CBSE Class 10 Science Acids Bases and Salts Assignment Set 07

This assignment for Class 10 Science Chapter 2, Acids, Bases and Salts, has 24 questions of 1 to 5 marks with complete answers, simple-words summaries and teacher's notes. It practises the ionisation of acids and bases, dilution, the reactions of acids with metals, carbonates and bases, the pH of everyday substances and two case based questions.

Very Short Answer Questions (1 mark)

Question 1. How will you test for the gas which is liberated when hydrochloric acid reacts with an active metal?
Answer:
Hydrogen gas is released when hydrochloric acid reacts with an active metal such as zinc: Zn + 2HCl → ZnCl₂ + H₂↑.
To test it, bring a burning matchstick (or candle) near the gas. The gas burns with a pop sound, which shows that it is hydrogen.
In simple words: Hydrogen burns with a pop sound.

Teacher's Note:
1. Mention both the gas and the test.
2. Do not use a large flame near the test tube.

Question 2. The pH of a sample of vegetable soup was found to be 6.5. How is this soup likely to taste?
Answer:
The pH is less than 7, so the soup is weakly acidic.
It is likely to taste slightly sour.
In simple words: pH below 7 is acidic.

Teacher's Note:
1. A pH close to 7 means very weak acidity.
2. Sour taste is a property of acids.

Question 3. What will be the action of liquid antacid on litmus paper? Is it acidic or basic?
Answer:
A liquid antacid is basic in nature, so it turns red litmus paper blue and has no effect on blue litmus.
In simple words: Antacids are bases.

Teacher's Note:
1. Antacid: milk of magnesia is magnesium hydroxide.
2. Bases turn red litmus blue.

Question 4. Name an indicator solution which shows different colours at different concentration of ions in the solution.
Answer:
Universal indicator. It is a mixture of several indicators and shows different colours at different pH values over the whole pH scale.
In simple words: Universal indicator gives a different colour for each pH value.

Teacher's Note:
1. Do not name litmus; litmus has only two colours.
2. Mention that it covers pH 0 to 14.

Question 5. State the name and chemical formula of the hydrated salt that changes from blue to white when it is heated.
Answer:
Hydrated copper sulphate (copper sulphate pentahydrate), CuSO₄·5H₂O. On heating it loses water of crystallisation and turns white: CuSO₄·5H₂O → CuSO₄ + 5H₂O.
In simple words: Blue copper sulphate crystals turn white when the water is lost.

Teacher's Note:
1. The formula must show 5H₂O.
2. Anhydrous copper sulphate is white.

Short Answer Questions (2 marks)

Question 6. Why do HCl, HNO₃, etc., show acidic characters in aqueous solutions while solutions of compounds like alcohol and glucose do not show acidic character?
Answer:
Acids such as HCl and HNO₃ ionise in water to give hydrogen ions: HCl → H⁺ + Cl⁻.
The H⁺ ions combine with water to form hydronium ions, H⁺ + H₂O → H₃O⁺, and these ions are responsible for acidic character.
Alcohol and glucose contain hydrogen but do not ionise in water to give H⁺ ions, so their solutions are not acidic.
In simple words: Only compounds that release H⁺ ions in water behave as acids.

Teacher's Note:
1. Having hydrogen is not enough; the hydrogen must be released as ions.
2. This is a standard NCERT question.

Question 7. Why does dry HCl gas not change the colour of the dry litmus paper?
Answer:
Dry HCl gas does not contain H⁺ ions because acids release H⁺ ions only in the presence of water.
In dry litmus paper and dry HCl gas no water is present, so HCl does not ionise and the colour of the litmus paper does not change.
In simple words: No water means no H⁺ ions, so no acidic action.

Teacher's Note:
1. Moist blue litmus would turn red with HCl gas because the gas dissolves and ionises.
2. Link every acidic property to H⁺(aq).

Question 8. How is the concentration of hydronium ions (H₃O⁺) affected when a solution of an acid is diluted?
Answer:
On dilution, the number of hydronium ions per unit volume decreases, so the concentration of H₃O⁺ ions decreases.
The solution becomes less acidic and its pH increases towards 7.
In simple words: Adding water makes the acid weaker in concentration.

Teacher's Note:
1. Dilution does not change the nature of the acid, only its concentration.
2. pH rises on dilution of an acid but never goes above 7.

