Selina Concise Solutions for ICSE Class 6 Chemistry Chapter 7 Water

POINTS TO REMEMBER

1. Water cannot be replanished and without water we cannot live.
2. 4 / 5 th of earth’s surface is covered with water and 1% of this available water is in the form of rivers, lakes, ponds and under ground water. It is utilized for agriculture and living beings consumption.
3. In the atmosphere water is present in the form of vapour, mist, clouds etc.
Sources of water:
Surface water: River and Lake water, Ocean and Sea water
Underground water: Well water, Spring water
Above surface water: Rain water and water vapour in air
4. Distillation : The process of removal of the dissolved salts from sea/ocean water.
5. Purest form of water is rain water as it is formed by evaporation and condensation.
6. Sea water is unfit for agriculture as plants do not tolerate saline water.
7. Change of water from water bodies (lakes, rivers and oceans) into vapours in air and condensing of vapours and fall in the form of snow, water is called water cycle and water cycle plays an important role in
(i) restoring the lost water from earth’s surface
(ii) in controlling the climatic conditions.
8. Water vapour is one of the invisible gases which makes up the air we breathe.
9. Drinking water may contain some minerals and salts but it must be free from suspended impurities, harmful bacteria and germs.
10. Sometimes water from a source look clear but may contain germs which cause water borne diseases like typhoid, cholera, jaundice, dysentry, gastroenteritis etc.
11. Water can be made fit for drinking by
(i) boiling,
(ii) sterilisation
(iii) exposing to air and sunlight
(iv) by chemical treatment like chlorination or ozonisation.
12. Water is universal solvent i.e. water can dissolve nearly in every substance.
13. Distilled water is the purest form of water but not good for drinking purposes. It is good for medicinal purposes, laboratories and batteries.
14. Both stirring and heating help in dissolving the substance in water.
15. “The amount of solute that dissolves in a given quantity of solvent at a given temperature is called solubility of the solute.”
16. Air dissolved in water helps aquatic animals and plants to survive.
17. Eutrophication: “The deficiency of oxygen in the water may cause death of aquatic animals. This loss of dissolved oxygen from water in water bodies is called eutrophication.”

EXERCISE-I

Question 1: Name:
(a) Three major sources of natural water
(b) Four sources of surface water.
(c) Two underground sources of water.

Answer:
(a) Three major sources of natural water :
1. Surface water (sea water)
2. Above surface water (rain)
3. Underground water (springs)
(b) Four sources of surface water sources :
1. Ocean water
2. River water
3. Lake water
4. Glaciers
(c) Two sources of underground water :
1. Well water
2. Spring water
Water is categorized based on where it is found, whether it is on top of the ground, deep inside the earth, or falling from the sky. These sources provide the hydration necessary for all biological ecosystems on our planet.
Teacher's Tip: Use the acronym "USA" to remember the types: Underground, Surface, and Above-surface water.
Exam Tip: Always provide exactly the number of examples asked for in the question to manage your time effectively.

Question 2: Answer the following questions in short:
(a) In which form is water present in the atmosphere ?
(b) Which source of water contains the highest concentration of salt in it ?
(c) Why is rain water considered the purest form of natural water?
(d) What possible impurities does rain water contain ?
(e) What is water table ?
(f) Why is spring water pure enough for drinking but unsuitable for laboratory use ?
(g) Why the taste of spring water differs at different places ?

Answer:
(a) Water is present in the form of water vapours in the atmosphere.
(b) Sea water contains highest concentration of salt.
(c) Rain water is the purest form of water as it is distilled water i.e. water from surface of earth has evaporated and then vapours condense in the atmosphere.
(d) The impurities present in rain water are dust and dissolved gases like oxygen, nitrogen and carbon dioxide and these gases are not poisonous, rain water is safe for drinking.
(e) Level of ground water is called water table.
(f) Spring water is free from suspended impurities and germs (harmful bacteria) as water has been filtered through different layers of soil and is fit for drinking. Spring water contains dissolved impurities which are due to the nature of soil surrounding the spring and cannot be used for laboratory purposes, where distilled water free from impurities is needed.
(g) Spring water contains unsolved impurities which have entered the water from surrounding soil and soil impurities (salts) differ from place to place. Hence taste of water differs from place to place.
The Earth acts like a giant filter, cleaning water as it moves through the soil but also adding minerals that change how it tastes. This explains why water from a mountain spring might taste very different from water in a valley.
Teacher's Tip: Remember that "purest" refers to natural sources; rain is nature's own distillation process.
Exam Tip: When defining the water table, mention that it represents the upper level of the underground water to show clear understanding.

Question 3: List three major impurities present in river water.

Answer:
Three major impurities present in river water :
1. Suspended impurities like clay and sand particles.
2. Harmful bacteria.
3. Mineral salts.
Rivers pick up a lot of materials as they flow across the land and rub against rocks and soil. These impurities must be removed before the water is safe for us to consume at home.
Teacher's Tip: Think of "B.S.M." - Bacteria, Suspended solids, and Minerals.
Exam Tip: Use scientific terms like "suspended" instead of just "floating" to describe solid dirt in water.

Question 4: Give the percentage of water in the following :
(a) Rice and wheat grains
(b) Eggs
(c) Tomatoes
(d) Bread
(e) Water melon

Answer:
Percentage of water in :
(a) Rice - 3 % to 4% Wheat grain - 3% to 4%
(b) Eggs - 75%
(c) Tomatoes - 95%
(d) Bread - 25%
(e) Water melon - 97%
Most of the food we eat is actually made up of water, which helps in our digestion and hydration. Even dry-looking things like grains contain a tiny amount of moisture hidden inside.
Teacher's Tip: Fruits usually have over 90% water, while dry grains have less than 5%.
Exam Tip: Memorize the values for common foods like tomatoes and watermelons, as they are frequently asked in multiple-choice questions.

Question 5: What are the three states of water ?

Answer:
Three states of water are :
1. Ice (solid)
2. Liquid (water)
3. Gaseous (steam)
Water is unique because it can naturally change between these three forms depending on the temperature of its surroundings. Each state has different physical properties even though they are all made of the same molecules.
Teacher's Tip: Solid, Liquid, Gas - just think of your freezer, your tap, and a boiling kettle.
Exam Tip: Mention both the scientific state (solid, liquid, gas) and the common name (ice, water, steam) for full marks.

Question 6: Why are ice, liquid water and steam considered to have the same chemical substance ?

Answer:
Ice, liquid water and steam have same composition i.e. 2 parts hydrogen and one part oxygen by volume and same formula H₂O, can be easily transformed from one state to other by heating or cooling.
Chemical identity is determined by what atoms are inside, not how the substance feels to the touch. Since all three forms are made of exactly the same H₂O molecules, they are chemically identical.
Teacher's Tip: Think of it like a person wearing different clothes; they look different, but they are still the same person.
Exam Tip: Mention the chemical formula H₂O and the ratio of hydrogen to oxygen to provide a scientific justification.

