Class 6 Science Notes Chapter 5 (Chapter 5) – Examplar Problem Book

Detailed Notes with MCQs of Chapter 5, "Separation of Substances," from your Science Exemplar book. This chapter is fundamental, not just for your class exams but also forms the basis for concepts you'll encounter in higher studies and various competitive government exams. Pay close attention to the principles behind each method.

Chapter 5: Separation of Substances - Detailed Notes for Exam Preparation

1. Why Separate Substances?

We often encounter mixtures in daily life. Separation is crucial for:

• Removing Impurities or Harmful Components: E.g., removing small stones from rice or pulses before cooking.
• Obtaining Useful Components: E.g., separating butter (useful component) from milk/curd.
• Obtaining Pure Substances: For scientific experiments, medicines, or industrial processes, highly pure substances are often required.

2. Methods of Separation

The choice of separation method depends entirely on the properties of the substances in the mixture (like size, weight, solubility, magnetic properties, state of matter etc.).

• Handpicking:

  • Principle: Difference in colour, size, or shape which is easily visible.
  • Used For: Separating slightly larger sized impurities like pieces of dirt, stones, and husk from wheat, rice, or pulses.
  • Limitation: Time-consuming and practical only when the quantity of impurity is small.

• Threshing:

  • Principle: Separating grain seeds from the dried stalks (chaff) after harvesting.
  • Methods:
    • Manually: Beating stalks against a hard surface.
    • Using Animals: Bullocks trampling over the harvested crop.
    • Using Machines: 'Threshers' are commonly used for large quantities.

• Winnowing:

  • Principle: Difference in weight. Lighter particles are carried away by wind or blowing air, while heavier particles fall down vertically.
  • Used For: Separating lighter husk (chaff) from heavier grains like wheat or rice. Farmers often stand on a raised platform and let the mixture fall, allowing the wind to blow away the husk.

• Sieving:

  • Principle: Difference in particle size. A sieve (mesh) with specific pore sizes is used.
  • Used For:
    • Separating fine flour particles from coarser bran.
    • Separating pebbles and stones from sand at construction sites.
    • Separating different sized components in a mixture.
  • Note: The size of the sieve holes depends on the size of the particles to be separated.

• Sedimentation, Decantation, and Loading: (Mainly for separating insoluble solids from liquids)

  • Sedimentation:
    • Principle: Heavier, insoluble components settle down at the bottom of the liquid over time when left undisturbed. The settled particles form 'sediment'.
    • Used For: Separating sand or mud from water.
  • Decantation:
    • Principle: Carefully pouring out the clear liquid (supernatant) from the top after sedimentation, without disturbing the sediment.
    • Used For: Separating the clear water after mud has settled down. Also used to separate two immiscible liquids (like oil and water) that form separate layers.
  • Loading:
    • Principle: Speeding up sedimentation of very fine suspended particles. A substance (like alum - 'phitkari') is added, which 'loads' onto the fine particles, making them heavier and causing them to settle down faster.
    • Used For: Purifying muddy water containing very fine clay particles that settle very slowly on their own.

• Filtration:

  • Principle: Separating very fine, insoluble solid particles from a liquid using a filter medium (e.g., filter paper, fine cloth, strainer). The filter allows the liquid to pass through but retains the solid particles.
  • Used For:
    • Separating tea leaves from tea using a strainer.
    • Separating pulp from fruit juice.
    • Laboratory separation of precipitates (insoluble solids) from solutions using filter paper.
  • Note: Filtration is more effective than sedimentation/decantation for separating very fine insoluble particles.

• Evaporation:

  • Principle: Converting a liquid into its vapour (gas) form, usually by heating. This method is used to separate a soluble solid dissolved in a liquid.
  • Used For: Obtaining salt dissolved in water (like recovering common salt from seawater). The water evaporates, leaving the solid salt behind.
  • Note: The liquid component is lost (turns into vapour) in this process unless collected separately.

• Condensation:

  • Principle: The process of converting a vapour (gas) back into its liquid form, usually upon cooling.
  • Used For: Often used in conjunction with evaporation to recover the liquid component (e.g., in distillation, although distillation itself is a more advanced concept). Examples include formation of dew, water droplets on the outer surface of a glass containing a cold drink.

• Using More Than One Method:

  • Often, a single method is not sufficient to separate all components of a mixture.
  • Example: Separating a mixture of sand and salt.
    1. Add water to the mixture (Salt dissolves, sand doesn't).
    2. Perform Sedimentation & Decantation OR Filtration to separate the insoluble sand.
    3. Perform Evaporation on the remaining salt solution to recover the salt (water evaporates).
    4. (Optional) If water also needs to be recovered, the evaporated water vapour can be collected and condensed.

• Solubility and Saturated Solutions:

  • Solute: The substance that dissolves (e.g., salt).
  • Solvent: The liquid in which the solute dissolves (e.g., water).
  • Solution: The homogeneous mixture formed when a solute dissolves in a solvent.
  • Solubility: The ability of a solute to dissolve in a solvent. Different substances have different solubilities.
  • Saturated Solution: A solution in which no more solute can be dissolved at a particular temperature. If more solute is added, it will remain undissolved at the bottom.
  • Effect of Temperature: Usually, the solubility of solids in liquids increases with an increase in temperature (i.e., you can dissolve more sugar in hot water than in cold water). Cooling a hot saturated solution often causes the excess dissolved solute to separate out, sometimes forming crystals.

Multiple Choice Questions (MCQs)

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

1. Which method relies on the difference in weight between components of a mixture?
   (a) Sieving
   (b) Handpicking
   (c) Winnowing
   (d) Filtration

2. To separate pebbles from sand at a construction site, the most common method used is:
   (a) Winnowing
   (b) Sieving
   (c) Handpicking
   (d) Sedimentation

3. The process of obtaining salt from seawater primarily involves:
   (a) Filtration
   (b) Evaporation
   (c) Decantation
   (d) Threshing

4. While preparing tea, a strainer is used to separate tea leaves from the liquid. This process is an example of:
   (a) Sedimentation
   (b) Decantation
   (c) Evaporation
   (d) Filtration

5. The settling down of heavier insoluble particles when mixed with a liquid is called:
   (a) Loading
   (b) Sedimentation
   (c) Decantation
   (d) Condensation

6. Which of the following methods would you use to separate sand from a mixture of sand and water?
   (a) Evaporation
   (b) Winnowing
   (c) Sedimentation and Decantation (or Filtration)
   (d) Threshing

7. A solution is said to be ___________ if it cannot dissolve more of the solute at a given temperature.
   (a) Dilute
   (b) Concentrated
   (c) Unsaturated
   (d) Saturated

8. Adding alum to muddy water helps the mud particles to:
   (a) Dissolve completely
   (b) Evaporate faster
   (c) Settle down faster
   (d) Float on the surface

9. Separating grains from stalks is achieved by the process called:
   (a) Winnowing
   (b) Sieving
   (c) Threshing
   (d) Handpicking

10. To separate a mixture of salt and chalk powder, which of the following sequence of steps is most effective?
    (a) Winnowing -> Sieving
    (b) Add water -> Filter -> Evaporate
    (c) Handpicking -> Evaporate
    (d) Add water -> Decant -> Sieve

Answer Key:

1. (c)
2. (b)
3. (b)
4. (d)
5. (b)
6. (c)
7. (d)
8. (c)
9. (c)
10. (b) [Chalk powder is insoluble, salt is soluble in water]

Study these notes thoroughly. Remember the principle behind each method and common examples. Good luck with your preparation!