Chapter 4.6 – Water | Chapter Solution Class 9

Water
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Publisher : Santra publication pvt. ltd.
Book Name : Madhyamik Physical Science And Environment
Class : 9 (Madhyamik)
Subject : Physical Science
Chapter Name : Water

Multiple Choice Questions (MCQ)

[Each of Mark-1]

Question 1

The mass ratio of hydrogen and oxygen in water is

  1. 2:1
  2. 1:2
  3. 1: 8
  4. 8: 1

Answer

1:8

Explanation

The mass ratio of hydrogen and oxygen in water is 2:16 or simplified as 1:8. This is because the molar mass of oxygen is 16 times that of hydrogen.

Question 2

Who first recognised that water is a compound of two elements (hydrogen and oxygen) in 1783?

  1. Lavoisier
  2. Cavendish
  3. J. Thomson
  4. Robert Boyle

Answer

Henry Cavendish

Explanation

The recognition that water is a compound of two elements, hydrogen and oxygen, is credited to the French chemist Antoine Lavoisier in 1783.

Question 3

What is the fundamental property that makes water so important?

  1. density
  2. its crystal structure
  3. its boiling point
  4. hydrogen bonds

Answer

hydrogen bonds

Explanation

The fundamental property that makes water so important is its ability to form hydrogen bonds, which contribute to its unique properties like high specific heat, solvent capabilities, and cohesion.

Question 4

In a pressure cooker, water boils at T°C; T is

  1. 100°C
  2. T > 100°C
  3. T < 100°C
  4. none of these

Answer

T > 100°C

Explanation

In a pressure cooker, water boils at a temperature greater than 100°C due to the increased pressure inside the cooker, which raises the boiling point.

Question 5

At 3.98°C (nearly 4°C), property. What is this?

  1. density is minimum
  2. volume is maximum
  3. density is maximum
  4. none of these

Answer

density is maximum

Explanation

At 3.98°C, the property of water is that its density is maximum. The water reaches its highest density at around 4°C before decreasing as it approaches the freezing point.

Question 6

Water can act as

  1. an acid only
  2. a base only
  3. both as acid and a base
  4. neither acid nor base

Answer

both an acid and a base

Explanation

Water can act as both an acid and a base. It can donate a proton (acting as an acid) or accept a proton (acting as a base) depending on the reaction conditions.

Question 7

ppm is a unit sometimes used to express the concentration level of pollutants in water. This ppm is equivalent to

  1. g/L
  2. kg/mL
  3. mL/L
  4. mg/dm3

Answer

mg/dm3

Explanation

ppm (parts per million) is equivalent to mg/dm3 (milligrams per cubic decimeter) as both represent the concentration of a substance in water.

Question 8

Hard water contains an appreciable amount of concentration of which ions?

  1. Cl– and NO3–
  2. Na+ and K+
  3. CO32- and Cl–
  4. Ca2+, Fe2+ and Mg2+

Answer

Ca2+, Fe2+ and Mg2+

Explanation

Hard water contains an appreciable amount of concentration of calcium ions (Ca2+), magnesium ions (Mg2+), and sometimes iron ions (Fe2+).

Question 9

Temporary hardness is due to the presence of which compound in water?

  1. NaCl
  2. CaSO4
  3. NaNO3
  4. Ca(HCO3)2

Answer

Ca(HCO3)

Explanation

Temporary hardness in water is due to the presence of calcium bicarbonate (Ca(HCO3)2), which can form insoluble calcium carbonate when heated, causing scale buildup.

Question

Answer in one word or in one sentence

[Each of Mark-1]

  1. What fraction of the earth’s surface is covered with water?
  2. What percentage of water is seawater?
  3. What is the amount of dissolved salt (in g/kg) in seawater?
  4. The % of fresh water or drinking water is very low. What is that low percentage?
  5. Can water boil at different temperatures?
  6. What particular feature makes the water too special?
  7. Name two unique properties of water.
  8. Give the value latent heat of the water in (i) melting and (ii) vaporization.
  9. What is a ‘hydrate’?
  10. What is MCL (maximum contamination level)?

