CBSE Class 10 Science Acids bases and salts Sure Shot Questions Set 06

These 14 sure shot questions for Class 10 Science Chapter 2, Acids, Bases and Salts, come with step-by-step answers and teacher's notes. They cover the board-style questions on bleaching powder, plaster of Paris, baking powder and baking soda, washing soda, the chlor-alkali process, hydrated salts and the pH scale.

Short Answer Questions

Question 1. What is bleaching powder? How is it prepared? List two uses of bleaching powder.
Answer:
Bleaching powder is calcium oxychloride, CaOCl₂, a pale yellow powder with the smell of chlorine.
Preparation: chlorine gas is passed over dry slaked lime: Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O.
Uses: bleaching cotton and linen in the textile industry and wood pulp in paper factories; disinfecting drinking water.
In simple words: Bleaching powder is made from chlorine and slaked lime and is used for bleaching and disinfecting.

Teacher's Note:
1. Use dry slaked lime.
2. Name two uses.

Question 2. A white powder is an active ingredient of antacids and is used in preparation of baking powder. Name the compound and explain how it is manufactured. Give chemical equation. Write a chemical equation to show the effect of heat on this compound.
Answer:
The compound is sodium hydrogencarbonate (baking soda), NaHCO₃.
It is manufactured by passing carbon dioxide and ammonia through a saturated solution of sodium chloride (brine): NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃.
Effect of heat: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂↑.
In simple words: Baking soda is made from brine, ammonia and carbon dioxide and breaks down on heating.

Teacher's Note:
1. Antacid action: it neutralises excess stomach acid.
2. Baking soda is NaHCO₃ and washing soda is Na₂CO₃·10H₂O.

Question 3. How is washing soda prepared from sodium carbonate? Give its chemical equation. State the type of this salt. Name the type of hardness of water which can be removed by it?
Answer:
Sodium carbonate is recrystallised from water to give washing soda (sodium carbonate decahydrate): Na₂CO₃ + 10H₂O → Na₂CO₃·10H₂O.
It is a basic salt (salt of a strong base and a weak acid).
Washing soda removes the permanent hardness of water.
In simple words: Recrystallisation of sodium carbonate gives washing soda.

Teacher's Note:
1. Sodium carbonate itself is made by heating baking soda.
2. Washing soda is a basic salt.

Question 4. List the important products of the Chlor-alkali process. Write one important use of each.
Answer:
When electricity is passed through brine: 2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g). It is called chlor-alkali because of the products chlor (chlorine) and alkali (sodium hydroxide).
Sodium hydroxide: making soap, paper and artificial fibres (degreasing metals).
Chlorine: water treatment (swimming pools), making bleaching powder, PVC and disinfectants.
Hydrogen: making ammonia for fertilisers, hydrogenation of oils, and as fuel.
In simple words: Chlor-alkali gives NaOH, Cl₂ and H₂, each with its own uses.

Teacher's Note:
1. Write one use for each product.
2. Chlorine is formed at the anode and hydrogen at the cathode.

Question 5. During electrolysis of brine, a gas G is liberated at anode. When this gas G is passed through slaked lime, a compound C is formed, which is used for disinfecting drinking water.
(i) Write formula of G and C.
(ii) State the chemical equation involved.
(iii) What is common name of compound C? Give its chemical name.
Answer:
(i) G is chlorine, Cl₂. C is CaOCl₂.
(ii) Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O
(iii) Common name: bleaching powder. Chemical name: calcium oxychloride.
In simple words: Chlorine plus slaked lime gives bleaching powder.

Teacher's Note:
1. Chlorine is formed at the anode.
2. Pass chlorine over dry slaked lime.

Long Answer Questions

Question 6. Identify the compound of calcium which is yellowish white powder and is used for disinfecting drinking water. How is it manufactured? Write the chemical equation for the reaction involved. What happens when it is left exposed to air? Mention its four important uses also.
Answer:
The compound is bleaching powder (calcium oxychloride), CaOCl₂.
It is manufactured by passing chlorine gas over dry slaked lime: Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O.
On exposure to air it reacts with carbon dioxide and slowly loses chlorine: CaOCl₂ + CO₂ → CaCO₃ + Cl₂↑.
Uses: (1) bleaching cotton and linen in the textile industry, (2) bleaching wood pulp in paper factories, (3) oxidising agent in chemical industries, (4) disinfecting drinking water.
In simple words: Bleaching powder is made from chlorine and dry slaked lime and loses chlorine in air.

Teacher's Note:
1. Keep it in an airtight container.
2. Bleaching powder is CaOCl₂, made from chlorine and dry slaked lime.

Question 7. The pH of a salt used to make tasty and crispy pakoras is 9. Identify the salt and write a chemical equation for its formation. List its two uses.
Answer:
The salt is baking soda, sodium hydrogencarbonate, NaHCO₃; its solution is mildly basic (pH about 9).
Formation: NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃.
Uses: (a) as an ingredient of antacids, (b) in soda-acid fire extinguishers (also in baking powder).
In simple words: Baking soda is a mildly basic salt used in cooking, antacids and fire extinguishers.

Teacher's Note:
1. Baking soda solution has pH about 8 to 9, so the pH value 9 fits.
2. Give two different uses.

Question 8. A compound X of sodium forms a white powder. It is a constituent of baking powder and is used in some antacid prescriptions. When heated, X gives out a gas and steam. The gas forms a white precipitate with limewater. Write the chemical formula and name of X and the chemical equation for its decomposition on heating. What is its role as baking powder and as an antacid?
Answer:
X is sodium hydrogencarbonate (sodium bicarbonate), NaHCO₃.
On heating: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂↑. The gas is CO₂; it gives a white precipitate with lime water: Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O.
As baking powder it releases CO₂, which makes cakes and bread soft and fluffy. As an antacid it is mildly basic and neutralises excess acid in the stomach.
In simple words: Baking soda gives CO₂ on heating and neutralises acids.