Question 7: How is a cloud formed ?

Answer:
Water from the surface of earth, lakes, seas, rivers, formed by plants (transpiration) etc. evaporates and rises high up in the atmosphere and condense into water droplets and collect to form cloud.
The sun's heat turns liquid water into invisible gas, which then cools down high in the sky and turns back into tiny droplets. These millions of droplets group together to create the visible clouds we see floating above.
Teacher's Tip: Clouds are formed by "E and C": Evaporation followed by Condensation.
Exam Tip: Don't forget to mention "transpiration" as a source of water from plants to show deeper knowledge.

Question 8: What is water cycle ? What is its importance ?

Answer:
Water cycle : The change of water from one form to another in nature which results in continuous circulation of water from earth’s surface to the atmosphere and from the atmosphere back to the earth’s surface is called water cycle.
Importance of water cycle:
1. It assures a continuous supply of water to us.
2. In controlling the climatic conditions all over the world.
The water cycle acts like a giant recycling machine that constantly cleans and moves water around our planet. This process is what creates weather patterns and makes sure that rivers never run completely dry.
Teacher's Tip: Imagine the water cycle as a big circle that never stops moving.
Exam Tip: When discussing importance, mention both "water supply" and "climate control" as these are the two main points.

Question 9: How are the following formed ?
(a) fog
(b) mist
(c) dew
(d) frost

Answer:
(a) Fog : When water vapours change into tiny droplets of water near the ground, fog is formed.
(b) Mist: When tiny droplets of water remain suspended in air it is called mist.
(c) Dew : When the water vapour condenses on cold objects like grass, leaves and flowers in winter in the form of tiny droplets of water called dew.
(d) Frost: When the dew freezes it is called frost.
These phenomena happen when water in the air comes into contact with cool temperatures, especially during the night or winter. They change the visibility of our surroundings and provide moisture to plants during dry periods.
Teacher's Tip: Fog is just a cloud that is touching the ground.
Exam Tip: Be clear about the difference between dew (liquid) and frost (solid/frozen) in your descriptions.

EXERCISE-II

Question 1: Name:
(a) Two chemicals used to destroy germs present in water.
(b) Two diseases which spread through impure water.
(C) A chemical used for loading.
(d) Two substances which add taste to water.
(e) Two household methods to get safe drinking water.

Answer:
(a) Two chemicals used to kill germs in water are:
1. Chlorine.
2. Potassium permanganate.
3. sterilizing water with ozone.
(b) Two diseases are cholera, dysentry.
(c) Chemical used for loading is potash alum.
(d) Minerals and carbon dioxide gas adds taste to water.
(e) Methods to get safe drinking water:
1. Adding chlorine tablets.
2. By adding potassium permanganate crystals.
We use science to protect ourselves from the tiny bacteria that live in untreated water sources. Chemicals like chlorine work by breaking down the cell walls of harmful germs, making the water safe to drink.
Teacher's Tip: Alum is for "Loading" (helping dirt sink) and Chlorine is for "Killing" (germs).
Exam Tip: When asked for diseases, always name common ones like Cholera or Typhoid as they are standard answers.

Question 2: Answer in brief:
(a) Why is river water unfit for drinking?
(b) Why is tap water a mixture?
(c) What is mineral water?
(d) What is the purpose of adding bleaching powder to water supplied to the town?
(e) How is chemically pure water obtained in the laboratory?
(f) how is water in swimming pool kept free a germs?

Answer:
(a) River water contains mineral salts, suspended impurities like clay, sand particles, organic matter and bacteria and is not fit for drinking.
(b) Tap water contains, minerals, air, chlorine and other dissolved impurities that varies from place to place, therefore it is a mixture.
(c) Mineral water is pure water fit for drinking. It is collected from natural source and contains air, minerals and salts free from suspended impuiities, harmful bacteria and germs.
(d) Adding bleaching powder to water, kills germs and harmful bacteria and viruses present in water.
(e) Chemically pure water for laboratories is obtained - by distillation. Distilled water does not contain any salt or mineral
(f) Water in swimming pool kept free from infections and germs by chlorination i.e. treating water with chlorine gas.
Water's role as a "universal solvent" means it's almost always a mixture because it picks up things like air and minerals wherever it goes. We must use processes like distillation if we need water that has absolutely nothing else in it.
Teacher's Tip: If it has more than one substance in it (like water + salt), it is a mixture.
Exam Tip: For the "river water" answer, make sure to list at least three specific types of impurities to be thorough.

Question 3: Define:
(a) Sterilisation
(b) Sedimentation
(c) Loading
(d) Aeration

Answer:
(a) Sterilisation : The process of removal of microorganisms including bacterial spores from water to avoid water borne diseases is known as sterilisation.
(b) Sedimentation : The setting of suspended solid matter at the bottom of a liquid is called sedimentation.
(c) Loading : The process of adding a chemical to an impure liquid in order to increase the speed of sedimentation of suspended particles is called loading.
(d) Aeration : To kill harmful micro-organisms present in filtered water, air underpressure is blown into the filtered water. This process is called aeration.
Water treatment uses physics (gravity for sedimentation) and chemistry (alum for loading) to make water clear and clean. Aeration helps by using air to kill certain germs and improve the freshness of the water.
Teacher's Tip: Loading "loads" weight onto tiny dirt particles to make them sink faster.
Exam Tip: Use words like "insoluble" or "suspended" when explaining sedimentation and loading.

Question 4: What is potable water ? List four characteristics of potable water.

Answer:
Potable water : Water fit for drinking purposes is called potable water.
Four characters of drinking water :
1. It should be transparent.
2. Should have no colour, no odour.
3. Should be free from harmful bacteria and germs.
4. It should contain same salts and minerals needed by the body CO₂ to add to taste.
Potable water must not only be clean but also healthy for our bodies to use for daily activities. Even if water looks clear, it must go through testing to ensure no invisible bacteria are hiding inside.
Teacher's Tip: "Potable" comes from the word "pot" - something you can put in your drinking pot!
Exam Tip: When listing characteristics, focus on physical traits (clarity) and safety traits (no germs).

Question 5: Why is water important for plants and animals?

Answer:
Importance of water for plants:
1. Plants need water to prepare their food, for germination and growth to produce fruits, flowers etc.
2. For conduction of food prepared by plants to other-parts of plant i.e., for translocation.
3. Large number of plants live in water; water provides nutrients and oxygen for their survival.
Crops need water for their growth. Water in the form of rain, washes the dust and smoke deposited on leaves by vehicles and helps the stomata in exchanging gases.
Importance of water for animals:
1. Lot of water is lost by the body of animals by sweating, in the form of urine and evaporation while doing various activities, so to make up for the lost water animals consume a lot of water in the form of drinking.
2. Water keeps the animals fresh and is natural medicine for their many ailments.
3. Water is good solvent and helps in the process of digestion, blood circulation, excretion etc. in the body of organisms.
4. Water is essential for the cleanliness of animals and their surroundings.
Water acts as a transport system inside living things, moving nutrients in and wastes out. Without it, the chemical reactions that allow plants to grow and animals to move would simply stop.
Teacher's Tip: Think of water as the "train tracks" for nutrients inside your body.
Exam Tip: Separate your answer into "Plants" and "Animals" sections to make it organized and easy for the teacher to grade.

