CBSE Class 10 Science HOTs Acids Bases And Salts Set 07

This Class 10 Science page gives 18 more higher order thinking questions from Chapter 2, Acids, Bases and Salts, each with a step-by-step answer, a simple-words summary and a teacher's note. The questions ask students to reason about sting treatment, dehydration of copper sulphate, milk turning to curd, activities with glucose and alcohol, and the products of salts on heating.

Reasoning Based HOTS Questions

Question 1. Account for the following:
(i) Rain water conducts electricity but distilled water does not.
(ii) We feel burning sensation in the stomach when we over eat.
(iii) A tarnished copper vessel regains its shine when rubbed with lemon.
(iv) The crystals of washing soda change to white powder on exposure to air.
(v) An aqueous solution of sodium chloride is neutral but an aqueous solution of sodium carbonate is basic.
(vi) State the relation between hydrogen ion concentration of an aqueous solution and its pH.
(vii) An aqueous solution has a pH value of 7.0. Is this solution acidic, basic or neutral?
(viii) Which has a higher pH, 1 M HCl or 1 M NaOH solution?
(ix) Tooth enamel is one of the hardest substance in our body. How does it undergo damage due to eating chocolates and sweets? What should we do to prevent it?
(x) How do H⁺ ions exist in water?
Answer:
(i) Rain water contains dissolved ionic substances (for example dissolved gases and dust), whose ions carry current. Distilled water has no ions.
(ii) On overeating, the stomach produces excess hydrochloric acid, which causes the burning sensation (acidity).
(iii) The dull coating on copper is basic copper carbonate. Citric acid in lemon neutralises and dissolves it, so the shine returns.
(iv) On exposure to air, washing soda crystals lose water of crystallisation (efflorescence) and change to white powder.
(v) Sodium chloride is a salt of a strong acid (HCl) and a strong base (NaOH), so it is neutral. Sodium carbonate is a salt of a weak acid (carbonic acid) and a strong base, so it is basic.
(vi) pH = −log[H⁺]. The higher the hydrogen ion concentration, the lower the pH.
(vii) Neutral.
(viii) 1 M NaOH has the higher pH (about 14); 1 M HCl has pH about 0.
(ix) Bacteria in the mouth change leftover sugar into acids. When the pH of the mouth falls below 5.5, tooth enamel (calcium phosphate) is corroded and decay starts. We should clean the mouth after eating and brush with a basic toothpaste to neutralise the acid.
(x) H⁺ ions cannot exist alone in water. They combine with water molecules to form hydronium ions: H⁺ + H₂O → H₃O⁺.
In simple words: Ions, acid-base neutralisation and water of crystallisation explain these everyday facts.

Teacher's Note:
1. Write a one or two line reason for each part.
2. Write the answer in short, clear steps.

Question 2. Bee sting should be treated with baking soda while wasp sting should be treated with vinegar, why?
Answer:
A bee sting injects an acid (formic acid), so a mild base such as baking soda is used to neutralise it.
A wasp sting is alkaline, so a mild acid such as vinegar is used to neutralise it.
In simple words: Neutralise an acid sting with a base and a basic sting with an acid.

Teacher's Note:
1. Acid and base cancel each other's effect.
2. Use the word neutralise.

Question 3. Explain why the pH in a person's mouth becomes lower after each meal.
Answer:
After a meal, bacteria in the mouth break down the sugar and food particles left on the teeth and form acids. These acids make the mouth more acidic, so the pH falls below 7 (and below 5.5 can damage enamel).
In simple words: Bacteria make acid from leftover food.

Teacher's Note:
1. Rinse the mouth after eating.
2. Toothpaste is basic and neutralises the acid.

Question 4. When CuSO₄·5H₂O is dehydrated, its characteristic blue colour disappeared. Why?
Answer:
The blue colour of copper sulphate is due to its water of crystallisation. On dehydration, the five water molecules are lost and anhydrous CuSO₄ is formed, which is white: CuSO₄·5H₂O → CuSO₄ + 5H₂O.
In simple words: The blue colour belongs to the hydrated form.

Teacher's Note:
1. Adding water brings the blue colour back.
2. Anhydrous copper sulphate is white.

Question 5. Name the acid present in ant sting and give its chemical formula. Also give the common method to get relief from the discomfort caused by the ant sting.
Answer:
The acid in an ant sting is formic acid (methanoic acid), HCOOH.
Relief is obtained by applying a mild base such as baking soda (sodium hydrogencarbonate) paste on the stung area, which neutralises the acid.
In simple words: Rub a mild base on an ant sting.

Teacher's Note:
1. Nettle stings also contain methanoic acid.
2. Baking soda or calamine can be used.

Question 6. Fresh milk has pH of 6. How do you think pH will change as it turns into curd? Explain your answer.
Answer:
The pH will decrease (become lower than 6).
When milk turns into curd, bacteria change lactose into lactic acid. More acid is formed, so the H⁺ concentration rises and the pH falls.
In simple words: Curd is more acidic than milk, so it has a lower pH.

