Selina Concise Solutions for ICSE 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.

 
Water is a unique substance that covers most of our planet and is necessary for the survival of every known organism. Its ability to exist in three different forms at once and dissolve many substances makes it the most important compound in chemistry.
Teacher's Tip: Think of "H-2-O" as "Two parts Hydrogen, One part Oxygen" to remember the formula easily.
Exam Tip: Always remember that water is densest at 4°C, not at its freezing point, which is a common trick question.

Exercise - I

Question 1: Name the four main sources of water.
Answer:
(i) Oceans (ii) Seas
(iii) Rivers (iv) Lakes 
These are the primary natural bodies where water is stored and transported across the Earth's surface. They provide habitats for marine life and are essential for the water cycle to function properly.
Teacher's Tip: Use the acronym "O.S.R.L." (Oceans, Seas, Rivers, Lakes) to keep them in mind.
Exam Tip: List all four items clearly with numbering to ensure you get full credit for the answer.

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. 
The water cycle acts as a giant recycling system that cleans and redistributes water globally. Without it, some parts of the world would be permanent deserts while others would be permanently flooded.
Teacher's Tip: Imagine the water cycle as the Earth's way of breathing water in (evaporation) and out (precipitation).
Exam Tip: Use terms like "regulation" and "availability" to sound more scientific in your descriptions.

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. 
Water is vital because it transports nutrients and oxygen to cells while removing waste products from the body. It also helps in maintaining body temperature through processes like sweating.
Teacher's Tip: Remember that even a tiny seed is "thirsty" and contains water to help it grow into a plant.
Exam Tip: Mention the high percentage of water in blood (90%) to provide a strong factual basis for your answer.

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 
Hydrogen and Oxygen are the building blocks that bond together to create the liquid we drink. The specific 2:1 ratio means there are twice as many hydrogen atoms as oxygen atoms in every single water molecule.
Teacher's Tip: Think of a Mickey Mouse head - the big head is Oxygen and the two small ears are Hydrogen!
Exam Tip: Always write the "2" in H₂O as a small subscript to show the correct chemical notation.

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. 
Changing the environment of water by adding substances or pressure alters how its molecules move and escape. Pressure forces molecules to stay together longer, while impurities get in the way of molecules trying to bond or escape.
Teacher's Tip: This is why a pressure cooker cooks food faster - the water gets much hotter than 100°C.
Exam Tip: For part (b), remember that impurities "expand" the liquid range by lowering the bottom and raising the top.

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. 
These phenomena are caused by water's unique physical traits, such as its density changes and its reaction to pressure. Understanding these helps explain everyday occurrences like why ice cubes float in your soda or why car engines need water.
Teacher's Tip: Remember that water is "weird" because it expands when it gets cold, unlike almost everything else which shrinks.
Exam Tip: When explaining why ice floats, always use the keyword "density" to get full marks.

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. 
Because ice forms on the top of a lake rather than the bottom, it creates an insulating "blanket" for the fish. This keeps the water at the bottom at a steady 4°C, allowing life to continue even when the air is freezing.
Teacher's Tip: Think of the ice layer as a warm winter coat for the whole pond.
Exam Tip: Clearly state that ice is a "bad conductor of heat" to explain why the water below stays warm.

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 
In a salt-water mixture, the salt is the solute because it is being dissolved, and the water is the solvent because it does the dissolving. Together, they form a uniform solution where you cannot see the individual parts.
Teacher's Tip: Solvent is the "Big Boss" (larger quantity) and Solute is the "Small Guest" (smaller quantity).
Exam Tip: Always include the simple formula "Solution = Solute + Solvent" to show a clear understanding.

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. 
A saturated solution is like a full bus; it cannot take any more passengers unless someone leaves or conditions change. Supersaturated solutions are special because they are "forced" to hold extra solute, often by heating and then cooling carefully.
Teacher's Tip: Think of "Unsaturated" as "Hungry," "Saturated" as "Full," and "Supersaturated" as "Overstuffed."
Exam Tip: Mention "at a given temperature" in your definitions, as solubility changes if the temperature changes.