Answer

  1. Approximately 71%.
  2. Approximately 97.5%.
  3. Approximately 35 g/kg.
  4. Approximately 2.5%.
  5. Yes, at different pressures.
  6. Its ability to form hydrogen bonds.
  7. High specific heat and solvent capabilities.
  8. (i) 334 J/g, (ii) 2260 J/g.
  9. A compound in which water molecules are bound to another substance.
  10. The maximum contaminant level (MCL) is the highest level of contaminant that is allowed in drinking water based on cost-benefit analysis and is enforceable.

Short answer type questions

[Each of Mark-2 or 3]

Question 1

Explain why water is an excellent (universal) solvent.

Answer

Water is an excellent universal solvent due to its ability to form hydrogen bonds, allowing it to dissolve a wide range of substances.

Question 2

Explain the ‘hydrogen bond’ that occurs in water molecules.

Answer

A hydrogen bond is a relatively strong attraction between a hydrogen atom in one water molecule and an oxygen atom in another water molecule, resulting in cohesive forces among water molecules.

Question 3

Mention the important physical properties of water.

Answer

Some important physical properties of water include high specific heat, high heat of vaporization, surface tension, and the ability to form hydrogen bonds.

Question 4

Illustrate with a diagram the variation of density of water with temperature. How this unique pattern is useful? 

Answer

variation of density of water

The density of water exhibits a unique pattern. As the temperature decreases from its maximum density at around 4°C, the density of water increases. However, as water approaches its freezing point (0°C), its density starts to decrease, causing ice to be less dense than liquid water.

This pattern is useful because it allows ice to float on the surface of water bodies, such as lakes and oceans. When water freezes, the less dense ice forms on the top, acting as an insulating layer that helps protect the underlying water from freezing completely. This enables aquatic life to survive during cold seasons, as the ice layer acts as a barrier to extreme temperature changes and provides a habitat for organisms beneath it.

Question 5

“Specific heat of water is 4.2 J/g”- Explain.

Answer

The specific heat of water is 4.2 J/g, which means it requires 4.2 joules of energy to raise the temperature of 1 gram of water by 1 degree Celsius.

Question 6

Explain with the chemical equation, what happens, when water reacts with (a) alkali metals (Na, K), (b) heavier alkaline earth metals (Ca, Ba), (c) halogens.

Answer

(a) When water reacts with alkali metals such as sodium (Na) or potassium (K), the following reaction occurs:

2M + 2H2O → 2MOH + H2

where M represents the alkali metal. The alkali metal displaces hydrogen from water, resulting in the formation of metal hydroxide and the liberation of hydrogen gas.

(b) When water reacts with heavier alkaline earth metals such as calcium (Ca) or barium (Ba), the reaction is as follows:

Ca + 2H2O → Ca(OH)2 + H2

or

Ba + 2H2O → Ba(OH)2 + H2

In these reactions, the alkaline earth metal displaces hydrogen from water, leading to the formation of metal hydroxide and the release of hydrogen gas.

(c) When water reacts with halogens like chlorine (Cl2) or iodine (I2), the following reactions take place:

Cl2 + H2O → HCl + HClO

or

I2 + H2O → 2HI + HIO

In these reactions, the halogen reacts with water to produce corresponding acids and hypohalous acids.

Question 7

Give the chemical equation for the reaction of fluorine with water. What is the speciality of this reaction?

Answer

The chemical equation for the reaction of fluorine (F2) with water (H2O) is:

F2 + H2O → 2HF + [O]

The speciality of this reaction is that fluorine is the most reactive halogen and it can react explosively with water, releasing hydrogen fluoride gas and oxygen as products.

Question 8

What do you mean by potable water? Mention a few health effects due to different pollutants in water.

Answer

Potable water refers to water that is safe and suitable for human consumption.

Health effects due to different pollutants in water:

  1. Chlorine and chlorination by-products: Linked to respiratory problems and an increased risk of bladder and colorectal cancer.
  2. Arsenic: Increases the risk of skin lesions, cardiovascular diseases, and various types of cancer.
  3. Radon: This can lead to lung cancer when consumed in drinking water.

Question 9

Briefly outline the sources of different water pollutants.

