ICSE – Grade 9 – Chemistry – Ch 03 – QA

1. Explain why water is called a universal solvent.
    Answer:
    Water is called a universal solvent because it can dissolve a large variety of substances, including ionic and many covalent compounds. Its polar nature allows it to break down ionic bonds and surround individual ions, thus dissolving them. This property of water makes it essential for biological, chemical, and industrial processes.

2. What are the anomalous properties of water?
   Answer:
    The anomalous properties of water include its expansion below 4°C, maximum density at 4°C, and high specific heat. Unlike other substances, water expands when cooled below 4°C, which is why ice is less dense than water. These properties help in maintaining aquatic life and regulate Earth’s climate.

3. State the physical properties of water.
   Answer:
    Water is colourless, odourless, and tasteless. It has a boiling point of 100°C, a freezing point of 0°C, and maximum density at 4°C. Its specific heat is 4.2 J/g°C, latent heat of fusion is 336 J/g, and latent heat of vaporization is 2260 J/g. Water expands on freezing, which is its anomalous expansion.

4. Describe the water cycle with all stages.
   Answer:
    The water cycle includes evaporation (water changes to vapour), condensation (vapour cools and forms clouds), precipitation (rain, snow, hail fall to Earth), percolation (water seeps into the ground), and run-off (water collects in water bodies). This continuous cycle maintains Earth’s water balance.

5. What is specific heat capacity? How is it significant for water?
   Answer:
    Specific heat capacity is the amount of heat required to raise the temperature of 1 gram of a substance by 1°C. For water, it is 4.2 J/g°C. Its high value means water heats up and cools down slowly, which helps in regulating body temperature and Earth’s climate.

6. Explain latent heat of fusion and latent heat of vaporization of water.
   Answer:
    Latent heat of fusion is the heat required to change 1 g of ice to water at 0°C without temperature change, and is 336 J/g. Latent heat of vaporization is the heat needed to convert 1 g of water to steam at 100°C without temperature rise, which is 2260 J/g. These help water absorb or release heat during state change.

7. Why does ice float on water?
   Answer:
    Ice floats on water because it is less dense than liquid water. When water freezes, it expands due to hydrogen bonding, making the ice occupy more volume and thus reducing its density. This allows it to float, forming an insulating layer that protects aquatic life during winter.

8. State the importance of dissolved salts in water.
   Answer:
    Dissolved salts in water provide essential ions like calcium, magnesium, sodium, and potassium which are necessary for physiological processes in humans, such as nerve transmission, muscle contraction, and bone development. They also maintain the electrolyte balance in cells.

9. Describe the importance of air dissolved in water.
   Answer:
    Air dissolved in water contains oxygen and carbon dioxide. Oxygen is essential for the respiration of aquatic animals, while carbon dioxide is used by aquatic plants for photosynthesis. Without dissolved air, aquatic ecosystems would not survive.

10. What is a true solution? List its characteristics.
    Answer:
     A true solution is a homogeneous mixture where the solute is completely dissolved in the solvent. Its characteristics include transparency, uniform composition, solute particles not visible, particles do not settle, and cannot be separated by filtration.

11. Define saturated, unsaturated, and supersaturated solutions.
    Answer:
     A saturated solution holds the maximum solute at a given temperature. An unsaturated solution can dissolve more solute. A supersaturated solution contains more solute than normally possible at that temperature and is unstable.

12. Explain the effect of temperature on solubility of solids and gases.
    Answer:
     For solids, solubility generally increases with temperature. For gases, solubility decreases with rising temperature. Hence, heating a solid solution can dissolve more solute, but gases escape more rapidly from heated liquids.

13. What is a solubility curve? What is its significance?
    Answer:
     A solubility curve is a graph showing how solubility changes with temperature. It helps determine how much solute can dissolve at a specific temperature and predict crystallization points. It’s also useful in comparing solubility of different substances.

14. What is crystallization? How is it useful?
    Answer:
     Crystallization is the process of forming crystals from a solution by cooling or evaporation. It is used to purify substances and obtain solid crystals from a saturated solution, as impurities remain in the solvent.

15. What are hydrated and anhydrous substances? Give examples.
    Answer:
     Hydrated substances contain water of crystallization, e.g., CuSO₄·5H₂O. Anhydrous substances do not contain water, e.g., NaCl. Heating a hydrated salt drives off water, converting it to its anhydrous form.

