Selina Concise Solutions for ICSE Class 9 Chemistry Chapter 6 Water

Exercise 6(A)

Question 1. In free state, water occurs in three states viz, solid, liquid and gaseous state.
Answer: 1. Solid state of water: In solid state, a large amount of fresh water is found in the form of snow or Ice.
2. Liquid state of water: Most of the water present in oceans, on land water found in streams, rivers, lakes, ponds, springs are also liquid state of water.
3. Gaseous state of water: In gaseous form, water vapours present in air. When these vapours condense, they form clouds, mist, fog etc. are examples of water in gaseous form.
In simple words: Water is a versatile substance found as solid ice in glaciers, liquid water in our taps and rivers, and invisible gas (water vapor) in the air around us.

📝 Teacher's Note: Use a globe or map to show students that while 70% of the Earth is water, most of it is in the liquid state in oceans, while the solid state is mostly at the poles.

🎯 Exam Tip: When asked about states of water, ensure you provide specific examples for each state, such as glaciers for solid and water vapor for gas.

Question 2. Water is considered as a compound because it is made up of two elements, hydrogen and oxygen combined in the ratio of 1 : 8 by mass.
Answer: Mass ratio of elements \( \text{H}_2\text{O} \)
\( \text{H} : \text{O}, 2 \times 1 : 16 \times 1 = 1 : 8 \)
(Atomic mass of \( \text{H} = 1, \text{O} = 16 \))
And also components of water cannot be separated by physical methods but can be separated by electrolysis of water.
In simple words: Water isn't just a mixture; it's a compound because its parts are chemically locked together in a fixed recipe and can only be pulled apart using electricity.

📝 Teacher's Note: This is a fundamental concept to distinguish between mixtures (like salt water) and compounds (like pure water). Emphasize that chemical properties of water are entirely different from Hydrogen and Oxygen.

🎯 Exam Tip: To prove water is a compound, always mention the fixed mass ratio (1:8) and the fact that it requires a chemical method (electrolysis) to separate.

Question 3.
Answer: (a) The temperature in Mumbai and Chennai do not fall as low as in Delhi because Mumbai and Chennai are situated at the bank of the oceans due to high specific heat capacity the presence of a large amount of water is able to modify the climate of the nearby land areas are warmer in winter and cooler in summer temperature does not fall wherever Delhi has not same condition.

(b) Our body is almost 65% of water and water has property of specific heat. Due high specific heat capacity the presence of large amount of water is able to modify the climate of the body and control the temperature of our body which is warm in winter and cooler in summer.
In simple words: Water acts like a giant temperature stabilizer. Because it takes a long time to heat up or cool down, it keeps coastal cities and our own bodies from getting too hot or too cold.

📝 Teacher's Note: Explain "Specific Heat Capacity" using the analogy of a battery—water "stores" heat energy very effectively, preventing sudden temperature spikes.

🎯 Exam Tip: The keyword here is "High Specific Heat Capacity." Mentioning this property is essential for scoring full marks in reasoning questions about climate or body temperature.

Question 4. Water dissolves many substances forming aqueous solution. It can dissolve solids, liquids and gases.
Answer: When a solid dissolves in water the solid is said to be solute, the water (the solvent) and the resultant liquid, the solution. So it is said that water is a universal solvent. In other words water can dissolve nearly every substance.
In simple words: Water is the world's best "melter." It can pull apart the molecules of almost anything, which is why we call it the universal solvent.

📝 Teacher's Note: Differentiate between solute (what is being dissolved), solvent (the liquid doing the dissolving), and solution (the final mixture) using a simple salt-water example.

🎯 Exam Tip: Remember the term "Universal Solvent." It is the most common way water is described in chemistry due to its polar nature.

Question 5. The sudden release of the latent heat of condensation causes the violence associated with torrential rain.
Answer: The sudden release of the latent heat of condensation causes the violence associated with torrential rain.
In simple words: When water vapor turns back into liquid rain, it lets out a huge burst of stored energy. This extra energy is what makes storms so powerful and windy.

📝 Teacher's Note: This is a bridge between chemistry and geography. Explain that "Latent Heat" is "hidden heat" that is released when a gas turns into a liquid.

🎯 Exam Tip: Associate "Latent heat of condensation" with the energy source for atmospheric phenomena like thunderstorms and cyclones.

Question 6.
Answer: (a) Specific heat

(b) Water has an unusual physical property. When cooled, it first contracts in volume, as do other liquids, but at 4°C (maximum density), it starts expanding, and continues to do so till the temperature reaches 0°C, the point at which it freezes into ice.
The property of anomalous expansion of water enables marine life to exist in the colder regions of the world, because even when the water freezes on the top, it is still liquid below the ice layer.
(c) Melting point: The constant temperature at which a solid changes into a liquid state, by absorbing the heat energy is called melting point.
Boiling point: The temperature at which water starts boiling under normal pressure is called boiling point of water. It is 100°C. Boiling point increases with increase in pressure and vice versa.
Specific heat capacity: The amount of heat required to raise the temperature of unit mass of that substance through 1°C.
Latent heat of vaporization of water: The energy required to change water into its vapour at its boiling point without any change in temperature is called latent heat of vaporization of water. Latent heat of vaporization of water is 2260 joules J/g or 540 cal/g. In the reverse process, 2260 joules of heat is released when 1 g of steam condenses to form 1 g of water at 100°C.
Latent heat of fusion of ice: The amount of heat energy required by ice to change into water is called latent heat of fusion of ice. Latent heat of fusion of ice is 336 J/g or 80 cal/g. In the reverse process, 336 joules of heat is released when 1 g of water solidifies to form 1 g of ice at 0°C.
In simple words: Water has many "superpowers." It expands when it freezes (which is weird for a liquid!), it takes a lot of energy to boil, and it can store "hidden" heat when changing from ice to water.

