Class 10 Science · Chapter 2

Acids, Bases and Salts

352 Class 10 Science questions on Acids, Bases and Salts from the NCERT textbook, NCERT Exemplar and CBSE board papers 2018–2026, each with its answer and a step-by-step solution.

CBSE Class 10 Science previous year papers, with answers →

Questions · page 15 of 18

1. Q281 CBSE 2023
   1. (i)Write the chemical name and Molecular formula of tooth enamel.
   2. (ii)​How does it get ​corroded ? ​What is the preventive ​measure ​for this ​?
   Show answer

   Enamel is calcium hydroxyapatite; mouth bacteria make acids that lower the pH below 5.5 and corrode it; brushing with basic toothpaste prevents decay.

   (i) Tooth enamel is calcium hydroxyapatite, ${\text{Ca}}_{10}({\text{PO}}_4{)}_6(\text{OH}{)}_2$, which NCERT describes as a crystalline form of calcium phosphate (calcium phosphate itself is ${\text{Ca}}_3({\text{PO}}_4{)}_2$). (ii) Bacteria present in the mouth break down the sugar and food particles left after eating and produce acids. When these acids lower the pH in the mouth below $5.5$, the enamel, which does not dissolve in water, is corroded and tooth decay begins. Preventive measure: clean the mouth after eating food, and brush the teeth with toothpaste, which is generally basic and neutralises the excess acid.

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2. Q282 CBSE 2023
   Write the chemical composition of tooth enamel. Under ​what conditions of pH ​it ​starts corroding ​? ​Explain the reason of ​tooth decay and suggest one method to ​prevent it.
   Show answer

   Enamel is calcium hydroxyapatite, which corrodes below pH 5.5 when mouth bacteria make acids; clean the mouth and brush with basic toothpaste.

   Composition: tooth enamel is calcium hydroxyapatite, ${\text{Ca}}_{10}({\text{PO}}_4{)}_6(\text{OH}{)}_2$, a crystalline form of calcium phosphate; it is the hardest substance in the body. pH: it does not dissolve in water, but starts corroding when the pH in the mouth falls below $5.5$. Reason for tooth decay: bacteria present in the mouth degrade the sugar and food particles left after eating and produce acids; these lower the pH in the mouth below $5.5$, and the acid corrodes the enamel. Prevention: clean the mouth after eating. Brushing with a toothpaste, which is generally basic, removes the food particles and neutralises the excess acid.

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3. Q283 CBSE 2023
   1. (i)Why ​is acidified water considered ​to ​be a ​good conductor of electricity ?
   2. (ii)Write a chemical equation showing ​the ionic products formed on dissolving potassium hydroxide in ​water.
   3. (iii)Care must ​be taken while diluting concentrated nitric acid with water. Why ?
   Show answer

   Acidified water has mobile ions; KOH dissociates into potassium and hydroxide ions; the acid is added to water since dilution is highly exothermic.

   (i) Pure water contains very few ions. When a little acid is added, the acid produces hydrogen ions, ${\text{H}}^{+}$ (present as ${\text{H}}_3{\text{O}}^{+}$), and its negative ions (for example ${\text{SO}}_4^{2-}$ from sulphuric acid). These ions are free to move and carry the electric current, so acidified water is a good conductor of electricity. (ii) $\text{KOH(s)}\stackrel{{\text{H}}_2\text{O}}{\to }{\text{K}}^{+}(\text{aq})+{\text{OH}}^{-}(\text{aq})$ (iii) Diluting concentrated nitric acid is a highly exothermic process. If water is added to the concentrated acid, the heat generated may cause the mixture to splash out and cause burns, and the glass container may break because of excessive local heating. The acid must therefore be added slowly to water, with constant stirring, never water to the acid.

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4. Q284 CBSE 2023
   1. (a)​How is a universal ​indicator better than litmus ​solution ?
   2. (b)State ​two features of pH ​scale.
   3. (c)Why ​should we not add water to a concentrated acid ?
   Show answer

   Universal indicator gives an approximate pH, unlike litmus; the pH scale runs from 0 to 14; adding water to concentrated acid is dangerous.

