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Selina Concise Chemistry Class 8 ICSE Solutions – Water

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Selina Concise Chemistry Class 8 ICSE Solutions – Water

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APlusTopper.com provides step by step solutions for Selina Concise ICSE Solutions for Class 8 Chemistry Chapter 8 Water. You can download the Selina Concise Chemistry ICSE Solutions for Class 8 with Free PDF download option. Selina Publishers Concise Chemistry for Class 8 ICSE Solutions all questions are solved and explained by expert teachers as per ICSE board guidelines.

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Selina Concise ICSE Solutions for Class 8 Chemistry Chapter 8 Water

Points to Remember:

  • Water is the source of life for all living beings.
  • Water occurs in all the three states of matter i.e. ice, liquid water and water vapours.
  • Water found in nature (i) oceans and seas (ii) rivers and lakes (iii) springs and wells (iv) rain.
  • Rainwater is the purest form of water. Sea water is very impure.
  • Potable water should be free from suspended impurities and harmful germs.
  • Water is a compound, with the molecular formula is H2O.
  • The boiling point of water is 100°C and the freezing point is 0°C.
  • 0°C is also called tripple point, because water can exist in all its three states.
  • The specific heat of water is higher than that of any other liquid. It is used as a cooling agent.
  • Water has minimum volume and maximum density at 4°C. This is called anomalous behaviour of water.
  • Water is an universal solvent. The gases dissolved in water have biological importance. They enable aquatic life to sustain itself.
  • Water may be ‘hard or soft’. Hardness of water can be removed by boiling or by chemical treatment.
  • Water pollution is a serious problem.
  • Industrial and agriculture processes, nuclear and thermal plants pollute water.

Exercise – I

Question 1.
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.
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.
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. 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 crystallisation. Examples:
selina-concise-chemistry-class-8-icse-solutions-water-6

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)
(f) Milk, blood, smoke, jellies, butter, ink etc.
(g) Acetone, ethanol, turpentive

Question 7.
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 ?
Answer:
(a) Blue vitriol is blue in colour as it contains 5 molecules of water of crystallisation (CuSO^HjO). When it is heated, it loses water of crystallisation and becomes an hydrous CuS04 which is grey-white in colour.
(b) Washing soda (Na2CO3.10H2O) is a white crystalline substance and on exposure to air it gets changed to white powder.
(c) Pure water is neutral to litmus which means that no change in the colour of blue or red litmus solution is observed when 1 treated with water.

Question 8.
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.
Answer:
(a) Silica get pouches are very commonly used to absorb moisture and to keep things dry. They are often kept in unused water bottles, with camera lenses etc. to keep them dry. These pouches are ideal to reuse throughout, in places at home where there is excess of moisture.

(b) On exposure to air, table salt (NaCl) turns moist and ultimately forms a solution especially during rainy season because it contains impurities like magnesium chloride and calcium chloride which are deliquescent. Sodium chloride is not deliquescent.

(c) The cold drink bottles contain carbon dioxide and are bottled under high pressure i.e. they contain a large amount of gas dissolved in them and on opening a bottle we hear a fizz sound, this is because of the solubility of CO2 gas in it and pressure in it.

Question 9.
Give balanced chemical equations for the reaction of water with
(a) Sodium (b) Iron
(c) Carbon dioxide (d) Sodium oxide
Answer:
selina-concise-chemistry-class-8-icse-solutions-water-1

Question 10.
What is metal activity series ?
Answer:
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.
selina-concise-chemistry-class-8-icse-solutions-water-2

Question 11.
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,
Answer:
(a) Water gas
(b) Oxygen
(c) Hydrogen
(d) Carbon dioxide

Exercise – III

Question 1.
Define:
(a) Soft water
(b) Hard water
Answer:
(a) Soft water: The water present in different natural sources has different substances dissolved in it. The water drawn from certain sources forms a lather with soap rather easily. Such water is called soft water.
(b) Hard water: Water obtained from various sources does not easily form a lather with soap, rather it forms a white sticky scum or a precipitate. This water is called hard water.

