Water Class-8 Dalal Simplified ICSE Chemistry Solutions Chapter-8

Water Class-8 Dalal Simplified ICSE Chemistry Solutions Chapter-8, Water Dr Viraf J Dalal Middle School Allied Publishers Solutions. Chapter-8. We Provide Step by Step Solutions to Questions and Answers of Dr Viraf J Dalal Middle School Chemistry Allied Publishers. Visit official Website CISCE for detail information about ICSE Board Class-8.

Water Class-8 Dalal Simplified ICSE Chemistry Solutions Chapter-8

Board	ICSE
Class	8th
Subject	Chemistry
Book Name	Dalal New Simplified
Chapter-8	Water
Unit-1	Water
Topic	Solution of exercise questions
Session	2023-24

Exercise-1

Water Class-8 Dalal Simplified ICSE Chemistry Solutions Chapter-8

Question: 1. Name a ‘pure form’ and an ‘impure form’ of water in the natural state.

Answer: Rain water – pure natural water.

Sea water, lake water – impure form of natural water.

Question: 2. State what is potable water. Give its qualities or characteristics which make it fit for human consumption.

Answer: Potable water is water which is fit for human consumption & for drinking purposes.

Characteristics of potable water which make it fit for human consumption :

1. Potable water should be clear, colourless & odourless.

2. It should be free from harmful bacteria, germs, suspended impurities & harmful salts such as nitrates, nitrites, cyanides & urea.

3. It should contain small amounts of dissolved gases & minerals which give it a good taste. Dissolved air & salts of sodium, calcium, magnesium etc. impart taste to the water & are useful for metabolic functions of the human body.

Question: 3. Give a reason why water is called a ‘universal solvent’ but, an alkali is not.

Answer: Water is called a ‘universal solvent’ because it has the ability to dissolve a wide variety of substances. On the other hand, an alkali is not a universal solvent because it can only dissolve certain types of substances, specifically acids and some other specific compounds.

Question: 4. Name the solute, solvent & solution in the statement – ‘sodium chloride dissolves in water to give sodium chloride solution’. Define each of the terms in italics.

Answer: Solute — potassium chloride

The substance which dissolves or disappears in the solvent i.e., liquid to form a solution is called a solute.Solvent — waterThe liquid or medium of dissolution which allows the solute to dissolve in it so as to form a solution is called a solvent.

Solution — potassium chloride solution.

A homogeneous mixture of a solute in a solvent is called a solution.

Question: 5. Explain how each of the following factors affects the formation of a solution

(a) Proportion of the solute & the solvent
(b) Particle size of the solute
(c) Temperature of the solvent

Answer: (a) Proportion of solute & solvent — The solute must be in a small amount compared to the solvent. To dissolve the solute, the solvent must be in a much greater amount.

(b) Particle size of the solute — The solute must be finely powdered and not be present as large crystals. This initiates quicker & complete dissolution of the solute in the solvent.

(c) Temperature of the solvent — The solvent may be slightly heated since the solute may not be completely soluble in the solvent at ordinary temperatures.

Question: 6. Define the term ‘solubility ’ of a solute in water. If 10 g. of a solute is added to 100 g. of water at 50°C and stirred, it is seen that the total solute dissolves at that temperature giving an unsaturated solution. Is 10 g, the solubility of the solute in water. Give reasons.

Answer: Solubility of a solute in water is defined as the maximum amount of solute in grams that will saturate 100g. of water at t°C.

No, 10g is not the solubility of the solute as the solution is unsaturated. Solubility will be attained when the solution is saturated.

Question: 7. Give a reason why –

(a) Alcohol and water form a miscible mixture while oil & water do not.

(b) Boiled water tastes flat,

(c) On opening a bottle of soda [carbon dioxide dissolved in water] the gas escapes out with a ‘fizz’.

Answer: (a) As alcohol is soluble in water hence they form a miscible mixture whereas oil is not soluble in water hence they both form an immiscible mixture.

(b) As soluble gases in water contribute to the taste of water and on boiling, the temperature of the water increases, thereby the solubility of the dissolved gases decreases and the dissolved gases are expelled out. Hence, boiled water tastes flat.

(c) Soda bottles contains carbon dioxide dissolved in water under pressure. On opening the bottle, the pressure decreases hence solubility of dissolved carbon dioxide decreases & the gas escapes out with a ‘fizz’.