Question 6: What are the three methods of removing germs from natural water. Explain.

Answer:
Three methods to remove germs :
1. By boiling : Boiling kills germs present in water.
2. Adding potassium permanganate: By adding potassium permanganate in the well the germs can be killed.
3. Chlorination: After filtration water is passed through chlorination tank here chlorine kills the germs.
Each of these methods uses heat or chemicals to attack the biological structure of invisible pathogens. Boiling is the simplest way for families to clean water at home without special equipment.
Teacher's Tip: Boiling for 10-20 minutes is the safest home method.
Exam Tip: Name the specific chemical (Chlorine or Potassium Permanganate) used in each method to get full credit.

Question 7: Name the steps involved in the purification of drinking water supplied in cities and towns.

Answer:
The water source for our towns and cities are river, lakes or underground water which contains suspended and dissolved impurities.
To remove these impurities steps involved are:
1. Loading and sedimentation : to settle the suspended impurities to form a sediment for this purpose potash alum is added.
2. Filtration : The water still contains lighter suspended impurities which are removed by filtration through sand and gravel. In cities ground water is drawn from tube well or submersible pumps which have filters fitted in them. This clear water still contains germs.
3. Chlorination : To kill germs water is passed into chlorination tank where it is treated with chlorine to kill germs.
Water is now potable i.e. safer for drinking and supplied to homes.
Cities use a multi-step process to ensure that even heavily polluted river water becomes safe for thousands of people to use. Each stage is designed to catch a different type of impurity, from large sand grains to tiny microscopic bacteria.
Teacher's Tip: S.L.F.C. - Sedimentation, Loading, Filtration, Chlorination.
Exam Tip: Describe what each step removes (sedimentation for heavy dirt, chlorination for germs) to show complete understanding.

Question 8: What is the taste of distilled water ? Why is it not potable?

Answer:
Taste of water is tasteless i.e. flat. It is because distilled water does not contain any salt or mineral required for our body. So it is not potable water.
Distilled water is chemically pure but biologically incomplete because our bodies need certain dissolved minerals like calcium. It tastes "flat" because it lacks the dissolved gases and minerals that give regular water its refreshing flavor.
Teacher's Tip: Distilled water is for batteries and labs, not for drinking bottles!
Exam Tip: Use the word "minerals" to explain why it is not healthy or tasty for regular drinking.

Question 9: Give reasons :
(a) Ice floats on water.
(b) Marine life is able to survive in colder regions.
(c) Water droplets can be seen outside a chilled water bottle.

Answer:
(a) Water has maximum density at 4°C. Ice is lighter than water therefore it floats on water.
(b) This anomalous property of water enables aquatic plants and animals to survive in colder regions of world because even when the water of ponds, lakes, river freezes it freezes on the top but remains a liquid below ice layer.
(c) Sometimes we see water droplets on the outer surface of the glass containing ice cold water, this is because the water vapour presents in air, on coming in contact with the cold glass of water, loses energy and gets converted into liquid state, which we see as water droplets.
Water is a strange substance because its solid form is less dense than its liquid form, which is the opposite of most other materials. This "anomalous" behavior acts like an insulating blanket for fish, keeping them warm underneath a layer of floating ice.
Teacher's Tip: Remember 4°C is the magic temperature where water is the heaviest.
Exam Tip: Use the scientific term "Anomalous expansion" to explain why ice behaves differently than other solids.

EXERCISE-III

Question 1: Why is water called a universal solvent ?

Answer:
As water can dissolve in most of the substances, solids, liquid and gaseous. Therefore it is called universal solvent.
Water's unique chemical structure allows it to pull apart the molecules of many different substances and mix them in. This is why water in nature is almost never pure; it always contains some dissolved materials from its surroundings.
Teacher's Tip: "Universal" means it can dissolve "almost everything" on Earth.
Exam Tip: Mention that it can dissolve all three states of matter (solids, liquids, and gases) to make your answer complete.

Question 2: Define:
(a) Solute
(b) Solvent
(c) Solution
(d) Saturated solutions
(e) Unsaturated solutions

Answer:
(a) Solute : A solute is a substance that dissolves in a medium which can be water or any other substance. Solute is in smaller quantity in a solution.
(b) Solvent : A solvent is a medium in which a solute dissolves. It is in large quantity in a solution. Water is the most common solvent. The other solvents are alcohol, carbon tetrachloride etc.
(c) Solution : A solution is a homogeneous uniform mixture formed by a solute and a solvent.
(d) Saturated solution : When a solution cannot dissolve any more of solute at a given temperature, it is called saturated solution.
(e) Unsaturated solution : A solution that can take up more of the solute at a given temperature, is said to be an unsaturated solution.
Solutions are all around us, from the tea we drink to the air we breathe. Knowing the difference between these terms helps us understand how chemicals mix and separate in both nature and industry.
Teacher's Tip: Solute (smaller word) is the smaller part; Solvent (bigger word) is the bigger part.
Exam Tip: Always mention "at a given temperature" when defining saturated or unsaturated solutions, as solubility changes with heat.

Question 3: State two factors by which solubility of a solute in a solvent can be increased.

Answer:
Two factors are :
1. Stirring.
2. Increase in temperature.
When you heat a liquid, the molecules move faster and create more space for the solute to enter. Stirring helps by physically breaking down the solute and spreading it quickly throughout the solvent.
Teacher's Tip: Think of making sweet tea: you stir it and use hot water to get the sugar to dissolve faster.
Exam Tip: Focus on "Temperature" as the most important factor for increasing solubility in most solids.

Question 4: Why do aquatic animals die in boiled water ?

Answer:
Boiled water is deprived of oxygen i.e. there is no oxygen. For want of oxygen animals die in boiled water.
Boiling causes dissolved gases to expand and escape into the air, leaving the water empty of life-sustaining oxygen. Even after the water cools down, it takes time for air to dissolve back into it from the atmosphere.
Teacher's Tip: Fish don't breathe the H₂O molecules; they breathe the extra O₂ bubbles trapped between them.
Exam Tip: Be specific and state that "dissolved oxygen" is removed by boiling.

Question 5: State three differences between water and air.

Answer:
Differences between water and air :
Water :
1. Water is liquid and can exist in three states i.e. solid-ice, liquid-water, gas - vapours.
2. Is a compound.
3. H₂O is its formula.
Air :
1. Air is gaseous and can exist in liquid state when cooled under pressure.
2. Is a mixture.
3. Has no formula.
Water is a chemical union of elements in a fixed ratio, whereas air is just a loose collection of gases that happen to be together. This means you can change the ratio of gases in air easily, but you cannot change the ratio in water without creating a new substance.
Teacher's Tip: Water is a "Married" couple (Compound); Air is a "Group of Friends" (Mixture).
Exam Tip: Mention the presence of a chemical formula (H₂O) for water vs the lack of one for air to show a strong chemical understanding.

