CBSE Class 10 Chemistry: More About Salts – Long Answer Questions
Medium Level (Application & Explanation)
Q1. Explain with examples how salts are classified into families. Why is this classification useful?
Answer:
Salts are classified into families based on their common positive (cation) or negative (anion) radicals.
For instance, sodium chloride (NaCl) and sodium sulphate (Na₂SO₄) have the same sodium ion (Na⁺) and thus, belong to the sodium salt family.
Similarly, sodium chloride (NaCl) and potassium chloride (KCl) contain the chloride ion (Cl⁻) and are part of the chloride salt family.
Benefits of this classification:
It helps to predict properties and reactions of salts inside a family.
Students and chemists can identify the result of replacing either cation or anion in different chemical reactions.
Makes it easier to understand and study the periodic patterns in compounds’ behavior.
Example: Sodium carbonate (Na₂CO₃) and sodium sulphate (Na₂SO₄) both form sodium salts but differ based on their anion, influencing their chemical uses.
Q2. Describe how the pH nature (acidic, basic, or neutral) of a salt is determined by the strength of its parent acid and base. Give suitable examples.
Answer:
The pH of a salt solution is determined by the relative strength of the acid and base from which the salt is formed:
If both the acid and base are strong, the salt is neutral (pH about 7), e.g., NaCl from HCl (strong acid) + NaOH (strong base).
If the acid is strong and the base is weak, the salt is acidic (pH < 7), e.g., NH₄Cl from HCl (strong) + NH₄OH (weak).
If the acid is weak and the base is strong, the salt is basic (pH > 7), e.g., Na₂CO₃ from H₂CO₃ (weak) + NaOH (strong).
This knowledge helps in:
Predicting the behavior of salts in various environments.
Understanding their application (like using basic salts in antacids).
Testing can be done by dissolving the salt in water and using litmus or pH paper for observation.
Q3. Explain the process of manufacturing sodium hydroxide (NaOH) from common salt. List at least three uses of this compound in daily life.
Answer:
Sodium hydroxide (NaOH) is produced using the chlor-alkali process:
Electrolysis of brine (aqueous NaCl) is carried out.
At cathode: Water is reduced to form hydrogen gas (H₂) and OH⁻ ions.
At anode: Chloride ions (Cl⁻) are oxidized to chlorine gas (Cl₂).
Net reaction: 2NaCl+2H2O→2NaOH+Cl2+H2
NaOH is collected from the solution.
Uses:
Soap and detergent production: A key ingredient in saponification.
Paper manufacturing: Helps to break down wood pulp.
Cleaning agents: Used as a drain cleaner and in laundries.
Q4. What is water of crystallization? Using copper sulphate as an example, explain how heating affects such salts and how the process is reversible.
Answer:
Some salts, like copper sulphate (CuSO₄·5H₂O), have fixed numbers of water molecules attached within their crystal structure. This is known as water of crystallization.
These water molecules:
Give the salt distinct color and shape.
Are not loosely held – they are part of the crystal lattice.
When heated:
CuSO₄·5H₂O loses water and turns into white anhydrous CuSO₄.
Water vapor is released; water droplets (condensation) may be seen in the test tube.
Reversibility:
If water is added back to the white powder, it regains its blue color, showing the process is reversible.
This property is important for testing the presence of water and identifying salts.
Q5. Describe the steps involved in testing the pH of an unknown salt. How can this information be used to infer the type of salts present?
Answer:
To test the pH of an unknown salt:
Dissolve a small amount of the salt in distilled water to make a solution.
Dip pH paper or use universal indicator in the solution.
Observe and record the resulting color and match it to the pH scale.
Based on pH:
pH ~7 means a neutral salt (from strong acid + strong base).
pH < 7 means an acidic salt (from strong acid + weak base).
pH > 7 means a basic salt (from weak acid + strong base).
This test helps to:
Identify unknown salts in laboratory or industry.
Determine proper handling and usage (some acidic or basic salts may need safety precautions).
High Complexity (Analytical & Scenario-Based)
Q6. Imagine you must design a water purification system for a village without access to modern filters. Which salt from common salt chemistry would you choose and how would you implement its use? Discuss the benefits and cautionary aspects.