📝 Teacher's Note: The anomalous expansion of water is a life-saving property. Use the diagram to show how the densest water (4°C) stays at the bottom, allowing fish to survive even when the surface is frozen solid.

🎯 Exam Tip: Be precise with the values: \( 336\text{ J/g} \) for fusion and \( 2260\text{ J/g} \) for vaporization. These are frequently asked in numerical and objective questions.

Question 7. Composition of water :- Hydrogen and Oxygen Volume Ratio = \( \text{H}_2 : \text{O} \)
Answer: \( \text{H}_2 : \text{O} = 2 : 1 \)
In simple words: In every molecule of water, there are twice as many hydrogen atoms as there are oxygen atoms.

📝 Teacher's Note: Remind students that the formula is \( \text{H}_2\text{O} \), which directly shows the 2:1 ratio of atoms (and thus volume at the same temperature and pressure).

🎯 Exam Tip: Don't confuse the mass ratio (1:8) with the volume ratio (2:1). Read the question carefully to see which one is asked.

Question 8. The properties of water are different from the properties of elements from which it is formed
Answer:

In simple words: Even though water is made of two gases (Hydrogen and Oxygen), the liquid result is completely different—it's a liquid, it dissolves things, and it has unique density rules.

📝 Teacher's Note: This table illustrates the law that a compound possesses entirely different properties from its constituent elements. For example, Hydrogen is combustible and Oxygen supports combustion, but water is used to extinguish fire.

🎯 Exam Tip: This table is great for "Distinguish between" questions. Memorize at least two differences, like state (liquid vs gas) and density.

Question 9. The property of anomalous expansion of water enables aquatic life to exist because of the water freezes on top of the surface of the water body, but it is still liquid below the ice layer.
Answer: The property of anomalous expansion of water enables aquatic life to exist because of the water freezes on top of the surface of the water body, but it is still liquid below the ice layer.
In simple words: Because ice is lighter than water and stays on top, it acts like a warm blanket for the lake, keeping the water underneath liquid so fish can survive the winter.

📝 Teacher's Note: Link this back to the diagram in Question 6. It's one of the most important environmental impacts of any physical property in chemistry.

🎯 Exam Tip: Explain that ice acts as an "insulator," preventing the heat of the water from escaping into the freezing air.

Question 10.
Answer: (a) Aim: To show that tap water contains dissolved salts.
Procedure: Put some tap water on a clean watch glass and place it over a beaker containing water as shown in fig. Boil the water in the beaker. When all the water has evaporated from the watch glass, remove the burner and let it cool. We see at the watch glass against light, a number of concentric rings of solids matter on it. These are dissolved impurities, left behind after evaporation of water. To show that water contains dissolved solids.

(b) Aim: To show that tap water contains dissolved gases.
Procedure: Take a round bottomed flask and filled it with the tap water. In its mouth fix a delivery tube, in such a way that its lower end of the delivery tube is in line with the under – surface of the cork. Arrange the apparatus according to diagram. Heat the flask with the help of a Bunsen burner. It is seen that tiny bubbles of gas are coming out, which stick to the sides of flask, heat it continuously, till the water is about to boil. It is seen that Bubbles of gas start coming out of beehive shelf. Now lower the flame, to keep the water just near its boiling point. Invert over the beehive shelf a graduated tube, completely filled with tap water. Gradually, the boiled off air, starts collecting in the flask. Collect at least two tubes of boiled off air.
In simple words: We can prove water isn't just "pure H2O" by boiling it away to find leftover salt rings, or by heating it to see bubbles of trapped air coming out.

📝 Teacher's Note: These two experiments are classic lab demonstrations. The "concentric rings" on the watch glass are a visual proof of solutes in a solution.

🎯 Exam Tip: For experiment-based questions, always clearly state the Aim, Procedure, and Observation. Use the term "concentric rings" when describing the residue on the watch glass.

Question 11. CO2 and O2 add taste to water for drinking purposes.
Answer: \( \text{CO}_2 \) and \( \text{O}_2 \) add taste to water for drinking purposes.
In simple words: Pure water is actually quite tasteless. It's the tiny amounts of dissolved oxygen and carbon dioxide that give it the fresh taste we enjoy.

📝 Teacher's Note: This is why boiled water tastes "flat"—the heat has driven out these dissolved gases.

🎯 Exam Tip: Dissolved gases and minerals are the two main factors that give drinking water its flavor.

Question 12. Oxygen is more soluble in water than nitrogen.
Answer: Oxygen is more soluble in water than nitrogen. Air dissolved in water contains a higher percentage of oxygen. That is, 30% – 35% and in ordinary air it is only 21 %. In this way air dissolved in water is different from ordinary air.
In simple words: Water "prefers" oxygen more than nitrogen. This is lucky for fish, as they get air that is richer in oxygen than the air we breathe!

📝 Teacher's Note: Use this to explain how aquatic life can survive on relatively small volumes of dissolved air—the air they get is "concentrated" oxygen.

🎯 Exam Tip: Compare the percentages: 21% \( \text{O}_2 \) in normal air vs. 30-35% \( \text{O}_2 \) in dissolved air.

Question 13.
Answer: Rivers and lakes have large amount of water and water has high specific heat capacity, due to which they do not freeze easily. Even if they freeze, they freeze at top layer. There is water below due to Anomalous expansion of water.
In simple words: Huge lakes don't turn into ice cubes overnight because water holds onto its heat for a long time. Even in deep winter, only the top layer freezes, leaving a safe liquid home for fish below.

