CBSE Class 9 Science Practical Skills – Separation of Mixture

CBSE Class 9 Science Practical Skills – Separation of Mixture

EXPERIMENT

AIM
To separate the components of a mixture of sand, common salt and ammonium chloride (or camphor) by sublimation.

MATERIALS REQUIRED
Sand, common salt (NaCl), ammonium chloride (NH₄Cl) or camphor, filter paper, funnel, china dish, tripod stand, wire gauze, beaker, glass rod, burner, cotton plug.

THEORY
In a mixture, the components retain their properties and can therefore, be separated by physical methods based upon their properties.

1. Camphor and ammonium chloride on heating directly convert into vapour. This is called sublimation. In contrast to this, sand and common salt do not sublime. So from a mixture, containing ammonium chloride (or camphor), common salt and sand, ammonium chloride (or camphor) can be easily separated by the process of sublimation. On heating ammonium chloride (or camphor) converts into vapour leaving behind the non-volatile components. On cooling the vapour of ammonium chloride (or camphor) changes into solid.
   
2. Common salt is soluble in water but sand is not soluble in water.
3. When a mixture of common salt and sand in water is passed through filter paper, sand particle remain on the filter paper but other components of mixture i.e., common salt and water pass through filter paper.
4. From a solution of common salt in water, common salt can be obtained by evaporation.
5. For separation of different components of a mixture of ammonium chloride (or camphor) common salt and sand, methods of sublimation, Alteration and evaporation are applied.

PROCEDURE
A. Steps for separation of ammonium chloride:

1. Take the mixture of sand, ammonium chloride and common salt in a china dish and cover it with an inverted funnel.
   
2. Plug the end of stero funnel with cotton.
3. Place the china dish over a tripod stand and heat it gently.
4. Observe the changes. It is observed that ammonium chloride undergoes sublimation, i.e. vapours of NH4C1 are formed which get condensed on the cooler sides of the funnel.
5.  Scrap the condensed ammonium chloride from the inner walls of the funnel (see figure). The non-sublimed mixture contains sand and common salt which remain behind in the china dish.
   

B. Steps for separation of Sand:

1. To the non-sublimate mixture add water.
2. Stir the mixture with a clean glass rod.
3. Filter the solution as shown in figure. Common salt solution is obtained as the filtrate, whereas sand is obtained as the residue on the filter paper.
   

C. Steps for separation of common salt (sodium chloride):

1. Take the filterate in a china dish.
2. Evaporate the filtrate to dryness, where water evaporates leaving behind sodium chloride (common salt) in the china dish (see figure).
   

RESULT
Sand, common salt and ammonium chloride mixture is separated by sublimation, Alteration and ‘ evaporation.

PRECAUTIONS

1. Heat ammonium chloride carefully as it is volatile.
2. Fix a cotton plug at the end of the funnel so that vapours of ammonium chloride do not escape.
3. Moisten the filter paper before filtering.
4. Wash your hands with soap after completing the experiment.

INTERACTIVE SESSION

Examiner :
What is a pure substance?
Examinee:
A pure substance is one which is made up of only one kind of atoms or molecules.

Examiner :
What is a mixture?
Examinee:
A mixture contains more than one element/compound, which can be separated by physical methods.

Examiner :
Based upon composition, how many types of mixtures are there?
Examinee:
Mixtures are of two types-homogeneous and heterogeneous.

Examiner :
What is a homogeneous mixture? Give an example.
Examinee:
A mixture which has uniform composition is called a homogeneous mixture, e.g. sugar solution. .

Examiner :
What is a heterogeneous mixture? Give an example.
Examinee:
A heterogeneous mixture is one in which the composition is not uniform, e.g. chalk and sand mixed together.
Examiner :

Why can a mixture be separated by physical methods?
Examinee:
In a mixture, the components retain their properties and based upon these properties the components can be separated using physical methods.

Examiner :
Can water (H₂O) be separated into its components hydrogen (H₂) and oxygen (O₂) using physical methods?
Examinee:
No, since water is a compound, it cannot be broken down into hydrogen and oxygen by physical methods.

