Selina ICSE Class 8 Chemistry Solutions Chapter 8 Water

Selina ICSE Solutions

Exercise – I

Q1. Name the four main sources of water.
(i) Oceans
(ii) Seas
(iii) Rivers
(iv) Lakes

Q2. State the importance of water cycle in nature.
(i) Water cycle helps in regulating weather on the earth.
(ii) Water cycle makes water available in various forms on the earth.

Q3. Why is water very precious for all living beings?
One of the most important elements for the continuation of life is water. Water is appropriately referred to as the source of life since it is essential for all living things to live and because life on earth first emerged in the oceans. 90% of human blood is made up of water, which makes up a significant portion of all living things’ bodily mass water. Numerous compounds can be dissolved by water. Consequently, it acts as the liquid. All internal biological reactions occur in a medium. Water is present in plants and fruits. Even seemingly dry materials like wood, peas, beans, grams, etc., contain some amount of water.

Q4. 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?
The two gases which formed water are Hydrogen and Oxygen. The chemical composition of these two in water is 11.11% and 88.89% respectively. Molecular formula is H2O.

Q5. What is the effect on boiling point of water when
(a) Pressure is increased
(b) Impurity is added
(a) As pressure rises, water’s boiling point rises as well.
(b) Water with impurities has a lower freezing point and a higher boiling point. For instance, salt is used to lower the melting point of ice. A freezing combination is one such mixture. About -15° C. is the freezing mixture, which is the melting point.

Q6. 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.
(a) The specific heat of water is high. Water does not warm up or cool down rapidly. This characteristic makes water a superior cooling agent.
(b) Water pipes freeze and expand, causing them to burst during particularly harsh winters.
(c) Because the atmospheric pressure is lower in the hills than it is in the plains, water boils at a lower temperature there. This explains why cooking takes longer in hilly areas.
(d) Ice has a lower density than water. At 4°C, water has its highest density. Ice floats on water due to this reason.
(e) The dissolved contaminants in sea water raise the freezing point. Sea water does not freeze at 0°C for this reason.

Q7. How does anomalous expansion of water help aquatic organisms in cold climates
Water species may survive in extremely cold temperatures thanks to the anomalous expansion of water. Water initially becomes heavier and sinks when the temperature drops. Up till 4°C, this process continues. After which this enlargement occurs. Water that is on the surface freezes. Due to ice’s poor thermal conductivity, heat cannot be lost from the water below, ensuring the existence of aquatic life.

Exercise – II

Q1. Explain the terms:
(a) Solution
(b) Solute
(c) Solvent
A solution is defined as “a homogenous mixture of two or more components that can be modified.”
(a) Solute: A solute is a substance that dissolves in another substance to form a solution. In a solution, solute amounts are less. Solute is defined as “Substance which is dissolved in solvent.” In a solution, solute amounts are less. A solvent is a substance that allows a salute to dissolve. It is present in solution in significant amounts.
Solvent + Solute = Solution

Q2. What is meant by
(a) Unsaturated
(b) Saturated and
(c) Supersaturated solutions.
(a) Unsaturated solutions – A solution is considered unsaturated if more of the solute can dissolve in it at a specific temperature.
(b) Saturated solutions – At a certain temperature, a solution is said to be saturated when it can dissolve no more of the solute.
(c) Supersaturated solutions – A supersaturated solution is one that contains more solute than it can store at ambient temperature.

Q3. How do the solubility of a solid and a gas affected by –
(a) Increase in temperature
(b) Increase in pressure
(a) A solid solute normally becomes more soluble as temperature rises. This enables the creation of supersaturated solutions. A gas becomes less soluble as its temperature rises.
(b) The solubility of a substance in water is mostly unaffected by pressure. In the case of gases, an increase in pressure causes an increase in the amount of a gas dissolved in water.

