Class 6 Science Notes Chapter 5 (Separation of Substances) – Science Book

Detailed Notes with MCQs of Chapter 5: Separation of Substances. This is an important chapter, not just for your class exams but also because the basic principles often appear in various government competitive exams. We need to understand why we separate substances and how we do it using different methods.

Why Separate Substances?

We often encounter mixtures in our daily lives. Separation is necessary to:

1. Remove undesirable or harmful components: Like removing stones from rice or pulses.
2. Obtain useful components: Like separating butter from milk (curd) or petrol/diesel from crude oil.
3. Obtain pure substances: For scientific experiments, medicines, or specific industrial uses.

What is a Mixture?
A substance containing two or more components physically mixed together, where each component retains its original properties. Mixtures can be solid-solid, solid-liquid, liquid-liquid, etc.

Methods of Separation

Based on the properties of the components in a mixture (like size, weight, solubility, state), we use different methods:

1. Handpicking:

   • Principle: Difference in colour, size, or shape, visible to the naked eye.
   • Used for: Separating slightly larger-sized impurities like stones, husk, or dirt from grains (wheat, rice, pulses) or separating different types of vegetables/fruits.
   • Type of Mixture: Solid-Solid (Heterogeneous).
   • Limitation: Time-consuming and practical only when the quantity of impurity is small.

2. Threshing:

   • Principle: Using mechanical force to separate grains from their stalks (harvested crops).
   • Process: Stalks are beaten or struck against a hard surface to loosen the grains. Nowadays, machines (threshers) are commonly used, especially for large quantities.
   • Used for: Separating grains like wheat, paddy (rice), mustard seeds from the dried stalks.
   • Type of Mixture: Solid (grain) attached to Solid (stalk).

3. Winnowing:

   • Principle: Difference in weight. Lighter components are separated from heavier ones using wind or blowing air.
   • Process: The mixture is dropped from a height. The wind carries away the lighter components (like husk or chaff), while the heavier grains fall vertically down.
   • Used for: Separating husk (lighter) from heavier grains (wheat, rice).
   • Type of Mixture: Solid-Solid (Heterogeneous), where components have significant weight differences.

4. Sieving:

   • Principle: Difference in particle size.
   • Process: The mixture is passed through a sieve (a mesh with specific pore sizes). Components smaller than the pore size pass through, while larger components remain on the sieve.
   • Used for: Separating fine flour from coarser bran or impurities; separating pebbles and stones from sand at construction sites.
   • Type of Mixture: Solid-Solid (Heterogeneous), where components have different particle sizes.

5. Sedimentation & Decantation: (Often used together)

   • Sedimentation Principle: Heavier, insoluble solid particles in a liquid settle down at the bottom due to gravity.
   • Decantation Principle: Carefully pouring out the clear liquid (supernatant) from the top after sedimentation, without disturbing the settled solid (sediment).
   • Used for: Separating insoluble solids from liquids (e.g., mud from water, sand from water). Also used to separate two immiscible liquids (liquids that don't mix, e.g., oil and water).
   • Type of Mixture: Solid-Liquid (Insoluble solid) or Liquid-Liquid (Immiscible).

6. Filtration:

   • Principle: Separating very fine, insoluble solid particles from a liquid by passing the mixture through a filter medium (like filter paper, fine cloth, or a strainer). The liquid passes through the pores, but the solid particles are retained.
   • Used for: Separating tea leaves from prepared tea; separating pulp from fruit juice; cleaning muddy water (finer separation than decantation). Used in water purification systems.
   • Type of Mixture: Solid-Liquid (Fine insoluble solid).

7. Evaporation:

   • Principle: Converting a liquid into its vapour (gas) by heating, leaving behind the dissolved solid.
   • Process: The solution is heated; the liquid component evaporates, and the solid component is left behind.
   • Used for: Obtaining a dissolved solid from a liquid (e.g., obtaining salt from seawater or salt solution).
   • Type of Mixture: Solid-Liquid (Soluble solid, forming a homogeneous solution).

