Chapter 4.6 – Water | Chapter Solution Class 9

Earth is called the “Living Planet” because water supports all forms of life, including plants, animals, and humans. It is also called the “Blue Planet” because a large portion of the Earth’s surface is covered with water, giving it a blue appearance from space.

Water has a high specific heat capacity, which helps in regulating environmental temperature. It allows large water bodies to store heat and release it slowly, preventing sudden temperature fluctuations in the surroundings.

The high surface tension of water allows it to form droplets and supports capillary action. This property is essential for the movement of water in plant roots and blood circulation in small capillaries in animals.

The high boiling point of water makes it suitable for sterilizing medical instruments by boiling, ensuring the destruction of harmful microorganisms.

Water is called a versatile solvent because it can dissolve a wide variety of substances, including salts, gases, and organic molecules, making it essential for biological and chemical processes.

The pH of drinking water should be between 6.5 and 8.5 to ensure safety and suitability for consumption.

The coliform count of drinking water should always be zero (0) to ensure it is free from harmful bacteria.

Municipal water is disinfected using methods like chlorination, filtration, and ultraviolet (UV) radiation to eliminate harmful microorganisms and make it safe for consumption.

UV radiation effectively kills bacteria and viruses without adding any chemicals to the water, making it a safe and efficient method for disinfection.

When hard water is treated with soap, it forms insoluble scum instead of producing lather due to the presence of calcium and magnesium ions.

• Ion that causes hardness: Ca2+ (Calcium ion)
• Ion that does not cause hardness: Na+ (Sodium ion)

Bicarbonate hardness is caused by dissolved bicarbonates of calcium and magnesium in water. It can be removed by boiling, which precipitates insoluble carbonates.

Hard water forms scale deposits in boilers, reducing heat transfer efficiency and increasing fuel consumption. It can also lead to pipe blockages and boiler damage.

Ion-exchange resins are used to remove hardness from water by replacing calcium and magnesium ions with sodium or hydrogen ions. They are regenerated by passing dilute H₂SO₄ (for cation resins) and NaOH solution (for anion resins) through them.

1. Used in laboratories for analytical work.
2. Used in wet batteries and boilers to prevent scaling.

(b) Liquid

Explanation:

Water in liquid form is essential for all life forms as it acts as a medium for biological reactions and transport of nutrients.

(a) Specific heat capacity

Explanation:

Water has a high specific heat capacity, allowing it to absorb and release heat without rapid temperature changes, stabilizing environmental temperature.

(a) 21% O2, 78% N2

Explanation:

The proportion of oxygen and nitrogen in dissolved air in water is similar to atmospheric composition, supporting aquatic life.

(b) 6.5 – 8.5

Explanation:

A pH range of 6.5-8.5 is considered safe for drinking water as it prevents corrosion and microbial growth.

(c) HCO₃^–

Explanation:

Bicarbonates of calcium and magnesium contribute to temporary hardness in water.

Increases

Explanation:

The boiling point of water increases with pressure because the molecules require more energy to escape into the gaseous phase. In contrast, the freezing point decreases under increased pressure as the molecules are forced closer together, making it harder for them to form a solid structure.

Increases

Explanation:

Eutrophication occurs when excessive nutrients, mainly nitrogen and phosphorus, accumulate in water bodies. This leads to excessive growth of algae (algal bloom), which depletes oxygen levels and harms aquatic life.

Ca(HCO3)2

Explanation:

Temporary hardness in water is caused by the presence of calcium bicarbonate Ca(HCO3)2. This type of hardness can be removed by boiling, as heat breaks down bicarbonates into insoluble carbonates, which can be filtered out.

Cation

Explanation:

Cation exchange resins contain negatively charged sites that attract and replace positively charged ions such as Ca²⁺ and Mg²⁺, which cause water hardness. These resins are used in water softening processes.

0.05 mg/L

Explanation:

The World Health Organization (WHO) and national guidelines set the safe limit of arsenic in drinking water at 0.05 mg/L. High levels of arsenic can cause severe health issues, including skin diseases and cancer.

True

Explanation:

Ancient civilizations such as the Egyptian, Mesopotamian, Harappan, and Chinese civilizations were established along riverbanks. Rivers provided a steady water supply, fertile land for agriculture, and a means for trade and transportation.

False

Explanation:

The density of water is maximum at 4°C (not 0°C). At 0°C, water starts freezing and expands, making ice less dense than liquid water. This is why ice floats on water.

False

Explanation:

In a pressure cooker, the boiling point of water increases due to the higher pressure inside the sealed vessel. This allows food to cook faster as water remains in liquid form at temperatures higher than 100°C.

Water exists in three states:

1. Solid (Ice)
2. Liquid (Water)
3. Gas (Water Vapor)

The liquid state provides a medium for life as it supports biological functions and serves as a solvent for chemical reactions in living organisms.

1. Capillary Action: Helps water move through plant roots and xylem vessels.
2. Support for Small Organisms: Allows insects like water striders to walk on water.

Dissolved substances in water are essential for:

1. Aquatic life, as gases like oxygen and carbon dioxide are needed for respiration and photosynthesis.
2. Human consumption, as minerals such as calcium and magnesium are necessary for health.

