Water

The Chapter on Water in 6th Standard ICSE Chemistry introduces students to the fundamental aspects of this essential substance. Here’s a summary of the key topics likely covered:

What is Water?

• Water is a liquid at room temperature and is crucial for all living things.
• The water cycle is introduced, explaining how water moves between these states through evaporation, condensation, precipitation, and collection.

Sources of Water:

• The chapter likely covers various natural sources of water like rain, rivers, lakes, ponds, and oceans.
• It might also touch upon groundwater and how we access it through wells.

Properties of Water:

• Physical Properties:
  • Water is generally colorless, odorless, and tasteless in its pure form.
  • It has a relatively high boiling point and freezing point.
  • The concept of density might be introduced, with the unique property of ice being less dense than liquid water.
• Chemical Properties:
  • Water is a relatively stable compound.
  • Simple reactions involving water might be mentioned.

Importance of Water:

• The chapter emphasizes the importance of water for life, including plants, animals, and humans.
• It highlights its role in various bodily functions (transport, temperature regulation) and natural processes (agriculture, weather).

Water Pollution:

• The concept of water pollution is likely introduced, explaining how harmful substances can contaminate water sources.
• Common sources of water pollution (sewage, industrial waste, agricultural runoff) might be mentioned.
• The importance of clean water and basic methods of water purification (like boiling or filtering) might be discussed.

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

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 a water table ?
(f) Why is spring water pure enough for drinking but unsuitable for laboratory use ?
(g) Why does the taste of spring water differ at different places ?

Answer:

(a) Water’s Many Forms in the Sky: It’s fascinating to think that the water all around us can take on so many different forms, especially up in the atmosphere. We have invisible water vapor, that gaseous form we feel as humidity. Then there are those visible clouds, which are actually collections of incredibly tiny liquid water droplets or, when it’s cold enough, delicate ice crystals. It’s like a constant dance between these states of water happening right above our heads.

(b) The Salty Seas: You’re absolutely right, the oceans are the saltiest places when it comes to water on Earth. Over vast stretches of time, rivers and streams carry dissolved minerals to the sea. As water evaporates from the ocean surface, these salts are left behind, leading to that characteristic salty taste we all know. It’s a testament to the Earth’s long history and the continuous movement of water.

(c) Rain: Nature’s Distilled Water (Almost!): There’s a good reason why rainwater is often thought of as the purest natural water. The process of evaporation acts like a natural distillation. When water turns into vapor, things like dissolved salts and other impurities are left behind on the Earth’s surface. So, the water that rises into the atmosphere to form clouds is essentially much cleaner to begin with.

(d) The Not-So-Pure Reality of Rain: However, while the source of rainwater is pure, its journey down to the ground can change things. As those water droplets fall, they can encounter all sorts of things present in the atmosphere. This includes natural gases like carbon dioxide, which dissolves to form a weak acid, as well as pollutants like sulfur dioxide and nitrogen oxides from industrial activities. Plus, it can pick up tiny dust particles floating in the air. So, while purer than many other sources, rainwater isn’t always perfectly pristine by the time it reaches us.

(e) The Underground Water Table: That boundary you mentioned, the water table, is a really important concept for understanding groundwater. Imagine digging down into the earth. At some point, you’ll hit a level where the soil and rocks are completely soaked with water – that upper surface of this saturated zone is the water table. Its depth can vary greatly depending on location, rainfall, and how the land slopes.

(f) Spring Water: Naturally Filtered, Not Always Pure Enough for Everything: It’s true that spring water often tastes so refreshing because it’s undergone a natural filtration process. As it seeps through layers of rock and soil, many impurities are filtered out. However, these same rocks and soil can also dissolve minerals and salts into the water. While these can be beneficial for drinking water (and contribute to its taste), they can definitely cause problems in sensitive laboratory experiments where even trace amounts of dissolved substances can interfere with results.

(g) The Unique Taste of Place in Spring Water: You’ve hit on a key point about spring water – its taste is so diverse! Think of it like this: water is a fantastic solvent. As it travels underground through different geological formations, it comes into contact with various types of rocks and minerals. Each of these can dissolve to a certain extent, adding its own unique cocktail of minerals and salts to the water. This is why spring water from one location can taste noticeably different from spring water found somewhere else – it truly carries the “taste” of the earth it has flowed through.