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. 
Heat helps solids dissolve by providing energy to break apart their crystals, but it makes gas molecules move too fast to stay in the liquid. Pressure acts like a lid, forcing gas molecules into the liquid, which is why sodas are fizzy when opened.
Teacher's Tip: Remember: Hot Tea dissolves more sugar (solid), but Cold Soda stays more fizzy (gas).
Exam Tip: Clearly distinguish between the behavior of solids and gases to avoid losing marks.

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^-10 m
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^-7 m.
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. 
These differences depend on the size of the particles mixed into the liquid and whether they stay mixed or sink. Solutions are perfectly blended, suspensions are clumpy and sink, and colloids are the "middle ground" like milk or fog.
Teacher's Tip: Remember: Solution = Clear, Colloid = Cloudy, Suspension = Chunky!
Exam Tip: Using particle size measurements like 10^-10 m in your answer shows a high level of scientific accuracy.

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:
Common name: Blue vitriol | Chemical name: Copper sulphate pentahydrate | Formulae: CuSO₄  5H₂O
Common name: Green vitriol | Chemical name: Iron (II) sulphate heptahydrate | Formulae: FeSO₄  7H₂O 
This "hidden" water is part of the solid crystal structure and gives some minerals their beautiful shapes and colors. If you heat these crystals, the water evaporates and the crystal often turns into a dull white powder.
Teacher's Tip: Think of it as water being "trapped" inside a cage made of salt molecules.
Exam Tip: Be sure to include the "dot" in the chemical formula to show the water is loosely attached.

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 \cdot 10H₂O)
(b) Common salt (NaCl), potassium nitrate (KNO₃), sugar (C₁₂H₂₂O₁₁) 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 10H₂O)
(f) Milk, blood, smoke, jellies, butter, ink etc.
(g) Acetone, ethanol, turpentive 
Chemicals interact with moisture in many ways: some hold onto it (hydrated), some pull it from the air (deliquescent), and some give it away (efflorescent). Learning these categories helps scientists predict how chemicals will behave when left out on a lab bench.
Teacher's Tip: "Deliquescent" substances are so thirsty they actually turn into a puddle of liquid by absorbing air moisture!
Exam Tip: For hydrated substances, always write the full formula including the water molecules (10H₂O) to get full marks.

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 (CuSO₄  5H₂O). When it is heated, it loses water of crystallisation and becomes an hydrous CuSO₄ 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 treated with water. 
These observations prove that chemical properties change when water is removed or added. Litmus paper is a great way to show that water is neither an acid nor a base, but perfectly neutral.
Teacher's Tip: Blue vitriol "faints" and turns white when it loses its water, but it "wakes up" and turns blue again if you add a drop of water!
Exam Tip: Mention the specific color change (blue to grey-white) to show you know the exact results of the experiment.

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. 
Moisture in the air is constantly looking for surfaces to cling to, and some chemicals are like magnets for water. The fizz in a soda is just the gas escaping because the high pressure keeping it inside has been released.
Teacher's Tip: Pure salt doesn't get wet; it's the "hitchhiking" impurities that cause the mess in your salt shaker!
Exam Tip: Be sure to clarify that it is the *impurities* in salt, not the salt itself, that are deliquescent.

Question 9: Give balanced chemical equations for the reaction of water with
(a) Sodium (b) Iron
(c) Carbon dioxide (d) Sodium oxide
Answer:
(a) Sodium + Water →fast Sodium hydroxide + Hydrogen (cold)
2Na + 2H₂O → 2NaOH + H₂(g) (cold)
(b) Iron with water
3Fe + 4H₂O → Fe₃O₄ + 4H₂(g) (hot/steam)
(c) Carbon dioxide + Water → Carbonic acid
CO₂ + H₂O → H₂CO₃
(d) Sodium oxide + Water → Sodium hydroxide (alkali)
Na₂O + H₂O → 2NaOH 
Chemical equations are like recipes that show exactly how atoms rearrange themselves when they meet. These reactions show that water can act as a partner to form acids, bases, or release hydrogen gas depending on what it touches.
Teacher's Tip: Notice how Sodium reacts with *cold* water, but Iron needs *steam*—this shows Sodium is much more reactive.
Exam Tip: Always include states like (g) for gas to show exactly what is produced in the reaction.