EXERCISE-IV

Question 1: State four ways by which water can be conserved.

Answer:
Four ways to conserve water:
1. Do not allow water to drip from defective taps.
2. More dams should be built.
3. More plantation should be done as plants help in bringing rain.
4. Wastage of water should be avoided.
5. Close the tap when you are brushing your teeth. Rather use a mug. Close the tap when you are washing clothes, open the tap only when you need it. Reduce, Reuse and recycle should be our mantra
Since only a tiny fraction of the Earth's water is fresh and usable, we must treat it as a precious resource. Every drop saved through small daily habits like using a mug for brushing adds up to millions of liters over time.
Teacher's Tip: Remember the three R's: Reduce, Reuse, Recycle.
Exam Tip: List practical, everyday actions like "fixing leaky taps" as they demonstrate an understanding of real-world conservation.

Question 2: Explain harvesting of water.

Answer:
When it rains heavily water runs into streets, drains and wasted we can not make use of it. Our purpose is to catch this rain water, store it for future use when we actually need it. For this harvesting of rain water should be done. Rain water is collected from the roofs and verandas of the buildings with the help of pipes and carried to tanks for storage and is used at the time of need when it is not raining.
Rainwater harvesting turns your home into a collection center for nature's cleanest water. This stored water can be used for gardening, washing cars, or even flushing toilets, saving your tap water for drinking.
Teacher's Tip: "Harvesting" water is like picking fruit; you catch it when it falls so you can use it later.
Exam Tip: Describe the path of the water: from the roof, through the pipes, to the storage tank.

Question 3: What are the three main causes of water pollution?

Answer:
Three main causes of water pollution :
1. Addition of waste products from homes.
2. Addition of waste products from agriculture.
3. Addition of waste from industries.
4. Addition of sewage in water bodies.
Pollution happens when we put substances into the water faster than nature can break them down. These chemicals and wastes can poison fish and make the water dangerous for humans to touch or drink.
Teacher's Tip: Pollution comes from "H.A.I." - Homes, Agriculture, and Industry.
Exam Tip: Mention "Industrial waste" and "Sewage" as they are the two largest sources of water pollution globally.

Question 4: State the main steps to be taken to prevent water pollution.

Answer:
Steps to be taken to prevent water pollution :
1. Trees and plants be planted along the banks of rivers and canals.
2. Bathing and cleaning of animals near or in water sources be not allowed.
3. Use of pesticides, insecticides, fungicides and fertilisers should be reduced.
4. The polluted water from industries should be treated first and then discharged into water bodies.
5. Use biodegradable detergents.
6. Water containing sewage should be passed through sewage treatment plants first and then this water should be used for irrigation.
7. Wells should be covered properly and washing and cleaning of clothes, utensils and animals should not be done near the well to keep them clean.
Protecting our water requires a change in how we treat the land around our rivers and lakes. By treating industrial waste before it touches nature, we can keep the entire ecosystem healthy and safe for everyone.
Teacher's Tip: Keep the "Dirty stuff" away from the "Drinking stuff."
Exam Tip: Suggest "Sewage treatment" and "Planting trees" as they are key infrastructure and biological solutions.

Question 5: What are the causes of floods and drought?

Answer:
Problems cause by floods :
1. A rise in the level of water in dams, rivers, lakes etc.
2. Heavy rainfall also causes floods.
3. Floods cause extensive damage to crops, property, animal and human life.
4. Crop-fields, villages and many low-lying areas get submerged under flood water.
5. Rains/floods also affect smaller animals living in the soil.
Problems cause by drought :
1. Crops may die, fodder may become scarce.
2. Living organisms of the soil die.
3. Animals may die, plants and trees will not survive.
4. Soil becomes dry, water level in rivers, lakes, dams etc. may fall. The ground water-level falls.
5. Drought displaces people from a large number of villages and towns.
Both floods and droughts are extreme weather events that happen when the water cycle gets out of balance. Too much water at once destroys homes, while too little water for a long time causes starvation.
Teacher's Tip: Floods = Too much, too fast. Drought = Too little, too long.
Exam Tip: Mention the impact on "Soil life" and "Crops" for both to show how these events affect agriculture.

Question 6: State some of the ways in which you as an individual can conserve water.

Answer:
The ways in which we can conserve water are as follows :
1. Use a bucket for taking it.
2. Make sure, water does not overflow from overhead tanks of your house.
3. Close the tap when you are brushing your teeth.
4. Wash fruits, vegetable in a bowl of water, rather than under a running tap. Water used for washing vegetables may be used to water plants in the garden.
Individuals have the power to save thousands of liters of water just by switching from a running tap to a bucket or bowl. This "greywater" from vegetables is actually full of nutrients that can help your home garden grow better.
Teacher's Tip: Reuse your vegetable washing water for your plants!
Exam Tip: Provide personal, actionable steps that a student can actually do themselves.

OBJECTIVE TYPE QUESTIONS

1. Fill in the blanks
(a) Water is a universal solvent.
(b) Rainwater is the purest form of natural water.
(c) Sand and dust are suspended impurities in water.
(d) Sea water has high concentration of salt (impurity).
(e) Water covers nearly 4 / 5 th of the surface of the earth.
(f) Evaporation of rain water leaves NO residue.
(g) Potash alum is the chemical added to water to remove the tiny suspended particles.
(h) A solution is a uniform mixture of a solute and a solvent.
(i) Ice, water and steam have different physical states but are chemically identical.
(j) Boiling kills most of the germs in water.
(k) The elements present in the molecules of water are hydrogen and oxygen.

2. Write True or False for the following statements

(a) Water is an element.
Answer: False.
Correct: Water is a compound.
Elements are single substances, but water is made of two different elements (Hydrogen and Oxygen) chemically bonded together.
Teacher's Tip: Elements are on the Periodic Table; Water is a mix of them.
Exam Tip: Always provide the "Correct" statement for any "False" answer to show you know why it's wrong.

(b) Tap water does not contain dissolved impurities.
Answer: False.
Correct: Tap water contain dissolved impurities.
Tap water often contains minerals and gases that it picked up from the ground or during the treatment process. These impurities are what give your local water its specific taste.
Teacher's Tip: Only distilled water is completely free of dissolved impurities.
Exam Tip: Remember that "impurities" isn't always a bad thing; minerals are technically impurities but are good for you.

(c) Alum is commonly used for removing suspended impurities.
Answer: False.
Correct : Alum is commonly used for settling down of suspended impurities.
Alum doesn't actually remove the dirt; it just makes it heavy so it sinks to the bottom where we can easily drain it away. This process is scientifically called "loading" for sedimentation.
Teacher's Tip: Alum makes tiny dirt "heavy" like a rock.
Exam Tip: Use the term "settling down" specifically when referring to the role of alum.