Teacher's Note:
1. Lactic acid is formed during curdling.
2. Lower pH means more acidic.

Question 7. Sodium hydrogen carbonate is a basic salt. Justify the statement. How is it converted into washing soda? Explain.
Answer:
Sodium hydrogencarbonate is the salt of a strong base (NaOH) and a weak acid (carbonic acid), so its aqueous solution is mildly basic (pH about 8) and it turns red litmus blue.
On heating it forms sodium carbonate: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂.
Recrystallisation of sodium carbonate from water gives washing soda: Na₂CO₃ + 10H₂O → Na₂CO₃·10H₂O.
In simple words: Baking soda is mildly basic and gives washing soda on heating and recrystallisation.

Teacher's Note:
1. Think of the acid and base that make the salt.
2. Two steps: heat and recrystallise.

Question 8. A green coloured hydrated metallic salt on heating loses its water of crystallisation and gives the smell of burning sulphur. Identify the salt and write down the reaction involved.
Answer:
The salt is hydrated ferrous sulphate (green vitriol), FeSO₄·7H₂O.
On heating, it first loses its water of crystallisation: FeSO₄·7H₂O → FeSO₄ + 7H₂O.
On further heating, it decomposes to give ferric oxide, and sulphur dioxide and sulphur trioxide gases (smell of burning sulphur): 2FeSO₄ → Fe₂O₃ + SO₂ + SO₃.
In simple words: Green ferrous sulphate crystals lose water and then give SO₂.

Teacher's Note:
1. SO₂ has the smell of burning sulphur.
2. The final solid is brown iron(III) oxide.

Question 9. State reasons for the following:
(a) A tarnished copper vessel begins to shine again when rubbed with lemon.
(b) All alkalis are bases but all bases are not alkalis.
(c) Use of a mild base like baking soda on the honey bee stung areas gives relief.
Answer:
(a) Copper vessels get a coating of basic copper carbonate (or oxide) in moist air. Lemon contains citric acid, which neutralises this coating, so the vessel shines again.
(b) Bases that dissolve in water are called alkalis. Many bases, such as Cu(OH)₂ and Fe(OH)₃, do not dissolve in water, so they are not alkalis.
(c) A honey bee sting contains methanoic (formic) acid. A mild base like baking soda neutralises it and gives relief.
In simple words: Acids and bases cancel each other; alkalis are soluble bases.

Teacher's Note:
1. Give one example of a base which is not an alkali.
2. Name the acid in a bee sting.

Question 10. Give reasons for the following:
(a) Only one half of water molecule is shown in the formula of Plaster of Paris.
(b) Sodium hydrogen carbonate is used as an antacid.
(c) On strong heating, blue coloured copper sulphate crystals turn white.
Answer:
(a) Two formula units of CaSO₄ share one molecule of water, so the formula is written with half a molecule: (CaSO₄)2·H₂O = CaSO₄·½H₂O.
(b) It is mildly basic (alkaline), so it neutralises the excess acid in the stomach and gives relief.
(c) On strong heating the crystals lose their water of crystallisation and become anhydrous copper sulphate, which is white: CuSO₄·5H₂O → CuSO₄ + 5H₂O.
In simple words: These facts follow from water of crystallisation and the basic nature of NaHCO₃.

Teacher's Note:
1. Hemi means half.
2. Give the equation for part (c).

Application and Analysis Questions

Question 11. Compounds such as alcohol and glucose also contain hydrogen but are not categorised as acids. Describe an activity to prove it.
Answer:
Materials: a beaker, a cork with two iron nails fixed in it, a 6 V battery, a bulb, a switch, connecting wires, and glucose or alcohol solution.
Procedure: 1. Connect the nails to the battery through the bulb and switch. 2. Pour glucose solution into the beaker so that the nail tips dip in the liquid. 3. Switch on and observe the bulb. 4. Repeat with alcohol solution (and then with dilute hydrochloric acid for comparison).
Observation: the bulb does not glow with glucose or alcohol, but it glows with dilute HCl.
Conclusion: glucose and alcohol do not give H⁺ ions in solution, so they are not acids.
In simple words: Only solutions that give H⁺ ions conduct and glow the bulb.

Teacher's Note:
1. Draw the circuit neatly in the board exam.
2. Acids give ions in water.

Question 12. A student prepared solutions of (i) an acid and (ii) a base in two separate beakers. She forgot to label the solutions and litmus paper is not available in the laboratory. Since both the solutions are colourless, how will she distinguish between the two?
Answer:
She can use another indicator: add a few drops of phenolphthalein to a sample of each solution. The solution that turns pink is the base; the other, which stays colourless, is the acid. (Turmeric, which turns red in a base, can also be used.)
Alternatively, add a little sodium hydrogencarbonate or a piece of zinc to each. The acid gives brisk effervescence; the base does not (zinc gives hydrogen only on warming with a strong base).
In simple words: Use phenolphthalein, turmeric or baking soda instead of litmus.