   (a) Litmus only shows whether a solution is acidic (blue litmus turns red) or basic (red litmus turns blue). Universal indicator is a mixture of several indicators that shows different colours at different concentrations of hydrogen ions, so its colour gives the approximate pH: it tells not only whether a solution is acidic, neutral or basic but also how strongly acidic or basic it is. (b) (1) The pH scale measures the hydrogen ion concentration in a solution, generally from $0$ (very acidic) to $14$ (very alkaline); pH $7$ is neutral, a pH below $7$ is acidic and a pH above $7$ is basic. (2) The higher the hydrogen ion concentration, the lower the pH, so acidic strength increases from $7$ towards $0$, and basic strength increases from $7$ towards $14$ as the hydroxide ion concentration rises. (c) Dissolving a concentrated acid in water is highly exothermic. If water is added to the acid, the little water is heated so strongly that the mixture may boil and splash out and cause burns, and the glass container may break from excessive local heating. So the acid is always added slowly to water, with constant stirring.

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5. Q285 CBSE 2023
   (i) An ​aqueous solution turns blue litmus red. ​Which of the following solutions when added in excess would reverse the change ?

   1. (1)​Lemon juice
   2. (2)Magnesium hydroxide
   3. (3)Vinegar
   4. (4)Calcium sulphate (ii) Out of the following, which compound/compounds will ​turn the phenolphthalein solution pink ? (1) ${\text{CH}}_3\text{COOH}$ (2) ${\text{Ca(OH)}}_2$ (3) $\text{HCl}$ (4) $\text{NaOH}$ (iii) Name a gas whose ​aqueous solution is basic. Write the formula/name of this solution. (iv) A basic substance is used to treat a honey-bee sting. Why ? (v) Name ​the acid which is present in (1) ​Tomato and (2) Tamarind.
   Show answer

   Magnesium hydroxide reverses the change; ${\text{Ca(OH)}}_2$ and $\text{NaOH}$ turn phenolphthalein pink; ammonia gives ${\text{NH}}_4\text{OH}$; tomato has oxalic acid, tamarind tartaric acid.

   (i) (2) Magnesium hydroxide - it is a base, and only a base added in excess turns the red litmus blue again. (ii) (2) ${\text{Ca(OH)}}_2$ and (4) $\text{NaOH}$ - both are bases, and phenolphthalein is pink only in a basic solution. (iii) Ammonia, ${\text{NH}}_3$. Its aqueous solution is ammonium hydroxide (ammonia solution), ${\text{NH}}_4\text{OH}$. (iv) A honey-bee sting leaves an acid in the skin, which causes the pain and irritation. A mild base such as baking soda neutralises this acid, so the pain is relieved; the base is kept mild so that it does not itself harm the skin. (v) (1) Tomato - oxalic acid; (2) Tamarind - tartaric acid.

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6. Q286 CBSE 2023
   1. (i)​Define water of crystallisation.
   2. (ii)Write the ​chemical name and formula of a compound having water of crystallisation in its molecule and appears blue.
   3. (iii)Write ​the chemical formula of bleaching powder. ​Write a balanced ​chemical equation of the reaction involved in its ​preparation. List ​its three ​uses.
   Show answer

   Water of crystallisation is the fixed water in a hydrated salt; blue ${\text{CuSO}}_4\cdot 5{\text{H}}_2\text{O}$; bleaching powder ${\text{CaOCl}}_2$ is made from chlorine and dry slaked lime.

   (i) Water of crystallisation is the fixed number of water molecules present in one formula unit of a salt, chemically combined in its crystal. It is written into the formula after a dot, and a salt carrying it is a hydrated salt. (ii) Copper sulphate pentahydrate (blue vitriol), ${\text{CuSO}}_4\cdot 5{\text{H}}_2\text{O}$. (iii) Bleaching powder is ${\text{CaOCl}}_2$ (calcium oxychloride). It is prepared by passing chlorine over dry slaked lime: ${\text{Ca(OH)}}_2+{\text{Cl}}_2⟶{\text{CaOCl}}_2+{\text{H}}_2\text{O}$. Three uses: (1) for bleaching cotton and linen in the textile industry and wood pulp in paper factories; (2) as an oxidising agent in many chemical industries; (3) for disinfecting drinking water, making it free of germs.

