Water Exercise Solutions | KalviGavani

How to Use This Page

Solve every exercise question on your own first, then compare your answers below. Focus on understanding why each answer is correct — that builds exam confidence.

Key Concepts (Quick Recap)

• Water covers about 71% of the Earth's surface, but only about 1% is available as freshwater for human use.
• Water cycle: The continuous circulation of water through evaporation, transpiration, condensation, precipitation, and infiltration.
• Evaporation: Conversion of water from liquid to vapour state (surface water, below boiling point).
• Transpiration: Release of water vapour from the leaves of plants through stomata.
• Condensation: Conversion of water vapour back into liquid water droplets forming clouds, fog, or dew.
• Precipitation: Falling of water from the atmosphere to the Earth's surface as rain, snow, sleet, or hail.
• Infiltration: Seeping of water into the soil and porous rocks to form groundwater.
• Hard water: Contains dissolved salts of calcium and magnesium (bicarbonates, chlorides, sulphates). Does not lather with soap easily.
• Soft water: Free from dissolved calcium and magnesium salts. Lathers easily with soap.
• Temporary hardness: Caused by dissolved bicarbonates of calcium and magnesium. Removed by boiling.
• Boiling: Ca(HCO3)2 → CaCO3↓ + H2O + CO2↑
• Permanent hardness: Caused by dissolved chlorides and sulphates of calcium and magnesium. Not removed by boiling. Removed by washing soda or ion exchange.
• Washing soda: CaSO4 + Na2CO3 → CaCO3↓ + Na2SO4
• Water treatment stages: Sedimentation → Filtration → Disinfection (chlorination).
• Water conservation: Rainwater harvesting, drip irrigation, reducing wastage, afforestation, and preventing water pollution.

I. Choose the Best Answer (MCQs)

**Q1. About 71% of the Earth's surface is covered by .............. (a) Air (b) Water (c) Land (d) None of these

**Q2. The process of conversion of water vapour into liquid water is called .............. (a) Evaporation (b) Condensation (c) Precipitation (d) Infiltration

**Q3. Which of the following processes releases water vapour from plants? (a) Evaporation (b) Transpiration (c) Respiration (d) Precipitation

**Q4. Hard water does not lather easily with soap because it contains .............. (a) dissolved sodium salts (b) dissolved calcium and magnesium salts (c) dissolved iron salts (d) dissolved potassium salts

**Q5. Temporary hardness of water is due to the presence of .............. (a) calcium and magnesium chlorides (b) calcium and magnesium sulphates (c) calcium and magnesium bicarbonates (d) sodium bicarbonate

**Q6. Permanent hardness of water can be removed by .............. (a) boiling (b) adding washing soda (c) adding common salt (d) adding lime

**Q7. Which of the following is a method of water conservation? (a) Rainwater harvesting (b) Deforestation (c) Over-irrigation (d) Industrial discharge

**Q8. The process by which rainwater seeps into the ground is called .............. (a) Evaporation (b) Infiltration (c) Condensation (d) Transpiration

II. Fill in the Blanks

1. About 71% of the Earth's surface is covered by water. 2. The continuous circulation of water from the Earth's surface to the atmosphere and back is called the water cycle. 3. The process of conversion of liquid water into water vapour is called evaporation. 4. Transpiration is the process by which plants release water vapour through stomata on their leaves. 5. The falling of water from the atmosphere to the Earth's surface is called precipitation. 6. Water that contains dissolved calcium and magnesium salts is called hard water. 7. Temporary hardness of water is caused by dissolved bicarbonates of calcium and magnesium. 8. Permanent hardness of water is caused by dissolved chlorides and sulphates of calcium and magnesium. 9. The chemical name of washing soda is sodium carbonate (Na2CO3). 10. The process of seeping of water into the ground is called infiltration.

III. True or False (with corrections for false statements)

IV. Match the Following

V. Short Answer Questions

Q1. What is the water cycle?

The water cycle (hydrologic cycle) is the continuous movement of water from the Earth's surface to the atmosphere and back again. It involves five main processes: evaporation (water from surface to vapour), transpiration (water from plants to vapour), condensation (vapour to liquid forming clouds), precipitation (rain, snow, etc.), and infiltration (water seeping into the ground). The Sun provides the energy that drives the water cycle.

Q2. What is evaporation? Give two examples.

Evaporation is the process by which water changes from the liquid state to the vapour state at temperatures below its boiling point. It is a surface phenomenon. Examples: 1. Drying of wet clothes in the sun. 2. Evaporation of water from puddles and lakes.