(d) Distillation is a good method for purifying water for town supply.
Answer: False.
Correct : Distillation is a good method - for purifying water for medicinal purpose.
Distillation is too slow and expensive to clean millions of liters of water for a whole city. It is only used when we need 100% pure water for medicine or science labs.
Teacher's Tip: Distillation is a "High Precision" method, not a "High Volume" one.
Exam Tip: Identify the cost and scale as reasons why distillation isn't used for town supplies.

MULTIPLE CHOICE QUESTIONS

1. Water content in human body is
1. 70%
2. 75%
3. 80%
4. 90%
Answer: 70%
Your body is mostly liquid, which helps move electricity and nutrients through your cells. This is why staying hydrated is the most important thing you can do for your health.
Teacher's Tip: Think of yourself as a 70% water balloon!
Exam Tip: Remember the "Rule of 70" for human biology questions.

2. The purest form of natural water is
1. seawater
2. river water
3. rainwater
4. lake water
Answer: rainwater
Rainwater is created by evaporation, which leaves all the dirt and salt behind on the ground. As long as the air is clean, the falling rain is the purest water nature provides.
Teacher's Tip: Rain is nature's own laboratory-style distilled water.
Exam Tip: Always pick "Rainwater" as the purest *natural* source, but "Distilled water" if the question asks for the purest *overall*.

3. When the water vapour changes into tiny droplets of water near the ground, it is called
1. mist
2. dew
3. fog
4. frost
Answer: fog
Fog happens when the air near the ground cools down enough for invisible gas to turn into visible liquid. It's essentially a cloud that you can walk through.
Teacher's Tip: Fog = Ground Cloud.
Exam Tip: Use the keyword "near the ground" to distinguish fog from regular clouds.

4. Water is a
1. compound
2. element
3. mixture
4. none of the above
Answer: compound
Water is formed when hydrogen and oxygen atoms chemically bond together in a strict 2-to-1 ratio. Because it has a fixed chemical formula (H₂O), it cannot be a mixture or an element.
Teacher's Tip: Compounds have fixed recipes; water's recipe is always H₂O.
Exam Tip: If it has a chemical formula, it is a compound!

5. Common salt is obtained from sea water by
1. distillation
2. crystallisation
3. evaporation
4. sublimation
Answer: evaporation
In large pans near the ocean, the sun's heat evaporates the water, leaving solid salt crystals behind. This is the primary way humans have harvested salt for thousands of years.
Teacher's Tip: Sun + Sea water = Salt.
Exam Tip: "Evaporation" is the process of losing liquid to keep the solid.

6. Jaundice affects
1. heart
2. lungs
3. liver
4. kidney
Answer: liver
Jaundice is a water-borne disease that causes the liver to stop working correctly, which often makes a person's skin look yellow. Drinking contaminated water is a major cause of this illness in many parts of the world.
Teacher's Tip: Yellow skin = Liver problem (Jaundice).
Exam Tip: Link "Jaundice" and "Liver" together in your study notes for quick recall.

7. Chlorination of water is done
1. to kill the germs
2. to remove the suspended impurities
3. to remove the dissolved impurities
4. none of the above
Answer: to kill the germs
Chlorine is a powerful chemical that acts as a disinfectant, wiping out harmful bacteria and viruses. It is added at the final stage of water treatment before water is sent to our homes.
Teacher's Tip: Chlorine is the "Germ-Killer."
Exam Tip: Remember that chlorination is for *biological* cleaning, not *physical* cleaning (like filtration).

ADDITIONAL QUESTIONS FOR PRACTICE

Question 1: State the sources of water
(a) on the earth’s surface
(b) below the earth’s surface.

Answer:
(a) Sources of water on the earth’s surface are -
1. Snow, frost - Snow and frost are the natural forms of water present. They are found in the solid state.
2. Rain water - The purest form of natural water almost free from impurities is rain water. Rain water may dissolve oxygen, nitrogen & carbon dioxide gas forming weak acids. In industrial regions, nitrogen dioxide and sulphur dioxide evolved may dissolve in rain water forming nitric acid & sulphuric acid which causes acid rain.
3. River water - It is one of the impure forms of natural water since most forms of surface water enters into river water. It contains impurities such as sand particles, organic matter, bacteria, mineral salts which dissolve after soil erosion & dissolved gases.
4. Lake water - It is another impure form of natural water which also contains impurities and other soluble salts.
5. Sea water - The most impure form of natural water containing over 3% soluble salts including sodium chloride. It also contains salts of calcium & magnesium.
(b) Sources of Water below the earth’s surface are -
1. Well water - Above the impervious rocky layers, of the earth’s surface is well water. which contains soluble impurities.
2. Spring water - Natural water accumulated above the rocky layers of the earth which forcefully comes out under pressure from the earth’s surface is spring water & contains soluble salts & minerals.
Water is constantly moving and changing quality depending on whether it is flowing through a forest or an industrial city. The impurities it picks up define whether we can use it for drinking, farming, or only industrial cooling.
Teacher's Tip: Surface water is usually dirtier than underground water because it's exposed to the air and ground traffic.
Exam Tip: Mention "3% soluble salts" for sea water and "dissolved gases" for rain to show specific knowledge.

Question 2: Give the importance of water in
(a) life processes
(b) household purpose
(c) fire fighting
(d) transportation.

Answer:
The importance of the Water is as follows :
(a) Life processes - Water is used by all plants, animals & humans for carrying out various metabolic processes including photosynthesis by plants & excretion by animals & humans.
(b) Household purposes - Water finds numerous applications, such as watering plants, washing clothes, cooking, bathing, cleaning etc.
(c) Fire fighting - Water is used for extinguishing fires either directly or as a constituent in a fire extinguisher.
(d) Transportation - Water serves as a habitat for marine life i.e. preferred place for an organism to live. (Note: Transportation also refers to boats/ships moving goods across oceans).
Water is the essential ingredient for civilization, allowing us to survive biologically and thrive economically. From the chemical reactions in our cells to the ships moving cargo across the sea, water is the medium for most global activities.
Teacher's Tip: Water is the "Universal Helper" for life and work.
Exam Tip: When discussing "transportation," you can also mention that water is used to carry heavy goods on ships and boats.

Question 3: Explain how water plays an important role in
(a) industry
(b) agriculture.

Answer:
(a) Uses of Water in Industry :
1. Water generates electricity in hydroelectric power stations.
2. Water generates steam in boilers, used for various industrial purposes.
3. Water finds application in chemical & other industries for cooling & cleaning operations.
(b) Uses of Water in Agriculture :
In agriculture water finds importance in irrigation, production of crops & as a medium for spraying pesticides.
Water is a source of power and a tool for cleaning in factories all over the world. In farming, it acts as the carrier for nutrients and protection, ensuring that plants have what they need to feed the global population.
Teacher's Tip: Industry uses water for "Steam and Electricity," and Agriculture uses it for "Growing and Spraying."
Exam Tip: Use the term "Hydroelectric" specifically when talking about electricity from water to get technical marks.