Answer

Sources of different water pollutants:

  1. Industrial Discharges: Factories and industrial facilities can release pollutants such as heavy metals, chemicals, and toxins into water bodies.
  2. Agricultural Runoff: Pesticides, herbicides, fertilizers, and animal waste from agricultural activities can wash into water sources through runoff, contaminating them.
  3. Municipal Wastewater: Untreated or inadequately treated sewage and wastewater from households and urban areas can introduce pathogens, nutrients, and other pollutants into water bodies.
  4. Mining Activities: Mining operations can release heavy metals, acids, and other harmful substances into water sources, contaminating them.
  5. Landfills and Waste Sites: Improperly managed landfills and waste disposal sites can generate leachate, a liquid that contains various pollutants, which can infiltrate groundwater and nearby water bodies.
  6. Oil and Fuel Spills: Accidental spills or leaks from oil and fuel storage facilities, transportation, and maritime activities can introduce petroleum hydrocarbons into water sources.
  7. Atmospheric Deposition: Airborne pollutants, such as heavy metals and acid rain, can settle onto the Earth’s surface, including bodies of water, through atmospheric deposition.

Question 10

Explain: Hardness of water and its types.

Answer

The hardness of water refers to the presence of dissolved minerals, primarily calcium and magnesium ions, in the water. Here’s a brief explanation of water hardness and its types:

  1. Hard Water: Water that contains a high concentration of calcium and magnesium ions is considered hard water.
  2. Temporary Hardness: Temporary hardness is caused by the presence of bicarbonate ions (HCO3–) in water, which can be removed by boiling or through the addition of lime (calcium hydroxide).
  3. Permanent Hardness: Permanent hardness is caused by the presence of non-carbonate minerals, such as sulfates and chlorides of calcium and magnesium. It cannot be removed by boiling and requires other methods like ion exchange or lime softening.

Question 11

Explain with a chemical equation how hard water can be softened by (i) boiling, (ii) use of sodium carbonate. 

Answer

(i) Boiling: Temporary hardness can be softened by boiling. The chemical equation is:

Ca(HCO3)2 (aq) → CaCO3 (s) + CO2 (g) + H2O (l)

When water is boiled, the bicarbonate ions (HCO3–) decompose, forming insoluble calcium carbonate (CaCO3) that precipitates out, along with the release of carbon dioxide gas (CO2) and water (H2O).

(ii) Use of Sodium Carbonate: Water hardness, both temporary and permanent, can be softened by adding sodium carbonate (Na2CO3). The chemical equation is:

Ca2+ (aq) + Mg2+ (aq) + Na2CO3 (aq) → CaCO3 (s) + MgCO3 (s) + 2Na+ (aq)

Sodium carbonate reacts with the calcium and magnesium ions, forming insoluble calcium carbonate (CaCO3) and magnesium carbonate (MgCO3) precipitates, which can be removed by filtration, thus softening the water.

Question 12

Explain with an equation how the method of ion exchange works for water treatment.

Answer

The method of ion exchange is used for water treatment to remove certain ions from water and replace them with other ions. It involves the use of an ion exchange resin. Here’s how it works:

Na+ (resin) + Ca2+ (water) → Ca2+ (resin) + 2Na+ (water)

In this example, the ion exchange resin initially contains sodium ions (Na+). As water containing calcium ions (Ca2+) passes through the resin bed, the sodium ions are exchanged for the calcium ions. The resin captures the calcium ions and releases sodium ions into the water, resulting in softened water with reduced calcium content.

The process can be regenerated by rinsing the resin with a concentrated salt solution, such as brine (sodium chloride). This displaces the calcium ions from the resin, and they are washed away, allowing the resin to be reused for subsequent ion exchange cycles.

Question 13

What is (i) soap scum, (ii) boiler scale? Give the related chemical equations.