Q3. Define transpiration.

Transpiration is the process by which plants lose water in the form of water vapour through tiny pores called stomata present on the surface of their leaves. It helps in the upward movement of water and minerals from roots to leaves and also cools the plant.

Q4. What is precipitation?

Precipitation is the process by which water falls from the atmosphere back to the Earth's surface in the form of rain, snow, sleet, or hail. It occurs when water droplets or ice crystals in clouds become heavy enough to fall due to gravity.

Q5. What is hard water?

Hard water is water that contains dissolved salts of calcium and magnesium in the form of bicarbonates, chlorides, and sulphates. It does not lather easily with soap and instead forms a sticky scum. It also causes scale formation in boilers and pipes.

Q6. What is soft water?

Soft water is water that is free from dissolved salts of calcium and magnesium. It lathers easily with soap and does not form scum or scale. Examples include rainwater and distilled water.

Q7. Name two methods by which hard water can be made soft.

1. Boiling: Removes temporary hardness by decomposing bicarbonates into insoluble carbonates. 2. Adding washing soda (Na2CO3): Removes permanent hardness by converting soluble chlorides and sulphates of calcium and magnesium into insoluble carbonates.

VI. Detailed Answer Questions

Q1. Describe the water cycle with a neat diagram.

The water cycle (hydrologic cycle) is the continuous circulation of water between the Earth's surface and the atmosphere. It is driven by the Sun's energy and consists of the following stages: 1. Evaporation: The Sun heats water in oceans, rivers, lakes, and ponds, converting it into water vapour. Since oceans cover most of the Earth's surface, they contribute the maximum amount of water vapour. 2. Transpiration: Plants release water vapour through stomata on their leaves. This process adds a significant amount of moisture to the atmosphere. 3. Condensation: As water vapour rises, it cools at higher altitudes and condenses into tiny water droplets or ice crystals, forming clouds, fog, or dew. 4. Precipitation: When the water droplets in clouds combine and become heavy enough, they fall to the Earth as rain, snow, sleet, or hail. 5. Infiltration and Run-off: Some of the precipitated water seeps into the ground (infiltration) to replenish groundwater. The rest flows over the land as surface run-off into rivers, lakes, and oceans, and the cycle continues. The water cycle ensures a continuous supply of freshwater on Earth and maintains the balance of the ecosystem.

Q2. Differentiate between hard water and soft water.

| Property | Hard Water | Soft Water | |---|---|---| | Dissolved salts | Contains calcium and magnesium salts | Free from calcium and magnesium salts | | Soap lather | Does not lather easily; forms scum | Lathers easily with soap | | Types | Temporary and permanent hardness | No hardness | | Boiling effect | Temporary hardness removed by boiling; permanent hardness remains | Already soft; no effect needed | | Scale formation | Forms scales in boilers and pipes | Does not form scales | | Washing | Wastes soap; clothes look dull | Cleans efficiently with less soap | | Examples | Groundwater, river water in limestone areas | Rainwater, distilled water | Hard water is not harmful for drinking but causes problems in industrial and domestic use. Soft water is preferable for washing and industrial applications.

Q3. Explain the methods of removing hardness of water.

A. Removal of Temporary Hardness: Temporary hardness is caused by dissolved bicarbonates of calcium and magnesium. It can be removed by: 1. Boiling: When hard water is boiled, the bicarbonates decompose into insoluble carbonates, which settle at the bottom. Chemical equations: - Ca(HCO3)2 → CaCO3↓ + H2O + CO2↑ - Mg(HCO3)2 → MgCO3↓ + H2O + CO2↑ The insoluble carbonates can be removed by filtration. 2. Clark's method (Adding slaked lime): Slaked lime [Ca(OH)2] is added to hard water. It reacts with bicarbonates to form insoluble carbonates. Chemical equation: - Ca(HCO3)2 + Ca(OH)2 → 2CaCO3↓ + 2H2O B. Removal of Permanent Hardness: Permanent hardness is caused by dissolved chlorides and sulphates of calcium and magnesium. It is not removed by boiling. Methods include: 1. Adding washing soda (Na2CO3): Washing soda reacts with soluble calcium and magnesium salts to form insoluble carbonates. Chemical equations: - CaSO4 + Na2CO3 → CaCO3↓ + Na2SO4 - MgCl2 + Na2CO3 → MgCO3↓ + 2NaCl 2. Ion exchange method (using zeolites/permutit): Hard water is passed through a column containing zeolite (sodium aluminium silicate). The sodium ions in the zeolite exchange with calcium and magnesium ions in the water. Chemical equation: - Na2Z + CaSO4 → CaZ↓ + Na2SO4 (where Z = zeolite) When the zeolite is exhausted, it can be regenerated by treating with concentrated sodium chloride solution: - CaZ + 2NaCl → Na2Z + CaCl2

Q4. Explain water conservation and its importance. Describe rainwater harvesting.

