Water

This chapter highlights water’s fundamental role in sustaining all living organisms, underpinning countless biological processes. We learn of its abundant presence across our planet, from the vast expanses of oceans, the meandering paths of rivers, and the tranquil surfaces of lakes, to its invisible yet vital presence as water vapor in the atmosphere.

The chapter then transitions into a detailed exploration of water’s physical attributes. Pure water is presented as a substance devoid of color, odor, or taste, a testament to its pristine nature. Key physical constants are introduced, such as its freezing point at 0∘C and its boiling point at 100∘C. A crucial concept introduced is its unique density characteristic, peaking at 4∘C. Perhaps one of water’s most remarkable physical properties, its ability to act as a “universal solvent,” is also thoroughly discussed, explaining its capacity to dissolve a broad spectrum of substances.

Moving beyond its physical characteristics, the chapter delves into the chemical reactivity of water. This section often covers its interactions with various elements, such as its reaction with certain metals to produce hydrogen gas alongside metal oxides or hydroxides. Its reaction with non-metals like carbon, forming water gas, might also be explored. The concept of electrolysis is typically introduced here, offering a practical demonstration of water’s fundamental composition as a compound of hydrogen and oxygen.

Finally, the chapter broadens its scope to address the critical environmental challenge of water pollution. Students learn about the diverse origins of pollution, ranging from industrial discharge and domestic sewage to agricultural runoff, and the detrimental impact these pollutants have on both the environment and living creatures. Crucially, the chapter outlines strategies for preventing water pollution, including responsible waste disposal and effective treatment methods. The overarching theme of water conservation is also strongly emphasized, underscoring the imperative for mindful utilization and management of this invaluable natural resource.

Exercise – I

Question 1.
Name the four main sources of water.
Answer:

1. Oceans
2. Seas
3. Rivers
4. Lakes

Question 2.
State the importance of the water cycle in nature.
Answer:
Here’s a way to explain why the water cycle is so important in nature, without sounding like a robot:

The water cycle is absolutely vital for keeping our natural world going. Think about it – it’s the way water constantly moves around our planet, going from the oceans and land up into the atmosphere, and then back down again as rain or snow. This continuous movement is what makes life possible.  

For starters, it’s how we get fresh water. Rain and snow that fall on land collect in rivers, lakes, and underground, giving us the water we need to drink and use for everything.

When water evaporates, it actually cools things down. So, the water cycle helps keep the Earth’s climate somewhat stable.  

It also shapes our landscapes over long periods, carving out valleys and creating fertile floodplains.  

So, in short, the water cycle isn’t just about water moving around. It’s the lifeblood of our planet, essential for fresh water, regulating climate, supporting ecosystems, and even shaping the land we live on. Without it, nature as we know it simply couldn’t exist

Question 3.
Why is water very precious for all living beings?
Answer:Water is essential for all life on Earth. It’s crucial for human health, supports diverse ecosystems, and enables plants to grow, providing both food and oxygen. Without water, life as we know it could not exist.

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:
water is formed when hydrogen and oxygen gases react chemically. Each water molecule is composed of two hydrogen atoms and one oxygen atom, hence its chemical formula, H₂O.

Question 5.
What is the effect on the boiling point of water when
(a) pressure is increased
(b) impurity is added
Answer:
(a) When the pressure is increased:

Think of it like this: the water molecules need enough energy to escape into the gaseous state and overcome the surrounding pressure to boil. If you crank up the pressure pushing down on the water’s surface, those molecules need to work harder – they need more energy, which means a higher temperature – to break free and boil. So, increasing the pressure actually makes the boiling point of water go up. It’s why things boil at higher temperatures in a pressure cooker.

(b) When an impurity is added:

Imagine you’ve got pure water, and its molecules are relatively free to escape as a gas when they get enough energy. Now, if you stir in something else, like salt or sugar (an impurity), these new particles get in the way of the water molecules trying to reach the surface and escape. This makes it harder for the water to turn into a gas at its usual boiling point. To get the water to boil, you need to give it more energy, which translates to a higher temperature. 

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) Seawater does not freeze at 0°C.
Answer:
(a) Water’s high specific heat capacity allows it to absorb significant thermal energy with minimal temperature change, making it an excellent coolant for engines and power plants.

(b) Pipes crack when water freezes because ice is less dense than liquid water, causing it to expand and exert immense pressure on the pipe walls.