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7. Q287 CBSE 2023
   The teacher while conducting practicals in the laboratory ​divided the ​students into three groups ​and gave them various solutions to find ​out their pH ​and classify them into acidic, basic and neutral solutions. Group A — Lemon ​juice, vinegar, colourless ​aerated drink Group B — Tomato juice, coffee, ginger juice Group C — Sodium ​hydroxide, sodium chloride, lime water

   1. (a)For the solutions provided, which group is/are likely to have pH value (i) less than 7, and (ii) greater than 7 ?[1]
   2. (b)List two ways of determining pH of a solution[1]
   3. (c)(i)Explain, why the sour substances such as lemon juice are effective in cleaning the tarnished copper vessels.[2]
   4. (c)(ii)OR : “pH has great importance in our daily life.” Justify this statement by giving two examples.[2]
   Show answer

   Groups A and B have pH below 7, group C above 7; citric acid in lemon juice dissolves the basic copper carbonate coating.

   (a) (i) pH less than 7: Group A and Group B — every solution in them is acidic. (ii) pH greater than 7: Group C, through its sodium hydroxide and lime water; note that the sodium chloride in that group is a neutral salt with a pH of exactly $7$. (b) By using universal indicator or pH paper and matching the colour produced against the standard colour chart, and by using a pH meter. (c)(i) The dull green coating on a tarnished copper vessel is basic copper carbonate, formed by the action of moist air. Lemon juice contains citric acid, which reacts with this basic coating and dissolves it as a soluble copper salt, with water and carbon dioxide; the shiny copper below is then exposed. (c)(ii) OR — Tooth decay: the acid made by bacteria from leftover sugar corrodes the enamel once the pH of the mouth falls below $5.5$, and a mildly basic toothpaste neutralises it. Bee sting: the acid left by the sting causes pain, and rubbing on a mild base such as baking soda neutralises it and gives relief.

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8. Q288 CBSE 2020
   Answer question numbers 3(a) – 3(d) ​on the basis of your understanding of the ​following paragraph and the ​related studied concepts. Taj Mahal, the great wonder of the world, is made of white marble which is composed of calcium carbonate. About 60 years ago it was ​discovered that this ​monument is being eaten away by acid rain. The Archaeological Survey of India, that looks ​after this building of historical importance is of the opinion that the atmospheric pollution due to vehicular traffic and industries, mainly Mathura Refinery may be a major cause of ​acid rain in and around the monument. Normal rain is slightly acidic because it absorbs some ${\text{CO}}_2$ from the atmospheric air. Acid rain is more acidic than normal ​rain because it also has absorbed oxides of nitrogen and sulphur.

   1. (a)Write the chemical formula of calcium carbonate.[1]
   2. (b)List two gases that are the main contributors of acid rain.[1]
   3. (b)OR : Name the gas that is liberated when acid rain falls on calcium carbonate.[1]
   4. (c)Rain is called acid rain when its pH value is (A) 3 (B) 5.5 (C) 7.8 (D) 9.7[1]
   5. (d)Select the correct statement about calcium : (A) It is a metal and its valency is one. (B) Its electronic configuration is (2, 8, 8, 2). (C) It is a non-metal with valency two. (D) It is more reactive than sodium and potassium.[1]
   Show answer

   Calcium carbonate is ${\text{CaCO}}_3$; acid rain comes from sulphur and nitrogen oxides; its pH is below 5.6; calcium is $(2,8,8,2)$.

