50 Objective Questions and Answers on Advanced Separation Techniques & Purity of Substances
In Chemistry, mastering separation techniques and understanding the concept of pure vs impure substances is essential for both theory and practical questions in WAEC, NECO, JAMB/UTME, and other O’Level exams. Topics such as centrifugation, evaporation, crystallization, fractional crystallization, precipitation, distillation, and tests for purity (melting point, boiling point, density, chromatography) appear regularly in objective and essay sections.
To help you revise effectively and score high, we’ve created another set of 50 high-quality multiple-choice questions strictly based on these concepts. These questions are designed to reflect the style and difficulty level seen in recent examinations. Attempt all 50 questions first, then check the correct answers provided at the end to evaluate your performance and identify areas for improvement.
Whether you’re preparing for upcoming exams or building a strong foundation in Chemistry, this practice set will sharpen your understanding. Let’s begin!
1. Centrifugation is primarily used to separate particles based on differences in:
A. Colour
B. Size, shape, density, viscosity and rotor speed
C. Taste
D. Odour
2. In a centrifuge, the solid particles are forced to:
A. Rise to the top
B. Settle at the bottom of the tube
C. Remain suspended
D. Evaporate
3. After centrifugation, the clear liquid above the settled particles is called:
A. Residue
B. Supernatant
C. Filtrate
D. Sublimate
4. In hospitals, centrifugation is commonly used to separate:
A. Salt from water
B. Blood cells from plasma
C. Sand from gravel
D. Alcohol from water
5. Evaporation is used to separate:
A. Two immiscible liquids
B. A dissolved solid from its solution by removing the liquid
C. Magnetic materials
D. Sublimable solids
6. During evaporation to dryness, the solvent is:
A. Collected and reused
B. Sacrificed (escapes as vapour)
C. Frozen
D. Filtered
7. Evaporation is commonly used in:
A. Petroleum refining
B. Salt-making industries
C. Blood separation
D. Dye separation
8. Crystallization is preferred over simple evaporation when the salt:
A. Has a very high boiling point
B. Decomposes easily on heating
C. Is magnetic
D. Is insoluble
9. Which of these compounds is commonly obtained by crystallization?
A. NaCl
B. CuSO₄·5H₂O
C. SiO₂
D. CaCO₃
10. In crystallization, the solution is first heated to become:
A. Dilute
B. Saturated
C. Supersaturated
D. Unsaturated
11. Fractional crystallization is used to separate:
A. Two or more solid solutes with different solubility behaviours
B. Immiscible liquids
C. Gases from liquids
D. Magnetic particles
12. Precipitation involves:
A. Heating the solution to dryness
B. Forming an insoluble solid from a solution
C. Cooling a saturated solution
D. Spinning a mixture at high speed
13. In the reaction AgNO₃(aq) + KBr(aq) → AgBr(s) + KNO₃(aq), AgBr is:
A. The filtrate
B. The precipitate
C. The supernatant
D. The distillate
14. The precipitate formed in a reaction is usually separated by:
A. Distillation
B. Filtration
C. Centrifugation only
D. Evaporation
15. Distillation is used to separate liquids based on differences in:
A. Density
B. Volatility / boiling points
C. Magnetic properties
D. Colour
16. The process of cooling vapour back to liquid in distillation is called:
A. Evaporation
B. Condensation
C. Sublimation
D. Precipitation
17. Ethanol (b.p. 78°C) and water (b.p. 100°C) can be separated by:
A. Filtration
B. Simple distillation
C. Sieving
D. Chromatography
18. Impurities generally cause the melting point of a substance to:
A. Increase
B. Decrease
C. Remain the same
D. Become zero
19. Impurities cause the boiling point of a liquid to:
A. Decrease
B. Increase
C. Remain unchanged
D. Become unpredictable
20. A pure solid substance melts:
A. Over a wide temperature range
B. At a fixed temperature
C. Only above 100°C
D. Below 0°C
21. Pure naphthalene melts at:
A. 78°C
B. 80°C
C. 100°C
D. 0°C
22. Pure ethanol boils at:
A. 78°C
B. 100°C
C. 80°C
D. 120°C
23. The boiling point of a liquid is affected by:
