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Crystals and crystallisation - class-VIII

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Crystallisation is used to purify :

  1. solids

  2. liquids

  3. gases

  4. all of the above


Correct Option: A
Explanation:

Crystallization is used to obtain pure solid from impure solid sample. It is used to purify solids.

The salt we get from sea water can have many impurities in it. These impurities can be removed by :

  1. sedimentation

  2. crystallisation

  3. centrifugation

  4. all of the above


Correct Option: B
Explanation:

Crystallisation is used to obtain pure solid from impure solid sample. It is used to purity solids, e.g., purification of salts that we get from sea.

Separation of crystals of alum from impure sample is carried out through :

  1. centrifugation

  2. distillation

  3. crystallisation

  4. none of the above


Correct Option: C
Explanation:

Pure solids are removed as crystal from the solution through crystallisation. Crystallisation is used to purify solids, e.g., seperation of crystals of alum from impure samples.

While obtaining pure copper sulphate from an impure sample what will finally be obtained in the china dish?

  1. Impurities of the sample are left behind in the china dish

  2. Copper sulphate

  3. Water droplets

  4. Chine dish remains empty


Correct Option: B
Explanation:

When crystallisation is performed to obtain pure copper sulphate from an impure sample, finally crystals of copper sulphate is obtained in the chine dish.

Pure $CuSO _4$ can be obtained from an impure sample by :

  1. crystallisation

  2. galvanisation

  3. centrifugation

  4. sedimentation


Correct Option: A
Explanation:

Crystallisation is a method used to purity solids. Pure $CuSO _4$ in the form of cystals can be obtained from an impure sample by crystallisation.

Why crystallisation is used as a method of obtaining pure $CuSO _4$ from impure sample?

  1. Because $CuSO _4$ is a liquid

  2. Because $CuSO _4$ is a solid

  3. Because $CuSO _4$ is a gas

  4. None of the above


Correct Option: B
Explanation:

Crystallisation is a method used to purify solids. $CuSO _4$ is solid, hence it can be obtained as in a pure cystal form from impure samples using cystallisation.

Crystallization is used to separate :

  1. solid from solid

  2. liquid from solid

  3. solid from liquid

  4. all of above


Correct Option: C
Explanation:

Crystallisation is a sophisticated form of evaporation technique in which crystals of the solute are encouraged to develop during the process of dissolving out from the solution as the solvent evaporates.

X is a solid. Y is a liquid. How can X be separated from a solution of Y?

  1. Sedimentation

  2. Filtration

  3. Crystallization

  4. All of above


Correct Option: D
Explanation:

Sedimentation is the process of separating an insoluble solid from a liquid in which it is suspended by allowing it to settle to the bottom of the container. Filtration is used for separating insoluble solids from a liquid. Crystallisation is a sophisticated form of evaporation technique in which crystals of the solute are encouraged to develop during the process of dissolving out from the solution as the solvent evaporates.

The copper sulphate solution is heated gently on a water bath for :

  1. proper mixing of the sample

  2. evaporation of water

  3. dissolving the copper sulphate in solution

  4. all of above


Correct Option: B
Explanation:

Pure copper sulphate is obtained from an impure sample by re-crystallization. The sample is mixed in water and filtered to remove impurities. It is then heated gently on a water bath for evaporation of water. Thus, a saturated solution is obtained.

While obtaining pure copper sulphate by crystallization, the solution is saturated is indicated by :

  1. change in color

  2. change in consistency

  3. formation of bubbles

  4. formation of crystals near rod


Correct Option: D
Explanation:

During purification of copper sulphate by crystallization, the impure sample is mixed with water and then heated to saturation. The saturation is indicated by dipping glass rod in solution from time to time. Small crystals are formed near the glass rods which indicates saturation.

To crystallize copper sulphate, it is dissolved in water and a small quantity of ................. is also added.

  1. concentrated nitric acid

  2. dilute hydrochloric acid

  3. dilute sulphuric acid

  4. dilute nitric acid


Correct Option: C
Explanation:

To crystallise copper sulphate, water is dissolved and a small quantity of dilute sulphuric acid is added to prevent the hydrolysis of copper sulphte. The impurities left behind in the solution are removed by filtration.