Question 9. Do basic solutions also have H⁺ (aq) ions? If yes, then why are these basic?
Answer:
Yes, basic solutions also contain H⁺(aq) ions because water ionises slightly.
But the concentration of OH⁻ ions is much more than that of H⁺ ions, and this makes the solution basic.
In simple words: Basic solutions have both ions, but OH⁻ ions are in excess.

Teacher's Note:
1. Neutral water has equal numbers of both ions.
2. Compare the two ion concentrations in your answer.

Question 10. Plaster of Paris should be stored in a moisture-proof container. Explain why?
Answer:
Plaster of Paris is a powder that absorbs moisture readily.
In the presence of water it combines with water and turns into hard gypsum, CaSO₄·2H₂O, so it cannot be used again. Hence it is stored in a moisture-proof container.
In simple words: Moisture sets POP into a solid, so it is kept dry.

Teacher's Note:
1. The reaction is CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O.
2. This is not hygroscopy of the gas type; it is a chemical change.

Question 11. Give the chemical name of the compound present in tooth enamel. What is the nature of this compound?
Answer:
Tooth enamel is made of calcium phosphate, Ca₃(PO₄)₂ (calcium hydroxyapatite).
It is basic in nature, so it is corroded by acids when the mouth pH falls below 5.5; toothpastes are basic to neutralise the acid.
In simple words: Enamel is basic calcium phosphate.

Teacher's Note:
1. Base reacts with acid, hence the corrosion.
2. The hardest substance in the body.

Question 12. What happens when nitric acid is added to egg shell?
Answer:
Egg shells contain calcium carbonate. With nitric acid, brisk effervescence of carbon dioxide gas is seen: CaCO₃ + 2HNO₃ → Ca(NO₃)₂ + H₂O + CO₂↑.
In simple words: Egg shell fizzes with acid because it is calcium carbonate.

Teacher's Note:
1. The gas turns lime water milky.
2. Write the balanced equation.

Short Answer Questions (3 marks)

Question 13. Give balanced chemical equation for:
(i) Reaction of acids with metals.
(ii) Reaction of acids with metal carbonate.
(iii) Reaction of acid with hydrogen carbonates (bicarbonates).
(iv) Reaction of acid with bases.
Answer:
(i) Acid + Metal → Salt + Hydrogen: Zn + 2HCl → ZnCl₂ + H₂↑
(ii) Acid + Metal carbonate → Salt + Carbon dioxide + Water: CaCO₃ + 2HCl → CaCl₂ + CO₂↑ + H₂O
(iii) Acid + Metal hydrogencarbonate → Salt + Carbon dioxide + Water: NaHCO₃ + HCl → NaCl + CO₂↑ + H₂O
(iv) Acid + Base → Salt + Water (neutralisation): NaOH + HCl → NaCl + H₂O
In simple words: Metals give hydrogen, carbonates and hydrogencarbonates give carbon dioxide, and bases give only salt and water.

Teacher's Note:
1. Learn the general word equation first and then add one example.
2. Write the general word equation first and then the example.

Question 14. Classify the following salts as acidic, basic or neutral salts: potassium sulphate, ammonium chloride, sodium carbonate, sodium chloride.
Answer:
Neutral salts (strong acid + strong base): potassium sulphate, sodium chloride.
Acidic salt solution (strong acid + weak base): ammonium chloride.
Basic salt solution (weak acid + strong base): sodium carbonate.
In simple words: The nature of the salt follows the strength of the acid and base it is made from.

Teacher's Note:
1. pH of NH₄Cl solution is below 7 and of Na₂CO₃ solution is above 7.
2. Strong acid + strong base gives a neutral salt; strong acid + weak base gives an acidic salt; weak acid + strong base gives a basic salt.

Question 15. Answer the following questions:
(i) Define olfactory indicators. Name two substances which can be used as olfactory indicator.
(ii) Choose strong acids from the following: CH₃COOH, H₂SO₄, H₂CO₃, HNO₃.
Answer:
(i) Substances whose smell changes in acidic or basic medium are called olfactory indicators. Examples: onion and vanilla essence (also clove oil).
(ii) Strong acids: H₂SO₄ and HNO₃. (CH₃COOH and H₂CO₃ are weak acids.)
In simple words: Olfactory indicators change their smell in acid or base.

Teacher's Note:
1. Onion loses its smell in a base, vanilla loses its smell in a base.
2. Strong acids ionise completely.

