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Pure substances, mixtures and chromatography

8th grade (Year 9 in England) · 16 questions · Common Core: NGSS.MS-PS1-2 · England: KS3 Chemistry: Pure and impure substances, KS4 Combined Science: Chemical analysis

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Date

Score/ 16

1
Question 1
How can you test whether a sample of water is pure?
- A. Smell it
- B. Check that it looks clear
- C. Check that it boils at exactly 100 °C (at sea-level pressure)
- D. Shake it
Question 2
In a chromatogram the solvent moved 5 cm and a dye spot moved 3.8 cm from the start line. What is the Rf value of the dye? (Give 2 decimal places.)
Answer
3
Question 3
Purity and chromatography: which statement is NOT true?
- A. Orange juice is a pure substance.
- B. In chromatography, a more soluble dye usually travels further.
- C. Mixtures can be separated by physical methods.
- D. A pure substance has a fixed melting point.
4
Question 4
Why is the start line on chromatography paper drawn in pencil, not pen?
- A. Pencil is easier to see
- B. Pencil makes the dyes move faster
- C. Pen is too expensive
- D. Pencil (graphite) does not dissolve in the solvent
Question 5
In a chromatogram the solvent moved 5 cm and a dye spot moved 2 cm from the start line. What is the Rf value of the dye? (Give 2 decimal places.)
Answer
6
Question 6
Purity and chromatography: which statement is NOT true?
- A. Air is a mixture of gases.
- B. An Rf value can be greater than 1.
- C. Mixtures can be separated by physical methods.
- D. A pure substance has a fixed melting point.
7
Question 7
In chemistry, what does "pure" mean?
- A. natural, with nothing added by people
- B. clean and safe to drink
- C. made of only one substance
- D. very expensive
Question 8
In a chromatogram the solvent moved 12 cm and a dye spot moved 9 cm from the start line. What is the Rf value of the dye? (Give 2 decimal places.)
Answer
9
Question 9
Purity and chromatography: which statement is NOT true?
- A. In chromatography, a more soluble dye usually travels further.
- B. Orange juice is a pure substance.
- C. A pure substance has a fixed melting point.
- D. Mixtures can be separated by physical methods.
10
Question 10
How can you test whether a sample of water is pure?
- A. Check that it boils at exactly 100 °C (at sea-level pressure)
- B. Shake it
- C. Check that it looks clear
- D. Smell it
Question 11
In a chromatogram the solvent moved 10 cm and a dye spot moved 7.5 cm from the start line. What is the Rf value of the dye? (Give 2 decimal places.)
Answer
12
Question 12
Purity and chromatography: which statement is NOT true?
- A. Mixtures can be separated by physical methods.
- B. Impurities have no effect on melting point.
- C. In chromatography, a more soluble dye usually travels further.
- D. A pure substance has a fixed melting point.
13
Question 13
An ink gives three separate spots on a chromatogram. What does this show?
- A. The ink is insoluble
- B. The ink is a mixture of at least three dyes
- C. The paper was dirty
- D. The ink is a pure substance
Question 14
In a chromatogram the solvent moved 10 cm and a dye spot moved 6 cm from the start line. What is the Rf value of the dye? (Give 2 decimal places.)
Answer
15
Question 15
Purity and chromatography: which statement is NOT true?
- A. An Rf value can be greater than 1.
- B. Air is a mixture of gases.
- C. A pure substance has a fixed melting point.
- D. In chromatography, a more soluble dye usually travels further.
16
Question 16
Why is the start line on chromatography paper drawn in pencil, not pen?
- A. Pencil is easier to see
- B. Pen is too expensive
- C. Pencil makes the dyes move faster
- D. Pencil (graphite) does not dissolve in the solvent

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