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.
K (Potassium)
Na (Sodium)
Ca (Calcium)
Mg (Magnesium)
Al (Aluminium)
Zn (Zinc)
Fe (Iron)
Pb (Lead)
(H) (Hydrogen)
Cu (Copper)
Hg (Mercury)
Ag (Silver)
Au (Gold)
Pt (Platinum) 
The reactivity series is a "leaderboard" for metals, showing which ones are aggressive and which ones are shy. Metals at the top will kick out metals at the bottom during a chemical "fight."
Teacher's Tip: Use a mnemonic: "Please Stop Calling Me A Zebra, I Like Her Calling Me Smart Goat" (Potassium, Sodium, Calcium, Magnesium, Aluminium, Zinc, Iron, Lead, Hydrogen, Copper, Mercury, Silver, Gold).
Exam Tip: Remember that Hydrogen is included in the list even though it's a gas, as a point of comparison.

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 
Different reactions result in the release of different invisible gases that we can identify through chemical tests. Knowing these gases helps us understand the outcome of industrial processes and environmental changes.
Teacher's Tip: "Water gas" is a special mix of Hydrogen and Carbon Monoxide used in industry.
Exam Tip: If an answer is a specific name like "Water gas," make sure to use that exact term rather than just listing the individual gases.

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. 
Hard water contains dissolved minerals that "fight" against soap, making it hard to get bubbles. Soft water is "cleaner" in terms of mineral content and makes bathing and washing clothes much easier.
Teacher's Tip: Think: "Soft" water gives "Soft" bubbles; "Hard" water gives "Hard" scum.
Exam Tip: Use the word "lather" to describe bubbles to get professional marks on your exam.

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.
Ca(HCO₃)₂ →boiled CaCO₃ + H₂O + CO₂(g)
(soluble calcium bicarbonate) → (ppt) (calcium carbonate) + (water) + (carbon dioxide)
(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.
Na₂CO₃ + CaSO₄ → CaCO₃ + Na₂SO₄
(sodium carbonate) + (calcium sulphate) → (insoluble calcium carbonate) + (soluble sodium sulphate) 
Temporary hardness is like a "visitor" that leaves if you turn up the heat, but permanent hardness is like a "resident" that needs chemicals to be removed. Removing these minerals is important for keeping our pipes from getting clogged and making our soap work better.
Teacher's Tip: Boiling "kills" temporary hardness, but you need Washing Soda to "wash away" permanent hardness.
Exam Tip: Be sure to mention "insoluble carbonates" as the result of these reactions, as they are what get filtered out.

Question 3: Name three water borne diseases.
Answer:
(i) Cholera (ii) Typhoid (iii) Hepatitis 
These illnesses are caused by tiny germs that live in dirty water and can make people very sick if they drink it. This is why filtering and boiling water is a crucial safety step in many parts of the world.
Teacher's Tip: Remember "C.T.H." - Clean The Hygeine!
Exam Tip: Spelling is important for disease names, so practice writing them out a few times.

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.

 
Pollution happens when we treat our water sources like trash cans for chemicals and sewage. By managing our waste better and protecting riverbanks, we can ensure there is enough clean water for everyone in the future.
Teacher's Tip: Think of trees on a riverbank as "nature's filter" that keeps dirt and trash out of the water.
Exam Tip: When listing control methods, divide them into "Industrial" and "Domestic" for a more organized answer.

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 in cold weather. Because it lowers the freezing point of water.
(b) Ice bergs float in ocean water because density of ice is less than water.
(c) Carbonated drinks are bottled under high pressure because the solubility of carbon dioxide increases with pressure. 
Alcohol acts as an "antifreeze" to keep engines running in snowy weather. The pressure in a soda bottle is what keeps the bubbles "hiding" inside the liquid until you are ready to drink it.
Teacher's Tip: It's the same reason we put salt on snowy roads—to stop the water from turning into slippery ice!
Exam Tip: For part (c), always mention the direct relationship: more pressure = more solubility.

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.
 
These blanks cover the foundational facts about water's behavior and its role in chemistry. Mastering these terms will help you understand more complex environmental and chemical problems later.
Teacher's Tip: Remember the 4°C rule—it's the magical "sweet spot" for water's density.
Exam Tip: Be careful with (c); usually solubility *decreases* with heat for gases, so double-check the logic in your head before writing.

