Class 8 SELINA Solutions Chemistry Chapter 8: Water

Correct option: (b) rain water

Solution:

The purest form of water is rainwater.

It is formed naturally by evaporation followed by condensation of water vapour in the atmosphere.

Correct option: (c) 1:8

Solution:

In water,

Correct option: (b) dissolved common salt

Solution:

Sea water contains a large amount of dissolved common salt.

Correct option: (d) condensation

Solution:

Water returns on earth’s surface from the atmosphere by the process of condensation.

Correct option: (c) underground water

Solution:

Spring water is the source of underground water.

The four sources of natural water are as follows:

1. Oceans and seas

2. Rivers and lakes

3. Springs and wells

4. Rainwater

The change of water from one state to another in nature, resulting in its continuous circulation from the earth's surface to the atmosphere and from the atmosphere back to the earth's surface, is called the water cycle.

Water is considered a renewable resource on the earth. It is capable of existing in all the three states of matter. From one state, it changes into another state easily.

This interchange of water from one state to another occurs constantly in nature. It involves evaporation, condensation, and precipitation (rainfall). This process of interconversion helps balance the amount of water in the atmosphere and on the earth's surface, thus supporting plant and animal life.

The natural processes by which water is added to the atmosphere are :

• Evaporation of water from oceans, seas, rivers, etc.

• Combustion, i.e. burning of fuels.

• Respiration by all living beings and transpiration by plants.

Gases from which water is formed: Hydrogen and oxygen

Chemical composition of hydrogen and oxygen gases in water by mass: 2:1 of hydrogen to oxygen by mass.

Molecular formula: H₂O

(a)  Water needs a large amount of heat to become hot and more time to lose a large amount of heat to become cold. Thus, water neither heats up nor cools down quickly. Due to this property, water is an excellent cooling agent. It is a coolant in car engines, nuclear reactors, factories, etc. It absorbs the heat produced in an engine, machine, or part of a factory and prevents it from becoming overheated.

(b)  When water is cooled, it contracts and its density increases, but only until the temperature reaches 4 °C. On further cooling, water starts expanding, with a decrease in its density. This phenomenon is termed an anomalous expansion of water. In the colder parts of the world, the temperature is below 0 °C. The water present in the pipes expands during freezing. As a result, ice exerts huge pressure on the pipes, and they burst.

(c)   The boiling point of water decreases with a decrease in pressure and increases with an increase in pressure. Therefore, water boils at a lower temperature in the hilly areas, where the atmospheric pressure is lower than in the plains, so cooking food in hilly regions takes longer.

(d)   When water is cooled, it contracts and its density increases, but only until the temperature reaches 4 °C. On further cooling, water starts expanding, with a decrease in its density. This phenomenon is termed an anomalous expansion of water. Thus, the solid form of water, i.e., ice, is lighter than its liquid form. Hence, ice floats on water.

(e)  When the water is cooled down, it contracts and its density increases, but only until the temperature reaches 4 °C. On further cooling, water starts expanding, with a decrease in its density. This phenomenon is termed an anomalous expansion of water. Thus, the solid form of water, i.e., ice, is lighter than its liquid form. Hence, ice floats on water. Thus, anomalous expansion of water is an excellent boon to aquatic life in cold regions. When the temperature falls, the topmost layer of water cools and freezes, while the bottom layers remain liquid at 4 °C. The surface changes into ice, which floats over the water below. Ice is a poor conductor of heat and acts like a blanket that protects the bottom layers of the water from the cold air above and refrains it from freezing.

(f)    When the water is cooled down, it contracts and its density increases, but only until the temperature reaches 4 °C. On further cooling, water starts expanding, with a decrease in its density. This phenomenon is termed an anomalous expansion of water. Thus, the solid form of water, i.e., ice, is lighter than its liquid form. Hence, ice floats on water. Thus, anomalous expansion of water is an excellent boon to aquatic life in cold regions. When the temperature falls, the topmost layer of water cools and freezes, while the bottom layers remain liquid at 4 °C. The surface changes into ice, which floats over the water below. Ice is a poor conductor of heat and acts like a blanket that protects the bottom layers of the water from the cold air above and refrains it from freezing. This is how fish and other living organisms live inside lakes during winter.

(g)  Water needs a large amount of heat to become hot and more time to lose a large amount of heat to become cold. Thus, water neither heats up nor cools down quickly.

a. The boiling point of water increases with an increase in pressure.

b. An impurity present in water raises its boiling point.

