🧪 Chemistry · Topic 2 of 10
Start with the study notes on Separation of Substances (definitions, examples and a quick revision), then work through 60 practice questions, each with the correct answer and a clear explanation of why it is right. When you are ready, take this topic as an interactive quiz to test yourself.
Topic overview
A mixture contains two or more substances that are not chemically joined. We separate mixtures:
Remember: Each method works because the parts of the mixture differ in some property: size, weight, solubility, magnetism or boiling point.
Handpicking is removing unwanted particles by hand. It works only when they are few and clearly different in size or colour.
Example: Picking small stones out of rice or pulses before cooking.
Threshing is beating harvested stalks to separate the grain from them.
Example: Farmers beat wheat stalks by hand, use animals, or use threshing machines.
Winnowing separates lighter husk from heavier grain using wind. The mixture is dropped from a height: the wind blows the husk away, and the grain falls straight down.
Example: After threshing, farmers winnow to get clean grain.
Sieving separates particles of different sizes. Small particles pass through the holes; large ones stay behind.
Examples:
Examples:
Adding alum (phitkari) to muddy water makes tiny particles clump together and settle faster. This is called loading.
Filtration removes insoluble solids from a liquid by passing it through filter paper or cloth.
Examples:
Remember: Filtration cannot remove dissolved substances. Salt or sugar particles are so small that they pass through the filter with the water.
Examples:
Distillation is evaporation followed by condensation. The liquid is boiled, and its vapour is cooled and collected separately as pure liquid, while the dissolved solid stays behind.
Examples:
Separates miscible liquids with different boiling points. The liquid with the lower boiling point boils off first.
Example: Petrol, kerosene and diesel are separated from crude oil this way.
In crystallisation, a hot saturated solution is cooled slowly, and the dissolved solid separates out as pure crystals. Impurities stay in the liquid, so crystals are purer than the solid from simple evaporation.
Example: Growing large blue crystals of copper sulphate.
Many mixtures need two or three methods, one after another.
Examples:
| Misconception | Correct understanding |
|---|---|
| Filtration can remove salt from salt water. | Dissolved salt passes through the filter; evaporation or distillation is needed. |
| Evaporation gives you pure water. | Evaporation recovers the solid. To collect the pure water as well, you need distillation. |
| Winnowing works because husk and grain differ in size. | Winnowing works on weight: wind carries away the lighter husk. |
| Any two liquids can be decanted. | Only immiscible liquids such as oil and water; miscible liquids need distillation. |
| One method can separate any mixture. | The method depends on how the parts differ, and many mixtures need several methods. |
Which method removes a few stones from rice?
Handpicking.
What property difference does winnowing use?
Weight: the husk is lighter than the grain.
Why can't filtration separate sugar from water?
Dissolved sugar particles are too small and pass through the filter.
How is salt obtained from sea water?
By letting the sun evaporate the water in shallow pits.
How would you get pure water from salt water?
By distillation: boil it and condense the vapour separately.
How would you separate sand and salt?
Dissolve the salt in water, filter out the sand, then evaporate the water.
Why is alum added to muddy water?
It makes fine particles clump together and settle faster (loading).
Name a method that separates iron filings from sand.
Magnetic separation, using a magnet.
Key takeaway: Every separation method uses a difference between the parts of a mixture, such as size, weight, solubility, magnetism or boiling point.
Notes written by Velnov for Class 6–8 students, following the topics of the NCERT science syllabus.
Q1. easy
Why do we need to separate the components of a mixture?
Answer: A — To remove impurities and obtain useful or pure substances
We separate mixtures to remove harmful or useless components (like stones from rice), to obtain a useful component (like butter from milk), or to get a pure substance (like salt from sea water).
Q2. easy
Picking out small stones from rice or wheat by hand is called:
Answer: A — Handpicking
Handpicking is used when the unwanted particles are few, and large enough in size or different enough in colour to be picked out by hand.