Water Conservation: Water conservation means using water carefully and avoiding its wastage. Freshwater available for human use is less than 1% of all water on Earth, making conservation essential. Importance of water conservation: 1. Ensures availability of clean water for future generations. 2. Reduces the energy required to process and distribute water. 3. Prevents depletion of groundwater aquifers. 4. Protects natural ecosystems that depend on water. 5. Reduces the cost of water treatment and supply. Methods of water conservation: 1. Rainwater harvesting — collecting and storing rainwater. 2. Drip irrigation — delivering water directly to plant roots, minimising wastage. 3. Afforestation — planting trees to increase infiltration and reduce run-off. 4. Preventing water pollution — avoiding discharge of industrial waste and chemicals into water bodies. 5. Reducing domestic wastage — turning off taps, fixing leaks, using water-efficient appliances. 6. Reusing and recycling water — using treated wastewater for industrial and agricultural purposes. Rainwater Harvesting: Rainwater harvesting is the technique of collecting and storing rainwater for future use. There are two main methods: 1. Surface run-off harvesting: Rainwater that flows over the ground surface is collected in storage tanks, reservoirs, or ponds. This water can be used for irrigation, washing, and after treatment, for drinking. 2. Rooftop rainwater harvesting: Rainwater falling on rooftops is collected through pipes, filtered, and directed to a storage tank or recharge pit. This method directly recharges groundwater or stores water for domestic use. Advantages of rainwater harvesting: - Reduces dependence on groundwater and municipal supply. - Recharges depleted groundwater aquifers. - Provides an alternative source of water in drought-prone areas. - Reduces surface run-off, preventing soil erosion and flooding. - Rainwater is relatively pure and free from chemicals.

Common Mistakes to Avoid

• Confusing evaporation and transpiration: Evaporation is the conversion of surface water into vapour; transpiration is specifically the release of water vapour from plant leaves through stomata.
• Saying boiling removes all hardness: Boiling removes only temporary hardness (bicarbonates). Permanent hardness (chlorides and sulphates) requires washing soda or ion exchange.
• Confusing temporary and permanent hardness: Temporary hardness is caused by bicarbonates; permanent hardness by chlorides and sulphates. Both make water hard, but their removal methods differ.
• Thinking hard water is unsafe for drinking: Hard water is not harmful for drinking; it contains dissolved minerals. It is, however, problematic for washing and industrial use because it wastes soap and forms scales.
• Confusing condensation and precipitation: Condensation is the process of water vapour turning into liquid (forming clouds); precipitation is when that liquid falls to Earth as rain, snow, etc.
• Writing incorrect chemical equations for hardness removal: Remember — boiling decomposes bicarbonates (Ca(HCO3)2 → CaCO3 + H2O + CO2), while washing soda reacts with sulphates/chlorides (CaSO4 + Na2CO3 → CaCO3 + Na2SO4). Do not mix up the reagents and products.
• Forgetting that the Sun drives the water cycle: The Sun is the primary energy source for evaporation and transpiration, which initiate the water cycle. Do not attribute it to gravity alone.

Exam Checklist

• [ ] Can you define the water cycle and list all its stages (evaporation, transpiration, condensation, precipitation, infiltration)?
• [ ] Can you differentiate evaporation from transpiration with examples?
• [ ] Can you define hard water and soft water, and list their differences?
• [ ] Can you distinguish between temporary hardness and permanent hardness (causes and removal methods)?
• [ ] Can you write balanced chemical equations for removing temporary hardness (boiling, Clark's method) and permanent hardness (washing soda, ion exchange)?
• [ ] Can you explain the three stages of water treatment (sedimentation, filtration, disinfection)?
• [ ] Can you describe rainwater harvesting and its methods (rooftop, surface run-off)?
• [ ] Can you list at least five methods of water conservation?
• [ ] Can you explain why rainwater is the purest form of natural water?
• [ ] Can you define infiltration and explain why deforestation reduces it?