Question: 8. Give the importance of-

(a) Dissolved minerals & salts
(b) Dissolved air – in water.

Answer: (a) Dissolved minerals & salts in water — Provides minerals for living bodies for their growth & imparts taste to water.

(b) Dissolved air [O₂, CO₂, & NO₂] in water Solutions — like :

• Dissolved O₂ is useful for respiration in aquatic plants and animals.
• Dissolved CO₂ for photosynthesis
• Dissolved NO₂ is converted into nitrogenous compounds by bacteria and provides nourishment for plants.

Question: 9. How are solutions generally classified. What is the basis of this classification.

Answer: Solutions are generally classified as:

(a) True solutions

(b) Colloidal solutions

(c) Suspensions

The basis of above classification is size of the dissolved or dispersed particles in the medium.

Question: 10. Draw a table to differentiate – True solutions, colloidal solutions & suspensions – with respect to

1. Type of mixture

2. Solute particles in the mixture

Answer:

Category	True solutions	Colloidal solutions	Suspensions
Type of mixture	Homogenous mixtures, clear & transparent in nature	Heterogenous mixtures, turbid in nature	Heterogenous mixtures of undissolved particles in dispersion medium.
Solute particles in the mixture	Dispersed substances – Can pass through filter paper. -Can pass through semipermeable membrane.	Dispersed substance – Can pass through filter paper. – Cannot pass through semipermeable membrane.	Dispersed substances – Cannot pass through filter paper. -Cannot pass through semipermeable membrane.

Question: 11. Give four examples of each of the above types of solutions i.e. true solution, colloidal solution & suspension. Give a reason why a true solution does not exhibit ‘Tyndall effect’.

Answer: (a) True solution : Salt solution, vinegar, copper sulphate solution, sugar in water solution, air, brass.
(b) Colloidal solution : Milk, ink, blood, soap solution, starch sol.
(c) Suspension : Milk of magnesia, flour in water, chalk- water solution, muddy water. A true solution cannot exhibit ‘Tyndall effect’ because ‘the size of particles is very small and cannot reflect light falling on them.”

Question: 12. If 5 g. of a solute is added to 50 ml. of a solvent at a particular temperature and the solution can dissolve more of the solute at that temperature – is the solution obtained a saturated or an unsaturated solution. Give reasons.

Answer: As more of solute can be dissolved at that temperature, it is an unsaturated solution. A saturated solution cannot dissolve more of the solute at that particular temperature.

Question: 13. Explain the meaning of the term ‘saturated solution’. State two methods to convert a saturated solution to an unsaturated solution.

Answer: A saturated solution is one which cannot dissolve more of the solute at a given temperature.

Two methods to convert a saturated solution to an unsaturated solution are:

1. By heating the saturated solution slowly it can dissolve more of the solute at a higher temperature.

2. By adding more solvent to the saturated solution. Increased amount of solvent can dissolve more of the solute at that temperature.

Question: 14. State how a ‘supersaturated’ solution differs from a ‘saturated solution’. State briefly how you would prepare a supersaturated solution using potassium chloride and water.

Answer: Super saturated solution contains more amount of solute than saturated solution in hot solution. On cooling, it gives out excess salt which separate out.

Preparation of supersaturated solution of potassium chloride in water:
Take 100 g of water and add little by little of KCl salt and stir it, if it gets dissolved, add a little more and stir it. We find that 35 g of KCl can make the solution saturated at 20°C. Now heat the saturated solution and add more of KCl. At 40°C it will dissolve 40 g and 46 g at 60°C and 54 g at 80°C. This super saturated solution at 80°C. On cooling this super saturated solution excess of solute [KCl] crystals separates out.

Question: 15. The process of separation or deposition of crystals from a hot saturated solution on gentle cooling of the solution is called ‘crystallisation’. With reference to the statement explain what is meant by ‘water of crystallisation’. State which physical properties of a crystal are correlated with the water of crystallisation.

Answer: Water of crystallisation — When a crystal is separated or deposited from a hot saturated solution on gentle cooling of the solution, it generally unites with a definite number of water molecules and enters into a loose chemical combination with them. This definite number of water molecules is called water of crystallisation.

The geometric shape and colour of the crystal is correlated with water of crystallization.

