Samacheer Kalvi 8th Science Unit 13 Book Back Answers:

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Samacheer Kalvi 8th Science Book Back Solutions PDF:

8th Science Subject 1 Mark and 2 Mark Solutions PDF available below. Click the Download option to download the book back 1 Mark & 2 Mark questions and answers. Take the printout and use it for exam purposes. Samacheer Kalvi 8th Science Unit 13 Answers are given below.

Science Book Back Answers

Unit 13 – Water

I. Choose the best answer:

1. Water changes to the ice at ………….
(a) 0°C
(b) 100°C
(c) 102°C
(d) 98°C
Answer: (a) 0°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
Answer: (b) pressure is high

3. The gas collected at the cathode on electrolysis of water is ………….
(a) oxygen
(b) hydrogen
(c) nitrogen
(d) carbon dioxide
Answer: (b) hydrogen

4. Which of the following is a water pollutant?
(a) Lead
(b) Alum
(c) Oxygen
(d) Chlorine
Answer: (a) Lead

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
Answer: (a) 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 …………..
5. Loading speeds up the process of …………..

Answer:

1. tasteless
2. 100°C
3. boiling
4. 4°C
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.
Answer: True.

2. Seawater is suitable for irrigation as it contains dissolved salts.
Answer: False.

Correct statement:
Seawater is not suitable for irrigation as it has high salinity.

3. Excessive use of chemical fertilizers depletes the soil and causes water pollution.
Answer: True.

4. Water unfit for drinking is called potable water.
Answer: False
Correct statement:
Water suitable for drinking is called potable water.

5. Soap lathers well in hard water.
Answer: False
Correct statement:
Soap lathers well in soft water.

IV. Match the following:

1. Universal solvent – Water Pollutant
2. Hard water – Kills germs
3. Boiling – Ozonisation
4. Sterilization – Water
5. Sewage – Stomach ailments

Answer:

1. Universal solvent
2. Hard water
3. Boiling
4. Sterilization
5. Sewage

V. Give reasons for the following:

1. Alum is added to water in sedimentation tank.
Reason:

1. Chemical substance potash alum is added to water to speed up the process of sedimentation.
2. This process is called loading.
3. The particles of potash alum combine with the suspended impurities and make them settle down at a faster rate.

2. Water is a universal solvent.
Reason:

1. Water has a unique property to dissolve more substances than any other liquids.
2. It can dissolve solids such as salt and sugar, liquids such as honey and milk and gases such as oxygen and carbon dioxide in it.
3. Water can dissolve more number of substances than any other solvent.
4. Therefore, it is called as universal solvent.

3. Ice floats on water.
Reason:

1. This is because ice is lighter than water.
2. It means that the density of ice is lower than that of water.
3. Since ice is a bad conductor of heat it does not allow heat to pass through it.
4. So, the water below the ice remains in liquid form, where most of the aquatic life lives.

4. Aquatic animals can breathe in water.
Reason:

1. Air dissolved in water is important for the aquatic animals to survive.
2. Aquatic animals extracts the oxygen form the water and expels water.
3. They can breathe in water only through the dissolved oxygen present in water.

5. Sea water is unfit for drinking.
Reason:

1. Every litre of sea water contains 35 grams of dissolved salts most commonly known as sodium chloride (NaCl).
2. Such water is called saline water.
3. It is not suitable for drinking and is said to be non-potable water.

6. Hard water is not good for washing utensils.
Reason:
Hard water damages the utensils and containers in which it is stored and forms a hard layer.

VI. Define the following:

1. Freezing Point.
Answer:
The temperature at which a liquid turns into solid when cooled is known as freezing point. The freezing point of water is 0°C.

2. Boiling point.
Answer:
The temperature at which a water boils and changes to steam is called as boiling point. The boiling point of water is 100°C at atmospheric pressure.

3. Specific heat capacity.
Answer:
The amount of heat that is needed to raise the temperature of a unit mass of a substance by 1°C is called specific heat capacity.

4. Latent heat of fusion.
Answer:
The amount of heat energy required by ice to change into water is called latent heat of fusion of ice.

5. Potable water.
Answer:
The water suitable for drinking is called potable water.

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