Question 2.
(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
Answer:
(a) (i) Temporary hard water— Water, which has . bicarbonates of calcium and magnesium dissolved in it, is temporary hard water. This kind of hardness is easily removed by boiling.
(ii) Permanent hard water— Water, which has sulphates and chlorides of calcium and magnesium dissolved in it, is called permanent hard water. This hardness cannot be removed by boiling.

(b) Removal of hardness of water:
(i) Temporary hardness—
By Boiling— This method helps to remove only the temporary hardness of water. When temporary hard water is boiled, the bicarbonates of calcium and magnesium break up to form their insoluble carbonates.
These can be filtered out so that water becomes soft
selina-concise-chemistry-class-8-icse-solutions-water-3
(ii) Removal of hardness of permanent hardness of water—
By Adding sodium carbonate (washing soda)
Permanent hardness of water is removed when water is treated with a small quantity of sodium carbonate.
It reacts with the soluble chlorides and sulphates of calcium and magnesium to form their 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.
selina-concise-chemistry-class-8-icse-solutions-water-4

Question 3.
Name three water borne diseases.
Answer:
(i) Cholera (ii) Typhoid (iii) Hepatitis

Question 4.
What are the main causes of water pollution ? How can it be controlled ?
Answer:
Main causes of water pollution are:

  • Chemical waste, industrial waste and agricultural processes.
  • Thermal waste from nuclear and thermal power plants.
  • Sewage and garbage.

Prevention 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.

Question 5.
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.
Answer:
(a) Alcohol is mixed with the water used in car radiators to prevent it from freezing ki 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 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.

selina-concise-chemistry-class-8-icse-solutions-water-7

3. Give one word/words for the following statements:

(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 crystallisation.

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

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

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

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 m

What Is Water Conservation

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Conservation Of Water

India is still not in the ‘water stressed’ category but the day is not far when we will be facing a water crisis. We need to think of various ways to strictly avoid wastage. Since only a small percentage of water on our planet is usable, it is very important that we use water carefully. Conservation of water can be done by building dams, avoiding wastage of water at homes, both indoors and outdoors, harvesting rainwater, and preventing pollution of water. To prevent further shortages, households, communities, and private industries can adopt water saving practices, some of which are given below.

Households can save water by promptly fixing leaks or dripping taps: Although the water lost from leaks may not seem like much, it adds up to quite a lot. A dripping tap can result in a loss of upto 72 litres of water every day.

Recycling of water is very important: We can start this even in our homes. For example, water used in cleaning utensils can be used to water plants in the garden instead of being poured down the drain.

A lot of water is lost during floods: Dams, reservoirs, and embankments will help to check floods and the flow of water.

What Is Water Conservation 1
Floods

By improving the methods of irrigating the fields: Instead of filling up the entire field with water, farmers can be taught the method of watering the roots of plants drop by drop. Israel, which is a water-scarce country, has come up with this innovative technique called drip irrigation. This type of irrigation employs the method of watering the roots of the plants drop by drop. This saves up to one-third of the water that would otherwise be lost during spray applications. Further, it enables farmers to double harvests using the same amount of water. In other countries, such as Egypt, some solutions suggested are improvements in water efficiency and moving away from water-intensive crops.

What Is Water Conservation 2
A typical irrigation dripper

Building Dam
A dam is a structure built to hold back water in order to prevent floods, and to provide water for irrigation and storage. Building a dam is a solution to both drought and flood. Dams are also used in producing electricity. Built on rivers, a dam has high walls and has many openings or gates to both let in and hold back water. During heavy rainfall, when the rivers fill up, water enters the dam. When water is needed later, the gates are opened to let out water.

What Is Water Conservation 3
A dam

Preventing Water Pollution
Another method to conserve water is to safeguard our fresh water bodies from pollution. Garbage and harmful chemicals pollute the water and make it unfit for use. Polluted water is also very bad for aquatic life. Plants and animals in and around polluted water may die or get infected. And when human beings consume the contaminated fish, etc., they are also put at risk of diseases.