📝 Teacher's Note: This question combines "High Specific Heat" (prevents cooling) and "Anomalous Expansion" (keeps liquid water at the bottom).

🎯 Exam Tip: Mention both properties to provide a complete scientific explanation for why large water bodies don't freeze solid.

Question 14. Importance of dissolved salts in water:
Answer: 1. Dissolved salts provide a specific taste to water.
2. Dissolved salts act as micro-nutrients for the growth and development of living beings.
In simple words: Salts in water aren't just for flavor; they also provide essential minerals that our bodies and plants need to grow healthy.

📝 Teacher's Note: Discuss the concept of "Mineral Water" vs. "Distilled Water." Distilled water is pure but lacks the healthy minerals found in tap or spring water.

🎯 Exam Tip: List two points: Taste and Nutrients. Use the term "micro-nutrients" for a more scientific answer.

Question 15.
Answer: (a) Boiled water tastes flat because boiled water does not contain matter like air, carbon dioxide and other minerals, So the boiled water tastes flat.

(b) Ice at zero degree centigrade gives more cooling effect than water at 0°C because, ice at 0°C absorbs 336J per gram of energy to melt to 0°C water and hence gives more cooling effect.

(c) Burn caused by steam is more severe than burn caused by boiling water because, 1 g of steam contains 2268J more energy than 1 g of boiling water and hence, cause more severe burns.

(d) Rain water does not leave concentric rings when boiled because rain water does not contain dissolved solid, so it does not form concentric rings.

(e) Air dissolved in water contains a higher percentage of oxygen because, solubility of oxygen in water is more than in air. So, air dissolved in water contains a higher percentage of oxygen.

(f) If distilled water is kept in a sealed bottle for a long time, it leaves etching on the surface of glass because, the substances which are insoluble in water, actually dissolve in minute traces in water. Even when we drink water from a glass, an extremely small amount of glass dissolves in water, so we see the etching on the surface of glass when a long time sealed bottle of distilled water poured into the glass.
In simple words: Science explains why steam burns more (hidden energy), why boiled water is boring (no bubbles), and even why water can slowly "eat" through glass over many years!

📝 Teacher's Note: Sub-part (c) is a very important safety lesson. Steam is dangerous because it carries the extra "Latent Heat of Vaporization." Sub-part (f) introduces the idea that "insoluble" often just means "very, very slightly soluble."

🎯 Exam Tip: For the steam burn question, always mention the specific energy difference (\( 2268\text{ J/g} \)) to show you understand Latent Heat.

Question 16.
Answer: (i) When solid changes with liquid, it absorbs heat equal to latent heat of fusion and when a liquid changes into solid, it loses heat equal to latent heat of solidification.
(ii) When a liquid changes into gas, it absorbs heat equal to latent heat of vaporization and when a gas condenses into liquid, it loses heat equal to latent heat of condensation.
In simple words: Changing state is like a financial transaction—you either have to "pay" heat energy to melt/boil, or you "get paid" heat energy when freezing or condensing.

📝 Teacher's Note: This reinforces the concept that phase changes happen at a constant temperature. The energy goes into breaking or forming bonds, not changing the thermometer reading.

🎯 Exam Tip: Remember: Solid to Liquid = Absorb. Liquid to Solid = Release. The amount of heat is always the same for the same mass.

Page No: 44

Exercise 6(B)

Question 1.
Answer: (a) Solution: Solution is a homogeneous mixture of two or more substances, components of which cannot be seen separately.
(b) Solute: A solute is the substance that dissolves in a solvent to form a solution.
(c) Solvent: A solvent is a medium in which the solute dissolves. Solution = Solute + Solvent
In simple words: A solution is a perfect mix where the parts vanish into each other. The "Solute" is the stuff you add (like sugar) and the "Solvent" is what does the work (like water).

📝 Teacher's Note: Use the word "homogeneous" to emphasize that every drop of the solution is identical to every other drop.

🎯 Exam Tip: Learn the equation Solution = Solute + Solvent. It's the simplest way to define these relationships.

Question 2. The solubility of nitrate decreases with the fall in temperature.
Answer: The solubility of nitrate decreases with the fall in temperature. Thus, when saturated solution of nitrate is cooled the excess of it separates from solution, in the form of crystals.
In simple words: Hot water can hold more salt than cold water. If you cool down a "full" solution, it can't hold everything anymore, and the extra salt pops back out as crystals.

📝 Teacher's Note: This is the basis for crystallization. It's like a bus that has fewer seats when it gets cold—some passengers have to get off!

🎯 Exam Tip: This is a common reasoning question about why crystals form when a hot saturated solution is cooled.

Question 3. The three factors on which the solubility of a solid depends are:
Answer: 1. Temperature
2. Nature of the solid
3. Nature of solvent
In simple words: How much stuff you can dissolve depends on how hot the water is, what the stuff is made of, and what kind of liquid you are using.

📝 Teacher's Note: Give examples: Salt dissolves better in hot water (Temp), Sugar dissolves better than sand (Nature of Solid), and salt dissolves in water but not in oil (Nature of Solvent).

🎯 Exam Tip: List these three factors clearly. "Temperature" is usually the most important one for solids.

Question 4. Take 100 g of distilled water in a beaker. To this add one gram of copper sulphate crystals.
Answer: Stir this mixture with the help of a glass rod and dissolve copper sulphate crystals. Similarly, go on dissolving more of copper sulphate, (1 gram) at a time with constant and vigorous stirring. A stage is reached when no more copper sulphate dissolves. It is called saturated solution at this temperature. Take this saturated solution of copper sulphate some solution in a test tube and add some copper sulphate crystals. The crystals do not dissolve but settle down. This indicates that the solution is really saturated.
In simple words: A "saturated" solution is like a sponge that is totally soaked. No matter how much more you try to add, it just won't take any more in.