Question: 16. Differentiate between ‘hydrated’ & ‘anhydrous’ crystals. Give the chemical formula of – the following hydrated crystals –

(a) Washing soda
(b) Gypsum
(c) Blue vitriol
(d) Epsom salt
(e) Glauber’s salt. State

Answer: Hydrated crystals : Contain definite number of water molecules in loose chemical combination with the crystals.

Anhydrous crystals : Do not contain any definite number of water molecules – in loose chemical combination with the crystals.

Formula of hydrated crystals:

(a) Washing soda : CaCO₃10H₂O
(b) Gypsum : CaSO₄.2H₂0
(c) Blue vitriol : CuSO₄.5H₂0
(d) Epsom salt : MgSO₄.7H₂0
(c) blue vitriol [CuSO₄.5H₂0] is a pentahydrate.

Question: 17. Differentiate between an efflorescent, deliquescent & ‘ hygroscopic substance with suitable examples.

Answer: Deliquescent substances:

1. Absorb moisture from atmosphere, absorb water and dissolve in it forming a saturated solution.
2. Deliquescence is maximized in dry conditions,
3. NaOH, KOH, CaCl₂, MgCl₂, FeCl₃ are the examples.
4. They are crystalline water soluble.
   FeCl₃ → FeCl₃ [Crystal changes to Solution]

Hygroscopic substances:

1. They are amorphous solids or liquids.
2. Absorb moisture from the air of atmosphere.
3. Do not change their original state.
   CaO (quick lime), cone. H₂SO₄, P₂O₅ and silica gel are the examples.

Conc.Conc.H2SO4Conc.⟶H2SO4Conc.

(State not changed)

is used for drying gases in laboratory.

Question: 18. Differentiate the function of concentrated sulphuric acid as a drying agent & as a dehydrating agent. Is fused calcium chloride a dehydrating or a desiccating agent. Give reasons.

Answer:

Drying agent	Dehydrating agent
Conc. sulphuric acid [as a drying agent] – absorbs only moisture from the substance and makes it dry without changing it’s composition.	Conc. sulphuric acid [as dehydrating agent] – removes water molecule from the composition of substance and reacts chemically and produces a new substance with new properties.
As a drying agent, conc. sulphuric acid is used to dry gases which are acidic in nature like chlorine, hydrogen chloride, etc.	As a dehydrating agent, conc. sulphuric acid can remove chemically combined water molecules from compounds like blue vitriol [CuSO4.5H2O] and glucose [C6H12O6].

Fused calcium chloride is deliquescent in nature, absorbs moisture and hence used as a desiccating agent.

Question: 19. Give a reason why metals are arranged in a series called – ‘activity series of metals’. Name a metal which reacts with

(a) Cold water
(b) Boiling water
(c) Steam to liberate hydrogen in each case.

Answer: Depending upon the reactivity of metals with water, they are arranged in a series called – ‘activity series of metals’ in a way such that the most reactive metal is at the top of the series, and the least reactive at the bottom.

(a) Potassium (K)(b) Magnesium (Mg)(c) Iron (Fe)

Question: 20. Differentiate between – (a) Hard & soft water (b) Temporary & permanent hard water – with suitable examples.

Answer: (a) Differences between Hard and soft water:

Hard water	Soft water
Water is said to be hard when it does not lather readily with ordinary soap.	Water is said to be soft when it lathers readily with ordinary soap.
Hard water contains dissolved calcium & magnesium salts, e.g. calcium and magnesium bicarbonates, sulphates and chlorides.	Soft water does not contain dissolved calcium & magnesium salts.
Sources of hard water include natural water i.e., water from springs and rivers.	Sources of soft water include rain water and distilled water.

(b) Differences between temporary and permanent hard water:

Temporary hard water	Permanent hard water
Temporary hard water is one whose hardness can be removed by boiling.	Permanent hard water is one whose hardness cannot be removed by boiling.
It is due to the presence of calcium & magnesium bicarbonates Ca(HCO3)2, Mg(HCO3)2.	It is due to the presence of calcium, magnesium, chlorides and sulphates [CaCl2, MgCl2, CaSO4, MgSO4].

​

Question: 21. Give balanced equations to show how –

(a) Temporary hardness enters into water,

(b) Temporary hardness in water can be removed by boiling,

(c) Permanent hardness in water can be removed by addition of washing soda.

Answer: 

– : End of Water Class-8 Dalal Simplified Solutions :–

Return to –  Dalal Simplified Chemistry for ICSE Class-8 Solutions

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