2. Match the statements in column A with that in column B.
Column A
(a) deliquescent substance
(b) hygroscopic substance
(c) efflorescent substance
(d) substance causing temporary hardness
Column B
(i) sodium bicarbonate
(ii) magnesium chloride
(iii) conc. sulphuric acid
(iv) washing soda
Answer:
(a) deliquescent substance - (ii) magnesium chloride
(b) hygroscopic substance - (iii) conc. sulphuric acid
(c) efflorescent substance - (iv) washing soda
(d) substance causing temporary hardness - (i) sodium bicarbonate
 
This matching helps you link chemical terms with real substances you might find in a lab or kitchen. Knowing which ones absorb water or lose it is vital for storing chemicals correctly.
Teacher's Tip: Use "Hygro-scopic" = "Huge thirst" to remember it absorbs water without melting.
Exam Tip: Draw straight lines or write the letters clearly to avoid confusion for the grader.

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.
 
One-word answers test your precise knowledge of scientific vocabulary. Being able to define a whole process or substance with a single term is a key skill for any scientist.
Teacher's Tip: "Potable" sounds like "portable," so imagine taking your safe water on a hike!
Exam Tip: Practice your spelling for words like "Anhydrous" and "Crystallisation" as they are commonly misspelled.

MULTIPLE CHOICE QUESTIONS

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

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

3. The ultimate source of all water on the earth is
(a) oceans and seas
(b) spring and wells
(c) rivers and lakes
(d) rain
Answer: (d) rain
 

4. Colloids have the particle size range between
(a) 10^-7 to 10^-10 m
(b) 10^-10 to 10^-12 m
(c) 10^-7 to 10^-5 m
(d) 10^-12 to 10^-15 m
Answer: (a) 10^-7 to 10^-10 m
 
These questions test your ability to pick the most accurate answer from a list. It covers everything from global water sources to the tiny scales of particle physics.
Teacher's Tip: For question 3, remember that although oceans are big, they wouldn't fill without rain!
Exam Tip: Read all options carefully before choosing, even if the first one seems right.

ADDITIONAL QUESTIONS

Question 1: What is the smallest particle of water ? Describe it’s structure.
Answer: The smallest particle of water is ‘molecule’. The molecule of water is made up of two atoms of hydrogen (H₂) and one atom of oxygen (O). Both of these atoms combine to form a compound called water i.e., H₂ + O → H₂O. 
A single molecule is the tiny unit that makes water behave the way it does. Even though it's too small to see, millions of these molecules stick together to form the liquid you drink.
Teacher's Tip: Think of the molecule as a LEGO set that only fits together in one specific way.
Exam Tip: Use the chemical formula H₂O to support your structural description.

Question 2: Describe the water-cycle in nature.
Answer: Water constantly moves from the earth to the air and again comes down to the earth in the form of rain. Water from seas, rivers lakes, ponds, puddles and streams evaporates due to the heat of the sun. Plants too give out large amounts of water from their leaves. Water vapours rise up. The air higher up in the atmosphere is cooler which cools the water vapours and condenses to form tiny droplets of water on small dust particles. These droplets combine to form clouds. The clouds get cooled further, the water drops become bigger. When they become too heavy, they fall on the earth as rain. The rainwater again forms seas, rivers, lakes, ponds and streams and it again evaporates to the air, thus the water-cycle in nature continues. 
This loop ensures that Earth's water is never truly lost, just moved and transformed. It is the reason we have weather, clouds, and fresh water to drink even from salty oceans.
Teacher's Tip: Remember the steps as: Evaporation (Up), Condensation (Cloud), Precipitation (Down).
Exam Tip: Be sure to mention "transpiration" (water from plants) for a more complete and advanced answer.

Question 3: How are natural springs of water formed ?
Answer: The percolated water gets collected beneath the earth on the non-porous rocks. This collected water forms the water-table. The ground water comes out of the surface at some places to form natural springs. 
Springs are like "pressure valves" for the water hidden deep underground. When the underground water-table gets too high or hits a crack in the rock, it pushes its way up to the sunlight.
Teacher's Tip: Think of underground water like a giant sponge under the soil.
Exam Tip: Use the term "water-table" to show you understand how underground water levels work.

