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Page No 49:
Question 1:
If a substance contains two or more kinds of molecules, what is it called?
Answer:
When a substance contains two or more kinds of molecules, it is called a mixture.
Page No 49:
Question 2:
Are most substances around us pure substances or mixtures?
Answer:
Most of the substances around us are mixtures.
As air contains a mixture of many gases like carbon dioxide, oxygen, nitrogen, sulphur dioxide, etc.
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Question 3:
Samples were taken from different parts of a mixture and tested. They were found to have exactly the same properties. What kind of mixture is it?
Answer:
The mixture will be homogeneous because in homogeneous mixture constituents are uniformly spread over the mixture.
Page No 49:
Question 4:
If a compound has fixed melting and boiling points, what can you say about the compound?
Answer:
If a compound has fixed melting and boiling point, it means the compound is pure.
Page No 49:
Question 5:
Why do we need to separate mixtures into its constituents?
Answer:
We need to separate mixtures into its constituents because of the following reasons:
1. To remove undesirable constituents
2. To obtain useful constituents
3. To obtain pure substances
​
Page No 54:
Question 1:
Which methods will you use to separate the following mixtures?
a. water and sugar
b. sand and water
c. sand and salt
Answer:
(a) By evaporation of water
(b) By filtration method
(c) By filtration and evaporation
Steps:
1. First, dissolve salt and sand in a beaker.
2. Salt will dissolve in water and sand will settle down.
3. After that filter, the solution and sand will be removed.
4. Then boil the water, boil it till water converts to vapours.
5. In the end, only pure salt crystals will be left in the solution.
Page No 54:
Question 2:
Name the method by which sedimentation can be made faster.
Answer:
Loading is the method of speeding up sedimentation.
Page No 54:
Question 3:
What kind of liquids is a separating funnel used to separate−miscible or immiscible?
Answer:
A separating funnel is used to separate a mixture of two immiscible liquids.
Page No 56:
Question 1:
When sugar is dissolved in water, why can't the individual particles of sugar be seen?
Answer:
When sugar is dissolved in water, it breaks up into simpler molecules. These simpler molecules comes in between the molecules of the liquid. These simpler molecules are too small to be seen, that's why individual particles of sugar can't be seen.
Page No 56:
Question 2:
Name two substances that are soluble and two that are not soluble in water.
Answer:
Substances that are soluble in water are sugar and salt.
Substances that are not soluble in water are sand and chalk powder.
Page No 56:
Question 3:
Can you dissolve more sugar in a saturated solution of sugar in water at a particular temperature?
Answer:
No, more sugar can not be dissolved in the saturated solution of sugar in water at a particular temperature.
A saturated solution is a solution that has dissolved as much solute as it is capable of dissolving. In a saturated solution, no more solute can be dissolved at a given temperature.
Page No 56:
Question 4:
A saturated solution of sugar in water is heated. Can you dissolve more sugar in it?
Answer:
A saturated solution is a solution that has dissolved as much solute as it is capable of dissolving at a particular temperature.
But if we raise the temperature, solubility of the solution rises and it can dissolve more sugar.
Page No 57:
Question 1:
Most of the substances we see around us are
(a) pure elements.
(b) pure compounds.
(c) impure elements.
(d) mixtures.
Answer:
(d) mixtures
Most of the substances we see around us are mixtures of different elements and compounds.
Page No 57:
Question 2:
A solution of salt in water is a
(a) compound.
(b) homogeneous mixture.
(c) heterogeneous mixture.
(d) element.
Answer:
(b) homogeneous mixture
When salt is dissolved in water, the salt is distributed uniformly in the solution. Thus, the salt solution is a homogeneous mixture.
Page No 57:
Question 3:
The property used in separating a mixture of two solids by winnowing is
(a) difference in colour.
(b) difference in size.
(c) difference in weight.
(d) attraction by magnet.
Answer:
(c) difference in weight
Winnowing is used to separate two solids of different weights, such as wheat and chaff.
Page No 57:
Question 4:
The process of settling down of particles of a solid in a liquid is
(a) decantation.
(b) sublimation.
(c) sedimentation.
(d) filtration.
Answer:
(c) sedimentation
The process of settling down of particles of a solid in a liquid is known as sedimentation.
Page No 57:
Question 5:
Which method is used to obtain a pure liquid from a solution?
(a) distillation
(b) condensation
(c) filtration
(d) loading
Answer:
(a) distillation
Distillation can be used to obtain a pure liquid from a solution.