16. What is water of crystallization? Give an example.
    Answer:
     Water of crystallization is chemically combined water in a salt’s crystal structure. For example, CuSO₄·5H₂O has five molecules of water per formula unit. It gives the compound its shape and colour.

17. What is efflorescence? Name two efflorescent substances.
    Answer:
     Efflorescence is the loss of water of crystallization from a substance on exposure to air. Examples include washing soda (Na₂CO₃·10H₂O) and Glauber’s salt (Na₂SO₄·10H₂O).

18. What are deliquescent substances? Give two examples.
    Answer:
     Deliquescent substances absorb moisture from the air and dissolve in it to form a solution. Examples include calcium chloride (CaCl₂) and sodium hydroxide (NaOH).

19. What are hygroscopic substances? How are they different from deliquescent substances?
    Answer:
     Hygroscopic substances absorb moisture but do not dissolve in it, e.g., concentrated H₂SO₄. Deliquescent substances absorb so much moisture that they dissolve in it. Hygroscopic substances remain solid.

20. Differentiate between drying and dehydrating agents.
    Answer:
     Drying agents remove moisture physically from surfaces (e.g., CaCl₂), while dehydrating agents remove chemically combined water from compounds (e.g., conc. H₂SO₄). Dehydration involves a chemical reaction.

21. What is a desiccator? What is its use?
    Answer:
     A desiccator is an airtight container with a drying agent (like silica gel) used to keep hygroscopic substances dry and free from moisture.

22. Define hard water. What causes hardness?
    Answer:
     Hard water does not lather easily with soap. It contains dissolved calcium and magnesium salts in the form of bicarbonates, chlorides, and sulphates.

23. What are the disadvantages of hard water?
    Answer:
     Hard water wastes soap, forms scum, causes scale formation in boilers, reduces efficiency of heating appliances, and damages plumbing systems.

24. What are the advantages of hard water?
    Answer:
     Hard water provides calcium and magnesium which are good for bones and teeth. It also contributes to the taste and mineral content of drinking water.

25. Differentiate between temporary and permanent hardness.
    Answer:
     Temporary hardness is caused by bicarbonates of calcium and magnesium and can be removed by boiling. Permanent hardness is caused by chlorides and sulphates and requires chemical treatment.

26. Explain Clark’s process for softening water.
    Answer:
     Clark’s process removes temporary hardness by adding slaked lime (Ca(OH)₂), which reacts with bicarbonates to form insoluble carbonates that can be filtered out.

27. How does washing soda soften hard water?
    Answer:
     Washing soda (Na₂CO₃) reacts with Ca²⁺ and Mg²⁺ to form insoluble carbonates, thus removing both temporary and permanent hardness.

28. What is ion exchange process?
    Answer:
     In the ion exchange process, hard water passes through a resin that exchanges calcium and magnesium ions with sodium ions, thus softening the water.

29. Explain the cleansing action of soap in soft and hard water.
    Answer:
     In soft water, soap forms lather easily. In hard water, soap reacts with Ca²⁺/Mg²⁺ to form insoluble scum, reducing its cleansing efficiency.

30. Why are detergents better than soaps in hard water?
    Answer:
     Detergents do not react with Ca²⁺ or Mg²⁺ ions and therefore do not form scum. They remain effective cleansers in both hard and soft water.

31. How are stalactites and stalagmites formed?
    Answer:
     In limestone caves, hard water drips and deposits calcium carbonate. Stalactites hang from the ceiling and stalagmites rise from the floor due to this deposition.

32. What is meant by scum? How is it formed?
    Answer:
     Scum is an insoluble precipitate formed when soap reacts with calcium or magnesium ions in hard water, reducing soap’s cleansing power.

33. What is the importance of water in daily life?
    Answer:
     Water is essential for drinking, cooking, cleaning, agriculture, industry, biological processes, and environmental balance. It supports all life forms.

34. Explain why water is used as a coolant.
    Answer:
     Water absorbs large amounts of heat without a significant rise in temperature due to its high specific heat capacity, making it ideal for cooling engines and reactors.

35. Why is solubility temperature-dependent?
    Answer:
     Solubility depends on kinetic energy. Higher temperature increases molecular motion, allowing more solute particles to interact and dissolve in the solvent.