📝 Teacher's Note: Saturated means "full." Emphasize that saturation is only valid at a *specific* temperature.

🎯 Exam Tip: Define saturated solution as one which cannot dissolve any more solute at a given temperature.

Question 5.
Answer: (a) (i) Henry’s law: It states that at any given temperatures, the mass of a gas dissolved in a fixed volume of a liquid or solution is directly proportional to the pressure on the surface of a liquid.
(ii) Crystallisation: It is the process by which crystals of a substance separate out on cooling its hot saturated solution.

(b) In the laboratory, crystals may be obtained by the following methods:
1. By cooling a hot saturated solution gently.
2. By cooling a fused mass.
3. By sublimation.
4. By evaporating slowly a saturated solution.
In simple words: Henry's law says more pressure means more gas dissolves (like in a soda can). Crystallization is the art of making perfect solid shapes by cooling down a chemical soup.

📝 Teacher's Note: Use a soda bottle to demonstrate Henry's law: when the pressure is released (cap opened), the "dissolved" gas escapes as bubbles.

🎯 Exam Tip: Henry's law applies specifically to gases. For solids, pressure has almost no effect on solubility.

Question 6. Action of heat on copper (II) sulphate crystals
Answer: When copper (II) sulphate crystals are heated in a hard glass test tube, the following observations are made:
1. The crystals are converted into powdery substance.
2. The crystals lose their blue coloration on further heating.
3. Steaming vapour are produced inside the tube which condense near the mouth of the tube to form a colourless liquid.
4. On further heating, steams escapes from the mouth of the tube and water gets collected in a beaker placed under the mouth of tube.
5. On further heating, the residue changes to a white powder and steam stops coming out.
\[ \text{CuSO}_4 \cdot 5\text{H}_2\text{O} \xrightarrow{\Delta} \text{CuSO}_4 + 5\text{H}_2\text{O} \]

Action of heat on iron (II) sulphate:
When iron (II) sulphate is heated in a test tube, the following observations are made:
1. The crystals crumble to white powder and a large amount of steam and gas are given out.
2. On strong heating, a brown residue of ferric oxide (\( \text{Fe}_2\text{O}_3 \)) is produced and a mixture of \( \text{SO}_2 \) and \( \text{SO}_3 \) is given off.
\[ \text{FeSO}_4 \cdot 7\text{H}_2\text{O} \xrightarrow{\text{Heat}} \text{FeSO}_4 + 7\text{H}_2\text{O} \]
\[ 2\text{FeSO}_4 \xrightarrow{\text{Ferric oxide (Brown residue)}} \text{Fe}_2\text{O}_3 + \text{SO}_2 + \text{SO}_3 \]
In simple words: Heating these blue and green crystals "cooks" the water out of them, turning them into plain white powders. If you keep heating the iron ones, they eventually turn brown and give off smelly sulfur gases.

📝 Teacher's Note: This experiment shows the difference between "Water of Crystallization" (the water that makes them blue/green crystals) and the chemical breakdown of the salt itself.

🎯 Exam Tip: Note the color changes: Blue \( \rightarrow \) White for Copper, and Green \( \rightarrow \) White \( \rightarrow \) Brown for Iron. These color shifts are frequently tested.

Question 7. Table salt becomes sticky on exposure during the rainy season.
Answer: Table salt becomes sticky on exposure during the rainy season because table salt generally contains a small percentage of Magnesium chloride, as an impurity. Since these impurities absorb moisture from air due to their deliquescent nature; therefore it gets wet in rainy season and becomes sticky.
In simple words: Your salt shaker clogs in the rain because of "impurities" that love water so much they suck it right out of the damp air!

📝 Teacher's Note: Pure Sodium Chloride (\( \text{NaCl} \)) is actually not deliquescent. It's the Magnesium and Calcium impurities that cause the problem.

🎯 Exam Tip: The keyword here is "Deliquescent." This means a substance absorbs so much water that it actually dissolves in it.

Question 8.
Answer: Potassium nitrate (\( \text{KNO}_3 \)): Increase in solubility of substances with rise in temperature.
Calcium sulphate (\( \text{CaSO}_4 \)): Decrease in solubility of substances with rise in temperature.
In simple words: Most salts dissolve more when things get hot, but some weird ones (like calcium sulphate) actually dissolve less!

📝 Teacher's Note: Use this to show that there are exceptions to every rule in chemistry. Most solutes are "endothermic" (need heat to dissolve), but some are "exothermic."

🎯 Exam Tip: Memorize \( \text{CaSO}_4 \) (Calcium Sulphate) as the classic example of a substance whose solubility *decreases* as temperature goes up.

Question 9. Solubility of \( \text{NaCl} \) at 40°C is 36.5 g.
Answer: Solubility of \( \text{NaCl} \) at 40°C is 36.5 g means 36.5 g of \( \text{NaCl} \) dissolves in 100 g of water at the temperature of 40°C.
In simple words: If you have 100 grams of water at 40°C, you can fit exactly 36.5 grams of salt inside it before it's full.

📝 Teacher's Note: Solubility is always measured per 100 grams of solvent. It's like the "percentage" capacity of water.

🎯 Exam Tip: Always specify the amount of water (100g) and the temperature when explaining a solubility value.

Question 10.
Answer: 1. A solution in which more of solute can be dissolved at a given temperature is an unsaturated solution.
2. A solution in which no more solute can be dissolved at a given temperature is a saturated solution at that temperature.
3. A solution in which some solute separates on cooling slightly is a supersaturated solution.
In simple words: Unsaturated = Room for more. Saturated = Totally full. Supersaturated = So over-full that it's about to burst into crystals.