During crystallization of copper sulphate, heating is stopped when :

  1. solution becomes saturated

  2. solution reaches boiling point

  3. solution is reduced to half of its quantity

  4. the solution is not heated at all


Correct Option: A
Explanation:

Crystallization process begins with dissolving the impure copper sulphate sample in water and then heating it. It is heated to saturation rather than dryness and once it is saturated the heating is stopped and the solution is allowed to cool.

Copper sulphate crystals are of which color after purification?

  1. White

  2. Black

  3. Blue

  4. Yellow


Correct Option: C
Explanation:

To crystallise copper sulphate, it is dissolved in water and a small quantity of dilute sulphuric acid is added to prevent the hydrolysis of copper sulphte. The impurities left behind in the solution are removed by filtration. The filtrate is concentrated to the crystallisation point and then cooled. On cooling, transparent blue crystals of copper sulphate separate.

When are the crystals of copper sulphate obtained?

  1. when hot acidic copper sulphate solution is cooled gradually.

  2. when cold acidic copper sulphate solution is heated gradually.

  3. when hot acidic copper sulphate solution is cooled suddenly.

  4. when cold acidic copper sulphate solution is heated suddenly


Correct Option: A
Explanation:

When hot acidic $ copper : sulphate$ solution is cooled gradually, the crystals of  $ copper : sulphate$ are obtained.

Soda water can be separated by lowering the pressure.

  1. True

  2. False


Correct Option: A
Explanation:

Soda water can be separated by lowering the pressure. When pressure is lowered, the solubility of soda ( or any gas) in water decreases. Hence, soda will escape out.

Which method is used to separate diastereomers?

  1. Distillation

  2. Fractional Crystallisation

  3. Sublimation

  4. Evaporation


Correct Option: B
Explanation:

Diastereomers have different physical properties. You can separate them by ordinary separation techniques such as fractional crystallization. 


Then you treat the separated diastereomers with appropriate reagents to regenerate the original enantiomers.

Option B is correct.

The method by which common salt is obtained from concentrated seawater is called:

  1. Crystallisation

  2. Catalyst

  3. Hydrolysis

  4. Evaporation


Correct Option: A
Explanation:

The method by which common salt is obtained from concentrated seawater is called crystallisation.


Crystallisation is a technique used to separate a solid from a solution. The solid is more soluble at a higher temperature and less soluble at a lower temperature.

Hence, option A is correct.

The crystals that are obtained after crystallization are:

  1. more than 90% pure

  2. between 40% and 80% pure

  3. between 10% to 40% pure

  4. 10% pure


Correct Option: A
Explanation:

Crystal is the pureset form of a compound, the method of forming crystals is known as crystallization, crystals are more than 90% pure and its purity can be increased upto 99.9% by recrystallization technique.  

Why crystallisation is used for obtaining pure copper sulphate from an impure sample rather than simple evaporation?

  1. Some impurities may remain dissolved in the solution even after filtration. On evaporation, these may contaminate the solid

  2. Because crystallisation is comparatively easier than evaporation

  3. Because crystallisation is comparatively faster

  4. All of the above


Correct Option: A
Explanation:

Crystallisation technique is better than simple evaporation technique as some solids decompose and some impurities may remain dissolved in the solution even after filtration. On evaporation, these contaminate the solid.

Crystals of pure copper sulphate are separated from the solution by :

  1. condenstaion

  2. distillation

  3. solidification

  4. filtration


Correct Option: D
Explanation:

Pure copper sulphate can be obtained from an impure sample by re-crystallization. The impure sample is dissolved in water, heated and then cooled which then later forms crystals. These copper sulphate crystals are then separated by filtration and drying.

Which of the following method is used for water treatment?

  1. Centrifugation

  2. Sedimentation

  3. Evaporation

  4. Crystallization


Correct Option: B
Explanation:

Sedimentation is a physical water treatment process using gravity to remove suspended solids from water. Solid particles entrained by the turbulence of moving water may be removed naturally by sedimentation in the still water of lakes and oceans.

