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Science [2 Mark Questions]

Water. · Tamil Nadu Board STD 8 (Tamil - English Medium). 15 questions.

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[2 Mark Questions]

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15 questions · timed · auto-graded

Question 12 Marks
Write the chemical equation for the production of water when zinc reacts with sulphuric acid.
Answer
$Zn + H _2 SO _4 \rightarrow ZnSO _4+ H _2$
$2 H _2+ O _2 \rightarrow 2 H _2 O$
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Question 22 Marks
What are the physical properties of pure water?
Answer
 Pure water is a clear and transparent liquid:
1. Pure water boils at $100^{\circ} C$ at one atmospheric pressure.
2. Pure water freezes at exactly $0^{\circ} C$ at one atmospheric pressure.
3. Pure water has a density of $1 gm / cm _3$.
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Question 32 Marks
Write a short note on catalytic nature of water.
Answer
Water acts as a catalyst in a number of reactions. Perfectly dry hydrogen and chlorine gases do not react in the presence of sunlight. However in the presence of traces of water, the reaction takes place with explosion to produce hydrogen chloride

$H _2+ Cl _2 \underset{\text { Sunlight }}{\stackrel{\text { Electrolysis }}{\longrightarrow}} 2 HCl$

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Question 42 Marks
Write the characteristics of potable water.
Answer
  1. Potable water should be colourless and odourless.
  2. II should be transparent.
  3. It should be free from harmful micro-organisms such as bacteria, virus and protozoa.
  4. It should be free from suspended impurities.
  5. I should contain some minerals and salts, necessary for our body and some dissolved gases to add taste.
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Question 52 Marks
What is soft water?
Answer
Water contains a number of dissolved salts and minerals. When these salts are present in very small quantities in water, it is called soft water.
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Question 62 Marks
Mention the disadvantages of hard water.
Answer
  1. It is not good for washing clothes. It forms scum with soap and detergents, which makes the soap ineffective and also spoils the clothes further.
  2. It damages the utensils and containers in which it is stored and forms a hard layer.
  3. It forms scales on the machine parts used in industries and decreases their efficiency.
  4. It results in stomach ailments if consumed for a long period.
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Question 72 Marks
What is the reason for hardness of water?
Answer
The hardness of water is due to the presence of dissolved salts of calcium and magnesium.
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Question 82 Marks
Write a note on ion-exchange.
Answer
  1. Another method used to remove the hardness of water is to pass it through a column of ion-exchange resins where calcium and magnesium ions get replaced by sodium ions.
  2. This converts hard water into soft water.
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Question 92 Marks
How plastic sheets affect the soil?
Answer
  1. Plastic sheets are used in agriculture to grow vegetables.
  2. At the end of the season, these plastic sheets are ploughed back into the soil.
  3. The plastic sheets break into tiny pieces and get eaten by earthworms, which is harmful to their health and that of soil.
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Question 102 Marks
What are the sources of water pollution?
Answer
  1. House hold detergents
  2. Domestic sewage
  3. Domestic waste and plastics
  4. Agricultural activities
  5. Oil spills
  6. Industrial waste.
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Question 112 Marks
What are the methods of removing hardness of water?
Answer
  1. Boiling – Temporary hardness is easily removed from water by boiling.
  2. Adding washing soda – Washing soda is used to remove the permanent hardness of the water.
  3. Ion-exchange – This converts hard water into soft water.
  4. Distillation – Temporary and permanent hardness both can be removed by the method of distillation.
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Question 122 Marks
Explain specific latent heat of vaporization of water.
Answer
  1. When water attains the temperature of 100°C, it starts changing its state from liquid to gaseous state, however, the temperature of the water does not rise above 100°C.
  2. It is because the heat energy supplied only changes the state of the boiling water.
  3. This heat energy is stored in steam and is commonly called the latent heat of vaporization of steam.
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Question 132 Marks
What are the causes of temporary hardness and permanent hardness of water?
Answer
  1. Temporary hardness is due to the presence of carbonate and bicarbonate salts of calcium and magnesium.
  2. Permanent hardness is due to the presence of chloride and sulphate salts of calcium and magnesium.
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Question 142 Marks
State the importance of dissolved oxygen and carbon dioxide in water.
Answer
  1. Fish extracts oxygen from the water and expels water through the gills. Fish can survive in water only through the dissolved oxygen present in water.
  2. Aquatic plants make use of dissolved carbon dioxide for photosynthesis.
  3. Carbon dioxide dissolved in water reacts with limestone to form calcium bicarbonate.
  4. Marine organisms such as snails, oysters, etc., extract calcium carbonate from calcium bicarbonate to build their shells.
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Question 152 Marks
Name the gas evolved at cathode and anode when water is electrolysed. State their ratio by volume.
Answer
  1. The gas which is evolved at the cathode: Hydrogen (H2).
  2. The gas which is evolved at the anode: Oxygen (O2)
  3. The ratio of H2 and O2 = 2:1.
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[2 Mark Questions] - Science STD 8 Questions - Vidyadip