36. What is meant by water of crystallization? How can it be removed?
    Answer:
     Water of crystallization refers to the fixed number of water molecules chemically bonded within a crystal structure of a salt. It can be removed by heating the hydrated salt, which drives off the water as vapour, leaving behind the anhydrous form of the salt. For example, heating CuSO₄·5H₂O removes its water of crystallization and turns it into white anhydrous copper sulphate.

37. Why is water essential for plants and animals?
    Answer:
     Water is crucial for plants as it is involved in photosynthesis, transport of nutrients, and maintaining cell turgidity. For animals, water acts as a medium for chemical reactions, regulates body temperature, transports nutrients and waste, and aids digestion. Without water, vital physiological processes would cease.

38. Describe how you would prepare a saturated solution in the lab.
    Answer:
     To prepare a saturated solution, take a fixed quantity of solvent (usually water) in a beaker and gradually add solute (e.g., salt) while stirring and gently heating. Continue until no more solute dissolves and undissolved particles settle at the bottom. Filter the solution at that temperature to obtain a saturated solution.

39. How is a supersaturated solution prepared? Explain with an example.
    Answer:
     A supersaturated solution is prepared by heating a saturated solution to a higher temperature, adding more solute until it dissolves completely, and then cooling the solution slowly without disturbing it. An example is a hot saturated solution of sodium acetate, which holds more solute than usual upon cooling.

40. What is the significance of solubility curves in chemical studies?
    Answer:
     Solubility curves help in understanding how much solute dissolves in a solvent at various temperatures. They are used to identify unknown substances, plan crystallization processes, determine the best conditions for reactions, and calculate the amount of solute needed at a specific temperature for industrial and lab applications.

41. Explain how temporary hardness is removed by boiling.
    Answer:
     Boiling water decomposes the bicarbonates of calcium and magnesium (which cause temporary hardness) into insoluble carbonates and carbon dioxide. The carbonates precipitate out and can be filtered, thus removing the hardness.
    
    
    
42. Write a short note on the use of water in industrial processes.
    Answer:
     In industries, water is used as a coolant due to its high specific heat, as a solvent in chemical production, in hydroelectric power generation, for cleaning and processing raw materials, and in boilers. It is also used in the production of paper, textiles, beverages, and pharmaceuticals.

43. What are the health concerns related to hard water consumption?
    Answer:
     Although not harmful in moderate amounts, prolonged consumption of very hard water may lead to digestive discomfort, urinary stones (due to excess calcium/magnesium), and scaling of pipelines that can harbor bacteria. However, it is generally safe and may provide essential minerals.

44. What changes occur when hydrated copper sulphate is heated?
    Answer:
     When CuSO₄·5H₂O is heated, it loses its water of crystallization and changes from blue to white anhydrous copper sulphate.
    
     The reaction is reversible; adding water restores the blue colour.

45. What is scum and how does it affect soap’s efficiency?
    Answer:
     Scum is a white, insoluble substance formed when soap reacts with calcium and magnesium ions in hard water.
    
     Scum reduces soap’s cleansing ability by using up soap molecules that could otherwise remove dirt.

46. Describe any two methods of removing permanent hardness.
    Answer:

Washing Soda: Na₂CO₃ reacts with Ca²⁺ and Mg²⁺ to form insoluble carbonates.

Ion Exchange Resin: Calcium and magnesium ions are replaced with sodium ions in an exchange column, softening the water.

47. Why does boiling not remove permanent hardness?
    Answer:
     Permanent hardness is caused by chlorides and sulphates of calcium and magnesium, which are heat-stable and do not decompose on boiling. Therefore, chemical methods like ion exchange or adding washing soda are required to remove it.

48. Why are stalactites and stalagmites found in limestone caves?
    Answer:
     Limestone caves contain calcium carbonate. As hard water drips through these caves, calcium bicarbonate decomposes into calcium carbonate which gets deposited. Over time, stalactites form from the ceiling and stalagmites from the floor due to continuous deposition.

49. How does water help regulate Earth’s temperature?
    Answer:
     Water’s high specific heat capacity enables it to absorb and store large amounts of heat during the day and release it slowly at night. This moderates temperature variations, stabilizing climate and weather patterns globally.

50. How does the polarity of water contribute to its solvent properties?
    Answer:
     Water’s polarity arises from its bent structure and unequal sharing of electrons. The slightly positive hydrogen atoms and slightly negative oxygen atom attract opposite charges of solutes, allowing water to break down ionic and polar compounds efficiently.