Get authentic, comprehensive, and scoring-oriented solutions for DAV Class 6 Science Chapter 4: Separation of Substances from the prescribed DAV Board textbook The Living World. Here you will find step-by-step verified answers to all questions in Something To Know (Sections A, B, C, D, E), structured separation techniques comparison tables, Value-Based Questions, and authentic DAV Board Examination Questions (PYQs).
🌾 Chapter 4 Quick Revision & Key Concepts Summary:
- Pure Substance vs Mixture: A pure substance contains only one kind of particle with fixed composition (e.g., distilled water, pure gold, common salt). A mixture contains two or more substances in any proportion without chemical bonding (e.g., air, lemon water, sea water).
- Types of Mixtures: Homogeneous (uniform composition throughout, e.g., sugar solution, air) and Heterogeneous (non-uniform composition with visible boundaries, e.g., sand in water, oil and water).
- Solid-Solid Separation: Hand-picking (visible differences in colour/size/shape), Threshing (separating grains from stalks by beating), Winnowing (separating heavier grains from lighter chaff by wind), and Sieving (separating particles of different sizes using a mesh screen).
- Insoluble Solid-Liquid Separation: Sedimentation (heavier particles settle down at bottom), Decantation (carefully pouring off top clear liquid), Loading (speeding up settling of fine clay particles using alum), and Filtration (passing mixture through a porous filter paper or sand-gravel beds).
- Soluble Solid-Liquid Separation: Evaporation (liquid vaporizes leaving behind solid, used to obtain salt from seawater) and Crystallisation (obtaining pure crystals from hot saturated solution).
- Sublimation: Direct transition from solid to vapour on heating without passing through liquid state (e.g., camphor, ammonium chloride, iodine, naphthalene).
- Centrifugation: Rapid spinning method used to separate suspended solid particles from liquids based on density difference (used in dairies to separate cream from milk, and in pathology labs).
Systematic Matrix of Separation Methods
| Separation Technique | Scientific Principle Involved | Standard Practical Example |
|---|---|---|
| 1. Winnowing | Difference in weight/density blown by wind | Separating wheat grains from light chaff |
| 2. Loading (with Alum) | Fine suspended particles become heavier and settle faster | Clearing muddy river/pond water |
| 3. Sublimation | One component sublimable, other non-sublimable | Separating camphor/ammonium chloride from salt |
| 4. Centrifugation | High-speed rotation forces heavier components to bottom | Separating butter/cream from curd or milk |
| 5. Evaporation | Liquid solvent evaporates, non-volatile solid remains | Obtaining common salt from sea water |
A. Fill in the blanks.
1. Housewives sometimes use the method of handpicking to separate stones from pulses.
Explanation: Small stone pebbles differ visibly in colour, shape, and size from pulses and are present in small quantities.
2. Common salt is prepared on a large scale through the process of evaporation of sea water.
Explanation: Seawater is collected in shallow pits (salt pans) where solar heat evaporates water, leaving behind crude common salt.
3. The method of winnowing is used to separate lighter components from heavier ones by wind or by blowing air.
Explanation: Farmers allow the mixture to drop from a height; wind blows away the lighter husk while heavier grain falls vertically.
4. In a solution of sugar and water, water is the solvent.
Explanation: The dissolving medium present in larger quantity is called the solvent, while the substance dissolving (sugar) is the solute.
5. The insoluble solid substance that settles down at the bottom of a liquid is called sediment.
Explanation: The settled layer of solid is sediment, and the process of settling under gravity is sedimentation.
B. Match the following:
| Column A (Mixture / Process) | Column B (Separation Technique) | Correct Matching Reason |
|---|---|---|
| 1. Separating butter from cream | Centrifugation | Rapid churning separates light cream/butter particles from liquid |
| 2. Separating camphor from salt | Sublimation | Camphor directly vaporizes into fumes on heating, leaving salt behind |
| 3. Separating tea leaves from tea | Filtration | Porous wire strainer retains insoluble tea leaves while liquid tea passes |
| 4. Separating grain seeds from stalks | Threshing | Beating stalks detaches mature grain seeds efficiently |
C. Tick the correct option.
1. Which of the following is an example of a heterogeneous mixture?
(a) air
(b) sugar solution
(c) brass
(d) chalk powder in water
Answer: (d) chalk powder in water
Reason: Chalk powder does not dissolve in water; its suspended particles remain visibly distinct, forming a heterogeneous mixture.
2. Which of the following is not an example of a pure substance?
(a) distilled water
(b) iron
(c) milk
(d) common salt
Answer: (c) milk
Reason: Milk is a complex heterogeneous mixture containing water, fats, proteins, carbohydrates, and minerals.