   (a) ${\text{CaCO}}_3$. (b) Sulphur dioxide (${\text{SO}}_2$) and nitrogen dioxide (${\text{NO}}_2$), the oxides of sulphur and nitrogen given out by vehicles and industries. OR (b) Carbon dioxide (${\text{CO}}_2$): ${\text{CaCO}}_3+{\text{H}}_2{\text{SO}}_4\to {\text{CaSO}}_4+{\text{H}}_2\text{O}+{\text{CO}}_2$. (c) Options (A) and (B): $3$ and $5.5$. Normal rain has a pH of about $5.6$, and rain is called acid rain when its pH falls below this value; both values are below it. (d) Option (B): Its electronic configuration is $(2,8,8,2)$.

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9. Q289 CBSE 2020
   Which ​one ​of the ​following natural sources contains ​Oxalic ​acid ​?

   1. (a)Tomato
   2. (b)Tamarind
   3. (c)Ant sting
   4. (d)Nettle sting
   Show answer

   (a) Tomato

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10. Q290 CBSE 2020
    The acid ​produced in our ​stomach during ​digestion of ​food ​is

    1. (a)Hydrochloric acid
    2. (b)Oxalic acid
    3. (c)Lactic acid
    4. (d)Acetic acid
    Show answer

    (a) Hydrochloric acid

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11. Q291 CBSE 2020
    Consider ​the ​following ​compounds : I. $\text{HCl}$ II. ${\text{C}}_2{\text{H}}_5\text{OH}$ III. ${\text{C}}_6{\text{H}}_{12}{\text{O}}_6$ IV. ${\text{H}}_2{\text{SO}}_4$ Which of these compounds do not ​conduct electricity in ​solution ​?

    1. (a)I and II
    2. (b)II and III
    3. (c)III and IV
    4. (d)I and IV
    Show answer

    (b) II and III

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12. Q292 CBSE 2020
    Baking soda is a mixture of :

    1. (a)Sodium carbonate and acetic acid
    2. (b)Sodium carbonate and tartaric acid
    3. (c)Sodium hydrogen carbonate and tartaric acid
    4. (d)Sodium hydrogen carbonate and acetic acid
    Show answer

    (c) Sodium hydrogen carbonate and tartaric acid

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13. Q293 CBSE 2020
    The ​chemical formula ​for ​plaster ​of Paris is ​:

    1. (a)${\text{CaSO}}_4\cdot 2{\text{H}}_2\text{O}$
    2. (b)${\text{CaSO}}_4\cdot {\text{H}}_2\text{O}$
    3. (c)${\text{CaSO}}_4\cdot \frac{1}{2}{\text{H}}_2\text{O}$
    4. (d)$2{\text{CaSO}}_4\cdot {\text{H}}_2\text{O}$
    Show answer

    (c) ${\text{CaSO}}_4\cdot \frac{1}{2}{\text{H}}_2\text{O}$

    Step-by-step solution →
14. Q294 CBSE 2020
    When a small amount of acid is added to water, the ​phenomena ​which occur are : (A) Dilution (B) ​Neutralisation (C) Formation ​of ${\text{H}}_3{\text{O}}^{+}$ ions (D) Salt formation The correct statements ​are ​:

    1. (a)(A) and (C)
    2. (b)(B) and (D)
    3. (c)(A) and (B)
    4. (d)(C) and (D)
    Show answer

    (a) (A) and (C)

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15. Q295 CBSE 2020
    Several ​factories were pouring their ​wastes in rivers A and B. Water samples were ​collected from these two ​rivers. It was observed that sample collected from river ​A ​was acidic ​while that of river B was basic. ​The factories located near A and B are

    1. (a)Soaps and detergents factories near A and alcohol distillery near B.
    2. (b)Soaps and detergents factories near B and alcohol distillery near A.
    3. (c)Lead storage battery manufacturing factories near A and soaps and detergents factories near B.
    4. (d)Lead storage battery manufacturing factories near B and soaps and detergents factories near A.
    Show answer

    (c) Lead storage battery manufacturing factories near A and soaps and detergents factories near B.

