Friday, May 4, 2012

Class 6 - Science - CH5 - Separation of Substances

NCERT Solutions and Study notes.


Q1. Why do we need to separate different components of a mixture? Give two examples.
Answer: We need to separate the components of a mixture for the following reasons:
  1. Remove the unwanted impurities.
  2. Remove the substances that are harmful for our health.
  3. Obtain pure substances.
  4. Separate two different, but useful components.
  1. Grain is separated from stalks, while harvesting.
  2. We filter tea leaves after preparing it.
  3. Milk or curd is churned to separate the butter.

Q2: What is winnowing? Where is it used?
Answer:Winnowing is a method to separate heavier and lighter components of a mixture by wind or by blowing air. This method is commonly used by farmers to separate lighter husk particles from heavier seeds of grain.

Q3. How will you separate husk or dirt particles from a given sample of pulses before cooking.
Answer: Husk or bigger pieces of dirt particles can be removed from a sample of pulses by handpicking.

Q4. What is sieving? Where is it used? 
Answer:Sieving is the process to filter components of a mixture of different sizes. Sieving allows fine particles to pass through the holes of the sieve, while the bigger impurities remain on the sieve.

Sieving is used in flour mills to separate broken particles of grains from flour. It is also used at construction sites to separate lumps, smaller stones from the mixture of sand and cement.

Q5. How will you separate sand and water from their mixture?
  1. Allow mixture to stand undisturbed for sometime in a container.
  2. Sand settles at the bottom of the container. It is called sedimentation.
  3. Gently pour the water in another container (called decantation).
  4. We may also use filter paper to remove fine particles of sand (called filtration).
Q6. Is it possible to separate sugar mixed with wheat flour? If yes, how will you do it? 

Answer: Through sieving we can separate sugar mixed with wheat flour.

Q7. How would you obtain clear water from a sample of muddy water?Answer:
  1. Allow muddy water to stand undisturbed in a container.
  2. After sometime, mud settles at the bottom of the container. This process is called sedimentation.
  3. Upper layer is clear water.
  4. Pour the clear water gently in another container. This process is called decantation.
  5. To remove finer impurities we can filter this water again with the help of filter paper. This process is called filtration.

Q8. Fill up the blanks  

(a) The method of separating seeds of paddy from its stalks is called __threshing_.

(b) When milk, cooled after boiling, is poured onto a piece of cloth the cream (malai) is left behind on it. This process of separating cream from milk is an example of _churning__.

(c) Salt is obtained from seawater by the process of _evaporation__.

(d) Impurities settled at the bottom when muddy water was kept overnight in a bucket. The clear water was then poured off from the top. The process of separation used in this example is called _sedimentation and _decantation__.

Q9. True or false? 

(a) A mixture of milk and water can be separated by filtration. : False

(b) A mixture of powdered salt and sugar can be separated by the process of winnowing. False

(c) Separation of sugar from tea can be done with filtration. False

(d) Grain and husk can be separated with the process of decantation. False

Q10. Lemonade is prepared by mixing lemon juice and sugar in water. You wish to add ice to cool it. Should you add ice to the lemonade before or after dissolving sugar? In which case would it be possible to dissolve more sugar?

We should add sugar before adding ice. Sugar dissolves in warm water more quickly than in cold water. We can dissolve more sugar in warm water.

Study Notes

Q11: Explain the process of distillation.

Distillation is a process of removing dissolved impurities from water. First of all take impure water in a round bottom flask. Set up the apparatus as shown in figure. Heat the water. Water will start evaporating and converts into water vapours. When these vapours  pass through the tube surrounded by cold water, they condense and convert into droplets of water. These are then collected in a beaker. This water is free from all impurities.


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