Question 3.

List three major impurities present in river water.

Answer:

• Sediment: Tiny particles of soil, sand, and organic matter that cloud the water, making it look murky. This happens due to erosion and runoff.
• Pathogens: Harmful microorganisms like bacteria, viruses, and protozoa that can cause diseases.
• Chemical pollutants: Substances from industrial discharge, agricultural runoff (like pesticides and fertilizers), and urban areas (like oil and heavy metals) that can contaminate the 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:

a) Rice and wheat grains: 10-15% 

(b) Eggs: 70-75%

 (c) Tomatoes: Around 95% 

(d) Bread: Around 35-40% 

(e) Watermelon: Around 92

Question 5.

What are the three states of water ?

Answer:

Water exists in three common states: solid (ice), liquid (water), and gas (steam or water vapor)

Question 6.

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

Answer:Ice, water, and steam are indeed all the same H₂O molecules, just behaving differently because of how hot or squeezed they are. Temperature and pressure call the shots, deciding if those molecules lock tightly into a solid (ice), slide past each other as a liquid (water), or zoom around as a gas (steam). The fundamental H₂O building block stays the same no matter what

Question 7.

How is a cloud formed ?

Moist air rises, cools, and becomes saturated. Water vapor then condenses onto tiny particles (condensation nuclei), forming visible water droplets or ice crystals, which we see as clouds.

What is the water cycle ? What is its importance ?

Answer:

The water cycle is nature’s way of continuously moving and recycling water on Earth. Driven by the sun and gravity, water changes from liquid to vapor (evaporation, transpiration), then back to liquid (condensation), falling as rain or snow (precipitation). This water then gathers in rivers, lakes, and underground (collection). This constant movement is vital for all life, provides fresh water, influences our weather, supports ecosystems, shapes the land, and even helps clean the water we rely on.

Question 9.
How are the following formed ?

(a) fog
(b) mist
(c) dew
(d) frost

Answer:

a) Fog: Ground-level cloud from air cooling and water vapor condensing.

(b) Mist: Less dense fog, you can see through it more easily, forms from water vapor condensing near the ground.

(c) Dew: Water droplets forming on cool surfaces at night when air cools below its dew point.

(d) Frost: Ice crystals forming on freezing surfaces when water vapor freezes directly onto them (deposition).

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) Chlorine and ozone. 

(b) Cholera and typhoid. 

(c) Alum. 

(d) Minerals and dissolved gases.

 (e) Boiling and filtering.

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 the swimming pool kept free of germs?

Answer:

(a) Why is river water unfit for drinking?

River water often contains dissolved salts, harmful germs, and other impurities picked up from the land and human activities, making it unsafe to drink directly.  

(b) Why is tap water a mixture?

Tap water comes from sources like rivers or groundwater and is treated with chemicals like chlorine to make it safe. It also naturally contains dissolved minerals, so it’s a mixture of water and these other substances.  

(c) What is mineral water?

Mineral water is water that naturally contains dissolved minerals or other dissolved substances that give it a specific taste or are believed to have health benefits.  

(d) What is the purpose of adding bleaching powder to water supplied to the town?

Bleaching powder is added to the town’s water supply to kill harmful germs and bacteria, disinfecting the water and making it safe for drinking.  

(e) How is chemically pure water obtained in the laboratory?

Chemically pure water is usually obtained in the laboratory through a process called distillation, where water is boiled and the steam is collected and condensed, leaving impurities behind.  

(f) How is water in a swimming pool kept free of germs?

Swimming pool water is kept free of germs by adding chemicals like chlorine or bromine, which act as disinfectants and kill bacteria and other microorganisms

Question 3.
Define:

(a) Sterilisation
(b) Sedimentation
(c) Loading
(d) Aeration

Answer:(a) Sterilisation: Making something completely free from living microorganisms.  

(b) Sedimentation: Allowing heavier particles in a liquid to settle down due to gravity.  

(c) Loading: Increasing the weight of suspended particles in water to help them settle faster.  

(d) Aeration: Mixing air into water

Question 4.