Crystallization is a process of :

  1. cooling a hot concentrated solution of a substance to obtain beads

  2. cooling a hot concentrated solution of a substance to obtain crystals

  3. heating a solution to obtain beads

  4. all of above


Correct Option: B
Explanation:

Crystallization is a process of cooling a hot, concentrated solution of a substance to obtain crystals. It is used to obtain pure solid from impure sample.

Common salt can be purified by a process of :

  1. crystallization

  2. homogenisation

  3. boiling

  4. distillation


Correct Option: A
Explanation:

Common salt obtained from sea water is an impure substance having many impurities. This common salt can thus be purified by crystallization.

The process of crystallization begins by:

  1. cooling in ice

  2. evaporation by heating

  3. passing to porous material

  4. none of the above


Correct Option: B
Explanation:

Crystallisation is a separation technique that is used to separate. a solid that has dissolved in a liquid and made a solution. The solution is warmed in an open container, allowing the solvent to evaporate, leaving a saturated solution. A solution that has as much solid dissolved in it.

After purification by crystallization, copper sulphate crystals can be obtained by :

  1. filtration

  2. passing through funnel

  3. evaporation to dryness

  4. all of above


Correct Option: A
Explanation:

Impure copper sulphate is dissolved in water and heated till saturation. It is then cooled and crystals of pure copper sulphate are formed. These crystals are then separated by filtration and then dried.

Why is crystallization better than evaporation to dryness?

  1. Solids get charred in evaporation

  2. Solids get decomposed in evaporation

  3. Impurities do not get removed in evaporation

  4. All of above


Correct Option: D
Explanation:

Purification by crystallization is a better technique as compared to evaporation by dryness. Some solids like sugar decompose or get charred in evaporation to dryness. Also impurities are not removed in this method. However, all these drawbacks are overcome in crystallization.

What are the characteristics of the solvent used during purification of copper sulphate by crystallization?

  1. Not dissolve the substance at room temperature

  2. Dissolve the impurities at room temperature

  3. Does not dissolve the impurities at high temperature

  4. All of the above


Correct Option: D
Explanation:

Choosing an appropriate solvent is the important process of crystallisation, as crystallisation works only when a proper solvent is used. It is important to choose a solvent that will not dissolve the substance at room temperature. But as the temperature of the solvent increases, the solubility of the solute also increases. At the same time, the impurities that are present must either be soluble in the solvent at room temperature or must be insoluble in the solvent at a high temperature. If the solvent is not hot when the dissolution is carried out, too much solvent will be used, leading to diminished yield.

The principle behind purification by crystallisation is that the :

  1. amount of solute that can be dissolved by a solvent increases with temperature

  2. amount of solute that can be dissolved by a solvent decreases with increase in temperature

  3. both a and b

  4. none of above


Correct Option: A
Explanation:

Crystallisation is one of the very important purification techniques, purifying substances by removing unwanted by-products. Crystalline compounds are generally purified via this crystallisation process. The principle behind the crystallisation is that the amount of solute that can be dissolved by a solvent increases with temperature.

During concentration (saturation), copper sulfate solution is constantly stirred to :

  1. ensure uniform evaporation

  2. prevent formation of a solid crust

  3. both a and b

  4. it is not stirred during saturation


Correct Option: C
Explanation:

To concentrate the filtrate of impure copper sulphate, the china dish containing filtrate is heated gently with constant stirring. This is done to ensure uniform evaporation and to prevent formation of a solid crust. 

Crystallization is better than evaporation to dryness. 

  1. True

  2. False


Correct Option: A
Explanation:

Crystallization is better than evaporation to dryness because some solids get decomposed or charred during evaporation.

Why is dilute sulphuric acid added during crystallization of copper sulphate?

  1. Prevent hydrolysis of copper sulphate

  2. Prevent crystallization of copper sulphate

  3. Enhance formation of large crystals

  4. All of the above


Correct Option: A
Explanation:

To crystallise copper sulphate, it is dissolved in water and a small quantity of dilute sulphuric acid is added to prevent the hydrolysis of copper sulphte. The impurities left behind in the solution are removed by filtration.