Q4. Differentiate between:
(a) Solution and suspension
(b) Suspension and colloid

(a) Suspension and solution

(b) Colloids and suspension

Q5. Define: ‘water of crystallization’. Give two examples with formulae.
The fixed volume of water that forms a loose chemical bond with a salt inside its crystal is referred to as crystallization water. Examples:

Q6. 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
(a) Examples of Hydrated substances are Washing soda (Na2SO4) and Glauber’s salt (10H2O).
(b) Examples of Crystalline anhydrous substances are Sugar (C12H22O11), Potassium nitrate (KNO3), and table salt (NaCl)
(c) Drying agents are Quicklime (CaO), phosphorus pentoxide (P2O5), and Concentrated sulphuric acid (H2SO4) (CaO)
(d) Deliquescent substances are Caustic soda (NaOH), calcium chloride (CaCl2), iron (III) chloride, crystalline magnesium chloride (MgCl2).
(e) Efflorescent substances are Glauber’s salt and washing soda (Na2SO4.10H2O)
(f) Colloids are milk, blood, ink, smoke, jellies, and butter.
(g) Solvents other than water are Acetone, ethanol, turpentive.

Q7. What do you observe when?
(a) Blue vitriol is heated?
(b) Washing soda is exposed to air?
(c) Blue litmus solution is added to water?
(a) Blue vitriol has a blue colour because it has five molecules of the crystallization water CuSO4H2O in it. It turns into a hydrous CuS04, which is a grey-white substance when heated because it loses the water that crystallization relies.
(b) Washing soda (Na2CO3.10H2O) is a white crystalline material that turns into white powder when exposed to air.
(c) The colour of the blue or red litmus solution does not change when handled with water because pure water is neutral to litmus.

Q8. Give reason:
(a) Silica gel pouches are kept in unused water bottles.
(b) Table salt becomes moist during rainy season.
(c) On opening a bottle of a cold drink, a fizz sound is heard.
(a) Silica gel pouches are frequently used to wick away moisture and preserve items’ dryness. To keep them dry, they are frequently placed in empty water bottles, along with camera lenses, etc. These bags are great to have around the house in locations where there is an excess of moisture.
(b) Because table salt (NaCl) contains deliquescent impurities like magnesium chloride and calcium chloride, it becomes moist when exposed to air and eventually forms a solution, especially during the rainy season. The substance sodium chloride does not melt.
(c) The carbon dioxide in the cold drink bottles is dissolved in them and is bottle under high pressure, which is why when we open a bottle of the beverage, we hear a fizzing sound.

Q9. Give balanced chemical equations for the reaction of water with
(a) Sodium
(b) Iron
(c) Carbon dioxide
(d) Sodium oxide

Q10. What is metal activity series?

The arrangement of metals in the decreasing order of their reactivity in the form of a series is called the activity or reactivity series of metals.

Q11. Name the gas produced when
(a) Steam is passed over hot coke.
(b) Chlorine is dissolved in water and exposed to sunlight
(c) A piece of calcium is added to water.
(d) When fossil fuel is burnt.
(a) Water gas
(b) Oxygen
(c) Hydrogen
(d) Carbon dioxide

Exercise – III

Q1. Define:
(a) Soft water
(b) Hard water
(a) Soft water: Water from various natural sources contains a variety of dissolved materials. Using soap and soft water makes lather easily. Soft water is a term for such water.
(b) Hard water: Hard water is defined as having a significant amount of dissolved minerals (like calcium and magnesium). It doesn’t produce any soap lather.

Q2. (a) Name the compounds responsible for
(i) Temporary hardness
(ii) Permanent hardness of water
(b) Suggest one method for the removal along with the reactions for
(i) Temporary hardness
(ii) Permanent hardness of water

(a) (i) Temporary hard water – Temporarily hard water that has calcium and magnesium bicarbonates dissolved in it. Boiling is an effective way to remove this kind of hardness.
(ii) Permanent hard water – Permanent hard water is water that has calcium and magnesium sulphates and chlorides dissolved in it. Boiling cannot get rid of this hardness.
(b) Removal of hardness of water:
(i) Temporary hardness—
By Boiling— Temporary hardness of the water is reduced boiling. The bicarbonates of calcium and magnesium separate into their insoluble carbonates when temporarily hard water is heated. These can be filtered out so that water becomes soft

