Samacheer Kalvi 8th Science Solutions Chapter 13 Pdf

Samacheer Kalvi 8th Science Solutions Chapter 13 Pdf

Tamilnadu Board Samacheer Kalvi 8th Science Solutions Chapter 13: Tamilnadu State Board Solution Class 8 Science Chapter 13 – Water.

Samacheer Kalvi 8th Science Solutions Chapter 13: Overview

Board

Samacheer Kalvi
Class

8

Subject

Science
Chapter

13

Chapter Name

Water


Samacheer Kalvi 8th Science Solutions Chapter 13 Pdf

(I) Choose the correct answer.

(1) Water changes to ice at

a ) 0°C     

b) 100° C

c) 102 °C

d) 98° C

(2) Solubility of carbon dioxide in water is high when the

a ) pressure is low

b) pressure is high

c ) temperature is high

d ) None of the above

(3) The gas collected at the cathode on electrolysis of water is

a) oxygen

b) hydrogen

c) nitrogen

d) carbon dioxide

(4) Which of the following is a water pollutant?

a ) Lead

b) Alum

c) Oxygen

d) Chlorine

(5) Permanent hardness of water is due to the presence of

a ) sulphates and chlorides

b) dust particles

c) carbonates and bicarbonates

d) Other soluble particles

( II) Fill in the blanks

1 ) Water is colorless, odourless and tasteless.

2 ) The boiling point of water is 100° C .

3) Temporary hardness of water can be removed by boiling of water.

4) The density of water is maximum at 4°C .

5) Loading speeds  up the process of sedimentation.

(III ) State true or false . If false, correct the statement.

1 ) Sewage should be treated well before being discharge it into water bodies.

This statement is true.

2 ) Sea water is suitable for irrigation as it contains dissolved salts.

This statement is false.

Explanation:

Due to its high salinity, sea water is unsuitable for the purpose of irrigation.

3) Excessive use of chemical fertilizers depletes the soil and causes water pollution.

This statement is true.

4) The density of water will not change at all temperature?

This statement is false.

Explanation :

The density of water will not remain constant, rather it will change at all temperature.

5) Soap lathers well in hard water .

This statement is false.

Explanation :

Soap does not lather well in hard water but instead, it lathers well in soft water.

(IV ) Match the following .

Universal solvent Water pollutant
Hard water Kills germs
Boiling Ozonisation
Sterilization Water
Sewage Stomach ailments

Answer

Universal solvent Water
Hard water Stomach ailments
Boiling Kills germs
Sterilization Ozonisation
Sewage Water pollutant

(V) Give reasons for the following.

1 ) Alum is added to water in sedimentation tanks.

Alum is a chemical substance that is added to water for the purpose of speeding up the process of sedimentation and this process is known as loading. The particles of potash alum performs an essential role by combining with the suspended impurities, so that they can settle down more rapidly. Due to this reason, alum is added to water in sedimentation tanks.

2 ) Water is a universal solvent.

As water possesses the ability to dissolve more substances in comparison to any other liquid, for this reason water is known as universal solvent. As for instance, sugar and salt which falls under the category of solids can dissolve rapidly in water, it also has the ability to dissolve milk and honey which are mainly in the form of liquids and gases like carbon dioxide and oxygen can also dissolve within it.

3) Ice floats on water.

As ice is lighter in comparison to water, for this reason ice can float on the surface of water. This clearly signifies that, the density of water is higher when compared to ice.

4) Aquatic animals can breathe in water.

The presence of dissolved air in water is extremely essential for the survival of the living organisms. Fish dies when taken out of water because fish contains gills, whose primary function is to absorb the oxygen that is dissolved in water. There is a discontinuation in the supply of oxygen, the moment they are taken out of water. As a result of which, they are unable to breathe and absorb the oxygen that are contained in the air. Due to this reason , they cannot survive when they are taken out of water.

5) Sea water is unfit for drinking.

As we all know that, 35 grams of dissolved salt are contained in every litre of seawater and these dissolved salts are mostly known by the term of sodium chloride. Such  water are high in salinity, for this reason they are known as saline water. Saline water is unsuitable for the purpose of drinking and so it is not potable water. Owing to this reason, sea water is considered unfit for drinking.

6) Hard water is not good for washing utensils.

Hard water not only damages the utensils but it also damages the containers in which it is kept. It  leads to the formation of hard layer. Due to this reason, hard water is not suitable for washing utensils.