Question 4: Give the occurrence of water in the three different states i.e. solid, liquid and gaseous.

Answer:
The Occurrence of water in three different states are:
1. Solid state - As snow and frost.
2. Liquid state - In sea water, river water and lake water.
3. Gaseous state - As water vapour in air the amount depending on climatic conditions.
Water's state in nature is determined by the temperature of the location, which is why it's solid at the poles and liquid near the equator. In the atmosphere, it exists as a gas that we can't see but can feel as humidity.
Teacher's Tip: Solid = Snow, Liquid = Lake, Gas = Vapor.
Exam Tip: Mention that the gaseous state depends on "climatic conditions" like temperature and humidity.

Question 5: Draw a labelled diagram to show the change of state of water from solid state to liquid state to gaseous state starting from ice.

Answer:
(Diagram description: The diagram shows ice cubes undergoing 'Melting' to become liquid water, then 'Vaporization' to become water vapour. Conversely, vapours undergo 'Condensation' (Liquefaction) to become liquid water, and liquid water undergoes 'Freezing' (Solidification) to become ice cubes.)
Heating provides energy to break the bonds between water molecules, moving them from a rigid structure (ice) to a loose flow (liquid) and then to total freedom (gas). Cooling does the opposite, taking energy away so the molecules slow down and huddle together again.
Teacher's Tip: Heating "melts and boils"; Cooling "condenses and freezes."
Exam Tip: In a diagram like this, always include arrows showing the direction of change and label the processes (like "Vaporization").

Question 6: Explain the term water cycle. State the main points to show how water moves from the earth’s surface to the atmosphere and back to the earth’s surface as rain.

Answer:
The water cycle is a natural process by which the circulation of water takes place from the earth’s surface to the atmosphere and back to the earth’s surface as rain water.
The Process of water cycle is discussed below :
FROM THE EARTH’S SURFACE - TO THE ATMOSPHERE
1. Evaporation - The sun’s rays fall on the earth & warm its surface & the air above it.
The heat evaporates the water from the streams, rivers & the sea.
2. Water [mainly in the form of water vapour] is also added to the atmosphere by -
(a) Respiration by living organisms: Glucose + Oxygen -> carbon dioxide + water + energy
(b) Burning of fossil fuels
3. Formation of clouds - The water vapour along with the warm air rises upwards, where at higher altitudes it condenses into small droplets of water forming clouds.
BACK TO THE EARTH’S SURFACE - AS RAIN WATER
1. Formation of rain - The clouds float in the atmosphere & when the size of the water droplets increases they fall down on the earth as rain water.
2. Rain water falls into streams - The rain water is absorbed by the soil collects underground & flows into streams.
3. Stream water enters rivers & seas - The stream water finds its outlets into rivers & later enters into the seas & oceans.
4. River & sea water evaporates forming clouds and thus continuing the water cycle.
Nature's water recycling system never stops, ensuring that the same water molecules used by dinosaurs millions of years ago are still moving through our taps today. This cycle is powered entirely by the energy from the sun and the pull of Earth's gravity.
Teacher's Tip: The water cycle has four big steps: Evaporation, Condensation, Precipitation, and Collection.
Exam Tip: Mention "Respiration" and "Burning of fossil fuels" as additional sources of water vapour to get extra marks.

Question 7: Give a reason why water is considered a universal solvent.

Answer:
Water is a polar covalent compound. When it comes in contact with any substance it breaks the electrostatic forces holding the molecules of that substance. Thus, the molecules break loose from the substance and hence dissolve in water. Thus, water is called a universal solvent and an alkali is not.
Water molecules act like tiny magnets that tug at other substances, pulling their pieces apart until they are completely mixed in. This "tug-of-war" is what allows water to dissolve minerals, nutrients, and even some gases effortlessly.
Teacher's Tip: "Polar" means water has a "North and South pole" like a magnet.
Exam Tip: Use the term "polar covalent compound" to give a professional chemical explanation for why water is such a good solvent.

Question 8: Define the term -
(a) solute
(b) solvent
(c) solution with reference to addition of sodium chloride to water.

Answer:
Solute- The substance which dissolves or disappears in the solvent i.e. liquid to form a solution is called a solute. e.g. sodium chloride.
Solvent- The liquid or medium of dissolution which allows the solute to dissolve in it, so as to form a solution is called a solvent, e.g. water.
Solution- A homogenous mixture of a solute in a solvent is called a solution.
When you add salt to water, you are performing a common chemical task: creating a liquid mixture where the salt is evenly spread out. In this case, the water is the "host" (solvent) and the salt is the "guest" (solute).
Teacher's Tip: Salt + Water = Salty Water (Solute + Solvent = Solution).
Exam Tip: Always provide an example like "sodium chloride in water" to illustrate these definitions clearly.

Question 9: Draw a neat labelled diagram of addition of copper sulphate to water. Labt. solute, solvent and solution in the same.

Answer:
(Diagram description: The image shows blue copper sulphate crystals being added to a beaker of clear water. It labels the copper sulphate crystals as the "SOLUTE," the clear water as the "SOLVENT," and the resulting blue liquid after stirring as the "SOLUTION.")
A diagram helps show how a solid "vanishes" into a liquid to create something that looks totally new but is really just a mixture. Even though the water turns blue, you can still get the copper sulphate back by evaporating the water.
Teacher's Tip: Copper sulphate turns water a beautiful bright blue!
Exam Tip: Always label the final beaker as a "homogeneous solution" to show you understand that it is uniform throughout.

Question 10: From the following substances given below state which will form a solution in the same.
(a) sodium carbonate
(b) calcium carbonate
(c) charcoal powder
(d) sodium sulphate
(e) table salt
(f) powdered particles of lead from a lead pencil
(g) iron powder
(h) copper filings
(i) sand particles
(j) honey

Answer:
(a) sodium carbonate
(d) sodium sulphate
(e) table salt
(j) honey
Only substances that can break apart into tiny molecules and hide between water molecules form a true solution. The other items, like sand or iron powder, are too heavy or insoluble and will just sink to the bottom or float.
Teacher's Tip: If it disappears completely and the water stays clear, it's a solution.
Exam Tip: Remember that minerals like "sodium carbonate" are salts that dissolve, unlike "calcium carbonate" (chalk) which does not.

Question 11: Name two gases each
(a) which are soluble in water
(b) which are insoluble in water.

Answer:
(a) The gases which are soluble in water are Carbon dioxide and chlorine
(b) The gases which are not soluble in water are Nitrogen, hydrogen
Soluble gases are essential for life; for instance, CO₂ dissolved in water allows aquatic plants to perform photosynthesis. Insoluble gases like nitrogen simply bubble out or stay in the atmosphere above the water.
Teacher's Tip: Fizzy drinks have CO₂ dissolved in them - that's a soluble gas!
Exam Tip: Mention "Carbon dioxide" for soluble gases as it's the most common and important example for this grade.