(ii) Permanent hardness of water—
By Adding sodium carbonate (washing soda):- When water is treated with a small amount of sodium carbonate, the water’s permanent hardness is eliminated.
It forms their insoluble carbonates by reacting with the soluble chlorides and sulphates of calcium and magnesium. Filtration can get rid of them, which makes the water softer. Sodium sulphate or sodium chloride formed after the reaction does not affect the soap.
        Na2CO3         +           CaSO4   →              CaCO3                +           Na2SO4
(sodium carbonate) (calcium sulphate) (insoluble calcium carbonate) (soluble sodium sulphate)

Q3. Name three water borne diseases.
(i) Cholera
(ii) Typhoid
(iii) Hepatitis

Q4. What are the main causes of water pollution? How can it be controlled?
Below are the main causes of water pollution:
• Chemical waste, industrial waste and agricultural processes.
• Thermal waste from nuclear and thermal power plants.
• Sewage and garbage.
Below are the preventions of water pollution:
• Domestic sewage should be treated before being discharged into rivers.
• Using of non-biodegradable substances like detergents should be stopped.
• Trees and plants must be planted along the banks of rivers.
• Purification of water bodies should be carried out.
• The waste products of industries should be treated before they are discharged into rivers.

Q5. Give reasons:
(a) Alcohol is mixed with water and is used in car radiators.
(b) Icebergs float on ocean water.
(c) Carbonated drinks are bottled under high pressure.
(a) Alcohol is mixed with the water used in car radiators to prevent it from freezing in cold weather. Because it lowers the freezing point of water.
(b) Ice bergs float in ocean water because density of ice is less than water.
(c) Carbonated drinks are bottled under high pressure because the solubility of carbon dioxide increases with pressure.

Objective Type Questions:

1. Fill in the blanks:
(a) Water has __________ density and __________volume at 4°C.
(b) Freezing mixture contains __________ and __________.
(c) The solubility of a gas in water __________ with rise in temperature and __________ with rise in pressure.
(d) __________ is the purest form of natural water.
(e) Use of excessive __________ by farmers causes water pollution.
(f) Boiling removes the __________ hardness of water.
(g) Water turns the colour of anhydrous copper sulphate __________.
(h) Water turns the colour of anhydrous copper sulphate __________.
(a) Water has maximum density and minimum volume at 4°C.
(b) Freezing mixture contains ice and salt .
(c) The solubility of a gas in water increases with rise in temperature and decreases with rise in pressure.
(d) Rain water is the purest form of natural water.
(e) Use of excessive fertilizers by farmers causes water pollution.
(f) Boiling removes the temporary hardness of water.
(g) Water turns the colour of anhydrous copper sulphate blue.
(h) Water turns the colour of anhydrous copper sulphate scum.

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


3. Give one word/words for the following statements:
(a) Water fit for human consumption ____________.
(b) The harmful substances dissolved in water ____________.
(c) The change of states of water from one form to another ____________.
(d) The gaseous form of water found in air – ____________.
(e) A mixture of common salt and ice – ____________.
(f) A substance which does not contain water ____________.
(g) A property due to which a substance absorbs water without dissolving ____________.
(h) Water molecules in loose chemical combination with other substances water of ____________.
(a) Water fit for human consumption – potable water.
(b) The harmful substances dissolved in water – impurities.
(c) The change of states of water from one form to another water cycle.
(d) The gaseous form of water found in air – water vapours.
(e) A mixture of common salt and ice – freezing mixture.
(f) A substance which does not contain water anhydrous substances.
(g) A property due to which a substance absorbs water without dissolving hygroscopic.
(h) Water molecules in loose chemical combination with other substances water of crystallization.

Multiple Choice Questions:

1. Two gases found dissolved in natural water are
(a) oxygen and carbon dioxide
(b) hydrogen and oxygen
(c) sulphur dioxide and hydrogen
(d) chlorine and ammonia
Answer: (a) Oxygen and carbon dioxide

2. Temporary hardness of water can be removed by
(a) filtering
(b) boiling
(c) loading
(d) none of the above
Answer: (b) boiling

3. The ultimate source of all water on the earth is
(a) Oceans and seas
(b) Spring and wells
(c) Rivers and lakes
(d) Rain
Answer: (d) Rain

4. Colloids have the particle size range between
(a) 10-7 to 10-10 m
(b) 10-10 to 10-12 m
(c) 10-7 to 10-5 m
(b) 10-12 to 10-15
Answer: (a) 10-7 to 10-10 m

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