📝 Teacher's Note: Use a sponge analogy: dry (unsaturated), damp (saturated), and dripping (supersaturated).

🎯 Exam Tip: A "supersaturated" solution is very unstable. Even dropping a tiny crystal into it will cause the whole thing to crystallize instantly.

Question 11.
Answer: • With the increase in pressure the solubility of a gas in water increases.
• With the increase in temperature, the solubility of a gas in water decreases.
• For example, The solubility of carbon dioxide in water under normal atmospheric pressure is low, but when the water surface is subjected to higher pressure, a lot more of \( \text{CO}_2 \) gas gets dissolve in it.
• Similarly, In the case of soda water, on opening the bottle, the dissolved gas rapidly bubbles out, since the pressure on the surface of the water suddenly decreases.
In simple words: Gases are the opposite of solids. They like pressure but hate heat. That's why cold soda stays fizzy, while warm soda goes flat quickly.

📝 Teacher's Note: This explains why fish die in warm water—as the temperature goes up, the dissolved oxygen they need to breathe escapes back into the air.

🎯 Exam Tip: Remember: Pressure \( \uparrow \) = Gas Solubility \( \uparrow \). Temperature \( \uparrow \) = Gas Solubility \( \downarrow \).

Question 12. A solubility curve is a line graph which shows changes in the solubility of a solute in a given solvent with a change in temperature.
Answer: To obtain this curve, values of temperature are plotted on X-axis and values of solubility on Y-axis.
Applications:
1. The variation in the solubility of any given substance with temperature can be studied with the help of solubility curve.
2. To compare the solubility of different substances, at a given temperature.
In simple words: A solubility curve is a map that tells you exactly how much salt will fit in your water at any temperature on the thermometer.

📝 Teacher's Note: This is a great way to introduce data interpretation. Different substances have curves with different "steepness"—the steeper the curve, the more sensitive the substance is to temperature.

🎯 Exam Tip: The X-axis is always Temperature, and the Y-axis is Solubility (g/100g water). Don't swap them!

Question 13.
Answer: (a) Water is an excellent liquid to use in cooling systems due to its ability to absorb large quantities of heat i.e. specific heat = 4.2J/g°C, so it is used in cooling system i.e. cooling agent.

(b) A solution is always clear and transparent because in a solution, solid disappeared in water and water has property – cleanliness and transparent. So, the solution is always clean and transparent.

(c) Lakes and rivers do not freeze suddenly in winters due to high specific latent heat of solidification. i.e. the amount of heat released when 1 g of water solidifies to form 1 g of ice at 0°C. It is about 336 J/g or 80 cal/g. Such enormous amount of heat leads to immediate freezing of lakes and rivers in winter.

(d) The component that actually dissolves in a solvent is known as solute. So it can separated from solution by filtration process. But filtration process is applicable only when solute is insoluble in solution. So the solute cannot be separated from solution by filtration.

(e) Fused \( \text{CaCl}_2 \) or concentrated \( \text{H}_2\text{SO}_4 \) is deliquescent in nature absorbs moisture and hence, these are used in desiccators or as drying agent.

(f) Carbon dioxide is dissolved in soda water under pressure. On opening the bottle, the pressure on the surface of water suddenly decreases, therefore, the solubility of \( \text{CO}_2 \) in water decreases and the gas rapidly bubbles out.
In simple words: Water is the ultimate multitasker. It cools car engines, stays clear when it holds salt, keeps fish from freezing, and makes our sodas fizzy.

📝 Teacher's Note: Sub-part (d) is a key test of understanding: you can't "filter" a solution because the particles are too small to get caught in the paper. Only "suspensions" can be filtered.

🎯 Exam Tip: In reasoning questions, always link the physical property (like high specific heat) to the practical application (like car radiators).

Question 14.
Answer: (a) Potassium nitrate
(b) Potassium chloride
(c) Sodium chloride
(d) Calcium sulphate
In simple words: These are names of common salts that dissolve in water to different degrees.

📝 Teacher's Note: These are standard examples used in solubility experiments. \( \text{KNO}_3 \) and \( \text{NaCl} \) are favorites for textbook problems.

🎯 Exam Tip: Learn the chemical formulas for these: \( \text{KNO}_3 \), \( \text{KCl} \), \( \text{NaCl} \), and \( \text{CaSO}_4 \).

Question 15.
Answer: These are the substances which can readily absorb moisture from other substances without chemically reacting with them. For example, Phosphorus pentoxide (\( \text{P}_2\text{O}_5 \)), quick lime (\( \text{CaO} \)).
In simple words: These are chemical sponges. They suck up water from the air or from other chemicals to keep things dry.

📝 Teacher's Note: These are called "Drying Agents" or "Desiccants." You've probably seen little silica gel packets in shoe boxes—those are industrial drying agents!

🎯 Exam Tip: Contrast these with "deliquescent" substances. Drying agents absorb moisture but don't necessarily turn into a liquid.

Question 16.
Answer:

In simple words: Chemicals have "common names" (like nicknames) and "chemical names" (scientific names). Some absorb water, some give it away (efflorescent), and some turn into a puddle!

📝 Teacher's Note: Correct the entry for Blue Vitriol—it is actually Hydrated Copper Sulphate (\( \text{CuSO}_4 \cdot 5\text{H}_2\text{O} \)). Efflorescent substances (like washing soda) lose their water of crystallization to the air.

🎯 Exam Tip: This table is a goldmine for MCQs. Make sure you know the difference between hygroscopic, deliquescent, and efflorescent.

Question 17.
Answer: (a) Increase in mass- Iron and conc. sulphuric acid
(b) Decrease in mass- Sodium carbonate crystals
(c) No change in mass- Sodium chloride
In simple words: Some things get heavier because they suck in water from the air. Others get lighter because they lose water. Ordinary salt stays the same.