Teacher's Note:
1. Link the observations to the compound.
2. Mention both roles.

Question 9. A white powder is added while baking breads and cakes to make them soft and fluffy. Write the name of the powder. Name its main ingredients. Explain the function of each ingredient. Write the chemical reaction taking place when the powder is heated during baking.
Answer:
The powder is baking powder.
Ingredients: sodium hydrogencarbonate (baking soda) and a mild edible acid such as tartaric acid.
Functions: baking soda releases carbon dioxide on heating or with the acid, which makes the cake rise and become fluffy; tartaric acid neutralises the bitter sodium carbonate formed.
Reaction on heating: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂↑
In simple words: Baking powder = baking soda + tartaric acid; CO₂ makes the cake rise.

Teacher's Note:
1. State the function of each ingredient separately.
2. Write the equation with balancing.

Question 10. Answer the following:
(a) Why is plaster of paris written as CaSO₄·½H₂O? How is it possible to have half a water molecule attached to CaSO₄?
(b) Why is sodium hydrogen carbonate an essential ingredient in antacids?
(c) When electricity is passed through aqueous solution of sodium chloride, three products are obtained. Why is the process called chlor-alkali?
Answer:
(a) The actual formula is 2CaSO₄·H₂O: one molecule of water is shared between two formula units of CaSO₄. It is simplified and written as CaSO₄·½H₂O.
(b) It neutralises the hydrochloric acid in the stomach: NaHCO₃ + HCl → NaCl + H₂O + CO₂.
(c) The three products are chlorine, hydrogen and sodium hydroxide. The process is called chlor-alkali because chlorine (chlor) and an alkali (sodium hydroxide) are formed: chlorine at the anode, hydrogen at the cathode and sodium hydroxide near the cathode.
In simple words: POP has one water per two CaSO₄; NaHCO₃ neutralises acid; chlor-alkali is from chlorine and alkali.

Teacher's Note:
1. Hemi means half.
2. Name the product at each electrode.

Question 11. (a) What are anhydrous and hydrated salts? Explain with a suitable example of each.
(b) How is plaster of Paris prepared? What reaction takes place when it sets to a hard mass?
Answer:
(a) Hydrated salts contain a fixed number of water molecules (water of crystallisation) in their crystals, for example CuSO₄·5H₂O (blue). Anhydrous salts do not contain water of crystallisation, for example CuSO₄ (white).
(b) Plaster of Paris is made by heating gypsum at 373 K: CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O. On mixing with water it sets into a hard mass of gypsum: CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O.
In simple words: Hydrated salts hold water in their crystals; POP sets back to gypsum.

Teacher's Note:
1. Heat gypsum only up to 373 K.
2. Give the colour of copper sulphate in each form.

Question 12. (a) You have three solutions A, B and C having a pH of 6, 2 and 9 respectively. Arrange these solutions in increasing order of hydrogen ion concentration. Which of the three is most acidic? What happens to the hydrogen ion concentration in A as it is diluted?
(b) If someone is suffering from a stomach problem called acidity, why is a solution of baking soda offered as a remedy?
(c) Write chemical name and formula of baking soda.
Answer:
(a) A lower pH means a higher H⁺ concentration. Increasing order of H⁺ concentration: C (pH 9) < A (pH 6) < B (pH 2). B is the most acidic. On dilution, the H⁺ concentration in A decreases (its pH rises towards 7).
(b) Baking soda is mildly basic, so it neutralises the excess hydrochloric acid in the stomach and gives relief.
(c) Sodium hydrogencarbonate, NaHCO₃.
In simple words: Lower pH means more H⁺; baking soda is a mild base.

Teacher's Note:
1. Arrange by pH from high to low.
2. Dilution reduces H⁺ concentration.

Question 13. (a) Define indicator. Name two indicators obtained from plants.
(b) Write a balanced chemical equation for the reaction taking place when sodium oxide reacts with water. How will this solution behave towards phenolphthalein and red litmus paper?
(c) State what happens when sodium hydroxide solution reacts with hydrochloric acid.
Answer:
(a) An indicator is a substance that shows different colours (or smell) in acidic and basic media. Two plant indicators: litmus (from lichen) and turmeric (also red cabbage).
(b) Na₂O + H₂O → 2NaOH. The solution is basic: it turns phenolphthalein pink and red litmus blue.
(c) Neutralisation takes place and sodium chloride and water are formed, with the evolution of heat: NaOH + HCl → NaCl + H₂O.
In simple words: Metal oxides give bases with water; bases give pink with phenolphthalein.

Teacher's Note:
1. Metal oxides are basic.
2. Learn the colour changes.

Question 14. (a) Given below are the pH values of four different liquids: 7.0, 14.0, 4.0, 2.0. Which of these could be that of (i) lemon juice, (ii) distilled water, (iii) sodium hydroxide solution and tomato juice.
(b) When blue litmus solution is added to soda water, what change will be observed and why?
Answer:
(a) (i) Lemon juice: 2.0. (ii) Distilled water: 7.0. (iii) Sodium hydroxide solution: 14.0, and tomato juice: 4.0.
(b) Soda water contains dissolved carbon dioxide, which forms carbonic acid, so the blue litmus turns red.
In simple words: More acidic liquids have lower pH.

Teacher's Note:
1. Match pH with the nature of each liquid.
2. Soda water is mildly acidic.