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16. Q296 CBSE 2020
    A visually challenged student, has to perform ​a ​lab ​test to detect the presence of acid in a ​given ​solution. The acid-base indicator ​preferred by him ​will be :

    1. (a)Blue litmus
    2. (b)Clove oil
    3. (c)Red cabbage extract
    4. (d)Hibiscus extract
    Show answer

    (b) Clove oil

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17. Q297 CBSE 2020
    An aqueous solution ‘A’ turns phenolphthalein solution pink. On addition of an aqueous solution ‘B’ ​to ‘A’, ​the pink ​colour disappears. The following statement is ​true ​for ​solution ‘A’ ​and ​‘B’.

    1. (a)A is strongly basic and B is a weak base.
    2. (b)A is strongly acidic and B is a weak acid.
    3. (c)A has pH greater than 7 and B has pH less than 7.
    4. (d)A has pH less than 7 and B has pH greater than 7.
    Show answer

    (c) A has pH greater than 7 and B has pH less than 7.

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18. Q298 CBSE 2020
    If ​10 mL of ${\text{H}}_2{\text{SO}}_4$ ​is mixed with 10 ​mL of ${\text{Mg(OH)}}_2$ ​of the same concentration, the ​resultant ​solution will give the following colour with universal ​indicator ​:

    1. (a)Red
    2. (b)Yellow
    3. (c)Green
    4. (d)Blue
    Show answer

    (c) Green

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19. Q299 CBSE 2020
    How do metal carbonates and ​metal hydrogen carbonates react with ​acids ? Taking magnesium as ​metal, write balanced chemical equations in both ​the cases. State the method ​to test ​the gas evolved ​in ​the reactions.
    Show answer

    Carbonates and hydrogen carbonates give salt, carbon dioxide and water with acids; the gas turns lime water milky.

    Metal carbonates and metal hydrogen carbonates react with acids to give a salt, carbon dioxide and water. ${\text{MgCO}}_3+2\text{HCl}⟶{\text{MgCl}}_2+{\text{CO}}_2+{\text{H}}_2\text{O}$ $\text{Mg}({\text{HCO}}_3{)}_2+2\text{HCl}⟶{\text{MgCl}}_2+2{\text{CO}}_2+2{\text{H}}_2\text{O}$ Test: pass the gas evolved through lime water; it turns milky because insoluble calcium carbonate forms, $\text{Ca}(\text{OH}{)}_2+{\text{CO}}_2⟶{\text{CaCO}}_3+{\text{H}}_2\text{O}$, which shows that the gas is carbon dioxide.

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20. Q300 CBSE 2020
    1. (a)You are provided ​with concentrated sulphuric acid. Describe the process of ​preparing a dilute solution of sulphuric acid.
    2. (b)What ​is the effect of ​dilution on $({\text{H}}_3{\text{O}}^{+}/{\text{OH}}^{-})$ ratio ?
    3. (c)If the ${\text{H}}_3{\text{O}}^{+}$ ion concentration is increased in a ​solution, will the pH increase or decrease ​? What are the probable colours of pH paper if ​the pH range ​is $0.5$ to $2.0$ ?
    Show answer

    Add acid to water with stirring; dilution lowers the ${\text{H}}_3{\text{O}}^{+}/{\text{OH}}^{-}$ ratio; more ${\text{H}}_3{\text{O}}^{+}$ means lower pH; pH 0.5 to 2.0 gives red paper.

    (a) Take water in a glass beaker, kept cool in a trough of cold water, and add the concentrated sulphuric acid to it slowly, a few drops at a time, stirring continuously with a glass rod. Never add water to the concentrated acid: the mixing is highly exothermic, and the heat could make the mixture splash out and cause burns, or crack the glass. (b) Dilution decreases the concentration of ${\text{H}}_3{\text{O}}^{+}$ ions per unit volume, so for the acid the ratio of ${\text{H}}_3{\text{O}}^{+}$ to ${\text{OH}}^{-}$ ions decreases (it moves towards $1$ as the pH rises towards $7$). (c) The pH decreases. For pH $0.5$ to $2.0$ (strongly acidic) the pH paper turns red (shades of red).

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