Importance of Water:

Water is needed in adequate amounts in or daily life for different purposes. It is important for:

1.      drinking in order to satisfy our biological needs.

2.      providing medium for all biochemical reactions inside our bodies to take place as water has the ability to dissolve a number or substances.

3.      regulation of body temperature.

4.      everyday uses like cooking, cleaning, bathing and washing, etc.

5.      growing crops, the germination of seeds in agriculture and in photosynthesis by plants.

6.      providing nutrients to aquatic plants and animals.

7.      controlling the earth's climate.

8.      various manufacturing processes.

9.      generating electricity.

P.S. : Write any four points.

Anomalous expansion of Water (Relative Volume and density of Ice and Water).

Water's solid form (ice) is lighter than its liquid form. Its density is maximum at 4°C. This is an anomalous phenomenon due to water's anomalous expansion.

Freezing lakes The anomalous expansion of water is a great boon to aquatic life in cold regions. When the temperature falls, the water at the surface changes into ice, which floats over the water below. Ice is a poor conductor of heat and acts like a blanket that protects the bottom layers of the water from the cold air above. This is how fish and other living organisms live inside lakes during winter.

Correct option: (d) chlorine and ammonia

Solution:

The gases dissolved in natural water are chlorine and ammonia.

Correct option: (a) 10^-7 to 10^-10 m

Solution:

Colloids have the particle size ranging between 10^-7 to 10^-10 m.

Correct option: (c) Blue vitriol

Solution:

The chemical formula of blue vitriol is CuSO₄.5H₂O.

Correct option: (a) Silica gel

Solution:

Silica gel absorbs moisture and is hence used as a drying agent.

Correct option: (a) hydrogen and oxygen

Solution:

Electrolysis of water can be represented by the following reaction:

(a) Water gas

(b) Oxygen gas

(c) Hydrogen gas

(d) Carbon dioxide gas

(a) Solution: A solution is a homogeneous mixture of two or more substances whose composition can be varied.

(b) Solute: A solute is a substance

(c) Solvent: A solvent is a medium in which a solute dissolve.

1. Unsaturated solution: A solution in which more of the solute can be dissolved at a given temperature is called an unsaturated solution.
2. Saturated solution: A solution that cannot dissolve anymore of the solute at a given temperature is called a saturated solution.
3. Supersaturated solution: A saturated solution that contains more than it can hold at room temperature is called supersaturated solution.

The fixed amount of water, which is in a loose chemical combination with one molecule of a substance in its crystal form, is “water of crystallisation”.

(a)  Hydrated substances: Blue vitriol (CuSO₄.5H₂O), Washing soda (Sodium carbonate decahydrate Na₂CO₃.10H₂O).

(b)  Crystalline anhydrous substances: Common salt (NaCl), Sugar (C₁₂H₂₂O₁₁).

(c)   Drying agents: Silica gel, Concentrated sulphuric acid.

(d)  Deliquescent substances: Caustic soda (NaOH), Iron (III) chloride (FeCl₃).

(e)  Efflorescent substances: Glauber’s salt (Na₂SO₄.10H₂O), Epsom salt (MgSO₄.7H₂O)

(f)   Colloids: Milk, Fog.

(g)  Solvents other than water: Ethanol, Acetone.

1. When blue vitriol (CuSO₄.5H₂O) or hydrated Copper sulfate solution is heated, it loses its water of crystallisation and becomes anhydrous.
   
2. Washing soda is efflorescent. It loses water of crystallisation when exposed to air. Thus, while crystalline washing soda changes into white powder on exposure to air.
3. Litmus is a visual acid-base indicator. Blue litmus changes to red in an acidic solution and red litmus changes to blue in an alkaline solution. When blue litmus solution is added to water, the colour of the litmus solution does not change since water is neutral.

The reactions of metals with water under different conditions indicate their reactive nature, enabling us to arrange them based on their reactivity, which is called a metal activity series.

Here, metals are arranged in decreasing order of their reactivity, with the most reactive metals at the top and the least reactive at the bottom of the series.