Page No 57:
Question 6:
The temperature of a saturated solution of sugar in water is raised by 10 °C. Which of these statements is now true?
(a) It will continue to be saturated.
(b) It will become unsaturated.
(c) Whether it remains saturated or becomes unsaturated depends on the amount of water taken.
(d) It will continue to be saturated, but if the temperature is increased beyond 10 °C it will become unsaturated.
Answer:
(b) it will become unsaturated
If the temperature of saturated sugar solution is increased by 10 oC, then more sugar can be dissolved in the solution and the solution becomes unsaturated.
Page No 57:
Question 7:
Which of the following can dissolve in water?
(a) only solids
(b) only solids and liquids
(c) solids, liquids and gases
(d) only liquids
Answer:
(c) solids, liquids and gases
Solids such as salt, liquids such as alcohol and gases such as nitrogen and oxygen can dissolve in water.
Page No 58:
Question 8:
Sedimentation and decantation are useful to separate
(a) immiscible liquids.
(b) soluble solid from liquid.
(c) insoluble solid from liquid.
(d) insoluble solid from a liquid where the solid is heavier than the liquid.
Answer:
(d) insoluble solid from a liquid where the solid is heavier than the liquid
Decantation and sedimentation can be used to separate solids from a liquid when the solid is heavier than the liquid and is insoluble in the liquid.
Page No 58:
Question 9:
A commonly used chemical for loading is
(a) hydrogen.
(b) alum.
(c) sulphur.
(d) common salt.
Answer:
(b) alum
Alum is a commonly used chemical for loading to speed up sedimentation.
Page No 58:
Question 10:
To get only the pure solid from its solution in water, which method will you use?
(a) evaporation
(b) distillation
(c) filtraction
(d) none of these
Answer:
(a) evaporation
Evaporation is the process used to get a pure solid from its solution in water.
Page No 58:
Question 1:
There are very few pure elements or compounds around us. Most substances are mixtures. True or false?
Answer:
True, most of the substances around us exist in the mixed state and not as pure elements or compounds.
Page No 58:
Question 2:
The constituents of all mixtures are uniformly spread throughout the mixtures. True or false?
Answer:
False, the constituents of a heterogeneous mixture are not uniformly spread throughout the mixture.
Page No 58:
Question 3:
Is a solution of sugar in water homogeneous or heterogeneous?
Answer:
In case of a sugar solution, the sugar is distributed uniformly in the water. Thus, sugar solution is a homogeneous solution.
Page No 58:
Question 4:
Name one property of a compound that remains fixed and can be used to test if the compounds has impurities in it.
Answer:
The melting point and boiling point are two properties of a compound that can be used to test its purity as they remain fixed.
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Question 5:
We have to separate a mixture into its constituents because only pure compounds are useful to us. Mixtures are not of much use. True of false?
Answer:
False, mixtures are also very useful to us. For example, the air we breathe is also a mixture of gases.
Page No 58:
Question 6:
Can a mixture of sawdust and water be separated by sedimentation and decantation?
Answer:
Sedimentation and decantation cannot be used to separate sawdust from water because sawdust is not heavier than water. Sedimentation and decantation can only be used to separate an insoluble solid from a liquid if the solid is heavier than the liquid.
Page No 58:
Question 7:
Which method gives a better separation of an insoluble solid from a liquid−sedimentation and decantation, or filtration?
Answer:
An insoluble solid can be separated from a liquid more efficiently by filtration. Filtration can separate out solids that are also lighter than the liquid and it is faster since, liquid need not be left to stand.
Page No 58:
Question 8:
Name the method which is used to speed up sedimentation.
Answer:
The method used to speed up sedimentation is known as loading.
Page No 58:
Question 9:
Which separation method is used to separate wheat grains from chaff?
Answer:
Wheat grains are separated from chaff using the process of winnowing.
Page No 58:
Question 10:
Name the method used to obtain salt from sea water.
Answer:
Salt is obtained from sea water by evaporation of sea water.
Page No 58:
Question 11:
Which method will you use to get a pure liquid from a solution containing several soluble solids in the liquid?
Answer:
Distillation can be used to get a pure liquid from a solution containing several soluble solids in the liquid.
Page No 58:
Question 12:
Which apparatus is used to cool the liquid vapours during distillation?
Answer:
A Liebig's condensor is used to cool liquid vapours during distillation.
Page No 58:
Question 13:
Which apparatus is used to separate two immiscible liquids?