What Is Water Conservation 4
Water pollution caused by harmful chemicals and garbage

Avoiding Wastage at Homes
Some ways to avoid wastage of water at homes are given below.

  • Inform the municipality or call a plumber if you spot any leaking pipes in your neighbourhood.
  • Take water in a bucket for your bath instead of using the shower. A bath in a tub uses the largest amount of water. Always remember to shut the tap tight after each use.
  • Do not leave a tap running while brushing your teeth.
  • Instead of using running water for washing utensils, soak them first in one tub or sink and wash them in another, using a thin trickle only to rinse off.
  • Use a wet cloth to clean the car instead of using a hose.
  • Water plants and grass early in the morning so that the water gets absorbed before the sun evaporates it. Watering by hand saves a lot of water as against using a hose.
  • Repair all leaky pipes and taps.
  • Try not to throw away used or extra water down the drain if it can be used for some other purpose, e.g., you could use water remaining in your water bottle to water the plants.

Rainwater Harvesting Systems

The process of collecting and storing rainwater from roofs or a surface catchment is called rainwater harvesting. Storing rainwater that collects on roofs instead of letting it go down the drain, is a practical solution in case of droughts. This technique is known as rooftop rainwater harvesting. This involves collecting rainwater from rooftops in dugout ponds, vessels, or underground tanks to store water for long periods. Another option is to allow water to go into the ground directly from the roadside drains that collect rainwater. The stored rainwater is treated before use because it may contain bird faeces, chemicals, and other pollutants, which need to be removed before use.
These help us to use rainwater directly. These structures are designed to help capture and store rainwater during the monsoon season, and serve as a source of drinking and irrigation water during the rest of the year.

What Is Water Conservation 5
Diagrammatic representation of a typical water harvesting system

In the southern states of India, tanks, artificial ponds, and reservoirs have been constructed as part of the water harvesting system. These do not make a significant contribution to the total freshwater resource in India, but water harvesting systems have a strong impact on a local scale.

What Is Water Scarcity And What Are Its Main Causes

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Water Scarcity and Its Main Causes

Scarcity of water is defined as a situation where there is insufficient water to satisfy normal requirements.
Though water is a renewable resource, we humans are using it at a faster rate than it is being replenished. There are various factors contributing to the depletion of the water table.

Increasing Population:
Growing population has resulted in a growing demand for houses, offices, shops, roads, etc. As a result, open areas like parks and playgrounds are used for construction. This reduces the seepage of water into the ground. Moreover, construction work requires more water for which underground water is used. As the rate of consumption of underground water is more than seepage, it results in a depletion of the water table.

Increasing Industries:
The rapidly growing population has also resulted in an increase in the number of industries. Water is used in almost every stage of production of things that we use. This has put a heavy load, especially on the freshwater ecosystem.

Agricultural activities: India is an agricultural country and farmers have to depend on rain for irrigating their fields. However, erratic monsoons result in excess use of groundwater thereby decreasing the underground water.

Deforestation: Trees have the ability to hold water. With deforestation, the number of plants decrease, hence the water keeps flowing towards the sea and the groundwater is not recharged.

In India, some causes of water scarcity are:

Uncontrolled use of borewell technology for extracting groundwater: Farmers have over exploited groundwater resources in our country due to uncertain monsoons and an unreliable state water supply.
Pollution of freshwater resources: This is due to the discharge of untreated sewage from homes, toxic chemicals from industries, and of pesticides and insecticides used by farmers into water bodies.
No effective measures for water conservation: Very little has been done for groundwater recharge and for care of freshwater ecosystems.

Effects of Water Scarcity on Plants

Water is very important for plant growth; its deficiency weakens the plant.
When plants do not get sufficient water, they do not grow to their expected height, have reduced photosynthetic activity, and produce much less harvest than expected. Without enough water, plants will also be susceptible to diseases, which can eventually lead to the death of the plant.
This may result in no food, no oxygen, no rain, and eventually end all forms of life on the Earth.