What is potable water ? List four characteristics of potable water.

Answer:

• Free from harmful microorganisms: It should not contain bacteria, viruses, or protozoa that can cause illness.  
• Absence of toxic substances: It should not have harmful levels of chemicals, heavy metals, or pesticides.
• Acceptable taste and odor: It should be palatable, meaning it doesn’t have unpleasant tastes or smells.  
• Reasonable clarity: While not necessarily perfectly clear, it should not be excessively cloudy or contain visible particles.

Question 5.

Why is water important for plants and animals ?

Answer:For plants, water is crucial for a few key reasons:

• Photosynthesis: It’s a main ingredient in the process where plants use sunlight to make their own food.  
• Transport: Water acts like a delivery truck, carrying nutrients from the soil up to all parts of the plant.
• Structure: It helps keep plants firm and upright.  

Animals also depend heavily on water:  

• Bodily Functions: It’s essential for digestion, circulation, regulating body temperature, and getting rid of waste.  

Question 6.

What are the three methods of removing germs from natural water? Explain.

Answer:

• Boiling: Heating water to a rolling boil for at least one minute (longer at higher altitudes) kills most harmful bacteria, viruses, and protozoa. 
• Filtration: Passing water through a filter with very small pores can physically remove larger germs like bacteria and protozoa, as well as sediment and debris. Different types of filters exist, with varying pore sizes to target specific contaminants.  
• Disinfection: This involves adding a disinfectant to the water to kill or inactivate germs. Common disinfectants include chlorine (in liquid or tablet form), iodine, or using ultraviolet (UV) light. The disinfectant damages the microorganisms, making the water safe to drink

Question 7.

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

Answer:

Water goes through several steps to become clean and safe! First, heavy stuff like sand settles out. Then, we use special “sticky” substances to clump tiny particles together into bigger, heavier bits that can also settle. Next, the water flows through layers of sand and gravel to filter out any remaining small particles. To kill harmful germs, we disinfect the water using things like chlorine or UV light. Finally, the clean water is stored and then piped to us. It’s quite a journey to make sure our water is just right!

Question 8.

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

Answer:

Distilled water tastes bland or flat. This is because the distillation process removes not only impurities but also the dissolved minerals and gases that give regular water its taste.

The lack of dissolved minerals means it doesn’t provide the trace elements our bodies need, and some argue it can even leach minerals from the body over time. ¹ For regular drinking, water with naturally occurring or added minerals is preferred for both taste and potential health benefits

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)Ice’s unique structure makes it less dense than liquid water, allowing it to float.

(b) Water’s maximum density at 4°C is crucial. It means that the bottom layers of a lake or ocean stay liquid even when the surface freezes, protecting aquatic life.

(c)Condensation is the culprit. The cold surface cools the nearby air, causing water vapor in the air to change back into liquid droplets.

EXERCISE-III

Question 1.
Why is water called a universal solvent ?
Answer:Water’s incredible dissolving power comes down to its polar structure. ¹ Think of water molecules as having little positive and negative charges. These charges act like tiny magnets, grabbing onto and separating the parts of many other substances, causing them to dissolve. ² 

Question 2.
Define:

(a) Solute
(b) Solvent
(c) Solution
(d) Saturated solutions
(e) Unsaturated solutions

Answer:

(a) Solute: The small part of a mixture that gets dissolved, like the sugar in your drink.

(b) Solvent: The main part that does the dissolving, like the water.

(c) Solution: A mixture where everything is evenly spread out, like that sweetened drink.

(d) Saturated solution: When you’ve added so much of the solute that no more will dissolve at that temperature – it’s holding all it can.

(e) Unsaturated solution: You can still add more solute and it will disappear; the solvent can still handle more.

Question 3.

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

Answer:

Heating: Warmer liquids have more energetic molecules that can do a better job of pulling apart the solid’s structure.

Stirring/Shaking: This constantly brings fresh, unsaturated solvent to the surface of the solid, allowing more solute particles to break away and dissolve.

Question 4.

Why do aquatic animals die in boiled water ?