3. The process of loading helps to—
(a) separate soluble solids from liquids
(b) speed up the settling down of fine suspended particles
(c) separate grains from stalks
(d) separate immiscible liquids
Answer: (b) speed up the settling down of fine suspended particles
Reason: Alum dissolves and adheres to tiny suspended clay particles, making them heavier so that they settle rapidly.
4. Transferring the clear liquid, into another container, leaving behind the residue, is called—
(a) sedimentation
(b) filtration
(c) decantation
(d) winnowing
Answer: (c) decantation
Reason: Decantation is the gentle pouring of the upper liquid without disturbing the settled sediment.
5. A substance, that sublimes easily, is—
(a) sugar
(b) common salt
(c) camphor
(d) chalk powder
Answer: (c) camphor
Reason: Camphor converts directly into gas when heated at normal atmospheric pressure without melting into liquid.
D. Answer the following questions in brief.
1. Name any three methods used for separating a solid-solid mixture.
Answer:
1. Hand-picking: Used when components differ in color, shape, or size and are in small quantities.
2. Threshing: Used to separate grains from dry stalks by beating.
3. Winnowing: Used to separate heavier grains from lighter husk/chaff using wind.
2. What is winnowing? Give an example of a mixture whose components can be separated by this method.
Answer: Winnowing is a separation method based on the difference in weight of the components of a solid mixture, carried out with the help of wind or blowing air.
Example: Separating wheat grains from lighter chaff/husk after threshing.
3. Some iron pins get scattered on the carpet when you are expecting guests. How can one collect all of them quickly and easily?
Answer: The iron pins can be collected quickly using a strong magnet (magnetic separation). Moving a bar or horseshoe magnet across the surface of the carpet will attract all magnetic iron pins to its poles immediately, preventing accidental injury to guests.
4. Name any three substances which sublime on heating.
Answer:
1. Camphor
2. Ammonium chloride
3. Naphthalene (or Iodine)
5. What is crystallisation? Name any three substances which can be purified using this technique.
Answer:
• Definition: Crystallisation is a process used to obtain a chemical substance in its pure solid crystalline form from a hot concentrated/saturated solution upon cooling.
• Three Substances Purified: Copper sulphate (blue vitriol), common salt, and alum (potash alum).
E. Answer the following questions.
1. Differentiate between a pure substance and a mixture. Give two examples of each.
Answer:
| Parameter | Pure Substance | Mixture |
|---|---|---|
| Composition | Definite and fixed chemical composition throughout | Variable composition; constituents mixed in any ratio |
| Properties | Has fixed melting and boiling points | Retains the individual properties of its constituents |
| Separation | Cannot be separated into simpler substances by physical methods | Constituents can be separated easily by simple physical methods |
| Examples | Distilled water, pure iron, oxygen | Air, sea water, brass, milk |
2. State the difference between heterogeneous and homogeneous mixtures. Give two examples of each type.
Answer:
| Feature | Homogeneous Mixture | Heterogeneous Mixture |
|---|---|---|
| Uniformity | Constituents are uniformly mixed throughout | Constituents are non-uniformly distributed |
| Visibility of Boundaries | No visible boundary of separation between components | Distinct visible boundaries of separation exist between components |
| Examples | Sugar dissolved in water, clean air, vinegar | Sand and water mixture, oil in water, muddy water |
3. Can a mixture of iodine and camphor be separated by the sublimation method? Give reason for your answer.
Answer: No, a mixture of iodine and camphor cannot be separated by the sublimation method.
Reason: Sublimation requires that one component must sublime on heating while the other must be non-sublimable. Since both iodine and camphor sublime readily when heated, both will vaporize together simultaneously, rendering physical separation impossible.
4. Why does the size of naphthalene balls, put in open, keep on reducing with time?
Answer: Naphthalene is a sublimable substance that possesses high vapour pressure at room temperature. When exposed to open air, naphthalene particles absorb ambient thermal energy and slowly change directly from solid to gaseous state through spontaneous sublimation without melting. As its solid mass escapes continuously into the surrounding air as vapour, the size of the naphthalene balls gradually shrinks until they disappear.
5. After a dust storm, how does rain help in making air clear?
Answer: Dust storms suspend vast quantities of minute, lightweight soil and dust particles in the atmosphere that do not settle easily due to air currents. When raindrops fall through this dusty air, water droplets collide with and wet the fine suspended dust particles. This process functions identically to natural loading: the loaded dust particles become significantly heavier, agglomerate together, and are pulled rapidly down to the ground by gravity, leaving the atmosphere clear and clean.