• Aquatic animals extract dissolved oxygen from water using gills for respiration.
• Aquatic plants absorb carbon dioxide for photosynthesis.

pH Level: Should be between 6.5 – 8.5 for safe consumption.

Bacterial Content: Should be zero coliform bacteria to prevent waterborne diseases.

Hard water forms scale deposits inside boilers, which reduce heat transfer efficiency, increase energy consumption, and can lead to equipment damage over time.

Detergents create foam layers on the water surface, blocking oxygen absorption from the atmosphere. This reduces dissolved oxygen, harming aquatic life.

Eutrophication is the excessive growth of algae due to high nutrient levels (mainly nitrogen and phosphorus) in water. It reduces dissolved oxygen, leading to the death of aquatic animals.

Sources: Natural minerals, industrial waste, and excessive use of groundwater.

Safe Limit: WHO recommends arsenic levels ≤ 0.05 mg/L in drinking water.

(a) In soft water, soap forms a rich lather easily.

(b) In hard water, soap forms an insoluble scum due to calcium and magnesium ions, reducing its effectiveness.

Water has several properties that allow it to regulate temperature in the environment and living beings:

1. High Specific Heat Capacity: Water can absorb and retain large amounts of heat without experiencing a significant rise in temperature. This property helps maintain stable temperatures in aquatic ecosystems and regulates body temperature in warm-blooded animals.
2. Latent Heat of Vaporization: Water requires a large amount of heat to convert from liquid to vapor. This property is responsible for cooling effects seen in sweating in humans and transpiration in plants, where heat is removed as water evaporates.
3. Latent Heat of Fusion: Water takes a considerable amount of energy to freeze. This prevents rapid freezing of water bodies, protecting aquatic life from extreme cold and maintaining the stability of ecosystems.

Water is often referred to as the “universal solvent” because of its ability to dissolve a wide range of substances, making it essential for various processes:

1. Role in Biological Processes: Water dissolves essential nutrients, minerals, and gases required for metabolic activities. It enables the transport of nutrients in blood and facilitates cellular reactions in all living organisms.
2. Role in Environmental Processes: Water dissolves gases like oxygen and carbon dioxide, allowing aquatic organisms to breathe and plants to perform photosynthesis. It also helps in the weathering of rocks and formation of soil.
3. Role in Chemical and Industrial Reactions: In industries, water is used as a solvent in chemical manufacturing, food processing, and cleaning applications. It is vital in pharmaceutical production and wastewater treatment processes.

Capillary action is the ability of water to move against gravity through narrow spaces without external force, which supports life in the following ways:

1. In Plants: Water moves upward from the roots to the leaves through the xylem due to capillary action. This ensures a continuous supply of water and minerals for photosynthesis and overall plant growth.
2. In Humans and Animals: Blood circulates through the smallest capillaries, supplying oxygen and nutrients to every cell. This process is crucial for maintaining bodily functions, including waste removal through the kidneys.

Water’s specific heat and high boiling point contribute to the stability of life and climate in the following ways:

1. Specific Heat: Water absorbs a large amount of heat before increasing in temperature. This property helps moderate Earth’s climate, preventing drastic temperature changes. It also keeps body temperatures stable in living organisms.
2. High Boiling Point: Water boils at 100°C, allowing it to exist as a liquid in most natural conditions. This ensures that water remains available for hydration, metabolic processes, and ecosystem stability. Industrially, the high boiling point allows water to be used effectively in power plants and heat exchange systems.

(a) Air dissolved in water has a different composition than atmospheric air. Water contains a higher percentage of oxygen (about 33%) compared to normal atmospheric air (21%). This dissolved oxygen is essential for aquatic life and respiration in fish and other organisms.

(b) Boiled water loses its pleasant taste because the dissolved gases (oxygen and nitrogen) escape when heated. The absence of these gases makes the water taste flat and different from fresh water.

Water hardness, caused by calcium and magnesium salts, can be removed using the following methods:

Temporary Hardness Removal:

• Boiling: When water is heated, bicarbonates of calcium and magnesium decompose into insoluble carbonates, which can be removed by filtration.

Permanent Hardness Removal:

• Ion-Exchange Resins: These resins replace calcium and magnesium ions with sodium or hydrogen ions, softening the water.
• Lime-Soda Process: Adding slaked lime (Ca(OH)₂) and sodium carbonate (Na₂CO₃) precipitates out the hardness-causing ions.

Water is called the elixir of life because of its fundamental role in sustaining life:

1. Essential for Life Functions: Water makes up about 70% of the human body and is involved in digestion, circulation, respiration, and excretion.
2. Supports Ecosystems: Aquatic life depends on water for oxygen, food, and reproduction. Water bodies regulate Earth’s climate by absorbing heat.
3. Key to Agriculture and Industry: Water is vital for irrigation, food production, and industrial applications such as cooling, cleaning, and energy generation.

Municipal water supply undergoes several purification processes to make it safe for consumption:

1. Sedimentation & Coagulation: Water is allowed to sit in large tanks so that suspended particles settle down. Chemicals like alum are added to speed up coagulation.
2. Filtration: Water passes through layers of sand and gravel to remove finer impurities.
3. Chlorination & UV Treatment: Chlorine is added to kill bacteria, and in some cases, UV radiation is used for additional disinfection.
4. Storage & Distribution: The purified water is stored in reservoirs and distributed through pipelines to homes and industries.