Examiner :
Define sublimation.
Examinee:
A process in which a solid directly gets converted into vapour phase below its melting point is called sublimation. Also the vapours on cooling directly get converted into the solid.

Examiner :
Give an example, other than NH₄Cl, which undergoes sublimation.
Examinee:
Iodine.

Examiner :
Give a method which can be used to separate two compounds with different solubilities in a solvent.
Examinee:
Recrystallization is used to separate two compounds with different solubilities in a solvent.

Examiner :
A mixture contains benzoic acid and common salt. How can this mixture be separated?
Examinee:
Sublimation can be used, as benzoic acid sublimes whereas common salt does not.

Examiner :
Can you separate a mixture of iodine and naphthalene by sublimation?
Examinee:
No, as both undergo sublimation.

Examiner :
Give the principle behind distillation.
Examinee:
Distillation is used to separate

1. volatile liquids from non-volatile impurities and
2. liquids having sufficient difference in their boiling points. Based upon the difference in their boiling points the components vapourize at different temperatures and the vapours are cooled and collected.

Examiner :
How would you separate cream from milk?
Examinee:
Cream can be separated from milk by centrifugation. Cream being lighter floats at the top.

Examiner :
What is the principle involved in fractional distillation?
Examinee:
If the difference in boiling points of liquids is not much, simple distillation cannot be used. In fractional distillation, vapours of liquid with lower boiling point condense first, the vapours rising up become richer in the more volatile component and can be separated from the less volatile component.

Examiner :
Can a mixture of sugar and common salt be separated by sublimation?
Examinee:
No, as neither sugar nor common salt undergoes sublimation.

Examiner :
What type of change is the formation of a mixture?
Examinee:
Formation of a mixture is a physical change and is reversible.

Examiner :
Out of a mixture of sand, common salt and ammonium chloride, which is volatile?
Examinee:
Ammonium chloride is volatile as it undergoes sublimation.

Examiner :
Iodine and ammonium chloride both undergo sublimation. How can you separate a mixture of these?
Examinee:
On adding water to a mixture of iodine and ammonium chloride, ammonium chloride being soluble in water dissolves in water and on filtering iodine will be obtained as residue. On evaporating the aqueous solution of ammonium chloride, water will evaporate, leaving behind ammonium chloride.

Examiner :
Give the principle involved in crystallization.
Examinee:
On heating a saturated solution, the concentration of the solution increases and on cooling the solubility decreases and pure crystals are taken out of the solution.

NCERT LAB MANUAL QUESTIONS

Question 1:
In your view, what could be the reason for direct conversion of some solids to vapours and vice-versa?
Answer:
It is because of the high vapour pressure of the liquid state of the solid. The liquid state is practically non-existent.

Question 2:
In the above experiment, you have observed conversion of solid to vapours. Is this a physical or a chemical change?
Answer:
Conversion of solid to vapours directly is a physical change.

Question 3:
Could you think of some applications of this change in daily life?
Answer:
Preparation of moth repellent balls, odonil, camphor, etc.

Question 4:
If in the first step the mixture had been dissolved in water what would have been the difficulty in separation?
Answer:
If in the first step the mixture had been dissolved in water, both ammonium chloride and sodium chloride being water soluble would have dissolved in water and their separation would not be possible.

Question 5:
Instead of common salt if one component in the given mixture is sulphur, how would you carry out separation process then?
Answer:
If instead of sodium chloride, one component of the mixture is sulphur, the following technique would be followed for separation

1. Sublimation – NH₄Cl undergoes sublimation, leaving behind sulphur and sand.
2. To the mixture of sulphur and sand, add CS₂ (carbon disulphide). Sulphur is soluble in CS₂ while sand is not.
3. Evaporate the mixture of sulphur in CS₂ to obtain solid sulphur.

Question 6:
Can the two components of a mixture that are soluble in water be separated by any technique? Justify your answer?
Answer:
Yes, two components of mixture, if solube in water, can be separated on the basis of difference of solubilities of the compound in water. The technique used will be crystallisation. Another technique that can be used is chromatography.