Question 4: Why is sea water salty ?
Answer: Rain water when pours down to earth, it dissolves salts and flows into the sea by rivers. The continuous discharge of saline water into the sea makes the sea water salty. 
As rain flows over rocks and through soil, it picks up tiny amounts of minerals and carries them to the ocean. Over millions of years, these minerals have built up, making the ocean extremely salty while the rivers stay fresh.
Teacher's Tip: The ocean is like a giant soup that has been simmering for billions of years, getting saltier and saltier!
Exam Tip: Mention that the process happens "continuously" over a long period of time for a thorough answer.

Question 5: Discuss the importance of water as a solvent for (a) our body (b) plants.
Answer: (a) Our body - Our body cannot absorb food substances unless they are soluble in water. In this process of digestion, food is reduced to simple substances that are soluble in water. They can then be dissolved in water and absorbed by the body. Extrements are defacated from our body by dissolving in water. Chemical reactions occur inside our body through water.
(b) Plants - In plants, mineral-salts are transported to the upper parts of the plants from the soil by means of water through roots. As a solvent, water helps the plants in preparing their food through photosynthesis process. 
Water acts like a "delivery truck" in both humans and plants, carrying nutrients where they are needed. Without its ability to dissolve these minerals, life would essentially stop functioning.
Teacher's Tip: If water wasn't a solvent, plants would just be standing in dry dirt without any "food" getting to their leaves.
Exam Tip: For plants, remember to link water specifically to the process of "photosynthesis."

Question 6: What do you mean by solubility of a solute in a solvent ?
Answer: The solubility of a solute in a solvent at a particular temperature is the amount of solute that can be dissolved in 100 mL of water at that temperature. 
Solubility tells us the "speed limit" for how much of a substance can fit into water. It changes based on how hot the water is and what kind of substance you are trying to dissolve.
Teacher's Tip: Imagine 100 mL as a "room" and the solute as "people" trying to fit inside.
Exam Tip: Don't forget to mention "100 mL" and "at a particular temperature" as these are the standard units for the definition.

Question 7: Give any three important uses of water.
Answer: Water is indispensable for all the living-organisms. 70% of human body consists of water. Water sustains several life processes in humans and animals. Water helps in controlling our body temperature. Water also generates electricity. Water keeps the environment and atmosphere mild when it is too hot. 
Beyond drinking, water powers our cities through hydroelectric dams and keeps our planet's climate stable. It is the most versatile resource we have, used in everything from cleaning to high-tech manufacturing.
Teacher's Tip: Remember: Life, Light (Electricity), and Lunch (Agriculture)!
Exam Tip: Pick three very different uses (like biological, industrial, and environmental) to show a broad understanding.

Question 8: Explain how the peculiar variation of density of water with temperature can (a) protect marine-life (b) cause water - pipes to burst in very cold places.
Answer:
(a) Due to steep fall in the temperature in coastal regions, the density of water in the sea varies and the top layer of the sea water turns into ice and floats on the surface of sea-water because of lesser density than that of water. Whereas, the bottom denser layers of water remain as usual, thus the marine-life gets protected from freezing.
(b) The peculiar variation of density of water with temperature causes water - pipes to burst in very cold places, it happens due to expansion of water on freezing. The water inside the pipes gets frozen and expands, pressing the inner walls of the water-pipes, causing them to burst. 
Water is unique because it expands as it gets colder than 4°C, which is the opposite of most other liquids. This protects fish in the winter but can destroy our plumbing if the water in the pipes turns to ice.
Teacher's Tip: Think of freezing water as a "strong man" pushing outward with huge force.
Exam Tip: Be sure to use the word "expansion" when explaining why pipes burst.

Question 9: A captain of ship will tell you that an iceberg is much bigger than it looks. Why is it so ?
Answer: Icebergs are the huge formation of water turned into ice, floating in the sea-water in the Arctic and Antarctic oceans Merely, 1/9th of the iceberg is visible above the water. The rest of the 8/9th is below the surface. Hence, an iceberg is very much bigger than it seems to be.
 
Because ice is only slightly less dense than water, it sits very low in the ocean. This makes them dangerous for ships because most of the massive mountain of ice is hidden "invisible" under the waves.
Teacher's Tip: This is where the phrase "the tip of the iceberg" comes from—it means you only see a tiny part of the whole problem!
Exam Tip: Using the fraction 1/9 above and 8/9 below will show you have a precise understanding of the topic.