📝 Teacher's Note: This is a practical application of the terms in the previous table. \( \text{H}_2\text{SO}_4 \) increases mass by absorbing moisture, while hydrated crystals lose mass as they dry out.

🎯 Exam Tip: If mass increases, the substance is hygroscopic or deliquescent. If mass decreases, it is efflorescent.

Question 18. Mass of solute = 136 g, Mass of solvent = 500 g.
Answer: \( \text{Solubility} = \frac{\text{Mass of solute}}{\text{Mass of solvent}} \times 100 \)
\( = \frac{136}{500} \times 100 = \frac{136}{5} = 27.2\text{ g} \)
In simple words: To find the solubility, we just calculate how much salt would fit in 100 grams of water if 136 grams fits in 500 grams.

📝 Teacher's Note: This is a simple ratio problem. Remind students that solubility is always "per 100g of solvent."

🎯 Exam Tip: Always state the final unit as "g/100g water" or simply "g" if the per-100g context is clear.

Question 19.
Answer: (a) Mass of sodium chloride = 15 g, Mass of water = 285 g
\( \text{Concentration of solution} = \frac{\text{Mass of solute}}{\text{Mass of solution}} \times 100 \)
\( = \frac{15}{(15 + 285)} \times 100 = \frac{15}{300} \times 100 = 5\% \)

(b) Volume of acetone (Solute) = 4 liter, Volume of solution = 90 liter
\( \text{Volume percent} = \frac{\text{Volume of solute}}{\text{Volume of solution}} \times 100 \)
\( = \frac{4}{90} \times 100 = 4.44\% \)
In simple words: Concentration tells you how much "stuff" is in the total mix. If you have 15g of salt in 300g of total liquid, it's a 5% solution.

📝 Teacher's Note: Be careful! In the concentration formula, the denominator is the mass of the Solution (Solute + Solvent), not just the Solvent.

🎯 Exam Tip: Read the question carefully to see if you are given the "mass of solvent" or "mass of solution" for the denominator.

Question 20.
Answer: (a) Solubility of \( \text{KNO}_3 \) at 313 K = 62
\( 62 = \frac{x}{50} \times 100 \implies x = \frac{62 \times 50}{100} = 31\text{g} \)

(b) Solubility of solid decreases with fall in temperature. A saturated solution on cooling, a part of dissolved solute separates out in the form of crystals.

(c) Solubility of salt at 293K:
\( \text{KNO}_3 \rightarrow 32\text{g} \)
\( \text{NaCl} \rightarrow 36\text{g} \)
\( \text{KCl} \rightarrow 35\text{g} \)
\( \text{NH}_4\text{Cl} \rightarrow 37\text{g} \)

(d) At 283K lowest solubility is of \( \text{KNO}_3 \rightarrow 21\text{g} \)

(e) Solubility of most of solids usually increases and of gas and liquid always decreases with rise in temperature.
In simple words: Different salts have different "limits." At room temperature, Ammonium Chloride is the most soluble of this group, but when it's cold, Potassium Nitrate is the hardest to dissolve.

📝 Teacher's Note: This compares several salts. Note that \( \text{KNO}_3 \) solubility changes drastically with temperature, while \( \text{NaCl} \) stays relatively flat.

🎯 Exam Tip: If a table of data is provided, use it to find specific values like "lowest solubility" or "most change."

Question 21.
Answer: (a) Wt. of empty dish = 50 gm, Wt. of dish and solution = 65 gm, Wt. of dish and residue = 54.3 gm
Wt. of saturated solution = \( 65 – 50 = 15\text{ gm} \)
Wt. of crystals = \( 54.3 – 50 = 4.3\text{ gm} \)
Wt. of water in saturated solution = \( 15 – 4.3 = 10.7\text{ gm} \)
\( \text{Solubility of KNO}_3 = \frac{\text{Wt. of crystals}}{\text{Wt. of water}} \times 100 \)
\( = \frac{4.3 \times 100 \times 10}{10.7 \times 10} = \frac{4300}{107} = 40.18\text{ g/100 g of water at 20°C} \)

(b) Wt. of water = 50 gm
Solubility at 50°C = 114 gm, Solubility at 30°C = 86 gm
Solubility change from 50°C to 30°C = \( 114 – 86 = 28\text{ gm} \)
\( 28 = \frac{\text{Wt. of crystals}}{50} \times 100 \implies \text{Wt. of crystals} \times 2 = 28 \implies \text{Wt. of crystals} = 14\text{ gm} \)
In simple words: By weighing a dish before and after drying out a solution, we can work backward to find exactly how much salt was dissolved in the water.

📝 Teacher's Note: This is a classic lab practical calculation. Ensure students understand that the "residue" is the dry salt left after the water has evaporated.

🎯 Exam Tip: Be very careful with the subtractions. Dish + Residue - Dish = Solute. Dish + Solution - Dish = Solution.

Page No: 46

Exercise 6(C)

Question 1.
Answer: Water is said to be soft when the water containing sodium salts easily gives lather with soap. Water is said to be hard when it does not readily form lather with soap. Water which contains only hydrogen carbonates of calcium and magnesium is called temporary hard water. Water containing sulphates and chlorides of magnesium and calcium is called permanent hard water.
In simple words: Soft water makes bubbles easily. Hard water is "stubborn"—it contains calcium or magnesium that stops soap from working correctly.

📝 Teacher's Note: Hardness is essentially "soap-destroying power." Use the terms "temporary" and "permanent" to describe how easily the hardness can be removed (boiling vs. chemicals).