(a)  Sodium with water:

(b)  Iron with water:

(c)   Carbon dioxide with water:

CO₂ + H₂O  ⟶     H₂CO₃

(d)  Sodium oxide with water:

       Na₂O + H₂O   ⟶  2NaOH

(a)  Solution and suspension

(b)  Suspension and colloid

1. Water is often termed the universal solvent because it dissolves almost all kinds of solids, liquids and gaseous substances. However, the amount of a substance that is dissolved may vary.
2. Silica gel pouches are commonly used to absorb moisture and keep things dry. They are often kept in unused water bottles, with camera lenses, shoe boxes, loose tablets, etc., to keep them dry. These pouches are ideal to reuse throughout the home in places with excess moisture.
3. On exposure to air, table salt (NaCl) turns moist and ultimately forms a solution, especially during the rainy season, because it contains deliquescent impurities like magnesium chloride and calcium chloride.
4. When the cold drink bottle is opened, dissolved carbon dioxide escapes into the atmosphere. As the gas escapes into the atmosphere, its partial pressure decreases, and its solubility also decreases, which results in carbon dioxide bubbles forming at the surface of the drink. Hence, gas comes out with a fizz.

Correct option: (a) Hydrated

Solution:

The salt which gives water vapour on heating is called Hydrated salt.

Example: Hydrated copper salt

Correct option: (a) Calcium sulphate

Solution:

Water with sulphates and chlorides of calcium and magnesium dissolved in it is called permanent hard water.

Correct option: (b) Sodium carbonate

Solution:

Sodium carbonate can remove the permanent hardness of water.

Correct option: (b) boiling

Solution:

The temporary hardness of water can be removed by boiling.

(a) maximum, minimum

(b) ice, salt

(c) decreases, increases

(d) Rainwater

(e) fertilizers

(f) temporary

(g) blue

(h) scum

(a) False – Soft water forms lather easily with soap.

(b) False – The permanent hardness of water is due to sulphates and chlorides of calcium and magnesium dissolved in it.

(c) True

(d) False – We get metal hydroxide if a metal reacts with cold water. If it reacts with steam, the metal oxide is formed. This is because the metal hydroxide thermally decomposes, or it splits upon heating to give the oxide and water. So, the correct statement would be, ‘The reaction of steam with magnesium produces hydrogen gas and magnesium oxide’.

Mg + H₂O ⟶ MgO + H₂(g)

(e) True

(a) Potable water

(b) Impurities

(c) Water cycle

(d) Water vapours

(e) Freezing mixture

(f) Anhydrous substances

(g) Hygroscopic

(h) Water of crystallization

(a)  Soft water - Water drawn from certain natural sources that easily forms a lather with soap is called soft water.

(b)  Hard water - Water drawn from certain natural sources that do not easily form a lather with soap, but form a white sticky scum or a precipitate, is called hard water.

Water-borne diseases – typhoid, hepatitis, cholera, diarrhoea, dysentery.

(a) Alcohol is mixed with water and is used in car radiators because it lowers the freezing point of water.

(b) Carbonated drinks are bottled under high pressure because the solubility of carbon dioxide increases with pressure.

i. Bicarbonates of calcium and magnesium

ii. Sulphates and chlorides of calcium and magnesium.

     i.            Method to remove temporary hardness of water:

Boiling: This method helps to remove only the temporary hardness of water. When temporary hard water is boiled, calcium and magnesium bicarbonates break up to form their respective insoluble carbonates. These can be filtered out so that water becomes soft.

  ii.            Method to remove permanent hardness of water:

By adding sodium carbonate (washing soda): When water is treated with a small quantity of sodium carbonate, its permanent hardness is removed. The carbonate reacts with the soluble chlorides and sulphates of calcium and magnesium to form their respective insoluble carbonates. These can be removed by filtration, and then the water becomes soft. Sodium sulphate or sodium chloride formed after the reaction does not affect the soap.

Causes of Water Pollution:

1. Chemical wastes of industrial and agricultural processes Water is required in large quantities in industries and agriculture for different purposes. But often, care is not taken to keep the water fit for consumption. Most industries dump their wastes in rivers and even the sea without prior treatment. Fertilisers, pesticides, insecticides, and other chemicals used in cultivated fields are dissolved in water when they flow across these fields. Acidic water from mines and acid rain also pollute the water in rivers and seas. These chemicals are dangerous for aquatic life, making the water unfit for human consumption.
2. Thermal wastes from nuclear and thermal power plants: Power plants use large amounts of water for cooling. They discharge the resultant hot water, often containing chemicals, into water streams. This increases the temperature of water, which is injurious to fish and other aquatic life forms.
3. Sewage and garbage: Discharging sewage and garbage into river water is another major cause of water pollution. Water becomes unsafe for drinking and other uses when people urinate, defecate, or wash in it.