solvent. Rf compares how far the substance travelled relative to the solvent.

5. A pure substance run in chromatography usually shows:

1. Multiple spots
2. A single spot
3. No spot
4. A colour change

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B: A single spot. A mixture would typically separate into more than one spot.

6. Why might a manufacturer test the purity of their product using melting point?

1. Impure substances melt at a single sharp temperature
2. Impure substances melt over a wider range at a lower temperature than the pure substance
3. Melting point is unrelated to purity
4. Pure substances have no melting point

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B: Impure substances melt over a wider range at a lower temperature than the pure substance. A pure substance gives a sharp, well-defined melting point.

7. A substance moves 3.6 cm up a chromatography paper while the solvent front moves 7.2 cm. What is its Rf value?

1. 3.6
2. 2.0
3. 0.50
4. 0.20

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C: 0.50. Rf = 3.6 ÷ 7.2 = 0.50. Rf is a ratio with no units. Dividing solvent distance by substance distance (giving 2.0) is the wrong way round.

8. Why is the starting line on chromatography paper drawn in pencil rather than ink?

1. Pencil is more soluble than ink
2. Pencil does not dissolve in the solvent
3. Pencil reacts with the paper
4. Pencil makes spots travel faster

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B: Pencil does not dissolve in the solvent. Pencil graphite is insoluble in the solvent so it stays put. Ink contains dyes that would dissolve and run up the paper, confusing the results.

9. Why must the starting line be drawn above the level of the solvent in the beaker?

1. So the paper stays dry
2. So the solvent rises faster
3. So the spots become darker
4. So the sample does not dissolve into the solvent pool

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D: So the sample does not dissolve into the solvent pool. If the spots sit below the solvent level they dissolve into the pool and nothing travels up the paper, so no separation occurs.

10. A solid melts over the range 112–118 °C. The pure compound melts sharply at 121 °C. What does this suggest?

1. The sample is pure
2. The sample is impure
3. The thermometer is faulty
4. The sample is an element

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B: The sample is impure. Pure substances melt at one sharp temperature. Impurities lower the melting point and make it melt over a range, so this sample is impure.