Answer:
The best choice is bleaching powder (CaOCl₂) because:
It releases chlorine, which kills bacteria and pathogens in water.
Easily available and simple to use.
Implementation:
Dissolve a controlled amount of bleaching powder in water.
Allow it to mix thoroughly and act for about 30 minutes to kill germs.
Benefits:
Provides safe drinking water by disinfecting it.
Low cost and does not require electricity.
Caution:
Excessive use can leave an unpleasant taste or form harmful by-products.
Correct dosage and thorough mixing are needed to ensure safety.
Users should be trained to avoid inhalation and skin contact.
Overall, bleaching powder is practical, but education and care are crucial for efficacy and safety.
Q7. Analyze why baking soda is used both in cooking and as an antacid. Discuss the chemical reactions involved in both applications.
Answer:
Baking soda (NaHCO₃) serves two key roles:
In cooking (baking):
On heating, NaHCO₃ decomposes to produce CO₂ gas:
2NaHCO3→Na2CO3+H2O+CO2
The release of CO₂ makes cakes and breads light and fluffy by creating bubbles in the dough.
As an antacid:
It neutralizes excess stomach acid (HCl):
NaHCO3+HCl→NaCl+H2O+CO2
Relieves indigestion and heartburn by reducing acidity.
Summary:
Releases CO₂ in both scenarios, but:
In food, it is for texture and taste.
In medicine, it is for neutralizing acid and providing comfort.
Thus, the versatility of baking soda comes from its mild basic property and ability to release CO₂ upon reaction.
Q8. A chemical factory produces large amounts of sodium chloride as a byproduct. Suggest three useful compounds that can be made from it, explain the processes involved, and mention at least one use of each product.
Answer:
From sodium chloride (NaCl), several valuable chemicals can be manufactured:
Sodium Hydroxide (NaOH) via Chlor-alkali process:
Electrolysis of brine gives NaOH (used in soap production, paper industry).
Bleaching Powder (CaOCl₂):
React NaCl with Ca(OH)₂ and Cl₂ gas: Ca(OH)2+Cl2→CaOCl2+H2O
Used in disinfecting water and bleaching fabrics.
Baking Soda (NaHCO₃):
React NaCl with ammonia, CO₂, and water (Solvay process):
Produces NaHCO₃ for baking, cleaning, and as an antacid.
Summary: Sodium chloride is not just a seasoning but the starting point for industries making soaps, disinfectants, and medicines.
Q9. How does the property of water of crystallization help in identifying whether a sample of copper sulphate is pure or not? Propose an experiment and interpret possible results.
Answer:
Pure copper sulphate is blue because it has water of crystallization (CuSO₄·5H₂O).
Experiment:
Take a few crystals of sample in a test tube.
Heat gently.
Observe the color change and look for water droplets.
Interpretation:
If the sample is blue and produces water, turning white:
It is pure, containing water of crystallization.
If the sample is already white and doesn't give water:
It’s anhydrous or impure (possibly used or exposed to air).
If the blue color returns after adding water to the white powder, it confirms the property.
This test helps in quality control in laboratories and industry.
Q10. A farmer is struggling with hard water in the irrigation system, leading to deposition in pipes and poor plant growth. Explain how washing soda (Na₂CO₃·10H₂O) can help solve this problem. Discuss the chemical principle behind its action.
Answer:
Hard water contains calcium (Ca²⁺) or magnesium (Mg²⁺) ions that form deposits (“scale”) and reduce water’s effectiveness for irrigation.
Washing soda (Na₂CO₃·10H₂O):
When added, reacts with Ca²⁺ or Mg²⁺ ions to form insoluble carbonates.
E.g., Ca2++Na2CO3→CaCO3↓+2Na+
Removes hardness by precipitating these ions as solid carbonates, making the water “soft.”
Benefits:
Prevents pipe clogging and improves water flow.
Enhances soil permeability and plant root health.
Thus, washing soda chemically treats the hard water, protecting both farm equipment and crops, demonstrating its importance in agriculture.