Question 12: If ‘X’ g. of potassium nitrate is added to 100 g. of water at 60°C and the salt dissolves completely then -
(a) is ‘X’ g. the solubility of potassium nitrate at 60°C.
(b) is the solution formed - saturated or unsaturated
(c) if on addition ‘X’ + ‘Y’ g. of potassium nitrate to the same amount of water at the same temperature and the solute now just remains behind after stirring then -
(d) is the solution now - saturated or unsaturated
(e) is ‘X’ + ‘Y’ g. the solubility of potassium nitrate.

Answer:
Add ‘X’ g. of solute i.e. potassium nitrate to 100 g. of water 60°C.
1. Stir the solute i.e. potassium nitrate in water thoroughly.
2. ‘X’ g. of the solute completely dissolves in water.
3. Add more solute and again stir thoroughly.
4. The solute continues to dissolves.
5. Water i.e. the solvent can dissolve more of the solute at the given temperature.
6. The solution is therefore is said to be unsaturated.
Add more solute to water till on adding an amount ‘X + Y’ g. of the solute i.e. potassium nitrate to 100 g. of water at 60°CC.
1. The solute just remains behind after stirring.
2. The solution is now saturated.
A saturated solution cannot dissolve more of the solute at a given temperature.
(e) Yes, ‘X’ + ‘Y’ g. is the solubility of potassium nitrate at 60°C.
Solubility is the "limit" of how much sugar or salt a cup of water can hold before it gives up and lets the extra solid sit at the bottom. This limit is like a sponge that can only soak up so much liquid before it starts dripping.
Teacher's Tip: Unsaturated = Can hold more. Saturated = Totally full.
Exam Tip: Use the diagrammatic steps to show you understand that a saturated solution has undissolved solute sitting at the bottom.

Question 13: State whether the following statements are true or false. If false write the correct statement.

(a) Solubility of most solids - decrease with increase in temperature.
Answer: False.
Correct - Solubility of most solids - increase in temperature.

(b) Distilled water is potable water.
Answer: False.
Correct - Drinking water is potable water.

(c) The process to remove germs in water is also called sterilization.
Answer: True.
Heat and energy help solvents expand, which is why hot water can hold much more sugar than cold water. Sterilization is a broad term that includes boiling and chemical cleaning to make things safe from bacteria.
Teacher's Tip: Hot tea dissolves sugar better than iced tea!
Exam Tip: When correcting a false statement, only change the key word (like changing "decrease" to "increase") to keep your answer focused.

Question 14: Differentiate between chemical pollution and thermal pollution.

Answer:
(a) Chemical pollution - A large number of industrial chemicals which include chemicals from paint, textile & dyestuff industry & various acids & salt solutions enter into water when discharged as industrial wastes.
Chemical pollutants include - Metallic salt solutions of mercury & lead which cause heavy devastation of marine & plant life.
Agricultural wastes include - Poisonous pesticides namely fungicides & insecticides which may also enter underground water through the soil.
(b) Thermal pollution - Certain industries such as the iron & steel industry & numerous chemical plants use large amounts of water for varied functions. The discharged waste water after going through technical processes - is rendered hot & on entering streams of natural water - enhance growth of harmful biological organisms.
Chemical pollution is about "toxic ingredients" that poison the water, while thermal pollution is about "unnatural heat" that ruins the habitat. Both can kill fish, but for very different reasons: one through poison and the other by removing oxygen.
Teacher's Tip: Chemical = Poison. Thermal = Heat.
Exam Tip: Mention "mercury and lead" as examples of chemical pollutants to show scientific depth.

Question 15: State some important steps to avoid pollution of water.

Answer:
Steps to avoid pollution of water are :
(a) Harmful wastes such as oils & chemicals should not enter into the water.
(b) Proper toilets & sewage systems should be used to prevent human excreta, containing disease causing organisms to enter into the water.
(c) Washing of clothes & bathing should be avoided near water sources.
(d) Planting of trees near water sources including river banks also minimizes pollution.
(e) To minimize thermal pollution the water should be cooled before being discharged as a waste.
(f) Man should be made aware through various awareness programmes & media about the harmful effects of water pollution & ways to control it.
Protecting water starts with keeping our personal and industrial "dirt" away from the places where we get our drinking water. Planting trees acts as a natural barrier that filters soil runoff before it reaches the river.
Teacher's Tip: "Prevention is better than cure" - it's easier to keep water clean than to clean it later.
Exam Tip: List "Sewage treatment" and "Cooling hot water" as two distinct technical solutions for preventing pollution.

Question 16: State what is meant by the term ‘conservation of water’. State a few water saving methods, which may be used in the home to conserve water.

Answer:
Conservation of water is the means of preventing wastage of water so that clean water can be obtained by preventing pollution of water and by protecting the sources of water.
Need for conservation - Inspite of large quantities of water on the earth’s surface only a small percentage is potable water fit for human consumption and household purposes. The need for water is ever increasing and hence all sources of water need to be conserved.
The various methods to conserve the water are :
1. Well should be covered and washing and cleaning should be prevented near a well.
2. Water saving devices must be used in homes
(a) such as closing running taps and using smaller cisterns in toilets.
(b) checking all leakages in household pipes.
(c) turning off the water tap while brushing teeth and while washing hands.
(d) using less electricity, since power plants also consume substantial amount of water.
Water conservation is about respecting the small amount of fresh water we have by not letting any of it go to waste. Simple habits at home, like fixing a leaky pipe, can save hundreds of gallons of water every year.
Teacher's Tip: Conserving water also conserves electricity! It takes power to pump water to your home.
Exam Tip: Define "Conservation" as both "preventing wastage" and "preventing pollution" for a complete answer.

Question 17: Give a reason why :
(a) Conservation of water is essential in spite of the fact that three fourth of the earth’s surface is covered by water.
(b) Polluted water causes disease.
(c) Drip irrigation helps in conservation of water.

Answer:
(a) Because only a small percentage is potable water fit for human consumption and household purposes.
(b) Polluted water acts as a carrier for germs which causes various diseases.
(c) Drip irrigation in agriculture utilizes supply of water in small quantities.
Most of the world's water is salty or frozen, leaving very little for humans to actually use. Techniques like drip irrigation target only the roots of plants, making sure not a single drop is wasted on the empty soil around them.
Teacher's Tip: Drip irrigation is like feeding a plant with a tiny dropper instead of a bucket.
Exam Tip: Be sure to mention that "only a small percentage is potable" to explain why three-fourths of water isn't enough.

Objective Type Questions

Q.l. Complete the statements by filling in the blanks with the correct words.

1. Rain water is the purest form of water. It may dissolve gases like carbon dioxide forming weak acids.
2. Water generates electricity in hydroelectric power stations and steam in boilers for industrial applications.
3. Water is added to the atmosphere by burning of fossil fuels and respiration by living organisms.
4. If potassium nitrate is added to water in a beaker to give a homogeneous mixture, then potassium nitrate is referred to as the solute, water as the solvent and the homogeneous mixture as the solution.
5. Water fit for human consumption and drinking purposes is called potable water.