PRACTICAL BASED QUESTIONS
Multiple Choice Questions/VSA (1 Mark)

Question 1:
A mixture of common salt and water can be separated completely by the process of
(a) filtration
(b) distillation
(c) sublimation
(d) all of these.

Question 2:
Recovery of salt from salt solution in water can be done by
(a) evaporation
(b) distillation
(c) filtration
(d) dissolving in more water.

Question 3:
Which of the following does not undergo sublimation?
(a) Camphor
(b) Sugar
(c) Iodine
(d) Benzoic acid

Question 4:
Look at the diagram and identify the method used for separating a mixture.

(a) Chromatography
(b) Distillation
(c) Crystallization
(d) Filtration

Question 5:
Which one of the following is a pure compound?
(a) Air
(b) Crude oil
(c) Sugar
(d) Tea

Question 6:
Identify the substance which sublimates and is used to protect warm clothes.
(a) Ammonium chloride
(b) Iodine
(c) Neem leaves
(d) Naphthalene

Question 7:
When a mixture of common salt and ammonium chloride is heated, it is observed that
(a) solid common salt gets deposited on the cooler parts of the funnel while solid ammonium chloride remains in the china dish
(b) mixture of common salt and ammonium chloride turns into greenish crystals when allowed to cool
(c) ammonium chloride gets deposited on the cooler parts of the funnel and solid common salt remains in the china dish
(d) droplets containing both common salt and ammonium chloride appear on the upper part of the funnel while some molten mixture of common salt and ammonium chloride remains in the china dish

Question 8:
The components of compound can be separated by using
(a) physical method
(b) chemical method
(c) cannot be separated by using any method
(d) none of the above.

Question 9:
Sublimation is used to separate
(a) two liquids having different boiling points
(b) two liquids having same melting point
(c) volatile and non volatile solids
(d) volatile solids having same melting point.

Question 10:
A liquid is said to be volatile
(a) if forces of interaction between the molecules are less
(b) if forces of interaction between the molecules is more
(c) forces of interaction first increase and then decrease
(d) all the above i.e. (a), (b), (c).

Question 11:
Brine solution is
(a) milk in water
(b) aqueous solution of potassium hydroxide
(c) concentrated solution of sodium chloride
(d) molten butter.

Question 12:
What is the order of methods applied to separate the components of a mixture of salt, sand and ammonium chloride?
(a) Dissolving in water, evaporation and sublimation.
(b) Dissolving in water, filtration, evaporation and sublimation.
(c) Sublimation, dissolving in water, filtration and evaporation.
(d) Moving a magnet, dissolving in water and sublimation.

Question 13:
In the given figure, identify the place at which you can find only pure ammonium chloride (NH4C1) after heating.

(a) B
(b) C
(c) A
(d) A and B both

Question 14:
How would you separate a mixture of sulphur, iron filings and salt?
(a) Magnet, dissolve in water, filter, crystallize.
(b) Magnet, filter, decant, crystallize.
(c) Dissolve in water, filter, crystallize, decant.
(d) Decant, crystallize, filter, dissolve in water.

Question 15:
When sodium chloride and ammonium chloride are dissolved in water and filtered, the residue is
(а) sodium chloride
(b) ammonium chloride
(c) both
(d) none

Question 16:
The aqueous mixture of salt and sand can be separated by the following method.
(a) Filtration
(b) Sublimation
(c) Condensation
(d) Melting

Question 17:
A mixture can be characterized by
(a) no fixed composition of the components
(b) homogeneity
(c) no occurrence of a chemical reaction
(d) both (a) and (c).

Question 18:
An example of a heterogeneous mixture is
(a) cream
(b) butter
(c) jelly
(d) all the three.

ANSWER KEY
MULTIPLE CHOICE QUESTIONS/VSA

1. (b)
2. (a)
3. (b)
4. (d)
5. (c)
6. (d)
7. (c)
8. (b)
9. (c)
10. (a)
11. (c)
12. (c)
13. (d)
14. (a)
15. (d)
16. (a)
17. (d)
18. (d)

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