Question 16. What is the colour of FeSO₄·7H₂O crystals? How does this colour change upon heating? Give balanced chemical equation for the changes.
Answer:
Ferrous sulphate crystals, FeSO₄·7H₂O, are pale green.
On heating they lose water of crystallisation and become a dirty white anhydrous solid: FeSO₄·7H₂O → FeSO₄ + 7H₂O.
If heated strongly, the anhydrous salt decomposes further to give brown ferric oxide, SO₂ and SO₃.
In simple words: Green crystals turn dirty white on losing water.

Teacher's Note:
1. Mention the colours before and after.
2. The hydrate has seven water molecules.

Question 17. Write the chemical formula of hydrated copper sulphate and anhydrous copper sulphate. Giving an activity to illustrate how these are inter convertible.
Answer:
Hydrated copper sulphate: CuSO₄·5H₂O (blue). Anhydrous copper sulphate: CuSO₄ (white).
Activity: heat a few blue crystals in a dry boiling tube. They lose water of crystallisation and turn white: CuSO₄·5H₂O → CuSO₄ + 5H₂O.
Add a few drops of water to the white powder (or cool it and moisten it). The blue colour returns, showing the change can be reversed.
In simple words: Heat removes the water from the blue crystals; water brings the blue colour back.

Teacher's Note:
1. Anhydrous copper sulphate is used to test for water.
2. Write the colour of each form.

Question 18. On adding dilute hydrochloric acid to copper oxide powder, the solution formed is blue-green.
(i) Predict the new compound formed which imparts a blue-green colour to solution.
(ii) Write a balanced chemical equation of the reaction which takes place.
(iii) On the basis of the above reaction, what can you say about the nature of copper oxide?
Answer:
(i) Copper(II) chloride, CuCl₂.
(ii) CuO(s) + 2HCl(aq) → CuCl₂(aq) + H₂O(l)
(iii) Copper oxide reacts with an acid to give salt and water, so it is basic in nature.
In simple words: Metal oxides are basic, so they react with acids.

Teacher's Note:
1. This activity is in the NCERT book.
2. Metal oxide + acid gives salt + water.

Question 19. How would you distinguish between baking powder and washing soda by heating?
Answer:
Heat a little of each in a dry test tube and pass the gas evolved through lime water.
Baking powder (baking soda) gives off CO₂, which turns lime water milky and extinguishes a burning matchstick: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂.
Washing soda crystals, Na₂CO₃·10H₂O, give out only water vapour (they melt in their water of crystallisation and then form a white powder), and no gas turns lime water milky.
In simple words: Baking soda gives CO₂ on heating; washing soda gives only water.

Teacher's Note:
1. Use the lime water test.
2. Mention what is observed in each case.

Question 20. Write uses of (i) Acids (ii) Bases (iii) Salts.
Answer:
(i) Acids: hydrochloric acid cleans boilers and sanitary ware; nitric acid is used to make fertilisers; sulphuric acid is used in car batteries.
(ii) Bases: sodium hydroxide is used to make soap, paper and textiles; calcium hydroxide reduces soil acidity; milk of magnesia, magnesium hydroxide, is an antacid.
(iii) Salts: sodium chloride is used as food seasoning and preservative; washing soda is used for cleaning clothes; baking soda is used in baking and as antacid; bleaching powder is used as a disinfectant; plaster of Paris is used for setting fractured bones.
In simple words: Acids, bases and salts have many everyday and industrial uses.

Teacher's Note:
1. Give at least two uses for each.
2. Mention the specific compound with each use.

Long Answer and Case Based Questions (4 and 5 marks)

Question 21. Solution A turns universal indicator blue to purple whereas solution B turns universal indicator orange to red.
(i) What will be the action of solution A on litmus?
(ii) What will be action of solution B on litmus?
(iii) Name any two substances which can give solutions like A.
(iv) Name any two substances which can give solutions like B.
(v) What sort of reaction takes place when solution A reacts with solution B?
Answer:
(i) A is basic (high pH), so it turns red litmus blue.
(ii) B is acidic (low pH), so it turns blue litmus red.
(iii) Sodium hydroxide solution, milk of magnesia (also lime water, ammonia solution).
(iv) Hydrochloric acid, lemon juice (also vinegar).
(v) Neutralisation reaction, giving salt and water.
In simple words: Blue-purple means a strong base; orange-red means an acid.

