Water ICSE Class-8th Concise Selina Chemistry Solutions

Water ICSE Class-8th Concise Selina Chemistry Solutions Chapter-8. We Provide Step by Step Answer of Objective, True False , Fill in the blanks, Match the following , Short / Long Answer Type , of Exercise-8 Water . Visit official Website CISCE for detail information about ICSE Board Class-8.

Water ICSE Class-8th Concise Selina Chemistry Solutions


Exercise – I

Question 1.

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Name the four main sources of water.
Answer:
(i) Oceans (ii) Seas
(iii) Rivers (iv) Lakes

Question 2.


State the importance of water cycle in nature.
Answer:
(i) Water cycle helps in regulating weather on the earth.
(ii) Water cycle makes water available in various forms on the earth.

Question 3.


Why is water very precious for all living beings ?

Answer:
Water is one of the most precious substances for the existence of life. Since life on the earth began in the oceans, and since no living thing can survive without water, it is rightly called the source of life.
Water forms a large part of the body mass of all living organisms — 90% of human blood is water. Water has the ability to dissolve a number of substances. Therefore, it serve as the liquid medium in which all reactions within the living body take place.
Fruits and vegetables contain water in them. Even dry-looking substances like wood, peas, beans, grams, etc., contain some amount of water.

Question 4.


Name the two gases from which water is formed. What is the chemical composition of these two gases in water ? Give the molecular formula of water ?
Answer:
Oxygen and hydrogen

Chemical composition = H2 and O2 proportion 2:1
Molecular formula = H2O

Question 5.

 

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What is the effect on boiling point of water when
(a) pressure is increased
(b) impurity is added
Answer:
(a) The boiling point of water increases with an increase, in pressure.
(b) Any impurity present in water lowers its freezing point and raises its boiling point.
For example, salt is added to ice to lower its melting point. Such a mixture is called a freezing-mixture. The melting is called a freezing mixture is about -15° C.

Question 6.


Give reasons:
(a) Water is used as a cooling agent

(b) Water pipes burst in severe winters.
(c) It is difficult to cook in hills compared to plains.
(d) Ice floats on water.
(e) Sea water does not freeze at 0°C.
Answer:
(a) Water has high specific heat. Water neither heats up nor cools down quickly. This property makes water as a excellent cooling agent.
(b) Water pipes bursts in severe winter because the water inside I the pipes freezes and increases its volume.
(c) Water boils at a lower temperature in the hills, where the atmospheric pressure is lower than in the plains. This is why it takes a longer time to cook in hilly regions.
(d) Ice has low density as compared to water. Water has maximum density at 4°C. That is why ice floats on water.
(e) There are impurities dissolved in sea water which increases the freezing point. That is why sea water does not freeze at 0°C.

Question 7.

 

How does anomalous expansion of water help aquatic organisms in cold climates ?
Answer:
The anomalous expansion of water helps in survivals of water animals in very cold climates. Initially when temperature of water falls, it becomes heavier and sinks down. This process continues till 4°C. Then after this expansion takes place. The surface layer of water gets freezed. Ice being bad conductor of heat does not allow loss of heat from the water below and results in survival of water animals.


Exercise – II

Question 1.


Explain the terms:
(a) Solution (b) Solute (c) Solvent.
Answer:
(a) Solution: “A homogeneous mixture of two or more substances can be varied is called a solution”.
(b) Solute: A substance which dissolves in a other substances to form a solution is called solute. Solute is smaller quantity in solution.

Or
“Substance which is dissolved in solvent.” is called Solute. Solute is smaller quantity in solution.
(c) Solvent: A solvent is a medium in which a solute dissolves. It is in large quantity in solution.
Solution = Solute + Solvent

Question 2.


What is meant by
(a) Unsaturated (b) Saturated and
(c) Supersaturated solutions.
Answer:
(a) Unsaturated solutions — A solution in which more of the solute can be dissolved at a given temperature is called an unsaturated solution.
(b) Saturated solutions — A solution that cannot dissolve any more of

the solute at a given temperature is called a saturated solution.
(c) Supersaturated solutions — A solution that contains more solute than it can hold at room temperature is called supersaturated solution.

Question 3.

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How do the solubility of a solid and a gas affected by –
(a) Increase in temperature
(b) Increase in pressure
Answer:
(a) Solubility of a solid solute generally increases with an increase in temperature. This makes it possible to prepare supersaturated solutions.
Solubility of a gas decreases with an increase in temperature.

(b) Pressure has practically no effect on the solubility of a solid (solute) in water.

In the case of gases, the amount of a gas dissolved in water increases with an increase in pressure.

Question 4.


Differentiate between:
(a) Solution and suspension
(b) Suspension and colloid
Answer:
(a) Solution and suspension

Solution

  1. It is an example of homogeneous mixture.
  2. Particle size less than 10-10m
  3. Transparent
  4. Solute particles can not be filtered. Solution pass easily through filter paper.

Suspension

  1. It is an example of heterogeneous mixture.
  2. Particle size greater team 10-7 m
  3. Opaque
  4. Particles of suspension do not pass through filter paper.

(b) Suspension and colloids

Suspension

  1. Heterogeneous
  2. Particle size greater than 10-7 m.
  3. Opaque.
  4. Particles are visible.
  5. and Particles of suspension settle at the bottom of the container.
  6. Particles of suspension do not pass through filter paper.

Colloid

  1. Heterogeneous.
  2. Particles size between 10-10 to 10-7m.
  3. Translucent.
  4. Particles can be seen with the help of a powerful microscope.
  5. Particles of colloids do not settle.
  6. Colloidal particles pass easily through ordinary filter paper but do not pass through ultra filters.

Question 5.


Define: ‘water of crystallisation’. Give two examples with formulae.
Answer:
The fixed amount of water which is in loose chemical combination with a salt in its crystal is called water of crystallization. Examples:

Ans 5 Water icse

Question 6.


Give two examples for each of the following:
(a) Hydrated substances
(b) Crystalline anhydrous substances
(c) Drying agents
(d) Deliquescent substances
(e) Efflorescent substances
(f) Colloids
(g) Solvents other than water.
Answer:
(a) Washing soda, Glauber’s salt (Na2SO4.10H2O)
(b) Common salt (NaCl), potassium nitrate (KNO3), sugar (C12H22O111) etc.
(c) Concentrated sulphuric acid (H2SO4), phosphorus pentoxide (P2O5), quicklime (CaO)
(d) Caustic soda (NaOH), crystalline-magnesium chloride (MgCl2), calcium chloride (CaCl2), Iron (III) chloride etc.
(e) Washing soda and glauber’s salt (Na2SO4.10 H2O)

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