(c) Cooking takes longer in mountains due to lower atmospheric pressure. This reduces water’s boiling point, meaning food cooks at a lower temperature and absorbs less heat energy.

(d) Why ice sits on top of water:

As water gets colder on its way to freezing, it actually becomes less dense. This happens because the water bits arrange themselves in a more spread-out, organized way when they turn into ice, creating tiny pockets of air inside. 

(e) Oceans freeze at lower temperatures than fresh water because dissolved salts interfere with the formation of the ice crystal lattice, requiring a colder environment for solidification.

Question 7.
How does anomalous expansion of water help aquatic organisms in cold climates?
Answer:
Water’s unusual expansion when colder than 4°C is a lifesaver for aquatic life in chilly regions. It causes colder water to float on top of warmer (but still above freezing) water. This means even if the surface freezes into ice, the water below stays liquid, providing a habitable environment for fish and other aquatic creatures throughout winter.

Exercise – II

Question 1.
Explain the terms:
(a) Solution (b) Solute (c) Solvent.
Answer:
(a) Solution: A solution is a uniform mixture formed when one or more substances completely dissolve into another, resulting in a single, clear phase. Think of it like sugar fully dissolving in water to create a consistent sugary liquid.

(b) Solute: The solute is the substance that dissolves in a solvent to form a solution. Using our example, the sugar is the solute because it’s the component that “disappears” into the water.

(c) Solvent: The solvent is the substance, usually a liquid, that dissolves the solute to create a solution.

Question 2.
What is meant by
(a) Unsaturated (b) Saturated and
(c) Supersaturated solutions.
Answer:
(a) Unsaturated Solution:

Picture making sweet tea. You keep adding spoonfuls of sugar, and each one effortlessly disappears into the water. That’s an unsaturated solution at work. The water hasn’t reached its dissolving capacity yet; it can still welcome more sugar at the current warmth. It’s as if the water has ample space to embrace more sweetness.

(b) Saturated Solution:

Now, imagine you’re making that same sweet tea, but you’ve stirred in a whole lot of sugar. You keep stirring, yet some grains stubbornly remain at the bottom, refusing to dissolve. You’ve reached the point of saturation. The water has dissolved the absolute maximum amount of sugar it can hold at that particular temperature. It’s like the water is declaring, “This is my limit! No more can fit inside.”

(c) Supersaturated Solution:

This is a slightly unusual scenario. Then, very carefully, you let it cool down without any disturbance. If you manage this just right, you might end up with a supersaturated solution. It’s as if the water is delicately clinging to more than it can comfortably manage. However, these solutions are often quite sensitive.

Question 3.
How do the solubility of a solid and a gas affected by –
(a) Increase in temperature
(b) Increase in pressure
Answer:
(a) Temperature’s Impact:

• Solids: Warmer temperatures typically boost the solubility of most solids. Imagine heating water to dissolve sugar faster – the extra heat helps the solid break apart and mix in.
• Gases: For gases, it’s the opposite: hotter liquids hold less dissolved gas. Think of a carbonated drink going flat faster when warm; the gas escapes more easily.

(b) Pressure’s Impact:

• Solids: Pressure has almost no effect on how much solid dissolves. Solids and liquids don’t compress much, so squeezing them doesn’t change their solubility.
• Gases: Higher pressure forces more gas into a liquid. This is why soda is bottled under high pressure to keep the fizz in; when you open it, the pressure drops, and the gas bubbles out.

Question 4.
Differentiate between:
(a) Solution and suspension
(b) Suspension and colloid
Answer:

(a) Solution and suspension

Feature	Solution	Suspension
Particle Size	Very small (less than 1 nm in diameter), typically individual ions or molecules.	Large (greater than 100 nm in diameter), visible to the naked eye.
Homogeneity	Homogeneous mixture; components are uniformly distributed and indistinguishable.	Heterogeneous mixture; components are not uniformly distributed and are distinguishable.
Visibility	Particles are invisible, even under a powerful microscope.	Particles are visible to the naked eye.
Transparency	Transparent; light passes through without scattering (does not show Tyndall effect).	Opaque or translucent; light is scattered (may show Tyndall effect, but often just blocks light).
Settling	Particles do not settle down on their own or on standing.	Particles settle down on standing due to gravity.
Filtration	Components cannot be separated by filtration (pass through filter paper).	Components can be separated by filtration.
Example	Saltwater, sugar water, air.	Muddy water, chalk in water, sand in water.