6. Sawdust, mixed in water, cannot be separated by the sedimentation method. Why?
Answer: Sedimentation operates on the principle that the solid particles must have a higher density than the liquid so that gravity can pull them down to settle at the bottom. Sawdust is lighter (less dense) than water. Instead of sinking to form a sediment, sawdust particles float on the water surface and remain suspended, making sedimentation ineffective. (Filtration must be used instead).
7. The filtration method cannot be used to separate sugar and salt mixed in water. Why?
Answer: Filtration can only separate an insoluble solid suspended in a liquid because the insoluble particles are larger than the microscopic pores of the filter paper and get trapped as residue. Both sugar and common salt are completely soluble in water. Their dissolved molecules break down to molecular size, forming a true homogeneous solution that passes completely through the pores of any filter paper along with the water filtrate.
Value-Based Questions (VBQ) & HOTS (Higher Order Thinking Skills)
💡 Value-Based Question (VBQ):
Scenario: During monsoon, the municipal tap water supplied to Raman’s locality became visibly turbid and muddy. While his neighbours complained and waited for days, Raman took a piece of alum, tied it to a clean string, swirled it gently in a bucket of turbid water twice, and allowed it to rest undisturbed for an hour. Clear water was obtained for boiling.
Values & Scientific Reasoning:
1. Problem-Solving Attitude: Raman demonstrated practical initiative and scientific thinking instead of remaining passive during a domestic crisis.
2. Scientific Principle: Alum performs loading by neutralising electrical charges on colloidal clay particles, causing them to clump together into larger, heavier flocs that sediment swiftly, providing clear water for purification.
🧠 Higher Order Thinking Skill (HOTS):
Question: You are provided with a mixture containing sand, common salt, and ammonium chloride. Outline a step-by-step procedure to separate each component in pure form.
Step-by-Step Procedure:
1. Sublimation (Separates Ammonium Chloride): Heat the dry mixture in a china dish covered with an inverted funnel plugged with cotton. Ammonium chloride sublimes and deposits on the cooler inner walls of the funnel as pure solid, leaving sand and salt behind.
2. Dissolution & Filtration (Separates Sand): Add water to the residue dish and stir. Salt dissolves completely while sand remains insoluble. Filter the mixture; sand is collected on filter paper as residue.
3. Evaporation / Crystallisation (Separates Common Salt): Heat the salt-water filtrate in an evaporating dish until water evaporates completely, leaving pure common salt crystals behind.
🎯 DAV Examination Corner: Authentic Board Exam Questions (PYQs 2022–23 & 2023–24)
Below are authentic questions directly retrieved from the DAV Public Schools Common Examination (Half-Yearly & Annual Board Papers) covering Chapter 4:
Q1. [DAV Half-Yearly Exam 2023–24 • 1 Mark VSA]
A rainfall, after a storm, removes the dust particles present in the air. State the process of separation applied here?
Model Answer: Loading. The falling raindrops attach to the light suspended dust particles, making them heavier so that they settle rapidly to the ground.
Q2. [DAV Half-Yearly Exam 2023–24 • 1 Mark VSA]
Water Works departments use the process of filtration to purify water. They use layers of __________ and __________ to filter very large quantities of water.
Model Answer: Layers of fine sand and gravel.
Q3. [DAV Half-Yearly Exam 2023–24 • 3 Marks SA-II]
Define centrifugation. Where is it widely used and state its underlying mechanism?
Model Answer:
• Definition: Centrifugation is a mechanical process of separating insoluble suspended solid particles from a liquid by spinning the mixture at high speed in a centrifuge machine.
• Mechanism: Rapid rotation subjects particles to outward centrifugal force. The denser (heavier) solid particles are forced to the bottom of the container, while the lighter liquid stays at the top.
• Applications: (1) Dairies and households to separate butter/cream from curd and milk; (2) Diagnostic pathology laboratories to separate blood cells from blood plasma.
Q4. [DAV Half-Yearly Exam 2023–24 • 4-Bit Case-Based Question]
Passage: Most materials in nature are mixtures whose components can be separated by simple physical methods based on differences in their physical properties.
(I) Choose the method of separating soluble solids from their liquid solution: (d) Crystallisation / Evaporation
(II) Method used when mixture quantity is small and components differ in colour/shape/size: (b) Hand-picking
(III) Process that helps in obtaining common salt from sea water: (c) Evaporation
(IV) Methods of separation used by farmers: (d) Both threshing and winnowing
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