Chapter 8 — Mixtures and Separation of Mixtures

1. Mixtures

A mixture contains more than one substance physically combined.

Its components retain their identities and can be separated using appropriate physical methods.

The key principle is:

Choose a separation technique according to the difference in a physical property of the components.

2. Chromatography

Chromatography separates substances because different components interact differently with a stationary phase and a mobile phase.

The stationary phase remains fixed, while the mobile phase moves through it.

A component that interacts more strongly with the mobile phase generally moves faster; one that interacts more strongly with the stationary phase moves more slowly.

Important terms:

• Stationary phase: fixed material through which separation occurs.
• Mobile phase: moving solvent or fluid.
• Eluent: solvent used to carry components through a column.
• Adsorbent: solid material used as the stationary phase in column chromatography.

The chapter also discusses common adsorbents and explains why different substances travel at different speeds.

3. Fractional distillation

Fractional distillation separates miscible liquids with different boiling points, especially when their boiling points are relatively close.

A fractionating column is placed between the distillation flask and condenser.

Inside the column, repeated:

vaporisation → condensation → vaporisation

enriches the vapour in the more volatile component.

This improves separation.

4. Simple vs fractional distillation

Simple distillation is more suitable when boiling points differ sufficiently.

Fractional distillation is preferred when boiling points are closer because the fractionating column provides repeated opportunities for separation.

The chapter gives ethanol-water as an example and explains the use of fractional distillation in petroleum refining.

5. Crude oil

Crude oil contains many hydrocarbons with different boiling points.

Fractional distillation separates it into useful fractions such as:

• petroleum gas;
• petrol;
• kerosene;
• diesel;
• fuel oils;
• lubricating/heavy oils.

Key idea

Separation does not change the substances chemically. It exploits differences such as boiling point, interaction with a stationary phase, solubility or other physical properties.

Chapter 8 — Mixtures and Separation of Mixtures

The source covers chromatography, stationary/mobile phases, column chromatography, fractional distillation and separation of crude oil into useful fractions.

Questions

A. Multiple Choice Questions

1. Chromatography separates substances primarily because different components:
a) Have identical movement through the medium
b) Move at different rates through stationary and mobile phases
c) Have the same boiling point
d) Have the same solubility

2. The phase that remains fixed in chromatography is the:
a) Mobile phase
b) Stationary phase
c) Vapour phase
d) Liquid phase

3. In column chromatography, silica gel commonly acts as the:
a) Mobile phase
b) Stationary phase
c) Eluent
d) Distillate

4. The solvent that flows through a chromatography column is called the:
a) Adsorbent
b) Eluent
c) Residue
d) Sediment

5. Different compounds move at different speeds through a chromatography column mainly because they have different:
a) Nuclear charges
b) Adsorption tendencies
c) Mass numbers only
d) Densities only

6. Fractional distillation is particularly useful for separating miscible liquids with:
a) Close boiling points
b) No boiling points
c) Identical boiling points
d) Very high densities only

7. The main function of a fractionating column is to provide:
a) Electrical energy
b) Repeated condensation and vaporisation
c) Chemical reactions
d) Filtration

8. Crude oil is separated into useful fractions mainly by:
a) Filtration
b) Fractional distillation
c) Sedimentation
d) Chromatography

B. Fill in the Blanks

9. Chromatography was first developed by Mikhail ______ in 1906.

10. Silica gel and alumina can act as ______ in column chromatography.

11. The phase that moves through the column is the ______ phase.

12. The solvent used to move components through a column is called the ______.

13. Fractional distillation depends on differences in ______ points.

14. Crude oil is a complex mixture of many ______.

C. True or False

15. The stationary phase moves continuously through a chromatography column.

16. Components that interact differently with the stationary phase may travel at different speeds.

17. A fractionating column improves separation through repeated condensation and vaporisation.

18. Crude oil is a pure substance.

D. Assertion–Reason

19. Assertion: Fractional distillation provides better separation than simple distillation for liquids with close boiling points.
Reason: The fractionating column permits repeated condensation and vaporisation.

20. Assertion: In chromatography, components can move at different rates.
Reason: Different components interact differently with the stationary and mobile phases.

E. Short Answer

21. What are the two main phases in chromatography?

22. What is the role of an adsorbent in column chromatography?

23. Why does the more volatile component rise more readily in fractional distillation?

24. Why is simple distillation less suitable for liquids having close boiling points?

F. Application Questions

25. In a chromatography column, compound A emerges before compound B. What does this suggest about their interaction with the stationary phase?

26. Ethanol and water have boiling points of about $78^\circ C$ and $100^\circ C$, respectively. Which separation technique is appropriate according to the chapter, and why?

---

Answers

A. MCQ Answers

1. b) Move at different rates through stationary and mobile phases
2. b) Stationary phase
3. b) Stationary phase
4. b) Eluent
5. b) Adsorption tendencies
6. a) Close boiling points
7. b) Repeated condensation and vaporisation
8. b) Fractional distillation

B. Fill in the Blanks

9. Tswett
10. adsorbents
11. mobile
12. eluent
13. boiling
14. hydrocarbons

C. True/False

15. False
16. True
17. True
18. False

D. Assertion–Reason

19. Both are true, and the reason correctly explains the assertion.
20. Both are true, and the reason correctly explains the assertion.

E. Answers

21. Stationary phase and mobile phase.
22. The adsorbent provides the stationary surface on which different components are retained to different degrees.
23. A more volatile liquid has a lower boiling point and enters the vapour phase more readily.
24. When boiling points are close, simple distillation does not provide sufficient separation. Repeated condensation and vaporisation in a fractionating column improves the separation.

F. Answers

25. A likely explanation is that A has a weaker interaction with the stationary phase and/or a stronger relative interaction with the mobile phase than B.
26. Fractional distillation is appropriate because the liquids are miscible and have relatively close boiling points. The fractionating column improves their separation.