(b) Suspension and colloid

Feature	Suspension	Colloid (Colloidal Solution)
Particle Size	Large (greater than 100 nm in diameter), visible to the naked eye.	Intermediate (between 1 nm and 100 nm in diameter).
Homogeneity	Heterogeneous mixture; components are distinguishable.	Heterogeneous mixture (appears homogeneous to the naked eye, but is heterogeneous at microscopic level).
Visibility	Particles are visible to the naked eye.	Particles are not visible to the naked eye, but can be seen under an ultramicroscope.
Transparency	Opaque or translucent; often blocks light.	Translucent; shows the Tyndall effect (scatters a beam of light passing through it).
Settling	Particles settle down on standing due to gravity.	Particles do not settle down on standing (remain dispersed due to Brownian motion and charge repulsion).
Filtration	Components can be separated by filtration.	Components cannot be separated by simple filtration (pass through ordinary filter paper, but can be separated by ultra-filtration).
Example	Muddy water, sand in water.	Milk, blood, fog, smoke, paint, jelly.

Question 5.
Define: ‘water of crystallisation’. Give two examples with formulae.
Answer:
Crystalline salts often trap a specific number of water molecules within their crystal structure; this is known as water of crystallisation. For example, blue vitriol (copper(II) sulfate pentahydrate) has 5 water molecules chemically bound per formula unit (CuSO₄⋅5H₂O), and washing soda (sodium carbonate decahydrate) contains 10 (Na₂CO₃⋅10H₂O). 

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) Hydrated: Washing soda and blue vitriol – they have water locked within their crystal structure.

(b) Anhydrous Crystalline: Common salt and sugar – they’re in crystal form but without any water molecules in their structure.

(c) Drying Agents: Silica gel and anhydrous calcium chloride – they’re good at soaking up moisture from their surroundings.

(d) Deliquescent: Sodium hydroxide and magnesium chloride – they grab so much moisture from the air that they actually dissolve and turn into liquids.

(e) Efflorescent: Washing soda and Epsom salt – they naturally lose the water that’s part of their crystals when exposed to air.

(f) Colloids: Milk (fat in water) and fog (water in air) – they’re mixtures where tiny particles are spread throughout another substance.

(g) Non-Water Solvents: Alcohol and acetone – liquids that can dissolve other things, just like water can.

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) Heating Blue Vitriol

When you warm up blue vitriol, that striking blue color starts to fade.. As this happens, you’ll also notice moisture escaping, often as visible steam, because the water molecules that give blue vitriol its color and structure are being driven off by the heat.

(b) Leaving Washing Soda Out in the Air

If you leave washing soda exposed to the open air, you’ll find it gradually turns more powdery. This is because it releases some of the water molecules it holds within its structure into the surrounding atmosphere, a process called efflorescence.

(c) Putting Blue Litmus into Water

Should you introduce blue litmus into plain water, you won’t observe any dramatic change. The blue litmus will simply retain its blue hue, indicating that the water is neutral.

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:
Why You Find Silica Gel in New Water Bottles

You know those small packets you often find tucked into new water bottles? They contain silica gel, which acts like a powerful sponge for moisture. During the manufacturing and storage of water bottles, a bit of dampness can get trapped inside. To prevent any unwanted mold or mildew from growing before you even get to use the bottle, these silica gel packets are added. Their purpose is to soak up any leftover moisture, making sure your new water bottle is completely dry and ready for use.

Why Table Salt Gets Damp on Rainy Days

Have you ever noticed how your table salt can become clumpy and damp on a rainy or very humid day? This means it naturally attracts and absorbs water from the air around it. When the air is heavy with moisture, like during a monsoon, the salt crystals on the surface actually pull in that water vapor. This absorbed moisture makes the salt stick together and feel wet.

The Fizz in Your Cold Drink

That enjoyable “fizz” you hear when you open a cold drink comes from carbon dioxide gas. These drinks are bottled under high pressure, which forces a lot of carbon dioxide to dissolve into the liquid. It’s almost as if the gas is being held captive within the drink. With the pressure gone, the carbon dioxide is no longer forced to stay dissolved and quickly escapes from the liquid, creating all those bubbles you see and the “fizz” sound you hear.

Question 9.
Give balanced chemical equations for the reaction of water with
(a) Sodium (b) Iron
(c) Carbon dioxide (d) Sodium oxide
Answer:

Question 10.
What is the metal activity series ?
Answer:
A more reactive metal will displace a less reactive one from its compounds. This principle allows us to predict reactions between a metal and a salt solution or acid: if the added metal is more reactive, it will displace the metal in the compound or acid.