Q3. medium
Handpicking is suitable only when the impurities are:
Answer: A — Few in number and clearly different in size or colour
Handpicking works only when the impurities are large enough to be seen and picked, and few enough to make it practical. Tiny or dissolved impurities need other methods.
Q4. medium
The process of beating out grain stalks to separate the grain from the stalk is called:
Answer: A — Threshing
Threshing separates grain seeds from the harvested stalks by beating them, either manually or using machines and animals. It is done after harvesting.
Q5. medium
The method of separating lighter husk from heavier grain using wind or air is called:
Answer: A — Winnowing
In winnowing, the mixture is dropped from a height in the wind. The lighter husk is blown away and forms a separate heap, while the heavier grain falls straight down.
Q7. medium
Which method is used to separate components of a mixture that differ in the SIZE of their particles?
Answer: A — Sieving
Sieving passes a mixture through a sieve with holes of a chosen size. Smaller particles pass through while larger ones stay behind - as when flour is sieved to remove bran.
Q8. medium
Sieving is commonly used at a construction site to separate:
Answer: A — Fine sand from pebbles and stones
At construction sites, sand is passed through a large sieve to separate the fine sand from pebbles and stones, since they differ greatly in particle size.
Q11. medium
Sedimentation followed by decantation is used to separate:
Answer: A — An insoluble solid from a liquid (like sand from water)
This pair of methods separates a heavier insoluble solid from a liquid. Dissolved substances like salt and sugar cannot be separated this way.
Q12. medium
Decantation can also separate two liquids that:
Answer: A — Do not mix with each other (like oil and water)
Immiscible liquids like oil and water form separate layers. The upper layer (oil) can be poured off, or a separating funnel can be used to drain the lower layer.
Q13. medium
The process of removing insoluble solids from a liquid by passing it through a filter is called:
Answer: A — Filtration
Filtration passes a mixture through filter paper or a cloth. The liquid passes through while the insoluble solid particles are trapped, giving a much cleaner separation than decantation.
Q16. medium
Which everyday activity is an example of filtration?
Answer: A — Straining tea leaves from tea using a strainer
Using a tea strainer traps the tea leaves (residue) while letting the liquid tea (filtrate) pass through - a common example of filtration at home.
Q18. medium
The process of converting a liquid into its vapour by heating is called:
Answer: A — Evaporation
Evaporation is the change of a liquid into vapour. It is used to recover a dissolved solid - the liquid evaporates away, leaving the solid behind.
Q19. medium
Salt is obtained from sea water mainly by:
Answer: A — Evaporation of the water using sunlight
Sea water is trapped in shallow pits and left in the sun. The water evaporates, leaving the salt behind as a solid, which is then collected and purified.
Q21. medium
Water droplets appear on the outside of a glass of cold water because of:
Answer: A — Condensation of water vapour from the air
The cold surface cools the air near it, causing the invisible water vapour in the air to condense into visible water droplets on the outside of the glass.
Q24. medium
Churning milk or curd to separate butter uses the method of:
Answer: A — Churning (centrifugation)
Churning spins the mixture so that the lighter butter (fat) particles separate and collect at the top, while the heavier liquid (buttermilk) stays below. This is a form of centrifugation.
Q26. medium
A mixture of iron filings and sand can best be separated using:
Answer: A — A magnet
Iron is magnetic, so a magnet attracts and removes the iron filings, leaving the non-magnetic sand behind. This is called magnetic separation.
Q30. medium
Which of these substances SUBLIMES?
Answer: A — Naphthalene (moth balls)
Naphthalene balls slowly turn directly into vapour and shrink over time - they sublime. Camphor, ammonium chloride and iodine also sublime. Salt, sand and sugar do not.
Q31. medium
A solution in which no more solute can dissolve at a given temperature is called:
Answer: A — A saturated solution
A saturated solution has dissolved the maximum possible solute at that temperature. Any extra solute added simply settles at the bottom undissolved.