Answer:
Two immiscible liquids can be separated using a separating funnel.
Page No 58:
Question 14:
The method that can be used to separate a liquid from a mixture of miscible liquids with different boiling points is ___________ distillation.
Answer:
The method that can be used to separate a liquid from a mixture of miscible liquids with different boiling points is fractional distillation.
Page No 58:
Question 15:
Solubility of most solids in water increases with rise in temperature. True or false?
Answer:
True. The solubility of most solids in water increases with rise in temperature.
Page No 58:
Question 16:
Can water dissolve liquids and gases also?
Answer:
Yes, water can dissolve liquids such as alcohol or milk. It can also dissolve gases such as oxygen, nitrogen etc.
Page No 58:
Question 17:
The solubility of common salt increases rapidly with rise in temperature. True or false?
Answer:
True. The solubility of common salt increases rapidly with rise in temperature.
Page No 58:
Question 1:
You are given a sample of water. How will you find out whether it is pure or not?
Answer:
To find out if water is pure or not, we can measure the boiling point of the sample. If the sample boils at 100 oC, then the water is pure.
Page No 58:
Question 2:
What is the principle used in the separation of mixtures?
Answer:
The principle used in the separation of a mixture employs a special property of one of the components of the mixture. For example, iron pieces mixed with sand can be separated using a magnet as iron gets attracted towards the magnet.
Page No 58:
Question 3:
To use a sieve to separate sand and rice seeds, what should be the size of the holes of the sieve in comparison to the size of sand particles and rice seeds?
Answer:
To separate sand from rice seeds, the holes in the sieve must be larger than the sand particles and smaller than the rice seeds so that, sand particles can easily pass through the sieves leaving behind the rice seeds.
Page No 59:
Question 4:
Which property of a filter paper is used to separate an insoluble solid from a liquid?
Answer:
An insoluble solid can be separated from a liquid using a filter paper as the filter paper allows the liquid to pass through it while the solid will not pass through the filter paper.
Page No 59:
Question 5:
We always talk about a saturated solution at a certain temperature? Why is the temperature specified?
Answer:
The solubility of a solid in a solution increases with increasing temperature. If the temperature of a saturated solution is increased, the solution will become unsaturated. Thus, the temperature plays an important role in specifying whether a solution is saturated or not.
Page No 59:
Question 6:
Under which condition can handpicking be used to separate the constituents of mixture?
Answer:
Handpicking can be used to separate the constituents of a mixture only when the constituents of the mixture are easily visible and can be separated. For example, stones can be separated from rice by handpicking.
Page No 59:
Question 7:
Why are objects at a distance seen more clearly after rain?
Answer:
When it rains, the dust particles in the air get wet and settle down on the ground. As a result, the air is clearer after it rains and objects at a distance are seen more clearly.
Page No 59:
Question 8:
The process of adding alum to water to hasten sedimentation is called 'loading'. Why has this name been given to the process?
Answer:
When alum is added to water, it dissolves in water and makes the suspended impurities heavier making them settle down quickly. Therefore, the addition of alum is known as loading.
Page No 59:
Question 1:
What is the difference between homogeneous and heterogeneous mixtures? Give an example of each mixture.
Answer:
| Homogeneous mixture | Heterogeneous mixture |
| The components of the mixture are uniformly distributed in the mixture. | The components of a mixture are not uniformly distributed in the mixture. |
| Example: Salt dissolved in water is a homogeneous mixture. | Example: Iron filings mixed in sand is a heterogeneous mixture. |
Page No 59:
Question 2:
List three properties of mixtures.
Answer:
Following are the properties of mixtures:
- The constituents of a mixture may be present in any ratio.
- The constituents of a mixture retain their individual properties The molecules of the constituents do not change. For example, when iron and sulphur are mixed together, their properties do not change.
- Constituents of a mixture can be separated easily. For example, stones in rice can be separated easily by visual observation.
Page No 59:
Question 3:
Give two situations where it is necessary to separate a mixture into its constituents.
Answer:
It is necessary to separate a mixture into its constituents for the following situations:
- Removing undesirable constituents: For example, it is desirable to remove small stones in rice, or separating tea leaves from tea.
- To obtain useful substances: Butter is obtained by churning curds. In the same way, crude oil can be separated into various oils such as petrol, kerosene, diesel etc.
Page No 59:
Question 4:
Draw a labelled diagram to show the arrangement used to filter a mixture of a liquid and an insoluble solid.