Question 10: Why is water used in hot-water bottles ?
Answer: Water takes a long time to cool down, so water is used in hot water bottles to prolong the warming-effect inside the hot- water bottle. 
Water has a high "heat capacity," meaning it can hold a lot of thermal energy without losing it quickly. This makes it the perfect "battery" for heat on a cold night.
Teacher's Tip: Water is like a "heat hoarder"—it grabs heat and doesn't want to let go!
Exam Tip: Use the phrase "high specific heat" for a more advanced and high-scoring answer.

Question 11: What do you mean by a saturated solution ?
Answer: When a solute stops dissolving into a solvent beyond a particular limit, the formed solution is said to be a saturated solution. 
In a saturated solution, the "gaps" between water molecules are completely filled with solute. If you add even one more grain of sugar, it will simply fall to the bottom because there is no more room for it to dissolve.
Teacher's Tip: Think of it as a sponge that is so wet it can't hold another drop of water.
Exam Tip: Remember to specify that the temperature must remain constant for the solution to be truly saturated.

Question 12: Answer the following questions:
1. Why is acid added to water before electrolysis ?
2. What is the chemical composition of water ? Give its molecular formula.
3. How does the anomalous expansion of water help aquatic organisms in cold climates ?
4. Name two gases dissolved in natural water.
Answer:
1. Acid is added to water before electrolysis to make it good conductor of electricity.
2. The constituents of water are hydrogen and oxygen. The molecular formula is H₂O.
3. When water is cooled it contracts and its density, increases until the temperature reaches 4°C, on further cooling water starts expanding with decrease in its density. This is anomalous phenomenon. The aquatic organisms can live there easily.
4. Hydrogen and oxygen gases are dissolved in water. 
Pure water is actually a poor conductor of electricity, so the acid "greases the wheels" for the reaction to happen. These four points summarize the most critical chemical and physical properties you need to know about water.
Teacher's Tip: For part 1, just remember: Acid = Electricity's Helper!
Exam Tip: Ensure you answer all parts of a multi-part question to get the full score.

Question 13: What do you mean by water pollution ? What are its causes and how can water pollution be controlled ?
Answer:
Water pollution - The water present in rivers, ponds, lakes and streams, comes from rain and the melted snow of the mountains. As it flows the down the plains, it picks up many dissolved and suspended impurities and gets polluted.
Causes of water pollution:

• Industrial and agricultural processes - Fertilizers, pesticides, insecticides and other chemicals used in cultivated fields, get dissolved in water when it flows over these fields. They make the water unfit for consumption.
• Nuclear and thermal power plants - Power plants discharge the hot water often containing chemicals into water streams. It is harmful for aquatic life.
• The discharge of sewage and garbage - The discharge of sewage and garbage into river water is one of the major causes of water pollution.

Prevention of water pollution -

• Make people aware of pollution.
• Make sanitary facilities available is rural areas and city slums.
• Domestic sewage should be treated before being discharged into rivers.
• The solid matter separated from sewage can be used to generate biogas.
• The waste products of industries should be treated before they are discharged into rivers and other water bodies.
• We should stop using substances like detergents, pesticides, polythene etc. 

Pollution is the "poisoning" of our most precious resource through human negligence. By using modern technology like sewage treatment plants and making better personal choices, we can keep our waterways safe and healthy.
Teacher's Tip: Pollution doesn't just "go away"—it travels downstream and affects everyone eventually.
Exam Tip: Be sure to define what pollution *is* before listing the causes and controls.

Question 14: Give balanced chemical equations to show the reaction between:
(a) Coke and steam
(b) Magnesium and water
Answer:
(a) Coke + Steam →1000°C Water gas
C + H₂O →1000°C [CO + H₂] (Water gas)
(b) Magnesium + Water → Magnesium oxide + hydrogen
Mg + H₂O →(steam) MgO + H₂ (g) 
These reactions require extreme conditions like 1000°C or steam because Carbon and Magnesium are not reactive enough to change water at room temperature. They show how scientists can force chemical changes using heat and pressure.
Teacher's Tip: Remember that "Coke" is just a fancy name for pure Carbon (C).
Exam Tip: Always write the temperature (1000°C) above the arrow to show the reaction conditions.