Answer

(i) Soap Scum: Soap scum is a solid residue that forms when the calcium and magnesium ions in hard water react with soap. The chemical equation for the reaction is:

Ca2+ (hard water) + 2C17H35COO- (soap) → (C17H35COO)2Ca (soap scum)

(ii) Boiler Scale: Boiler scale refers to the hard, insoluble deposits that form on the internal surfaces of boilers and heat exchangers due to the precipitation of calcium and magnesium salts from hard water. The chemical equation involves the reaction between calcium and magnesium ions and bicarbonate ions:

Ca2+ (hard water) + 2HCO3- (bicarbonate) → CaCO3 (scale) + CO2 + H2O

Mg2+ (hard water) + 2HCO3– (bicarbonate) → MgCO3 (scale) + CO2 + H2O

Question 14

What are the chemicals used in potable water treatment? Mention their role in the treatment.

Answer

Chemicals used in potable water treatment and their roles:

  1. Chlorine: Used for disinfection to kill or inactivate harmful microorganisms.
  2. Coagulants: Aid in the removal of suspended particles by causing them to clump together, allowing for easier filtration.
  3. Flocculants: Assist in the formation of larger particles from smaller ones, aiding in the removal of fine particles during sedimentation or filtration.
  4. pH Adjusting Chemicals: Control and adjust the pH level of water to optimize treatment processes.
  5. Activated Carbon: Removes organic contaminants, taste, and odor compounds through adsorption.
  6. pH Indicator Chemicals: Help monitor and maintain the desired pH range during treatment.

Question 15

Sketch the outline to explain the “working” of a potable water treatment plant.

Answer

Water Treatment

  1. Coagulation and Flocculation: Chemical coagulants are added to the water to destabilize and clump together suspended particles. Flocculants are then added to form larger particles called flocs.
  2. Sedimentation: The water is allowed to settle in large tanks, where flocs settle to the bottom as sediment, forming a sludge layer.
  3. Filtration: The water passes through layers of sand, gravel, and activated carbon filters to remove smaller particles, microorganisms, and dissolved impurities.
  4. Disinfection: To kill or inactivate harmful microorganisms, disinfectants like chlorine or chloramines are added to the water.
  5. pH Adjustment: Chemicals are used to adjust and stabilize the pH level of the water within the desired range.
  6. Storage and Distribution: The treated water is stored in clean reservoirs and then distributed through a network of pipes to consumers.

Question 16

Mention how F is harmful as a water pollutant. Mention its source, maximum contamination level (MCL) and its removal process.

Answer

Fluoride (F) as a Water Pollutant:

  • Source: Fluoride can enter water from natural sources, industrial discharges, and agricultural runoff. It’s also intentionally added to water supplies for dental health.
  • Maximum Contamination Level (MCL): The U.S. EPA sets the MCL for fluoride in drinking water at 4.0 mg/L (milligrams per liter).
  • Removal Process: Fluoride can be removed from water using various methods, including activated alumina, reverse osmosis, and ion exchange. Treatment depends on the concentration and the specific removal needs of the water source.

Question 17

Mention how arsenic is a source of toxicity. Mention its source, MCL and its removal process.

Answer

Arsenic is a toxic substance with harmful effects:

  • Source: Natural occurrence in groundwater, industrial activities, mining, and agricultural practices.
  • MCL: The maximum contamination level (MCL) for arsenic in drinking water is typically around 10 μg/L or lower.
  • Harmful Effects: Arsenic ingestion can lead to various health issues, including skin lesions, cancer (skin, lung, bladder), cardiovascular diseases, and neurological effects.
  • Removal Process: Arsenic removal methods include activated alumina filtration, coagulation/flocculation, adsorption with iron-based media, and membrane filtration (such as reverse osmosis).

Question 18

What is deionized water? How will you prepare deionized water from a sample of hard water?

Answer

Deionized water is water that has had its mineral ions, such as calcium, magnesium, sodium, and chloride, removed through a process called deionization.

To prepare deionized water from a sample of hard water, you can use an ion exchange process. Here’s a simplified explanation:

  1. Pass the hard water sample through an ion exchange resin, typically in a column or tank.
  2. The resin selectively binds and removes the mineral ions from the water, exchanging them with hydrogen (H+) or hydroxide (OH–) ions.
  3. Continue passing the water through the resin until the desired level of deionization is achieved.
  4. Collect the deionized water that has been stripped of mineral ions.

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