Answer:
The arrangement used to filter a mixture of a liquid from an insoluble solid is shown below:
Page No 59:
Question 5:
Draw a labelled diagram of the apparatus used to distil out pure water from a solution.
Answer:
Diagram of apparatus used to distil out pure waterfrom a solution
Page No 59:
Question 6:
How will you separate a mixture of grass, pebbles and sand?
Answer:
To separate a mixture of grass, pebbles and sand, the first step is to pass the mixture through a fine filter. Sand will pass through the filter leaving grass and the pebbles on the filter.
Then, we can winnow the grass and the pebbles since, they have different weights to separate them from each other.
Page No 59:
Question 7:
Explain with the help of a diagram how can a mixture of oil and water be separated.
Answer:
Oil and water can be separated using a separation funnel. The experimental set up is shown in the figure:
The mixture of oil and water is poured into the separating funnel and allowed to stand for some time. The oil accumulates at the top of the funnel and water near the bottom as shown in the figure. If the stop cock at the bottom is opened, water flows out of the funnel and can be collected in a beaker. If the stop cock is closed when the oil reaches the level of the stop cock, we will have oil in the funnel and water in the beaker.
Page No 59:
Question 8:
Why is water considered as an important solvent?
Answer:
Water is considered an important solvent because:
- Water can dissolve a large number of substances that are important for life.
- Waste products are also dissolved in water before they can be excreted from the body.
- Most of the chemical reactions occurring in the body take place in the presence of water.
- Gases such as oxygen and carbon dioxide are soluble in water. This makes aquatic life possible.
Page No 59:
Question 1:
A bottle contains either salt or chalk. How will you find out what it has, without tasting it?
Answer:
Salt is soluble in water but chalk is not. So, if you add salt to water it will dissolve and form a clear liquid. However, chalk will not dissolve in water and the chalk dust remains in the water.
Page No 59:
Question 2:
Think and write how you will separate a mixture of water, sand, salt and iron filings. You may have to use a method not given in the chapter to separate sand and iron filings. Think−which property of iron can be used for this?
Answer:
To separate the constituents of the mixture we can use the following steps:
- The first step is to pass the mixture through a filter paper. This will separate out a mixture of sand and iron filings from the salt solution.
- Now, pass a magnet over the mixture of sand and iron filings. The iron filings will be attracted to the magnet.
- Finally, evaporate the salt solution and collect the vapours. Condense the vapour to get water, while salt will be left behind.
Page No 59:
Question 3:
A mixture of salt and sugar is prepared by grinding them together. Do the molecules of the salt and sugar undergo any change?
Answer:
No, the molecules of salt and sugar remain as they are even when salt and sugar are crushed to make a mixture.
Page No 59:
Question 4:
A saturated solution of sugar in water is prepared. The temperature of water is lowered by 5 °C. Will the solution continue to be saturated? Give reasons.
Answer:
If a solution is saturated, it means that more solid will not dissolve in the solution at that temperature. If the temperature is lowered, then the solubility of sugar will be reduced even further and the solution will continue to remain saturated.
Page No 59:
Question 5:
Sea water has salt dissolved in it. You know that pure water can be obtained from sea water by distillation. However, this method is not used to get drinking water from sea water. What do you think is teh reason?
Answer:
Sea water has salt dissolved in it and pure water can be obtained by distillation. However, the energy required to heat large quantities of sea water to make this process widely applicable is too expensive. Also, the condensers required are very expensive if they are to be sized large enough to distill vast quantities of sea water. As a result, distillation is not used to get drinking water from sea water.
Page No 59:
Question 1:
In a factory, holes need to be drilled in steel to manufacture car parts. While doing this, the drill and steel get very hot due to friction and this can damage both. To solve this problem, a mixture of oil and water is sprayed onto the drill bit. The mixture acts as both a coolant and a lubricant. At the end of the process, there is a waste mixture of fine steel bits, dirty oil and water. To avoid wastage and pollution, it is advisable to separate the waste mixture and reuse the components.
Answer these questions.
1. Suggest two methods of removing the bits of steel from the waste mixture in order to recycle them.
2. How can the oil and water be separated?
Answer:
1. We can remove the bits of steel from the waste mixture in order to recycle them:
(i) By filtration
(ii) By using a magnet
2. Oil and water are two immiscible liquids. They form two separate layers in solution, the oil comes on top of solution and water remains on the bottom of the surface. By simply pouring the oil from the container, we can separate water and oil.
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