Water Distribution System In India

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Natural Distribution Of Water In India

Water availability in India depends greatly on the seasonal monsoons. The monsoons bring heavy rain over most of the country between June and October. Only Tamil Nadu is the exception and receives over half of its rain from October to November. India ranges from places having waterless conditions (Thar Desert) to places with rainforest climate (North Eastern States). In general, the northern half of the country sees greater extremes in rainfall.

India has a large network of rivers too. The three ARABIAN major rivers, Indus, Ganga and the Brahmaputra originate in the Himalayas and drain nearly two- thirds of the land area. These rivers also account for nearly the same proportion of India’s total water resources. Water in these rivers is strongly influenced by the monsoons.

Water Distribution System In India 1
Rainfall map of india

During the monsoons, water levels in rivers increase greatly and may result in floods. On the other hand, during the dry season, water levels go down quite a bit in most large rivers. Smaller tributaries and streams generally dry off completely. To regulate water flow in these rivers and distribute water more evenly throughout the year, large dams have been built on a number of rivers.

On the other hand, the mountainous and hilly regions of north and west do not allow adequate seepage. So, groundwater is mostly limited to lower-lying areas such surrounded by water on three sides as valleys. The peninsular areas of our country also do not allow proper absorption of water, and therefore do not have large continuous aquifers. Though the coastal plains are a rich source of groundwater, they can be contaminated by saltwater inflow caused by the over-pumping of groundwater. The Gangetic plains are, however, rich in groundwater as the aquifers keep getting recharged and restored.

What Are The Different Uses Of Water

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What Are The Different Uses Of Water

The uses of water can be broadly divided into agricultural, domestic, and industrial uses. The major portion of water in India is used in agriculture, as India is mainly an agricultural country. Besides being essential for life, water is used for many other purposes. In India, about 70% of the total water available is used for agriculture, 20-22% by industries, and only 8% is used for personal or domestic needs. Let us learn more about the various uses of water.

What Are The Different Uses Of Water 1
Percentage use of water

3 Major Uses of Water

Agricultural
Our country depends a lot on agriculture. Farmers rely on water to sustain their agricultural crops, e.g., wheat, paddy, etc. Many a times, rainfall is not sufficient to water these crops, and farmers have to use artificial watering systems, referred to as irrigation. Nearly 60% of the Indian population depends on agriculture for their living, and uses nearly 90% of all water for irrigation. Irrigation systems are necessary because the monsoons are unpredictable and in regions where cultivation completely depends on the rain, no rain can result in reduced yields or even total crop failure.

What Are The Different Uses Of Water 2Domestic
We need water to drink. Water that is suitable for drinking is called potable water. We also need water to bathe, wash clothes and dishes, clean our house, and to water plants. It also regulates the climate of a place and provides homes to many animals. Water required for household use is called domestic water. It is required for a variety of purposes such as drinking, preparing food, bathing, washing clothes and dishes, watering plants, etc. We generally get water in our homes in one of the following two ways. It is either delivered by the municipal water distribution system, or we extract it from an underground source through a motor-driven tube well. Apart from these uses, water is also used for transportation and recreation.

What Are The Different Uses Of Water 4
The water distribution and water treatment system in a city

In cities, municipal water treatment plants supply homes with treated water that matches drinking water standards. In rural areas, however, over 80% of the domestic water comes from underground sources. In areas where water is scarce or where wells, ponds, and rivers dry up in summers, people have to travel long distances to fetch water for their daily needs.

What Are The Different Uses Of Water 3
Industrial
Industry depends on water at all levels of production. It is used as a raw material, a solvent, a coolant, a transport agent, and for generating electricity. Due to rapid industrial development, the demand for water in this area has increased in the past several years. Industrial needs Factories use a large amount of water every day—as raw material, for cleaning, heating, cooling, generating electricity (e.g., water turbines), etc. The amount of required depends on the kind and size of the factory, and water.