# Samacheer Kalvi Class 8 Science Unit 13 Water Solutions

## Samacheer Kalvi Class 8 Science Unit 13 Water Solutions

Welcome to NCTB Solutions. Here with this post we are going to help 8th class students by providing Solutions for Samacheer Kalvi Class 8 Science Unit 13 Water. Here students can easily find all the solved solution for Water Exercise questions. Also here our Expert Science Teacher’s solved all the questions with easily understandable language with proper guidance so that all the students can understand easily. Here on this post students will get Unit 13 Water exercise solutions. Here all the solutions are based on Tamil Nadu State Board latest syllabus.

#### Unit 13 Water Exercise Questions Solutions :

(1) Water changes to ice at

(a) 0 degree C

(b) 100 degree C

(c) 102 degree C

(d) 98 degree C

Correct Option → (a)

Water changes to ice at 0 degree Celsius.

(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 this

Correct Option → (b)

Solubility of carbon dioxide in water is high when the Pressure is high.

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

(a) Oxygen

(b) Hydrogen

(c) Nitrogen

(d) CO2

Correct Option → (b)

The gas collected at the cathode on electrolysis of water is Hydrogen.

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

(b) Alum

(c) Oxygen

(d) Chlorine

Correct Option → (a)

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

(a) Sulphates and chlorides

(b) Dust particles

(c) Carbonates and biocaonates

(d) Other soluble particles

Correct Option → (a)

Permanent hardness of water is due to the presence of Sulphates and chlorides.

(II) Fill in the blanks

(1) Water is colourless, odourless and ____

(2) The boiling point of water is ____

(3) Temporary hardness of water can be removed by ____ of water.

(4) The density of water is maximum at ____

(1) → tasteless

(2) → 100 degree Celsius

(3) → Boiling

(4) → 4 degree Celsius

(5) → Sedimentation

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

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

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

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

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

(5) Soap lathers well in hard water.

(1) → True

(2) → False

(3) → True

(4) → False, the density of water will change at all temperature.

(5) → True

(V) Give reasons for the following

(1) Alum is added to water in sedimentation tanks.

Reason :

Alum is added to water in sedimentation tanks because Alum is a chemical substance which speed up the process of sedimentation. The particles of Alum combine with the suspended impurities and make them settle down at a faster rate therefore Alum is used.

(2) Water is a universal solvent

Reason :

Water is the universal solvent because water has unique property to dissolve more substances than any other solvents. Water can dissolve solids like salts, sugars liquids like honey, milk and gases like carbon dioxide in it therefore it is called universal solvent.

(3) Ice floats on water

Reason :

Ice floats on water because ice is lighter than water. Means the density of ice is lower than of that water, therefore Ice floats on water.

(4) Aquatic animals can breathe in water

Reason :

Aquatic animals can breath in water because air dissolved in water is important for living organism to survive and because of this oxygen is present in water therefore aquatic animals can breathe in water.

(5) Sea water is unfit for drinking

Reason :

Sea water is unfit for drinking because it contains salt and this salty water is not suitable for drinking therefore sea water is unfit for drinking.

(6) Hard water is not good for washing utensils.

Reason :

Hard water is not good for utensil because hard water contains the salt of calcium and magnesium which is stored and forms hard layer on utensil therefore hard water is not good for utensil.

(VI) Define the following

(1) Freezing point

The point at which water freezes at 0 degree C and forms ice this is known as freezing point.

(2) Boiling point

At the temperature at which water boils and changes into steam is known as boiling point. Boiling point of water is 100 degree Celsius.

(3) Specific heat capacity

The amount of heat that is needed to rise the temperature of a unit mass of a substance by 1 degree Celsius is called specific heat capacity of substance.

(4) Latent heat of fusion

The heat energy is utilized in changing the state of ice from solid to liquid, the amount of heat energy required by ice to change into water is called latent heat of fusion of ice.

(5) Potable water

The water is suitable for drinking is called potable water.

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

The electrolysis of water involves hydrogen and oxygen gas. At cathode hydrogen gas is evolved and at anode oxygen gas is evolved. The ratio by volume of hydrogen and oxygen is 2 : 1. Two moles of hydrogen collected at the cathode and one volume of oxygen is collected at anode.

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

Oxygen dissolved in water is important for the living organisms to survive. It gives oxygen to aquatic animals for respiration.

Carbon dioxide dissolved in water reacts with lime stone to form calcium bicarbonate, marine organisms like oyster, snail etc extract calcium carbonate from calcium carbonate to build their shells.