🎯 Exam Tip: Define soft water by its ability to form "lather" and hard water by its inability to do so.

Question 2.
Answer: The presence of hydrogen carbonates of calcium and magnesium makes water temporarily hard. The presence of sulphates and chlorides of magnesium and calcium makes water permanently hard.
In simple words: Bicarbonates cause temporary hardness. Sulphates and chlorides cause permanent hardness.

📝 Teacher's Note: Have students learn the four main salts: \( \text{Ca(HCO}_3)_2 \), \( \text{Mg(HCO}_3)_2 \) (temporary) and \( \text{CaCl}_2 \), \( \text{MgSO}_4 \) (permanent).

🎯 Exam Tip: If a question asks why water is *temporarily* hard, your answer must include "bicarbonates of calcium and magnesium."

Question 3.
Answer: Advantages of soft water:
When the water is soft, you use much less soap and fewer cleaning products. Your budget will reflect your savings. Plumbing will last longer. Clothes last longer and remain bright longer if they are washed in soft water.
Advantages of hard water:
Water free from dissolved salts has a very flat taste. The presence of salts in hard water makes it tasty. So, hard water is used in making beverages and wines. Calcium and magnesium salts present in small amounts in hard water are essential for bone and teeth development. Hard water checks the poisoning of water by lead pipes by forming a layer of insoluble lead sulphate inside the lead pipe.
In simple words: Soft water is better for your wallet and your laundry. Hard water is better for your health (bones) and actually tastes better in drinks!

📝 Teacher's Note: This is a rare case where "hard" isn't always bad. It's important for health and prevents the leaching of lead from old pipes.

🎯 Exam Tip: List two advantages for each. For hard water, mention bone development and protection from lead poisoning.

Question 4. In some limestone caves, conical pillar-like objects hang from the roof and some rise from the floor.
Answer: These are formed by water containing dissolved calcium hydrogen carbonate continuously dropping from the cracks in the rocks. Release of pressure results in the conversion of some hydrogen carbonate to calcium carbonate.
\( \text{Ca(HCO}_3)_2 \rightarrow \text{CaCO}_3 + \text{CO}_2 + \text{H}_2\text{O} \)
This calcium carbonate little by little and slowly deposit on both roof and floor of the cave. The conical pillar which grows downwards from the roof is called stalactite and the one which grows upward from the floor of the cave is called stalagmite. These meet after a time. In a year, some grow less than even a centimetre, but some may be as tall as 100 cm.
\( \text{CaCO}_3 + \text{CO}_2 + \text{H}_2\text{O} \rightarrow \text{Ca(HCO}_3)_2 \)
\( \text{MgCO}_3 + \text{CO}_2 + \text{H}_2\text{O} \rightarrow \text{Mg(HCO}_3)_2 \)
If the water flows over beds of gypsum (\( \text{CaSO}_4 \cdot 2\text{H}_2\text{O} \)), a little bit of gypsum gets dissolved in water and makes it hard.
In simple words: Stalactites (ceiling) and Stalagmites (floor) are like stone icicles. They form over hundreds of years as dripping water leaves behind tiny bits of calcium carbonate "dust."

📝 Teacher's Note: Use the mnemonic: StalacTite comes from the Top (ceiling), and StalagMite comes from the Mud (floor). The chemical reaction is the reverse of what happens when rocks dissolve.

🎯 Exam Tip: Know the difference between the two terms. Remember that it's the *insoluble* calcium carbonate (\( \text{CaCO}_3 \)) that actually builds the pillars.

Question 5.
Answer: Hydrogen carbonates of calcium and magnesium. Sulphates and chlorides of magnesium and calcium.
In simple words: These are the specific "culprit" salts that make water hard.

📝 Teacher's Note: This is a repeat of Question 2 for reinforcement. It's the core of the hard water topic.

🎯 Exam Tip: Be ready to write the chemical names and formulas for these salts on command.

Question 6.
Answer:
\( \text{Ca(HCO}_3)_2 \xrightarrow{\text{Boil}} \text{CaCO}_3 + \text{H}_2\text{O} + \text{CO}_2 \uparrow \)
\( \text{Mg(HCO}_3)_2 \xrightarrow{\text{Boil}} \text{MgCO}_3 + \text{H}_2\text{O} + \text{CO}_2 \uparrow \)
\( \text{Ca(HCO}_3)_2 + \text{Ca(OH)}_2 \xrightarrow{\text{Boil}} 2\text{CaCO}_3 + 2\text{H}_2\text{O} \)
\( \text{Mg(HCO}_3)_2 + \text{Ca(OH)}_2 \xrightarrow{\text{Boil}} \text{MgCO}_3 + 2\text{H}_2\text{O} \)
In simple words: Boiling turns the dissolved, invisible hardness into solid chalk that settles out. This is why boiling "softens" temporary hard water.

📝 Teacher's Note: These equations show why temporary hardness is called "temporary"—a simple heat source can drive the reaction to remove the minerals.

🎯 Exam Tip: The arrow \( \uparrow \) for \( \text{CO}_2 \) and \( \downarrow \) (or implied for the solid salt) are important symbols to show that the hardness is leaving the water.

Question 7.
Answer: It is more difficult to form lather with soap. Scum may form in a reaction with soap, wasting the soap. Carbonates of calcium and magnesium form inside kettles. This wastes energy whenever you boil a kettle. Hot water pipes ‘fur up’. Carbonates of calcium and magnesium start to coat the inside of pipes which can eventually get blocked.
In simple words: Hard water is a nuisance—it ruins soap by making "scum" instead of bubbles, and it coats the inside of kettles and pipes with a thick layer of "fur" or scale.

📝 Teacher's Note: Show a picture of a "scaled" kettle element. This layer of calcium carbonate acts as an insulator, meaning it takes more electricity/gas to heat the water.