Teacher's Note:
1. Any one correct method earns the marks.
2. Phenolphthalein is pink in base.

Question 13. When zinc metal is treated with a dilute solution of a strong acid, a gas is evolved which is utilised in the hydrogenation of oil. Name the gas evolved. Write the chemical equation of the reaction involved and also write a test to detect the gas formed.
Answer:
The gas is hydrogen.
Zn + 2HCl → ZnCl₂ + H₂↑ (or Zn + H₂SO₄ → ZnSO₄ + H₂↑)
Test: bring a burning splinter near the mouth of the test tube. The gas burns with a pop sound.
In simple words: Hydrogen burns with a pop sound.

Teacher's Note:
1. Hydrogenation converts vegetable oil to vanaspati.
2. Name the gas first.

Question 14. A metal compound A reacts with dilute sulphuric acid to produce a gas which extinguishes a burning candle. Identify the compound A and the gas produced. Write a balanced chemical equation for the reaction if one of the compounds formed in the reaction is sodium sulphate.
Answer:
Compound A is sodium carbonate and the gas is carbon dioxide.
Na₂CO₃(s) + H₂SO₄(aq) → Na₂SO₄(aq) + H₂O(l) + CO₂(g)
In simple words: A carbonate with acid gives CO₂, which puts out a flame.

Teacher's Note:
1. Sodium sulphate is the clue to sodium carbonate.
2. Use the given clues one by one to identify each compound.

Question 15. A gas is produced when conc. H₂SO₄ is added to solid sodium chloride taken in a test tube. The gas coming out through the delivery tube is passed over a dry blue litmus paper and then over a moist blue litmus paper. Record your observations and explain reason with the help of chemical equation.
Answer:
The gas is hydrogen chloride: 2NaCl + H₂SO₄ → Na₂SO₄ + 2HCl↑.
On dry blue litmus there is no change, because dry HCl gas has no H⁺ ions.
On moist blue litmus the paper turns red, because HCl dissolves in the moisture and gives H⁺ ions.
In simple words: HCl gas is acidic only when water is present.

Teacher's Note:
1. This is an NCERT activity.
2. Use a guard tube to dry the gas.

Question 16. Dry ammonia gas has no action on litmus paper, but a solution of ammonia in water turns red litmus paper blue. Why is it so?
Answer:
Dry ammonia gas has no OH⁻ ions, so it does not change the colour of litmus.
In water, ammonia forms ammonium hydroxide, which ionises to give OH⁻ ions: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻. These OH⁻ ions make the solution basic, so red litmus turns blue.
In simple words: Bases show their nature only in water.

Teacher's Note:
1. Similarly, dry HCl gas does not show acidic character.
2. Name the ion responsible.

Question 17. The following table lists the pH values of some substances: Hydrochloric acid 1, Milk 6, Pure water 7, Baking soda 9, Sodium hydroxide 14.
(A) What would happen to the pH of an acid and a base when each one is diluted (pure distilled water is added to it)?
(B) We say that greater the pH than 7, more basic is the solution. Whereas lesser the pH than 7, more acidic is the solution. Justify the above statement.
Answer:
(A) On dilution, the pH of an acid increases (moves towards 7) and the pH of a base decreases (moves towards 7).
(B) A pH greater than 7 means a lower H⁺ concentration and a greater OH⁻ concentration, so the solution is more basic. A pH lower than 7 means a greater H⁺ concentration and a lower OH⁻ concentration, so the solution is more acidic.
In simple words: Dilution moves the pH of both acid and base towards 7.

Teacher's Note:
1. Diluting never makes an acid basic.
2. Link pH to ion concentration.

Question 18. The pH of three solutions is given in the table: P has pH 1, Q has pH 7, R has pH 14.
(A) Which of these solutions could possibly react with zinc metal to produce hydrogen gas?
(B) Which of these solutions could be formed by the reaction of a metal oxide with water?
(C) Which of these solutions could be the raw material for the industrial manufacture of chlorine?
Answer:
(A) P and R. P (pH 1) is a strong acid, and zinc reacts with acids to give hydrogen. R (pH 14) is a strong base, and zinc also reacts with strong alkalis such as NaOH to give hydrogen.
(B) R. A metal oxide reacts with water to form a metal hydroxide (a base): metal oxide + water → metal hydroxide.
(C) Q. Brine (sodium chloride solution) is neutral (pH 7), and its electrolysis gives chlorine, hydrogen and sodium hydroxide.
In simple words: Acids and strong bases react with zinc; metal oxide plus water gives a base; brine is neutral.

Teacher's Note:
1. Match each pH with acid, neutral or base first.
2. Write the metal reactions for P and R.