(VI) Define the following.

1 ) Freezing point

Freezing point refers to the temperature which enables a liquid to transform into a solid state when cooled. 0 °C is the freezing point of water.

2) Boiling point

Boiling point refers to the temperature which facilitates the boiling of water to such an extent that it transforms into steam. At atmospheric pressure, 100 °C is the boiling point of water.

3) Specific heat capacity

Specific heat capacity of a substance refers to the quantity of heat that is necessary for the purpose of raising the temperature of a unit mass of a substance by 1° C.

4) Latent heat of fusion

Latent heat of fusion of ice refers to the quantity of heat energy that is necessary for ice to convert into water.

5) Potable water

Potable water refers to the water that is suitable for the purpose of drinking. 1-2 grams of dissolved salts (common salt) are contained in every litre of potable water.

(VII ) Answer in brief.

1 ) Name the gas evolved at cathode and anode when water is electrolysed. State their ratio by volume.

Hydrogen is the gas which is evolved at cathode and oxygen is the gas which is evolved at anode when water is electrolysed. 2:1 is the ratio of hydrogen and oxygen.

2) State the importance of dissolved oxygen and carbon dioxide in water.

Fish contains gills, whose primary function is to absorb the oxygen that is dissolved in water. There is a discontinuation in the supply of oxygen, the moment they are taken out of water. As a result of which, they are unable to breathe and absorb the oxygen that are contained in the air. Due to this reason , they cannot survive when they are taken out of water.  So, the survival of the fish is only possible through the presence of dissolved oxygen in water. Dissolved carbon dioxide are used by aquatic plants for the purpose of photosynthesis. There is an occurrence of reaction of dissolved carbon dioxide in water with limestone for the purpose of forming calcium bicarbonate. Snails and oysters makes use of calcium carbonate for the purpose of building their shells and this calcium carbonate are extracted from calcium bicarbonate. So, the above points clearly depicts the importance of dissolved oxygen and carbon dioxide in water.

3) What are the causes of temporary hardness and permanent hardness of water?

The presence of dissolved salts of calcium and magnesium are the primary reason behind the causation of hardness of water. Hardness can either the temporary or permanent. On one hand, the presence of carbonate and bicarbonate salts of calcium and magnesium leads to the causation of temporary hardness of water. On the other hand, the presence of chloride and sulphate salts of calcium and magnesium leads to the causation of permanent hardness of water.

4) Explain specific latent heat of vaporization of water.

After attaining the temperature of 100 °C , water begins to transform from liquid state into gaseous state. But, 100 ° C is the maximum limit, beyond which the temperature of water does not rise. This is due to the fact that, only the state of boiling water is changed by the supplied heat energy. This heat energy is known as latent heat of vaporization of steam and it is mainly the steam which reserves this heat energy. However, the maximum latent heat of vaporization are possessed by the steam and its value is represented as 540 calories /g or 2268 J/g.

5) What are the methods of removing hardness of water?

Distinct methods are utilised for the purpose of removing hardness from water and it mainly depends on the fact that, whether the hardness is temporary or permanent.

  • Boiling

Boiling is a method that can be used for the purpose of removing temporary hardness of water.

  • Adding washing soda

In order to remove permanent hardness of water, washing soda can be used.

  • Ion- exchange

This facilitates in converting hard water into soft water.

  • Distillation

Distillation is an effective method that can be  used in order to remove both temporary and permanent hardness.

(VIII ) Answer in detail.

1) How is water purified at a water purification plant?

Water is subjected to distinct processes for purification , in conventional water treatment plants.

Sedimentation

Large sedimentation tanks contains water that are collected from lakes or rivers. Then, the suspended impurities tends to settle down at the basal of the tank, as water is allowed to stand without any kind of disturbance. Alum is a chemical substance that is added to water for the purpose of speeding up the process of sedimentation and this process is known as loading. The particles of potash alum performs an essential role by combining with the suspended impurities, so that they can settle down more rapidly. Due to this reason, alum is added to water in sedimentation tanks.

Filtration

Then water from the sedimentation tanks is transferred to the filtration tanks through pumping. The filter beds that are present in filtration tanks are mainly composed of gravel, sand, pebbles, activated charcoal and concrete. Then, the water becomes completely free from any kind of suspended impurities because it has passed through these layers.