Q.2. State whether the following statements are true or false. If false, write the correct statement.

1. Sea water contains salts of calcium and magnesium.
Answer: True. The ocean is full of these minerals, which is what makes it salty and "hard" water. These salts are part of the reason you shouldn't drink sea water.

2. Water finds application as a means of transporting goods.
Answer: True. Ships and barges use rivers and oceans to carry massive amounts of food and products across the world. It is often the cheapest way to move very heavy cargo.

3. On boiling water exists in the liquid state.
Answer: False.
Correct - On boiling water exists in the gaseous state.
When you see steam coming out of a kettle, you are seeing water turn into gas. This happens when the water reaches its boiling point of 100°C.

4. Respiration uses up water from the atmosphere.
Answer: False.
Correct - Respiration add up water to the atmosphere.
When we breathe out, we release moisture from our lungs into the air. This moisture becomes part of the atmospheric water vapor in the water cycle.

5. Well water exists below the impervious rocky layers of earth.
Answer: False.
Correct - Well water exists above the impervious rocky layers of earth.
Water soaks through the soil until it hits rock that it can't pass through; it then sits on top of that rock like a giant puddle. This "puddle" is what we tap into when we dig a well.

6. Sodium carbonate and sodium sulphate do not form an aqueous solution.
Answer: False.
Correct - Sodium carbonate and sodium sulphate form an aqueous solution.
"Aqueous" simply means "in water." Both of these minerals dissolve perfectly in water to create a clear, uniform mixture.

7. If ‘X’ g. of solute is added to 100 g. of water at t°CC and the solution formed is a saturated solution, then ‘X’ g. is the solubility of the solute.
Answer: True. Solubility is defined as the maximum amount of a substance that can dissolve in a specific amount of liquid. If it's saturated, it has reached that maximum limit.

8. Purification of water is carried out to remove – dissolved gases e.g. carbon dioxide and dissolved minerals like magnesium salts.
Answer: True. While some minerals are good, too many can make water "hard" or bad for drinking. We use treatment to make sure the minerals are at just the right levels for health.

9. Water from rivers and lakes is – potable water.
Answer: False.
Correct - Water from rivers and lakes is impure water.
Natural surface water is almost always dirty because of soil, animals, and bacteria. It only becomes "potable" after it goes through a treatment plant to be cleaned.

10. Chemical pollutants include metallic salt solution of mercury and lead.
Answer: True. These metals are very toxic and can stay in the water for a long time. They are very dangerous to fish and to the humans who eat those fish.

Q.3. Match the statements in List I with their correct answer in List II.

List I
1. Sodium carbonate and water
2. Chemical used in sedimentation during purification of water
3. Vapourisation
4. Kerosene and water
5. A solution which can dissolve more of solute at a given temperature
6. Water as a habitat for marine life
7. A water-borne disease
8. Existence of water between 0°C and 100°C
9. Taste in water is due to
10. The most impure form of water

List II
A : Sea water
B : Preferred place for an organism
C : Liquid state
D : Aqueous solution
E : Dissolved gases and minerals
F : Unsaturated solution
G : Immiscible mixture
H : Liquid to gaseous state
I : Dysentery
J : Alum

Answer:

1. Sodium carbonate and water - D : Aqueous solution
2. Chemical used in sedimentation during purification of water - J : Alum
3. Vapourisation - H : Liquid to gaseous state
4. Kerosene and water - G : Immiscible mixture
5. A solution which can dissolve more of solute at a given temperature - F : Unsaturated solution
6. Water as a habitat for marine life - B : Preferred place for an organism
7. A water-borne disease - I : Dysentery
8. Existence of water between 0°C and 100°C - C : Liquid state
9. Taste in water is due to - E : Dissolved gases and minerals
10. The most impure form of water - A : Sea water

Q.4. Explain the meaning of the following terms :
1. Rain water harvesting
2. Drip irrigation in agriculture
3. Saturated solution
4. Conservation of water
5. Gaseous state of water in air

Answer:
1. Rain water harvesting which is a means of utilizing rain water instead of allowing it to be wasted to be conducted by building tanks or pits in low lying areas and collecting roof top rain water through pipes into tanks.
2. Drip irrigation in agriculture utilizes supply of water in small quantities.
3. A saturated solution cannot dissolve more of the solute at a given temperature.
4. Conservation of water is the means of preventing wastage of water so that clean water can be obtained by preventing pollution of water and by protecting the sources of water. Polluted water acts as a carrier for germs which causes various diseases.
5. Gaseous state of water in air : Gaseous State as water vapour in air the amount depending on climatic conditions.
These terms describe how we manage water resources and how water behaves in different chemical and environmental situations. Proper water management, like harvesting and conservation, is the only way to ensure we have enough water for the future.
Teacher's Tip: Use "Drip" for "Direct to Roots" in irrigation.
Exam Tip: For "Saturated solution," always emphasize that no more solute can dissolve at that *specific* temperature.

Q.5. Name the following

Question 1: A chemical used during chlorination of water.
Answer: Chlorine.
Chlorine is a strong disinfectant that kills germs by breaking down their biological structure. It is widely used in both city water systems and swimming pools to keep water safe.
Teacher's Tip: Chlorine is the "Germ Terminator."
Exam Tip: Spell "Chlorine" correctly as it is a common chemistry term.

Question 2: An agricultural pollutant in water.
Answer: Insecticides.
Insecticides are chemicals used to kill pests on crops, but they can wash into rivers when it rains. This causes pollution that can harm fish and birds far away from the farm.
Teacher's Tip: Pesticides and Fertilizer are also good answers.
Exam Tip: Mention that these pollutants are "poisonous" to explain why they are harmful.

Question 3: A solid, ‘natural form of water.
Answer: Snow.
Snow is formed when water vapour in the sky freezes directly into ice crystals without becoming a liquid first. It acts as a massive storage of fresh water for many rivers in the spring.
Teacher's Tip: Hail and Frost are also solid forms of water.
Exam Tip: Identify "solid" as the state of matter to match the question's requirement.

Question 4: The natural process by which circulation of water takes place from earth’s surface to atmosphere and back to earth’s surface.
Answer: Water cycle.
The water cycle is the Earth's way of constantly cleaning and moving its water supply. It relies on the sun's energy to lift water up and gravity to pull it back down.
Teacher's Tip: Water cycle = Nature's Recycling Program.
Exam Tip: Use the full term "Water Cycle" or "Hydrological Cycle" for your answer.

Question 5: The liquid or medium of dissolution which allows the solute to dissolve in it.
Answer: Water (solvent).
The solvent is the part of the mixture that "does the work" of dissolving the other substance. Because water is so good at this, we see it used as the solvent in almost all biological and industrial processes.
Teacher's Tip: Solvent is the "Host" that welcomes the "Solute" guest.
Exam Tip: Identify the "Solvent" as the component present in larger quantity in a solution.