Exercise Solutions Available
Step-by-step answers for all textbook questions in this chapter
I. Choose the Best Answer (MCQs)
Q1. Soap is a sodium or potassium salt of
- (a) Sulphonic acid (b) Fatty acid (c) Hydrochloric acid (d) Nitric acid
Q2. Which of the following works well in hard water?
- (a) Soap (b) Detergent (c) Both soap and detergent (d) Neither soap nor detergent
Q3. The process of making soap is called
- (a) Hydrogenation (b) Saponification (c) Fermentation (d) Polymerisation
Q4. Which of the following is a nitrogenous fertilizer?
- (a) Super phosphate (b) Urea (c) Muriate of potash (d) Bone meal
Q5. Glass is mainly composed of
- (a) Calcium silicate (b) Sodium silicate (c) Sodium borate (d) Calcium carbonate
Q6. Cement is made from
- (a) Limestone and clay (b) Sand and clay (c) Limestone and sand (d) Clay and gypsum
Q7. Which fertilizer provides all three major nutrients (N, P, K)?
- (a) Urea (b) NPK (c) Super phosphate (d) Ammonium nitrate
II. Fill in the Blanks
1. The process of making soap from fat and alkali is called saponification. 2. Soaps are sodium or potassium salts of fatty acids. 3. Detergents are sodium salts of sulphonic acids. 4. Soaps do not work well in hard water. 5. Glycerol is obtained as a by-product in soap making. 6. The chemical name of baking soda is sodium bicarbonate (NaHCO₃). 7. The chemical name of washing soda is sodium carbonate (Na₂CO₃). 8. Portland cement is the most commonly used cement. 9. The raw materials for making cement are limestone and clay. 10. Glass is made by heating a mixture of sand, soda ash, and limestone.
III. True or False
IV. Match the Following
| Column I | Column II | Answer |
|---|---|---|
| Soap | Sodium salt of fatty acid | Soap — Sodium salt of fatty acid |
| Detergent | Sodium salt of sulphonic acid | Detergent — Sodium salt of sulphonic acid |
| Baking soda | NaHCO₃ | Baking soda — NaHCO₃ |
| Washing soda | Na₂CO₃ | Washing soda — Na₂CO₃ |
| Urea | Nitrogenous fertilizer | Urea — Nitrogenous fertilizer |
| Super phosphate | Phosphatic fertilizer | Super phosphate — Phosphatic fertilizer |
V. Short Answer Questions
Q1. What is the difference between soap and detergent? Answer:
| Property | Soap | Detergent |
|---|---|---|
| Made from | Sodium/potassium salts of fatty acids | Sodium salts of sulphonic acids |
| Works in hard water | No (forms scum) | Yes (no scum) |
| Biodegradability | Biodegradable | Generally non-biodegradable |
| Cleansing action | Mild | Strong |
| Cost | Cheaper | More expensive |
| Environmental impact | Less polluting | More polluting |
Q2. What is saponification? Write the reaction. Answer: Saponification is the process of making soap by heating a fat or oil with an alkali (sodium hydroxide or potassium hydroxide).
Reaction: Fat + NaOH → Soap + Glycerol
Example: Sodium stearate (soap) is formed when stearin (a fat) reacts with sodium hydroxide. C₃H₅(OOC₁₇H₃₅)₃ + 3NaOH → 3C₁₇H₃₅COONa + C₃H₅(OH)₃
Glycerol (C₃H₅(OH)₃) is a valuable by-product used in cosmetics and medicines.
Q3. Why do soaps not work well in hard water? Answer: Hard water contains dissolved calcium and magnesium salts (like calcium sulphate, magnesium chloride). When soap is added to hard water, the sodium salts of fatty acids react with these calcium and magnesium salts to form insoluble calcium and magnesium salts (scum).
Reaction: Sodium stearate (soap) + Ca²⁺ → Calcium stearate (scum, insoluble)
This scum sticks to clothes and surfaces, reducing the cleaning action of soap and wasting soap.
Q4. What is baking soda? Give two uses. Answer: Baking soda is sodium bicarbonate (NaHCO₃). It is a white crystalline powder.
