Question 1 Medium
What name is given to a mixture designed as a useful product, made with carefully measured amounts?
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Answer: formulation
- Paints and medicines are designed mixtures.
- They are formulations.
Purity, formulations and chromatography: GCSE Combined Science Chemistry practice questions
A second set of questions on purity, formulations and chromatography. They are different from the ones on the topic page. Try each, then open the answer and compare your working.
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Question 2 Medium
Should the solvent level in the beaker be above or below the pencil line at the start?
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Answer: below
- If the spots were under the solvent they would dissolve.
- The solvent level is below the line.
Question 3 Hard
A spot moves 2.8 cm. The solvent front moves 8.0 cm. Calculate the Rf value.
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Answer: 0.35
- Rf = 2.8 ÷ 8.0.
- Rf = 0.35.
Question 4 Hard
A compound has an Rf value of 0.60. The solvent front moves 7.5 cm. How far does the compound move? Give your answer in cm.
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Answer: 4.5 cm
- Distance moved by substance = Rf × distance moved by solvent.
- 0.60 × 7.5 = 4.5 cm.
Question 5 Hard
A solid melts between 95 °C and 99 °C. Is it pure or impure?
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Answer: impure
- A pure solid melts at one sharp temperature.
- Melting over a range shows it is impure.
Question 6 Hard
Which of these is a formulation: pure water, a tin of paint or a copper wire?
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Answer: a tin of paint
- A formulation is a designed mixture.
- Paint is a mixture of several components with a purpose.
Question 7 Hard
Two spots in the same solvent have the same Rf value. What might this suggest about the two substances?
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Answer: they are the same substance
- Different compounds usually have different Rf values.
- The same Rf suggests the same substance.
Question 8 Hard
A chromatogram of an ink shows 3 spots. What is the smallest number of substances in the ink?
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Answer: 3
- Each spot is a different substance.
- There are at least 3 substances.
Quick checks
Short questions with options. Choose one before you open the answer.
Check 1
What does pure mean in chemistry?
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A single element or compound, not mixed with any other substance.
Check 2
How does melting point show whether a substance is pure?
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A pure substance melts at one sharp temperature. A mixture melts over a range.
Check 3
Does an impure substance usually melt at a lower or higher temperature than the pure one?
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Lower, and over a range.
Check 4
What is a formulation?
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A mixture designed as a useful product, made with carefully measured quantities of each component.
Check 5
Give three examples of formulations.
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Fuels, cleaning agents, paints, medicines, alloys, fertilisers and foods.
Check 6
What is chromatography used for?
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To separate mixtures of soluble substances and help identify them.
Check 7
In paper chromatography, what is the stationary phase?
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The paper.
Check 8
In paper chromatography, what is the mobile phase?
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The solvent.
Check 9
Why is the start line drawn in pencil?
Show answer
Pencil does not dissolve in the solvent. Ink would run.
Check 10
Why must the solvent level be below the start line?
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So the spots do not dissolve into the solvent in the beaker.
Check 11
What does one spot on a chromatogram show?
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The sample is a pure substance.
Check 12
What does more than one spot show?
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The sample is a mixture of at least that many substances.
Check 13
How do you identify a substance on a chromatogram?
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Compare its position and colour with known substances run on the same paper.
Check 14
What is the formula for the Rf value?
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Rf = distance moved by substance ÷ distance moved by solvent.
Check 15
Where do you measure the substance distance to?
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The centre of the spot, from the start line.
Check 16
What is the unit of Rf?
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It has no unit.
Check 17
What is the range of possible Rf values?
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Between 0 and 1.
Check 18
Why can an Rf value help identify a compound?
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Different compounds usually have different Rf values in the same solvent.
Check 19
Why do you mark the solvent front straight away?
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The solvent evaporates from the paper, so you would lose the position.
Check 20
Why do substances move different distances?
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They spend different amounts of time dissolved in the mobile phase and held on the stationary phase.
Check 21
Is carton orange juice pure in the chemical sense?
Show answer
No. It is a mixture of several substances, so it is not pure in chemistry.
Check 22
Can two different substances have the same Rf?
Show answer
Yes, so Rf alone cannot prove the identity. Use more than one test.
Check 23
What can simple distillation do in the inks practical?
Show answer
Separate the solvent (water) from the dyes in the ink.
Check 24
What happens to the Rf value if the solvent changes?
Show answer
It changes. Only compare Rf values for the same solvent.
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