Teacher's Note:
1. Match colour to pH range of universal indicator.
2. Neutralisation is acid + base.

Question 22. Sodium chloride's main source is sea water. It is also found in the form of rocks and is called rock salt. It is used for preparing sodium hydroxide, baking soda, washing soda etc. Sodium Hydroxide is made by passing electricity through an aqueous solution of sodium chloride known as chlor-alkali process. It is white translucent solid. Crystals of sodium hydroxide are readily soluble in water and gives strong alkaline solution. Washing Soda is made by heating baking soda. Recrystallisation of sodium carbonate gives a compound, which is used for removing permanent hardness of water. Plaster of Paris is obtained by heating Gypsum up to 373 K. On treatment with water it is again converted into gypsum and sets as a hard mass. It is used by doctors for setting fractured bones. It is used for making statues, models and other decorative materials.
(i) What is the chemical formula of plaster of paris?
(ii) Name the chemical compound obtained by the recrystallization of sodium carbonate.
(iii) Name the solution that is used to prepare caustic soda.
(iv) Which of these statements is incorrect?
(a) On treatment with water plaster of paris is again converted into gypsum.
(b) Crystals of sodium hydroxide are deliquescent.
(c) Sodium chloride is known as common salt.
(d) Plaster of Paris is calcium sulphate dihydrate.
Answer:
(i) CaSO₄·½H₂O.
(ii) Washing soda, Na₂CO₃·10H₂O.
(iii) Brine (concentrated sodium chloride solution).
(iv) (d). Plaster of Paris is calcium sulphate hemihydrate, not a dihydrate (the dihydrate is gypsum). The statements (a), (b) and (c) are correct.
In simple words: Plaster of Paris is the hemihydrate of calcium sulphate; gypsum is the dihydrate.

Teacher's Note:
1. Recall the three sodium compounds made from common salt.
2. Read the passage once before answering.

Question 23. Bases are bitter in taste, have soapy touch, turn red litmus blue and give hydroxide ions in solution. Chemical properties of bases are reaction with metals where certain metals react with alkali solutions on heating a gas is evolved. The reaction with non-metallic oxides which are generally acidic in nature form salt and water. pH Scale: The concentration of hydrogen ion in solution is expressed in terms of pH. The pH of a solution is defined as the negative logarithm of hydrogen ion concentration in moles per litre. For water or neutral solutions, pH = 7; for acidic solutions, pH < 7; for basic solutions, pH > 7.
(i) What happens when a base reacts with acid?
(ii) What is the general name of reaction between a base and acid?
(iii) Which of these statements is incorrect?
(a) Bases react with acids to form salt and water.
(b) Metals with alkali solutions produce hydrogen gas.
(c) Non-metallic oxides are basic in nature.
(d) The concentration of hydrogen ion in solution is expressed in terms of pH.
(iv) What colour change occurs to litmus when it is dipped in basic solution?
Answer:
(i) A base reacts with an acid to form salt and water.
(ii) Neutralisation reaction.
(iii) (c). Non-metallic oxides are acidic in nature, not basic.
(iv) Red litmus turns blue in a basic solution.
In simple words: Bases neutralise acids to give salt and water, and non-metal oxides are acidic.

Teacher's Note:
1. Read the passage once before answering.
2. For part (iii) pick the false statement.

Question 24. When electricity is passed through an aqueous solution of a common salt, a substance Z is produced along with the evolution of gases X and Y. When a burning matchstick is brought near the gas Y it burns with a pop sound, whereas X is used for disinfecting drinking water. When gas X is passed through a solution of slaked lime, an insoluble substance A is produced.
(A) Write the name of gases X and Y.
(B) Write the balanced chemical equation for the formation of substance A.
(C) Write your observations: (i) if a drop of blue litmus solution is added to the aqueous solution of substance Z. (ii) if methyl orange is added to substance Z.
Answer:
(A) X is chlorine and Y is hydrogen. Z is sodium hydroxide, formed by the chlor-alkali process.
(B) Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O. A is bleaching powder.
(C) (i) NaOH is a base, so blue litmus does not change colour (it remains blue). (ii) Methyl orange turns yellow in the base.
In simple words: Electrolysis of brine gives chlorine, hydrogen and NaOH; NaOH is basic.

Teacher's Note:
1. Hydrogen burns with a pop sound.
2. Methyl orange is yellow in base.