Uses: 1. Used in baking — it releases CO₂ when heated, making cakes and breads soft and fluffy. 2. Used as an antacid — it neutralises excess stomach acid (HCl) to provide relief from acidity. NaHCO₃ + HCl → NaCl + H₂O + CO₂
Q5. What is washing soda? Give two uses. Answer: Washing soda is sodium carbonate (Na₂CO₃·10H₂O). It is a white crystalline substance.
Uses: 1. Used as a cleaning agent for removing grease and stains from clothes. 2. Used for softening hard water — it reacts with calcium and magnesium salts to form insoluble precipitates, removing hardness.
Q6. What are fertilizers? Name two types. Answer: Fertilizers are chemical substances that provide essential nutrients (nitrogen, phosphorus, potassium) to plants for their growth and increased yield.
Types: 1. Nitrogenous fertilizers: Provide nitrogen. Examples: Urea (NH₂CONH₂), Ammonium sulphate ((NH₄)₂SO₄), Ammonium nitrate (NH₄NO₃). 2. Phosphatic fertilizers: Provide phosphorus. Examples: Super phosphate (Ca(H₂PO₄)₂), Triple super phosphate. 3. Potassic fertilizers: Provide potassium. Examples: Muriate of potash (KCl), Potassium sulphate (K₂SO₄). 4. Mixed/NPK fertilizers: Provide N, P, and K. Examples: NPK mixtures.
Q7. Distinguish between manure and fertilizer. Answer:
| Property | Manure | Fertilizer |
|---|---|---|
| Origin | Organic (animal/plant waste) | Inorganic (chemical) |
| Nutrient content | Low | High |
| Effect on soil | Improves soil texture and water retention | Does not improve soil texture |
| Absorption | Slow release of nutrients | Fast release of nutrients |
| Environmental impact | Eco-friendly | Can cause pollution if overused |
| Examples | Cow dung, compost, vermicompost | Urea, super phosphate, NPK |
VI. Detailed Answer Questions
Q1. Describe the manufacture of cement. Answer: Raw materials: Limestone (CaCO₃) and clay (aluminium silicate).
Process: 1. Mixing: Limestone and clay are mixed in a definite proportion (about 3:1) and ground into a fine powder. 2. Heating: The mixture is heated in a rotary kiln at about 1500°C. This causes chemical reactions and forms clinker (small, hard, grey nodules). 3. Cooling and grinding: The clinker is cooled and then mixed with 2-3% gypsum (CaSO₄·2H₂O). Gypsum is added to slow down the setting of cement. 4. Packaging: The ground mixture is packed in bags as Portland cement.
Chemical composition of cement: Cement contains calcium silicates, calcium aluminates, and calcium ferrites. When mixed with water, it undergoes hydration and sets hard.
Uses of cement:
- Construction of buildings, bridges, dams, roads
- Making concrete (cement + sand + gravel + water)
- Making mortar (cement + sand + water) for plastering
Q2. Describe the manufacture of glass. Answer: Raw materials:
- Sand (SiO₂) — provides silica
- Soda ash (Na₂CO₃) — provides sodium oxide
- Limestone (CaCO₃) — provides calcium oxide (stabiliser)
- Cullet (broken glass) — reduces melting temperature and saves energy
Process: 1. Mixing: The raw materials are mixed in proper proportions and ground into a fine powder. 2. Melting: The mixture is heated in a furnace at about 1500°C until it melts into a viscous liquid. 3. Shaping: The molten glass is shaped by blowing, pressing, or rolling into desired forms (bottles, sheets, etc.). 4. Annealing: The shaped glass is slowly cooled to remove internal stresses and prevent cracking. This process is called annealing.
Chemical reaction: Na₂CO₃ + SiO₂ → Na₂SiO₃ + CO₂ CaCO₃ + SiO₂ → CaSiO₃ + CO₂
Types of glass: 1. Soft glass (soda glass): Made from Na₂CO₃, CaCO₃, and SiO₂. Used for windows, bottles, and jars. 2. Hard glass (potash glass): Made using K₂CO₃ instead of Na₂CO₃. Used for chemical apparatus. 3. Borosilicate glass (Pyrex): Contains boron trioxide (B₂O₃). Heat-resistant, used for laboratory glassware. 4. Lead glass (flint glass): Contains lead oxide. Used for decorative items and optical lenses. 5. Safety glass: Laminated or tempered glass that does not shatter into sharp pieces.