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

Temporary hardness is due to the presence of carbonate and bicarbonate salts of calcium and magnesium and Permanent hardness results due to the presence of chloride and sulphate slats of calcium and magnesium.

(4) Explain specific latent heat of vaporization of water.

When water attains the temperature of 100 degree C it stars changing its state from liquid to gaseous state. The temperature of water dose not rise above 100 degree C .it is because the supplied heat only changes the state of the boiling water this heat energy is stored in stem and this stem is known as latent heat of vaporization of stem i.e latent heat of vaporization of water.

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

There are various methods of removing of water :

(a) Boiling method : it easily remove hardness of water.

(b) Adding washing soda method : it removes permanent hardness of water.

(c) Ion-exchange method : in this method ion:- exchange resin where calcium and magnesium ions get replaced by sodium ions and converts hard water into soft water.

(d) Distillation methods : temporary and permanent hardness of water can removed by this method.

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

The following process is used at water purification plant –

(a) Sedimentation : water from lake or rivers is collected in large sedimentation tanks. There water is allowed to stand undisturbed so that suspended impurities settle down at the bottom of the tank. Sometimes chemical substance is used to speed up the process if sedimentation.

(b) Filtration : water from the sedimentation tanks is then pumped to the filtration tanks. Filtration tanks contain filter beds made up of gravel, sand, pebbles, activated charcoal and concrete. Water passes through these layers and becomes free from any remaining dissolved or suspended impurities completely.

(c) Sterilization : the filtered water is treated chemically to remove the remaining germs or bacteria by sterilization method. Chlorine and ozone is used for sterilization method. Water from filtration tanks is pumped into chlorination tanks, where chlorine is added to remove harmful bacteria and germs. Then ozone gas is used to kill the germs present in it. After this process pure water is ready. From This process water purified at water purification plant.

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

Water contains number of dissolved salts and minerals, sometimes minerals and salts are present in water in large quantity this type of water is known as hard water. Hardness of water is due to the presence of dissolved salts of calcium and magnesium. Hardness is sometimes temporary or permanent. Permanent hardness is due to the presence of chloride and sulphate salts of calcium and magnesium.

Removal of hardness : There are different methods for removal of permanent hardness of water as following :-

(a) Adding washing soda : it is used to remove permanent hardness of water. Adding washing soda converts chlorides and sulfates into insoluble carbonates. These insoluble carbonates are removed by filtration.

(b) Ion-exchange : this method is also used for removal of hardness of water. In this method water is passed through a column of ion- exchange resins where calcium and magnesium ions gets replaced by sodium ions and this converts hard water into soft water.

(c) Distillation : in this method distillation is used .water is obtained after distillation is known as distilled water and it is purest water.

This are some different methods used for removal of hardness of water.

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

Electrolysis is the process of flowing of current from anode to cathode externally and internally conducting solution current is flowing from cathode to anode this process is known as electrolysis. In the electrolysis process current is passes through anode and cathode ions.

Anode is the positive charge electrode and cathode is negative charge electrode.

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

Water pollution occurs when harmful substances such as chemicals, sewage and waste are released into water. This substance produced change in the quality of water. They decrease the quality of water and make water polluted.

There are some sources which makes water polluted :

(a) Household detergents : household and cleaning detergents are a major source of water pollution. Many detergents have the different chemical and those are harmful for the aquatic animals when this chemical mixed water is added in sea or river water they make that sea or river polluted and affects on aquatic animals.

(b) Domestic sewage : wastewater that is disposed of form household is known as domestic sewage. It should be treated before mixed in water bodies like sea, river because untreated sewage contains impurities like food waste, toxic chemicals from household products and it may also contain disease causing microbes and they also affects on aquatic animals if it is mixed in water bodies without any treatment .

(c) Industrial waste : many industries release toxic wastes such as lead ,mercury, cyanides, cadmium etc. if this waste is released into water bodies it affects on plants, aquatic life and humans.

(d) Agricultural activities : fertilizers, pesticides and insecticides used in agriculture can dissolve in rainwater and flow into water bodies such as rivers and lakes. This causes an excess of nutrients like nitrates and phosphates as well as toxic chemicals in water bodies. This substance can also affects to aquatic life.

(e) There are some other sources of water pollution such as thermal pollution, oil spills, domestic waste and plastics etc this are some different sources of water pollution.

Next Chapter Solution :

👉 Acids and Bases

Updated: July 31, 2023 — 2:11 pm