Q33. medium
The amount of a substance that can dissolve in a given amount of solvent is called its:
Answer: A — Solubility
Solubility measures how much of a solute can dissolve in a fixed quantity of solvent at a given temperature. Different substances have different solubilities.
Q35. medium
The process of adding alum to speed up the settling of suspended impurities is called:
Answer: A — Loading
Loading is the addition of a substance (usually alum) that makes fine suspended particles heavier so they settle faster, making the water easier to clarify.
Q37. medium
Which method separates two MISCIBLE liquids with different boiling points?
Answer: A — Distillation (fractional distillation)
Miscible liquids that mix completely cannot be decanted. Distillation separates them by boiling: the liquid with the lower boiling point vaporises first and is condensed and collected separately.
Q39. medium
To separate tea leaves from prepared tea, we use:
Answer: A — Filtration
A strainer acts as a filter: it holds back the insoluble tea leaves (residue) and lets the liquid tea (filtrate) pass through.
Q41. medium
Which method would you use to separate stones from a small quantity of pulses?
Answer: A — Handpicking
With a small quantity and clearly visible stones, handpicking is the simplest and most practical method.
Q43. medium
Which of these mixtures can be separated using a sieve?
Answer: A — Flour and bran (particles of different sizes)
Sieving works when components differ in particle size. Fine flour passes through the sieve while coarse bran is left behind. Dissolved or liquid mixtures need other methods.
Q45. medium
Which method is used to separate cream from milk?
Answer: A — Centrifugation (churning)
A cream separator spins milk rapidly. The lighter cream (fat) collects toward the centre while the heavier skimmed milk moves outward - centrifugation at work.
Q47. medium
The water that evaporates from seas and lakes later forms clouds and returns as rain. This shows the natural use of:
Answer: A — Evaporation and condensation (the water cycle)
In the water cycle, water evaporates from water bodies, condenses in the cooler upper atmosphere to form clouds, and falls back as rain - nature's own distillation, giving fresh water.
Q49. medium
Which of the following CANNOT be separated by filtration?
Answer: A — Sugar dissolved in water
Dissolved sugar passes through the filter with the water because its particles are far too small to be trapped. Only undissolved solids like sand, chalk and tea leaves are separated by filtration.
Q51. medium
A separating funnel is best suited for separating:
Answer: A — Two immiscible liquids like oil and water
In a separating funnel, immiscible liquids form two layers. The tap at the bottom is opened to drain the lower (denser) liquid, and closed just as the upper layer reaches it.
Q53. medium
Which of these separation methods requires HEATING?
Answer: A — Evaporation, distillation and sublimation
Evaporation, distillation and sublimation all require heat to change a substance into vapour. The other methods are purely physical and need no heating.
Q55. medium
Crystallisation gives purer crystals than simple evaporation because:
Answer: A — Impurities remain dissolved in the liquid instead of drying onto the solid
In simple evaporation, everything dissolved dries out together, including impurities. In crystallisation, only the pure substance forms crystals while impurities stay in the remaining solution.
Q57. medium
Which of these best describes why separation methods differ from mixture to mixture?
Answer: A — The method depends on the physical properties of the components
We choose a method based on how the components differ - in size, weight, solubility, magnetism, boiling point or state. Understanding these differences lets us pick the right technique.
Q59. medium
Which method would separate a mixture of ammonium chloride and common salt?
Answer: A — Sublimation - ammonium chloride sublimes and salt is left behind
Ammonium chloride changes directly from solid to vapour on heating and re-deposits on a cool surface, while common salt does not sublime and remains in the dish.
Q6. hard
Winnowing works because the two components differ in their:
Answer: A — Weight (mass)
Winnowing separates a lighter component from a heavier one using air. The wind carries away the light husk while the heavy grain falls nearby, so the difference in weight is the key.