Q3. Explain the cleansing action of soap with a diagram description. Answer: A soap molecule has two ends with different properties:
1. Hydrophilic end (water-loving): The ionic (carboxylate) head of the soap molecule is attracted to water. It is also called the polar end.
2. Hydrophobic end (water-repelling): The long hydrocarbon chain (tail) of the soap molecule is attracted to oil and grease. It is also called the non-polar end.
Cleansing action: 1. When soap is added to water, the hydrophobic tails attach to the grease/dirt on the fabric, while the hydrophilic heads remain in the water. 2. The soap molecules surround the grease droplet, with hydrophobic tails embedded in the grease and hydrophilic heads facing outward toward the water. 3. This structure is called a micelle. Each grease particle is surrounded by many soap molecules forming a micelle. 4. When the fabric is agitated or rubbed, the micelles break away from the fabric and enter the water. 5. The grease is washed away with the dirty water, leaving the fabric clean.
In hard water: Soap reacts with calcium and magnesium ions to form an insoluble scum (calcium/magnesium stearate), which reduces cleaning efficiency and wastes soap.
Q4. Write about the uses of chemistry in everyday life with at least five examples. Answer: Chemistry plays a vital role in everyday life. Here are five key examples:
1. Soaps and detergents: Used for cleaning clothes, utensils, and our bodies. Soaps are made by saponification of fats with alkali. Detergents are synthetic cleaning agents that work well in hard water.
2. Fertilizers and pesticides: Chemical fertilizers (urea, super phosphate, NPK) provide essential nutrients to crops and increase agricultural yield. Pesticides and insecticides protect crops from pests.
3. Cement and construction: Cement (made from limestone and clay) is the foundation of modern construction — buildings, bridges, dams, and roads all use cement and concrete.
4. Glass: Used in windows, bottles, mirrors, laboratory equipment, and optical instruments. Different types of glass serve different purposes (borosilicate for labware, safety glass for vehicles).
5. Medicines and healthcare: Chemical substances like aspirin, antibiotics, antiseptics, and antacids are all products of chemistry. Baking soda neutralises stomach acid, and disinfectants kill germs.
6. Food preservation: Chemical preservatives, refrigeration (using CFCs or alternatives), and food additives are all chemistry applications that keep food fresh and safe.
7. Cosmetics: Toothpaste, shampoo, creams, and lotions are all chemical formulations designed for personal care.
Common Mistakes
- Confusing soap and detergent: Soaps are salts of fatty acids and don't work well in hard water. Detergents are salts of sulphonic acids and work in hard water.
- Forgetting that soaps are biodegradable but detergents are not: This is an important environmental distinction.
- Confusing baking soda and washing soda: Baking soda is NaHCO₃ (used in cooking and as an antacid); washing soda is Na₂CO₃ (used in cleaning and water softening).
- Confusing nitrogenous, phosphatic, and potassic fertilizers: Urea = nitrogenous, super phosphate = phosphatic, muriate of potash = potassic.
- Saying cement only contains limestone: Cement is made from limestone AND clay (plus gypsum added later).
- Forgetting glycerol as a by-product of saponification: Soap-making produces both soap and glycerol.
Exam Checklist
- [ ] Can you distinguish between soap and detergent with examples?
- [ ] Can you explain saponification and write the reaction?
- [ ] Can you explain why soaps don't work well in hard water?
- [ ] Can you describe the cleansing action of soap (micelle formation)?
- [ ] Can you list uses of baking soda and washing soda?
- [ ] Can you describe the manufacture of cement?
- [ ] Can you describe the manufacture of glass and its types?
- [ ] Can you distinguish between manure and fertilizer?
- [ ] Can you name and classify different types of fertilizers (N, P, K)?
- [ ] Can you give at least five examples of chemistry in everyday life?