Chapter 8 – Water

Selina Concise Chemistry 2023 solutions for class 8 Chemistry. Chapter 8 – Water is provided here with simple step-by-step explanations. These solutions for Water (8th Chapter) are extremely popular among class 10 students. The Water solution is handy for quickly completing your homework and preparing for exams.

EXERCISE- I

Question 1

Name the four main sources of water.

Answer

Question 2

State the importance of the water cycle in nature.

Answer

The water cycle is important because it ensures the continuous circulation and availability of water on Earth, regulates the Earth’s climate and weather patterns, supports plant growth, and helps to shape the Earth’s surface.

Question 3

Why is water very precious for all living beings?

Answer

Water is very precious for all living beings because it is essential for survival. It is necessary for regulating body temperature, transporting nutrients and oxygen, and eliminating waste.

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

The two gases from which water is formed are hydrogen and oxygen.

The chemical composition of these two gases in water is two atoms of hydrogen for every one atom of oxygen, forming the molecule H₂O.

The molecular formula of water is H₂O.

Question 5

What is the effect on the boiling point of water when

1. pressure is increased?
2. impurity is added?

Answer

1. When pressure is increased, the boiling point of water also increases.
2. When an impurity is added to water, the boiling point is elevated as well.

Question 6

Give reasons:

1. Water is used as a cooling agent.
2. Water pipes burst in severe winters.
3. It is difficult to cook in hills as compared to plains.
4. Ice floats on water.
5. Sea water does not freeze at 0°C.

Answer

1. Water is used as a cooling agent because of its high specific heat capacity.
2. Water pipes burst in severe winters due to the expansion of water on freezing.
3. Cooking in hills is difficult due to the lower boiling point of water at higher altitudes.
4. Ice floats on water due to its lower density than liquid water.
5. Sea water does not freeze at 0°C due to the presence of dissolved salts that lower its freezing point.

Question 7

How does the anomalous expansion of water help aquatic organisms in cold climates?

Answer

The anomalous expansion of water causes ice to float on top of liquid water, creating an insulating layer that helps to regulate the temperature of the water below. This helps aquatic organisms in cold climates to survive by preventing the water from freezing completely and allowing them to continue living in liquid water, even when the surface has frozen over.

EXERCISE- II

Question 1

Explain the terms:

1. Solution
2. Solute
3. Solvent

Answer

1. A solution is a homogeneous mixture of two or more substances.
2. A solute is a substance that is dissolved in a solution.
3. A solvent is a substance that dissolves the solute to form a solution.

Question 2

What is meant by

1. Unsaturated
2. Saturated and
3. Supersaturated solutions.

Answer

1. An unsaturated solution is a solution that contains less solute than the maximum amount that can dissolve at a given temperature and pressure.
2. A saturated solution is a solution that contains the maximum amount of solute that can dissolve at a given temperature and pressure.
3. A supersaturated solution is a solution that contains more solute than it would normally be able to dissolve at a given temperature and pressure.

Question 3

How does the solubility of a solid and gas gets affected by

1. Increase in temperature?
2. Increase in pressure?

Answer

1. For a solid solute, the solubility generally increases with an increase in temperature. For a gas solute, the solubility generally decreases with an increase in temperature.
2. For a gas solute, the solubility increases with an increase in pressure. For a solid solute, the effect of pressure is negligible.

Question 4

Differentiate between :

1. Solution and suspension
2. Suspension and colloid

Answer

(a) Differentiate between Solution and suspension

(b) Differentiate between Suspension and colloid

Question 5

Define ‘water of crystallisation’. Give two examples with formulae.

Answer

Water of crystallization refers to water molecules that are incorporated into the crystal structure of a compound during its formation.

Two examples of compounds with water crystallization are:

1. CuSO₄·5H₂O – Copper(II) sulfate pentahydrate
2. Na₂CO₃·10H₂O – Sodium carbonate decahydrate

Question 6

Give two examples for each of the following:

1. Hydrated substances
2. Crystalline anhydrous substances
3. Drying agents
4. Deliquescent substances
5. Efflorescent substances
6. Colloids
7. Solvents other than water.