8. Condensation:

   • Principle: The process of converting vapour (gas) back into its liquid form upon cooling.
   • Use Case: Often used in conjunction with evaporation if we need to recover the liquid component as well (e.g., obtaining pure water from salt solution - this combined process is essentially distillation, though the term might not be heavily used in Class 6). Example: Water vapour touching a cold lid condenses back into water droplets.

Using More Than One Method:
Often, a single method is insufficient to separate all components from a complex mixture. For example, to separate a mixture of sand, salt, and water:
* First, Sedimentation & Decantation (or Filtration) can be used to separate the insoluble sand.
* Then, Evaporation can be used to separate the dissolved salt from the water.
* If pure water is also needed, Condensation can be used to collect the water vapour produced during evaporation.

Solubility and Saturated Solutions:

• Solute: The substance that dissolves (e.g., salt).
• Solvent: The substance in which the solute dissolves (e.g., water).
• Solution: A homogeneous mixture formed when a solute dissolves in a solvent.
• Solubility: The ability of a solute to dissolve in a solvent.
• Saturated Solution: A solution in which no more solute can be dissolved at a specific temperature. If you keep adding solute to a solvent, a point comes when it stops dissolving and starts settling at the bottom – the solution is now saturated.
• Effect of Temperature: Generally, more solute can be dissolved in a solvent by increasing the temperature. Cooling a saturated solution often causes the excess dissolved solute to separate out as crystals.

Key Takeaways for Exams:

• Know the principle behind each separation method.
• Understand what kind of mixture each method is suitable for (e.g., solid-solid, solid-liquid, soluble/insoluble, particle size difference, weight difference).
• Be familiar with common examples for each method.
• Understand the concept of a saturated solution and the effect of temperature on solubility.
• Recognize that multiple methods might be needed for complex mixtures.

Multiple Choice Questions (MCQs)

Here are 10 MCQs based on this chapter for your practice:

1. Which method is most suitable for separating stones from rice?
   (a) Winnowing
   (b) Sieving
   (c) Handpicking
   (d) Threshing

2. Separating husk from wheat grains is commonly done by which method?
   (a) Sieving
   (b) Winnowing
   (c) Sedimentation
   (d) Filtration

3. The process of settling down of heavier, insoluble particles in a liquid is called:
   (a) Decantation
   (b) Filtration
   (c) Sedimentation
   (d) Evaporation

4. To obtain salt from seawater, the most appropriate method is:
   (a) Filtration
   (b) Decantation
   (c) Evaporation
   (d) Sieving

5. Which of the following methods uses the difference in particle size for separation?
   (a) Winnowing
   (b) Sieving
   (c) Evaporation
   (d) Threshing

6. A solution in which no more solute can be dissolved at a given temperature is called:
   (a) Unsaturated solution
   (b) Concentrated solution
   (c) Saturated solution
   (d) Dilute solution

7. Filtration is used to separate:
   (a) A soluble solid from a liquid
   (b) An insoluble solid from a liquid
   (c) Two immiscible liquids
   (d) Two miscible liquids

8. The process of converting vapour back into liquid upon cooling is known as:
   (a) Evaporation
   (b) Condensation
   (c) Sedimentation
   (d) Filtration

9. Threshing is a method used to separate:
   (a) Grains from stalks
   (b) Husk from grains
   (c) Pebbles from sand
   (d) Tea leaves from tea

10. To separate a mixture of sand and water, which sequence of methods could be used effectively?
    (a) Evaporation followed by Condensation
    (b) Sieving followed by Handpicking
    (c) Sedimentation and Decantation (or Filtration)
    (d) Winnowing followed by Threshing

Answer Key for MCQs:

1. (c) Handpicking
2. (b) Winnowing
3. (c) Sedimentation
4. (c) Evaporation
5. (b) Sieving
6. (c) Saturated solution
7. (b) An insoluble solid from a liquid
8. (b) Condensation
9. (a) Grains from stalks
10. (c) Sedimentation and Decantation (or Filtration)

Study these notes thoroughly. Understanding the underlying principle of each method is crucial. Good luck with your preparation!