Q9. hard
The settling down of heavier insoluble particles at the bottom of a container is called:
Answer: A — Sedimentation
Sedimentation is the settling of heavier insoluble solid particles (the sediment) at the bottom when a mixture is left undisturbed - for example, sand settling in muddy water.
Q10. hard
Pouring out the clear liquid from the top after the sediment has settled is called:
Answer: A — Decantation
Decantation is carefully pouring off the clear upper liquid without disturbing the settled sediment. It usually follows sedimentation.
Q14. hard
In filtration, the clear liquid that passes through the filter paper is called the:
Answer: A — Filtrate
The liquid collected after passing through the filter is the filtrate. The solid left behind on the filter paper is the residue.
Q15. hard
In filtration, the solid left behind on the filter paper is called the:
Answer: A — Residue
The residue is the insoluble solid trapped by the filter paper. For example, filtering muddy water leaves mud as the residue and gives clear water as the filtrate.
Q17. hard
Why can filtration NOT separate salt from salt water?
Answer: A — Dissolved salt particles are too small and pass through the filter with the water
Filtration removes only undissolved (insoluble) particles. Dissolved salt particles are extremely tiny and pass through the filter paper along with the water, so evaporation must be used instead.
Q20. hard
The process of converting vapour back into a liquid on cooling is called:
Answer: A — Condensation
Condensation is the change of vapour into liquid on cooling. Water droplets forming on a cold glass, and dew or clouds forming in nature, are examples.
Q22. hard
The method that combines evaporation and condensation to obtain pure water from salt water is called:
Answer: A — Distillation
In distillation, the solution is boiled so the water evaporates, and the vapour is then cooled and condensed back into pure liquid water in a separate container, leaving the salt behind.
Q23. hard
Distillation is used to separate:
Answer: A — A dissolved solid from a liquid when the pure LIQUID is also needed
Simple evaporation recovers only the dissolved solid, letting the liquid escape. Distillation recovers both - the pure liquid is condensed and collected, and the solid stays behind.
Q25. hard
A washing machine's spin cycle separates water from clothes using the principle of:
Answer: A — Centrifugation
Rapid spinning throws the heavier water outward and out of the clothes through the holes in the drum. Centrifugation is also used to separate cream from milk and in laboratories to separate blood components.
Q27. hard
Magnetic separation works only when one component is:
Answer: A — Magnetic (like iron, nickel or cobalt)
Magnetic separation depends on one component being attracted by a magnet. Iron, nickel and cobalt are magnetic; if neither component is magnetic, this method cannot be used.
Q28. hard
The change of a solid directly into vapour on heating, without becoming a liquid, is called:
Answer: A — Sublimation
Sublimation is the direct change from solid to vapour (and back). Camphor, naphthalene balls, ammonium chloride and dry ice sublime. It can separate such substances from a mixture.
Q29. hard
A mixture of common salt and camphor can be separated by:
Answer: A — Sublimation (camphor sublimes, salt does not)
On heating, camphor changes directly into vapour (sublimes) and can be collected on a cool surface, leaving the salt behind. Only one component must be a subliming solid.
Q32. hard
A saturated solution can usually dissolve MORE solute if you:
Answer: A — Heat it (raise the temperature)
For most solids, solubility increases with temperature. Heating a saturated solution allows it to dissolve more solute; on cooling, the excess solute may crystallise out again.
Q34. hard
Muddy river water is often treated with ALUM (phitkari) before filtering because alum:
Answer: A — Makes fine suspended particles clump together and settle faster
Adding alum causes tiny suspended clay particles to stick together into larger, heavier clumps that settle quickly. This process is called loading, and it speeds up sedimentation before filtration.
Q36. hard
Which sequence is used to purify muddy water at a water treatment plant?
Answer: A — Loading/sedimentation, then filtration, then disinfection (chlorination)
Muddy water is first treated with alum so particles settle (loading and sedimentation), then filtered through sand and gravel beds, and finally disinfected with chlorine to kill germs.