Answer

(a) Examples of hydrated substances:

• CuSO₄·5H₂O – Copper(II) sulfate pentahydrate
• CaCl₂·2H₂O – Calcium chloride dihydrate

(b) Examples of crystalline anhydrous substances:

• Na₂SO₄ – Anhydrous sodium sulfate – Na₂SO₄
• MgSO₄ – Anhydrous magnesium sulfate

(c) Examples of drying agents:

• CaO – Calcium oxide
• SiO₂·nH₂O – Silica gel

(d) Examples of deliquescent substances:

• NaOH – Sodium hydroxide
• KOH – Potassium hydroxide

(e) Examples of efflorescent substances:

• Na₂CO₃·10H₂O – Sodium carbonate decahydrate
• MgSO₄·7H₂O – Magnesium sulfate heptahydrate

(f) Examples of colloids:

• Milk
• Blood

(g) Examples of solvents other than water:

• Ethanol
• Acetone

Question 7

What do you observe when :

1. Blue vitriol is heated?
2. Washing soda is exposed to air?
3. Blue litmus solution is added to water?

Answer

1. When blue vitriol (copper sulfate) is heated, it loses its water of crystallization and turns from blue to white.
2. When washing soda (sodium carbonate decahydrate) is exposed to air, it effloresces and loses its water of crystallization, turning into a white powder.
3. When blue litmus solution is added to water, it remains blue as water is neutral and does not affect the blue litmus.

Question 8

Give reason:

1. Silica gel pouches are kept in unused water bottles.
2. Table salt becomes moist during the rainy season.
3. On opening a bottle of a cold drink, a fizzing sound is heard.

Answer

1. Silica gel pouches are kept in unused water bottles to absorb any moisture that may be present in the bottle, as moisture can promote the growth of bacteria and other microorganisms.
2. Table salt becomes moist during the rainy season because of the high humidity in the air. The moisture in the air can dissolve some of the salt, leading to the formation of a moist layer on the surface.
3. On opening a bottle of a cold drink, a fizzing sound is heard due to the release of carbon dioxide gas, which is dissolved in the liquid under high pressure. When the pressure is released upon opening the bottle, the gas bubbles out, causing the fizzing sound.

Question 9

Give balanced chemical equations for the reaction of water with

1. Sodium
2. Iron
3. Carbon dioxide
4. Sodium oxide

Answer

1. 2Na + 2H₂O → 2NaOH + H₂
2. 3Fe + 4H₂O → Fe₃O₄ + 4H₂
3. CO₂ + H₂O → H₂CO₃
4. Na₂O + H₂O → 2NaOH

Question 10

What is the metal activity series?

Answer

The metal activity series is a list of metals arranged in order of their reactivity, with the most reactive metals at the top and the least reactive metals at the bottom.

Question 11

Name the gas produced when

1. steam is passed over hot coke.
2. chlorine is dissolved in water and exposed to sunlight.
3. a piece of calcium is added to water.
4. when fossil fuel is burnt.

Answer

1. The gas produced when steam is passed over hot coke is hydrogen gas (H₂).
2. The gas produced when chlorine is dissolved in water and exposed to sunlight is oxygen gas (O₂) and chlorine gas (Cl₂).
3. The gas produced when a piece of calcium is added to water is hydrogen gas (H₂).
4. The gas produced when fossil fuel is burnt is carbon dioxide (CO₂), water vapour (H₂O), and other gases such as nitrogen (N₂) and sulfur dioxide (SO₂), depending on the type of fuel being burnt.

EXERCISE – III

Question 1

Define :

1. Soft water
2. Hard water

Answer

1. Water is said to be soft water if it readily gives foam with soap.
2. Water is said to be hard water if it does not readily give foam with soap.