Sterilization

The remaining germs or bacteria are then removed, by treating the filtered water chemically. This process is known as sterilization. Chlorine and ozone are the two main chemicals that are used in this process. The water is again transferred from filtration tanks into chlorination tanks with the help of pumping, where most of the harmful bacteria and germs are removed by adding chlorine . Chlorination refers to the process in which an adequate quantity of chlorine is added to water. The process of treating water with ozone gas in order to destroy the presence of germs in it is called  Ozonisation . Water can also be sterilized by exposing it to air and sunlight. The presence of germs in water gets destroyed by the  oxygen from air and sunlight. Aeration refers to a process which also helps to destroy the germs, by blowing air which is under pressure into the filtered water.

2) What is permanent hardness of water? How can it be removed?

The presence of chloride and sulphate salts of calcium and magnesium leads to the causation of permanent hardness of water.

Permanent hardness of water can be removed in the following ways :

  • Adding washing soda

In order to remove permanent hardness of water, washing soda can be used. Washing soda can help in converting chlorides and sulphates into insoluble carbonates and then there is  removal of these insoluble carbonates through the process of filtration.

  • Distillation

Distillation is an effective method that can be    used in order to remove both temporary and permanent hardness. Distilled water is the purest form of water, that is obtained after the process of distillation.

3) What is Electrolysis? Explain the electrolysis of water.

Electrolysis of water refers to the process in which the water molecules are broken down by the passage of electric current.

Electrolysis of water

Two carbon electrodes are fixed in a glass beaker and it is poured with water upto one-third of its volume. The positive carbon electrode functions like anode and the negative carbon electrode functions like cathode. On the electrodes, two test tubes are placed. The electrodes are connected with the help of a battery and current will pass in a continuous manner, till the test tubes are filled with a specific gas. Now, if we test the gas with the help of a burning splint we will observe that , the moment the extinguish splint is brought close to the mouth of the test tube which is in the cathode side, the gas starts to burn with a popping sound . However, this property is depicted by hydrogen gas. So, we can confirm that the test tube contains hydrogen gas. In a similar manner, if the burning splint is brought close to  another test tube which is in the anode side, the gas will burn more brightly.  So we can affirm that, this test tube contains oxygen gas. So, this experiment clearly depicts that water is composed of hydrogen and oxygen. 2:1 is the ratio of hydrogen and oxygen. Therefore, whenever there will be a collection of two volumes of hydrogen at the cathode ,there will always be a collection of one volume of oxygen at the anode.

2H2O Electrolysis2H2|  + O2|

————->

4) Explain the different ways by which water gets polluted.

Natural sources and man- made sources are mainly the two sources of water pollution.

a ) Household detergents

Household and cleaning detergents are considered to be the main reason of water pollution. The chemicals that are present in synthetic detergents does not tend to break down, so as a result of which they pollute both surface and groundwater. Extensive use of detergents can pose harmful effects to the life of aquatic animals.

b) Domestic sewage

Domestic sewage refers to the waste water that are disposed from households. Before disposing the domestic sewage into water, these sewage should be properly treated. A slots of impurities, toxic chemicals and disease causing -microbes are contained in these untreated sewage.

c) Domestic waste and plastics

When solid waste like plastics are disposed in water bodies,  these plastics tends to block the drains ,which ultimately leads to the spreading of vector borne diseases like malaria and dengue.

d) Agricultural activities

Fertilizers ,pesticides and insecticides that are mainly used for the purpose of agricultural activities can also prove to be harmful for the aquatic life because these pesticides and insecticides mixes with rainwater and flows into the water bodies, which ultimately leads to the causation of nitrates, phosphates and toxic chemicals in water bodies. It is also known as Eutrophication.

e) Industrial waste

Toxic wastes like lead, mercury, cyanides and cadmium are released from several industries and if these wastes are released into the water bodies without regulating it ,then it can negatively impact the life of humans, plants, animals and other aquatic organisms.

f) Oil spills

Water pollution is caused by oil spills, which poses a harmful effect to the aquatic life. The oil that floats on the water surface tends to block the sunbeams and decreases the oxygen that are dissolved in water and this in turn leads to the suffocation of aquatic organisms.

g) Thermal pollution

Water that is mainly used for the purpose of cooking is also discharged into the water bodies and the temperature of these water are usually quite high and there are also certain chemicals present in it. This ultimately leads to an increase in temperature and reduces the quantity of dissolved oxygen in water , which poses an adverse effect to the life of marine organisms.

Updated: December 1, 2021 — 1:08 pm

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