Which of the following can be separated by crystallization?

  1. Copper sulphate

  2. Benzoic acid

  3. Potash alum

  4. All of above


Correct Option: D
Explanation:

Crystallization is a method of purification by the formation of crystals. 


The above samples can be separated by crystallization by heating the solution and then cooling them to form crystals.

Hence the correct option is D.

Technique utilized to separate a mixture of dissolved solids by evaporation according to individual solubilities.

  1. Distillation

  2. Chromatography

  3. Fractional crystallization

  4. Filtration

  5. Titration


Correct Option: C
Explanation:

Technique utilized to separate a mixture of dissolved solids by evaporation according to individual solubilities is fractional crystallization. Two solutes have different solubilities in the same solvent at the same temperature. Fractional crystallization is step-wise crystallization. First, less soluble solute is crystallized out. Later, more soluble solute is crystallized out.

Usually crystals are dried with help of ________.

  1. dryer

  2. autoclave

  3. filter paper

  4. fan


Correct Option: C
Explanation:

Usually crystals are dried with help of filter paper.
The moisture present in the crystals is absorbed by filter paper.

Why does crystallisation seperate mixtures?

  1. Because the components do not chemically combine

  2. Because the components have different boiling point

  3. Because the components have different solubilities at high and low temperatures

  4. None of these


Correct Option: C
Explanation:

The crystallization can separate mixtures because the components have different solubilities at high and low temperatures. The solute to be crystallized is more soluble in hot solution than in cold solution.

Crystals can be grown by _________.

  1. cooling a saturated solution

  2. heating a saturated solution

  3. stirring a saturated solution

  4. planting in a well watered fertile soil


Correct Option: A
Explanation:

Cooling a saturated solution. (Heating is stopped at the point when a hot saturated solution is formed. If the resulting solution is allowed to cool to room temperature, the dissolved solid will precipitate out as pure crystals.)

How would you know if a solution is saturated?

  1. When a sample is taken out and cooled, it forms small crystals rapidly

  2. When the sample starts to evaporate

  3. When more solid is added, they continue to dissolve in the solution

  4. When solution changes colour


Correct Option: A
Explanation:

When a sample is taken out and cooled, it forms small crystals rapidly.(At a given temperature, a solution can only hold a certain amount of solute.A saturated solution is one that contains the maximum amount of solute at a given temperature.)

The most suitable method of separation of $1$ mixture of ortho and para-nitrophenols is:

  1. crystallisation

  2. steam distillation

  3. sublimation

  4. chromatography


Correct Option: B
Explanation:

The most suitable method of separation of 1:1 mixture of ortho and para-nitrophenols is steam distillation because they have a great difference in their boiling points. Hence, option B is correct.

What would you observe when a saturated solution of potassium chloride prepared at $60^0 C$ is allowed to cool to room temperature?

  1. Crystallization happens

  2. Evaporation happens

  3. Sublimation happens

  4. Condensation happens


Correct Option: A
Explanation:

Here crystallization happens and solid potassium chloride will separate out. (Crystallization is a chemical solid-liquid separation technique, in which mass transfer of a solute from the liquid solution to a pure solid crystalline phase occurs)

Arrange the following steps in crystallisation of copper sulphate.
A. Dissolve it in minimum amount of water.
B. Take some (approximately 5 g) impure sample of copper sulphate in a china dish.
C. Cover the solution with a filter paper and leave it undisturbed at room temperature to cool slowly for a day.
D. Evaporate water from the copper sulphate solution so as to get a saturated solution.
E. Filter the impurities out.

  1. A, C, B, D, E

  2. B, D, A, C, E

  3. B, A, D, E, C

  4. B, A, E, D, C


Correct Option: C
Explanation:

The correct order of steps in crystallisation of copper sulphate is B, A, D, E, C.
B. Take some (approximately 5 g) impure sample of copper sulphate in a china dish.
A. Dissolve it in minimum amount of water.
D. Evaporate water from the copper sulphate solution so as to get a saturated solution.
E. Filter the impurities out.
C. Cover the solution with a filter paper and leave it undisturbed at room temperature to cool slowly for a day.

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