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) When steam meets hot coke, a significant industrial reaction takes place, resulting in what’s known as “water gas.” This isn’t just a single substance but rather a valuable mixture primarily composed of carbon monoxide and hydrogen. Essentially, the intense heat provides the energy for the water molecules to react with the carbon in the coke, breaking down and rearranging into these two key gases.

(b) If you take chlorine dissolved in water and then let sunlight shine on it, you’ll observe the production of oxygen gas. The process is a bit of a chain reaction: first, the chlorine water itself begins to break down into hydrochloric acid and hypochlorous acid. It’s the hypochlorous acid that’s the key player here; it’s unstable in sunlight and readily decomposes further, liberating oxygen gas while also forming more hydrochloric acid.

(c) Dropping a piece of calcium into water leads to a rather energetic reaction, with the immediate release of hydrogen gas. Calcium, being a reactive metal, readily interacts with the water molecules. This reaction forms calcium hydroxide, a slightly soluble compound, and simultaneously displaces hydrogen atoms from the water, which then combine to form gaseous hydrogen bubbles that you can see escaping.

(d) Burning fossil fuels, whether it’s coal, oil, or natural gas, primarily results in the production of carbon dioxide. This is because these fuels are rich in carbon, and combustion is essentially the rapid reaction of this carbon with oxygen from the air. While carbon dioxide is the main product, other gases like sulfur dioxide and various nitrogen oxides can also be formed. The presence and quantity of these “other” gases depend largely on the specific chemical makeup of the particular fossil fuel being burned, as some contain more sulfur or nitrogen impurities than others.

Exercise – III

Question 1.
Define:
(a) Soft water
(b) Hard water
Answer:
(a) Soft water is the kind of water that makes soap really happy and sudsy. Have you ever used water that just won’t lather no matter how much soap you add? It’s low in dissolved minerals, especially calcium and magnesium, which are the culprits that interfere with soap’s ability to create a good lather. So, when you use soft water, you get those lovely, rich bubbles with ease.

(b) It contains a significant amount of dissolved minerals, primarily calcium and magnesium. These minerals have a knack for reacting with soap, preventing it from lathering properly and often leaving behind a sort of sticky scum or residue. You might also spot hard water leaving chalky deposits or scale on surfaces like taps and kettles.

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 Hardness: Bicarbonates of calcium (Ca(HCO3​)2​) and magnesium (Mg(HCO3​)2​). (ii) Permanent Hardness: Sulfates (CaSO4​, MgSO4​) and chlorides (CaCl2​, MgCl2​) of calcium and magnesium.

(b) (i) Removal of Temporary Hardness (Boiling): Boiling converts soluble bicarbonates into insoluble carbonates, which then precipitate out. Ca(HCO3​)2​Heat—->​CaCO3​↓+H2​O+CO2​↑ Mg(HCO3​)2​Heat——>​MgCO3​↓+H2​O+CO2​↑

(ii) Removal of Permanent Hardness (Washing Soda Method): Adding washing soda (sodium carbonate) precipitates the calcium and magnesium ions as insoluble carbonates. CaSO4​+Na2​CO3​→CaCO3​↓+Na2​SO4​ MgSO4​+Na2​CO3​→MgCO3​↓+Na2​SO4​

Question 3.
Name three water-borne diseases.
Answer:
Cholera, caused by Vibrio cholerae, manifests as severe, watery diarrhea, rapidly leading to life-threatening dehydration. It spreads primarily through water tainted with infected waste.

Typhoid fever, attributed to Salmonella Typhi, is characterized by a high fever, exhaustion, and headache, with the potential for serious health issues. Transmission occurs via contaminated food or water, often linked to inadequate sanitation.

Dysentery is an intestinal infection marked by bloody diarrhea, abdominal cramps, and fever. It can stem from bacteria like Shigella or the amoeba Entamoeba histolytica, circulating through tainted water, food, or direct contact with infected bodily fluids.

Question 4.
What are the main causes of water pollution? How can it be controlled?
Answer:
What mainly pollutes water?

Mostly, it’s what we humans do: factories dumping bad chemicals, sewage from homes, farm chemicals washing away, oil spills from accidents, and just throwing trash into water.

How can we stop water pollution?

We need to clean factory and home wastewater properly, farm smarter with fewer chemicals, be super careful with oil, stop littering, and teach everyone why clean water matters.