🎯 Exam Tip: The terms "scum" and "furring of pipes" are specific technical terms you should use in your answers.

Question 8. Soap is chemically a sodium salt of stearic acid.
Answer: Soap has the formula \( \text{C}_{17}\text{H}_{35}\text{COOH} \) (as acid) and has the formula \( \text{C}_{17}\text{H}_{35}\text{COONa} \). Soap is used for washing purposes.
In simple words: Soap is basically a long-chain fatty salt that acts like a bridge between water and grease.

📝 Teacher's Note: Explain that soap has two ends: one that loves water and one that loves oil. This is how it pulls dirt off your skin.

🎯 Exam Tip: Memorize the formula \( \text{C}_{17}\text{H}_{35}\text{COONa} \) for sodium stearate (soap).

Question 9.
Answer: Detergents are more soluble in water than soap and are unaffected by the hardness of water as their calcium salts are soluble in water.
In simple words: Detergents are "smart" soaps. They work perfectly even in hard water because they don't form that sticky scum.

📝 Teacher's Note: This is why laundry powders are usually detergents, not real soap. They are engineered to work in any water condition.

🎯 Exam Tip: The main advantage of detergents is that they do not form scum with hard water.

Question 10. Steam is usually made in boilers which are made of a number of narrow copper tubes surrounded by fire.
Answer: As the cold water enters these tubes, it is immediately changed into steam, while the dissolved solids incapable of changing into vapour deposit on the inner walls of the tubes. This goes on and makes the bore of the tubes narrower. The result is that less water flows through the tubes at one time and less steam is produced. When the bore of the tube becomes very narrow, the pressure of the steam increases so much that at times the boiler bursts. If hard water is used, calcium and magnesium ions of the water combine with the negative ions of the soap to form a slimy precipitate of insoluble calcium and magnesium usually called soap curd (scum). Formation of soap curd will go on as long as calcium and magnesium ions are present. Till then, no soap lather will be formed and cleaning of clothes or body will not be possible. Moreover, these precipitates are difficult to wash from fabrics and sometimes form rusty spots if iron salts are present in water.
In simple words: Hard water is dangerous for big machines. It creates "clogs" in boiler pipes that can make them explode, and it makes "soap curd" that leaves your clothes gray and stiff.

📝 Teacher's Note: This explains why industrial plants always use water softeners. The "bursting" of boilers is a serious industrial safety concern caused by hardness.

🎯 Exam Tip: Mention "Boiler Scale" or "Furring" as the reason for reduced efficiency and potential explosions in boilers.

Question 11.
Answer: Slaked lime:
\( \text{Ca(HCO}_3)_2 + \text{Ca(OH)}_2 \xrightarrow{\text{Boil}} 2\text{CaCO}_3 + 2\text{H}_2\text{O} \)
\( \text{Mg(HCO}_3)_2 + \text{Ca(OH)}_2 \xrightarrow{\text{Boil}} \text{MgCO}_3 + \text{CaCO}_3 + 2\text{H}_2\text{O} \)
Lime is first thoroughly mixed with water in a tank and then fed into another tank containing hard water. Revolving paddles thoroughly mix the two solutions. Most of the calcium carbonate settles down. If there is any solid left over, it is removed by a filter. This is known as Clarke’s process.
Washing soda:
When washing soda or soda ash is added to hard water, the corresponding insoluble carbonates settle down and can be removed by filtration.
\( \text{Ca(HCO}_3)_2 + \text{Na}_2\text{CO}_3 \xrightarrow{\text{Boil}} \text{CaCO}_3 + 2\text{NaHCO}_3 \)
\( \text{Mg(HCO}_3)_2 + \text{Na}_2\text{CO}_3 \xrightarrow{\text{Boil}} \text{MgCO}_3 + 2\text{NaHCO}_3 \)
In simple words: Clarke's process uses lime to "kick out" the hardness. Washing soda does a similar job by turning the hardness into a solid "dust" that we can filter out.

📝 Teacher's Note: Clarke's process is a large-scale way to soften temporary hard water. Note that adding *too much* lime will actually make the water hard again, so the amount must be precise.

🎯 Exam Tip: Remember the name "Clarke's Process" for the use of lime in softening water. It's a common exam question.

Question 12. Permutit is an artificial zeolite.
Answer: Chemically, it is hydrated sodium aluminium orthosilicate with the formula \( \text{Na}_2\text{Al}_2\text{Si}_2\text{O}_8 \cdot \text{XH}_2\text{O} \). For the sake of convenience, let us give it the formula \( \text{Na}_2\text{P} \). A tall cylinder is loosely filled with lumps of permutit. When hard water containing calcium and magnesium ions percolates through these lumps, ions exchange. Sodium permutit is slowly changed into calcium and magnesium permutit, and the water becomes soft with the removal of calcium and magnesium ions. When no longer active, permutit is regenerated by running a concentrated solution of brine over it and removing calcium chloride formed by repeated washing.
\( \text{CaP} + 2\text{NaCl} \rightarrow \text{Na}_2\text{P} + \text{CaCl}_2 \)
In simple words: Permutit is like a "chemical sponge" that swaps good sodium for bad calcium. When it's full, you just wash it with salt water (brine) to make it like new again!

📝 Teacher's Note: This is "Ion Exchange." The zeolite trades its \( \text{Na}^+ \) ions for the \( \text{Ca}^{2+} \) and \( \text{Mg}^{2+} \) ions in the water. It's the most modern and efficient way to soften water.

🎯 Exam Tip: Explain the "Regeneration" process—mention that brine (\( \text{NaCl} \)) is used to bring the permutit back to its active sodium state.