A. Only temperature
B. Atmospheric pressure
C. Colour of the container
D. Taste of the liquid
24. Density is represented by the Greek letter:
A. Delta (Δ)
B. Rho (ρ)
C. Sigma (σ)
D. Gamma (γ)
25. The density of ultra-pure water at 20.00°C is approximately:
A. 1.000 g/cm³
B. 0.998203 g/cm³
C. 0.900 g/cm³
D. 1.200 g/cm³
26. Which factor does NOT affect density determination?
A. Temperature
B. Air bubbles
C. Colour of the liquid
D. Viscosity
27. In paper chromatography, a pure coloured substance usually produces:
A. Multiple spots
B. A single spot
C. No spot
D. A streak
28. Adding salt to icy roads helps to:
A. Increase the melting point of ice
B. Lower the melting point of ice
C. Freeze the road
D. Make ice harder
29. The separation technique illustrated in the 1991 question (drop of universal indicator separated into dyes) is:
A. Distillation
B. Chromatography
C. Filtration
D. Evaporation
30. In the chromatography of universal indicator, how many components were resolved?
A. Two
B. Three
C. Four
D. Five
31. The adsorbent medium normally used in laboratory paper chromatography is:
A. Sand
B. Filter paper
C. Cotton wool
D. Glass wool
32. In centrifugation, the rotor speed affects:
A. The colour of particles
B. The rate of separation
C. The taste of the liquid
D. The odour
33. Which method is used to obtain pure alum crystals from an impure sample?
A. Distillation
B. Crystallization
C. Sieving
D. Magnetic separation
34. Fractional crystallization depends mainly on differences in:
A. Boiling points
B. Solubility at different temperatures
C. Density
D. Magnetic properties
35. Which technique is used in gin distilleries?
A. Filtration
B. Distillation
C. Centrifugation
D. Precipitation
36. The main purpose of a condenser in distillation is to:
A. Heat the mixture
B. Cool the vapour to liquid
C. Filter impurities
D. Increase pressure
37. A substance is likely impure if its melting point is:
A. Sharp and fixed
B. Lower than expected and not sharp
C. Exactly 100°C
D. Higher than 200°C
38. Which of these is a hydrated salt commonly crystallized?
A. NaCl
B. FeSO₄·7H₂O
C. C (carbon)
D. O₂
39. Precipitation is based on differences in:
A. Boiling points
B. Solubilities
C. Densities
D. Melting points
40. In the determination of melting point, the substance is placed in a:
A. Beaker
B. Capillary tube
C. Separating funnel
D. Fractionating column
41. Air bubbles affect density measurement by:
A. Increasing the true mass
B. Giving a falsely high volume
C. Changing colour
D. Increasing viscosity
42. Crystallization is very important in the manufacture of:
A. Petrol
B. Drugs and sugar
C. Concrete
D. Glass
43. Which process sacrifices the solvent?
A. Distillation
B. Evaporation to dryness
C. Crystallization
D. Chromatography
44. In industry, fractional crystallization is used to separate:
A. Crude oil
B. Multi-component salt solutions
C. Blood cells
D. Dyes
45. The boiling point of a mixture is usually:
A. Fixed
B. A range of temperatures
C. Always 100°C
D. Always 0°C
46. Adding antifreeze to car radiators:
A. Raises the freezing point
B. Lowers the freezing point of water
C. Increases boiling point only
D. Makes water magnetic
47. The point where the sample is applied in paper chromatography is called the:
A. Solvent front
B. Point of application / baseline
C. Rf point
D. Residue point
48. Pure substances have:
A. Wide melting/boiling ranges
B. Fixed melting and boiling points
C. No fixed physical properties
D. Variable density only
49. Which technique is NOT used for separating solids from liquids?
A. Filtration
B. Centrifugation
C. Distillation
D. Evaporation
50. In the universal indicator chromatography example, the solvent used was:
A. Water
B. Ethanol
C. Ethyl ethanoate
D. Tetrachloromethane
Answers (Correct Options)
1. B
2. B
3. B
4. B
5. B
6. B
7. B
8. B
9. B
10. B
11. A
12. B
13. B
14. B
15. B
16. B
17. B
18. B
19. B
20. B
21. B
22. A
23. B
24. B
25. B
26. C
27. B
28. B
29. B
30. B
31. B
32. B
33. B
34. B
35. B
36. B
37. B
38. B
39. B
40. B
41. B
42. B
43. B
44. B
45. B
46. B
47. B
48. B
49. C
50. C