Q38. hard
In many cases we need to use MORE THAN ONE method of separation because:
Answer: A — A mixture may contain several different components requiring different methods
A complex mixture may need a combination - for example, muddy salt water needs filtration to remove mud and then evaporation to recover the salt. The methods chosen depend on the components' properties.
Q40. hard
To separate a mixture of sand and salt, the correct sequence is:
Answer: A — Add water to dissolve the salt, filter out the sand, then evaporate to recover the salt
Salt dissolves in water but sand does not. Filtration removes the sand as residue, and evaporating the filtrate leaves the salt behind - a classic combination of methods.
Q42. hard
Farmers separate grain from stalks and then husk from grain. The correct order of methods is:
Answer: A — Threshing, then winnowing
First, threshing beats the harvested crop to free the grain from the stalks. Then winnowing uses wind to blow away the lighter husk, leaving the clean grain.
Q44. hard
Why does hot water dissolve sugar faster than cold water?
Answer: A — Higher temperature increases both the rate of dissolving and the solubility
At higher temperatures, particles move faster and interact more, so sugar dissolves more quickly, and more of it can dissolve overall. This is why sugar dissolves easily in hot tea.
Q46. hard
Which pair of methods is used one after the other to obtain clear water from muddy water at home?
Answer: A — Sedimentation (letting mud settle), then decantation or filtration
Leaving muddy water undisturbed lets the mud settle (sedimentation). The clear water on top is then poured off (decantation) or passed through a cloth/filter for a cleaner result.
Q48. hard
Rainwater is naturally quite pure because the process that forms it is essentially:
Answer: A — Distillation (evaporation followed by condensation)
When water evaporates, dissolved salts and impurities are left behind. The vapour condenses into rain, so rainwater is naturally distilled - though it can pick up pollutants from the air as it falls.
Q50. hard
Which property difference is used in each method - match correctly:
Answer: A — Sieving - size; Winnowing - weight; Magnetic separation - magnetism; Evaporation - solubility/volatility
Each separation method exploits a difference in properties: sieving uses particle size, winnowing uses weight, magnetic separation uses magnetic behaviour, and evaporation uses the fact that the solvent turns to vapour while the solute does not.
Q52. hard
Petrol and kerosene are obtained from petroleum by:
Answer: A — Fractional distillation (separating liquids by boiling point)
Petroleum is a mixture of many liquids with different boiling points. In fractional distillation, it is heated in a tall column, and each fraction (petrol, kerosene, diesel) condenses at a different height and is collected separately.
Q54. hard
Crystallisation is a method used to obtain PURE solid crystals from a solution. It works because:
Answer: A — As the hot saturated solution cools, the dissolved solid separates out as pure crystals
In crystallisation, a hot saturated solution is allowed to cool slowly. Since solubility falls as the temperature drops, the excess solute separates out as well-formed, pure crystals, leaving impurities in the solution.
Q56. hard
Which method would you choose to separate a mixture of common salt, sand and iron filings?
Answer: A — Magnet for iron, then water + filtration for sand, then evaporation for salt
First a magnet removes the iron filings. Then water dissolves the salt, and filtration removes the sand as residue. Finally, evaporating the filtrate recovers the salt - a three-step combination.
Q58. hard
Which overall statement about separation of substances is CORRECT?
Answer: A — Mixtures are separated by physical methods that exploit differences in the components' properties
The components of a mixture keep their own properties, so they can be separated by physical methods - handpicking, sieving, winnowing, filtration, evaporation, distillation, sublimation, magnetic separation and centrifugation - each based on a specific property difference.
Q60. hard
A mixture of chalk powder and water is BEST separated by:
Answer: A — Filtration, because chalk is insoluble in water
Chalk does not dissolve in water, so it stays suspended. Filtration traps the chalk as residue on the filter paper and lets clear water pass through as the filtrate.