Answer:

Aquatic animals die in boiled water primarily because the high temperature drastically reduces the amount of dissolved oxygen in the water, making it impossible for them to breathe. Additionally, the heat denatures essential proteins in their bodies and damages their delicate tissues and organs, leading to rapid death.

Question 5.

State three differences between water and air.

Answer:

Composition: Water is a chemical compound with a fixed ratio of hydrogen and oxygen (H₂O). Air is primarily a physical mixture of gases like nitrogen, oxygen, and argon, with variable proportions.  

State at Room Temperature: Water is typically a liquid at room temperature (around 25°C). Air is a gas at the same temperature.  

Compressibility: Water is relatively incompressible, meaning its volume is hard to reduce by pressure. Air is highly compressible, and its volume can be significantly reduced by applying pressure.

EXERCISE -IV

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

• Stop those drips! Fixing leaky taps and toilets makes a big difference.
• Choose efficient machines: Look for washing machines and dishwashers that use less water.
• Quick showers are better: Shorten your shower time to save water.
• Water smart outside: Water plants when it’s cooler and think about plants that don’t need much water.

Question 2.

Explain harvesting of water.

Answer:

Water harvesting is essentially about capturing and saving rainwater right where it lands, preventing it from just flowing away. Think of it as intentionally collecting rainwater, whether from your rooftop into a tank or through larger projects that help replenish underground water. The big idea is to use the rain we receive effectively, particularly in water-scarce regions, and to manage this precious resource thoughtfully.

Question 3.

What are the three main causes of water pollution ?

Answer:

1. Industrial Discharge: Factories and industries often release wastewater containing harmful chemicals, heavy metals, and other toxic substances directly into rivers, lakes, and oceans.  
2. Sewage and Domestic Waste: Untreated or poorly treated sewage from households and commercial establishments introduces pathogens (bacteria and viruses), nutrients (like nitrates and phosphates), and organic matter into water bodies.  
3. Agricultural Runoff: The use of fertilizers, pesticides, and herbicides in agriculture can lead to these chemicals being washed off fields by rain into nearby water sources. Animal waste from farms also contributes to nutrient pollution and pathogens.  

Question 4.

State the main steps to be taken to prevent water pollution.

Answer:To curb water pollution, we need to:

• Clean up factory discharge: Treat industrial wastewater to remove nasty stuff before it’s released.
• Farm smarter: Use less fertilizer and pesticides and stop them from running off into our water.
• Fix our sewage: Make sure household wastewater is properly collected and treated.
• Beat plastic: Cut down on plastic and make sure it doesn’t end up in our waters.
• Guard natural filters: Protect wetlands and riverside plants that naturally clean water.
• Get the word out: Teach people about water pollution and encourage good habits.
• Enforce the rules: Strictly apply and uphold laws that limit pollution.

Question 5.

What are the causes of floods and drought?

Answer:

Floods:

Floods happen when there is more water for the land to handle. This can be from really heavy rain, rivers overflowing their banks, rapid snow or ice melt, or even the sea surging onto the land during storms. Sometimes, human actions like deforestation, poor drainage, and building in flood-prone areas can make flooding worse.

Droughts:

Droughts occur when there’s a long period with much less rainfall than usual. This lack of water can be caused by natural weather patterns, but it’s often made worse by things like climate change, deforestation, and overuse of water by people. Hot temperatures can also worsen droughts by drying out the soil faster.

Question 6.

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

Answer:

I’ll take shorter showers.

I’ll turn off the tap while brushing my teeth or washing dishes.

I’ll fix any leaky faucets promptly.

I’ll only run the washing machine and dishwasher with full loads.

MULTIPLE CHOICE QUESTIONS

Tick (√) the correct alternative from the choice given for the following statements:

1. Water content in human body is

1. 70%
2. 75%
3. 80%
4. 90%

Answer : 70%

2. The purest form of natural water is

1. seawater
2. river water
3. rainwater
4. lake water

Answer : Rainwater

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

4. Water is a

1. compound
2. element
3. mixture
4. none of the above

Answer : Compound

5. Common salt is obtained from sea water by

1. distillation
2. crystallisation
3. evaporation
4. sublimation

Answer : Evaporation

6. Jaundice affects

1. heart
2. lungs
3. liver
4. kidney

Answer : Liver

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