Question 2

(a) Name the compounds responsible for

1. temporary hardness
2. permanent hardness of water

(b) Suggest one method for the removal along with the reactions for

1. temporary hardness
2. permanent hardness of water

Answer

(a) The compounds responsible for hardness

1. The compounds responsible for the temporary hardness of water are calcium hydrogen carbonate (Ca(HCO₃)₂) and magnesium hydrogen carbonate (Mg(HCO₃)₂).
2. The compounds responsible for the permanent hardness of water are calcium sulfate (CaSO₄) and magnesium sulfate (MgSO₄).

(b) Removal of hardness

Question 3

What are the main causes of water pollution? How can it be controlled?

Answer

Main causes of water pollution:

• Industrial waste
• Agricultural runoff
• Sewage discharge
• Oil spills

Ways to control water pollution:

• Proper waste disposal practices
• Enforcing regulations to limit industrial and agricultural pollution
• Treating sewage and wastewater before discharge
• Public education and awareness campaigns promoting responsible water use
• Implementing water conservation measures
• Regular monitoring and testing of water quality.

Question 4

Name three waterborne diseases.

Answer

Three waterborne diseases are:

• Cholera
• Typhoid fever
• Hepatitis A

Question 5

Give reasons:

1. Alcohol is mixed with water and is used in car radiators.
2. Icebergs float on ocean water.
3. Carbonated drinks are bottled under high pressure.

Answer

1. Alcohol is mixed with water and is used in car radiators to lower the freezing point and increase the boiling point of the coolant solution.
2. Icebergs float on ocean water because their density is lower than that of seawater. This is because ice has a crystal structure that leaves more space between its molecules than liquid water.
3. Carbonated drinks are bottled under high pressure to keep the carbon dioxide gas dissolved in the liquid. The high pressure helps to keep the gas in the solution and prevents the drink from going flat.

Objective Type Questions

Question 1

Fill in the blanks :

1. Water has ____ density and ____ volume at 4°C.
2. Freezing mixture contains ____ and ____
3. The solubility of a gas in water ____ with the rise in temperature and ____ with the rise in pressure.
4. ____ is the purest form of natural water by farmers
5. Use of excessive ____ causes water pollution.
6. Boiling removes the ____ hardness of the water.
7. Water turns the colour of anhydrous copper sulphate ____
8. The sticky substance formed when soap is added to hard water is ____

Answer

1. Maximum density, minimum volume
2. Ice, common salt
3. Decreases, increases
4. Rainwater
5. Fertilizers and pesticides
6. Temporary
7. Blue
8. Scum

Question 2

Match the statements in column A with that in column B.

Answer

Question 3

Give one word/words for the following statements:

1. Water fit for human consumption ____
2. The harmful substances dissolved in water ____
3. The change of state of water from one form to another ___
4. The gaseous form of water found in air ____
5. A mixture of common salt and ice ____
6. A substance which does not contain water ____
7. A property due to which a substance absorbs water without dissolving ____
8. Water molecules in loose chemical combination with other substances ____

Answer

1. Potable water
2. Pollutants
3. Phase change
4. Water vapour
5. Freezing mixture
6. Anhydrous
7. Hygroscopicity
8. Hydrates

Multiple Choice Questions

Question 1

Two gases found dissolved in natural water are

1. oxygen and carbon dioxide
2. hydrogen and oxygen
3. sulphur dioxide and hydrogen
4. chlorine and ammonia

Answer

oxygen and carbon dioxide

Explanation:

Two gases found dissolved in natural water are oxygen and carbon dioxide.

Question 2

The temporary hardness of water can be removed by

1. filtering
2. boiling
3. loading
4. none of the above

Answer

boiling

Explanation:

The temporary hardness of water can be removed by boiling.

Question 3

The ultimate source of all water on the Earth is

1. oceans and seas
2. spring and wells
3. rivers and lakes
4. rain

Answer

rain

Explanation:

The ultimate source of all water on the Earth is rain.

Question 4

Colloids have a particle size range between

1. 10^-7 to 10^-10 m
2. 10^-10 to 10^-12 m
3. 10^-7 to 10^-5 m
4. 10^-12 to 10^-15 m

Answer

10^-7 to 10^-10 m

Explanation:

Sizes of the colloid particulars: 10^-7 to 10^-10 m in diameter.