Class 10 Science · Chapter 2

Acids, Bases and Salts

208 Class 10 Science questions on Acids, Bases and Salts from the NCERT textbook, NCERT Exemplar and CBSE board papers 2024–2026, each with its answer and a step-by-step solution.

CBSE Class 10 Science previous year papers, with answers →

Questions · page 2 of 11

1. Q21 NCERT · Intext Questions (Page 25)
   Why ​does an aqueous ​solution of an acid ​conduct ​electricity?
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   An acid ionises in water to give free, mobile ions, and their movement towards the electrodes carries the current.

   An acid dissolved in water ionises to give hydrogen ions, ${\text{H}}^{+}(aq)$ (hydronium ions, ${\text{H}}_3{\text{O}}^{+}$), and the corresponding negative ions — for example $\text{HCl}+{\text{H}}_2\text{O}\to {\text{H}}_3{\text{O}}^{+}+{\text{Cl}}^{-}$. These ions are charged and free to move; when the electrodes are connected, the positive ions move to the negative electrode and the negative ions to the positive electrode, and this movement of ions carries the electric current. So an aqueous solution of an acid conducts electricity (the bulb glows in Activity 2.8).

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2. Q22 NCERT · Intext Questions (Page 25)
   Why does ​dry $\text{HCl}$ ​gas not ​change the ​colour ​of the dry ​litmus paper?
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   Dry $\text{HCl}$ gas cannot ionise without water, so it gives no ${\text{H}}^{+}$ ions and dry litmus does not change colour.

   Litmus changes colour in response to hydrogen ions, ${\text{H}}^{+}(aq)$, and it is these ions that give $\text{HCl}$ its acidic character. $\text{HCl}$ produces hydrogen ions only when it dissolves in water: $\text{HCl}+{\text{H}}_2\text{O}\to {\text{H}}_3{\text{O}}^{+}+{\text{Cl}}^{-}$. Dry $\text{HCl}$ gas and dry litmus paper contain no water, so the $\text{HCl}$ molecules cannot ionise, no ${\text{H}}^{+}(aq)$ ions are formed, and the litmus does not change colour. With moist litmus the water present dissolves the gas, ions form, and the paper turns red (Activity 2.9).

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3. Q23 NCERT · Intext Questions (Page 25)
   While diluting an acid, ​why ​is ​it ​recommended that the acid should be ​added to water ​and ​not water ​to the acid?
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   Dissolving acid in water is highly exothermic; a large volume of water absorbs the heat, whereas water added to acid boils and splashes.

   Dissolving a concentrated acid in water is a highly exothermic process — a large amount of heat is released. When the acid is added slowly to a large volume of water with constant stirring, the heat is absorbed by the large mass of water and the temperature rises only slightly. If water is added to the concentrated acid instead, the small amount of water added receives all the heat at once; it boils, the mixture may splash out and cause serious acid burns, and the glass container may crack because of excessive local heating. Hence the acid must always be added to water, slowly and with stirring, and never water to the acid.

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4. Q24 NCERT · Intext Questions (Page 25)
   How ​is the concentration of ​hydronium ions (${\text{H}}_3{\text{O}}^{+}$) ​affected ​when a solution of ​an acid ​is ​diluted?
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   Dilution lowers the concentration of hydronium ions, because the same number of ions is spread through a larger volume.

   On dilution, water is added to the acid solution. The number of hydronium ions stays the same but the volume of the solution increases, so the same ions are spread through a larger volume: the concentration of hydronium ions (${\text{H}}_3{\text{O}}^{+}$) per unit volume decreases. Doubling the volume with water, for example, halves the concentration.

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5. Q25 NCERT · Intext Questions (Page 25)
   How is the concentration ​of ​hydroxide ions (${\text{OH}}^{-}$) ​affected when excess base ​is dissolved in ​a solution ​of sodium ​hydroxide?
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   The ${\text{OH}}^{-}$ concentration increases, because more hydroxide ions are added to practically the same volume, making the solution more strongly basic.

   Sodium hydroxide gives hydroxide ions in water: $\text{NaOH}(s)\stackrel{{\text{H}}_2\text{O}}{\to }{\text{Na}}^{+}(aq)+{\text{OH}}^{-}(aq)$. When excess base is dissolved in the solution, more ${\text{OH}}^{-}$ ions are added to practically the same volume of solution, so the number of hydroxide ions per unit volume — the concentration of ${\text{OH}}^{-}$ ions — increases, and the solution becomes more strongly basic (its pH rises).

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6. Q26 NCERT · Intext Questions (Page 28)
   You have two ​solutions, A and B. The pH of solution ​A is $6$ and pH of solution B is $8$. ​Which solution has more hydrogen ion ​concentration? Which of this is ​acidic ​and ​which one is basic?
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   Solution A (pH $6$) has the higher hydrogen-ion concentration and is acidic; solution B (pH $8$) is basic.

   Solution A (pH $6$) has the higher hydrogen-ion concentration, because the lower the pH, the higher the concentration of ${\text{H}}^{+}$ ions. Solution A (pH $6$, less than $7$) is acidic and solution B (pH $8$, more than $7$) is basic.

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7. Q27 NCERT · Intext Questions (Page 28)
   What ​effect does ​the concentration of ${\text{H}}^{+}(aq)$ ​ions ​have ​on the nature of ​the ​solution?
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   The higher the ${\text{H}}^{+}(\text{aq})$ concentration, the more acidic the solution and the lower its pH.

   The concentration of ${\text{H}}^{+}(aq)$ ions decides how acidic a solution is. The higher the concentration of ${\text{H}}^{+}(aq)$ ions, the more acidic (the stronger) the solution and the lower its pH; the lower the concentration of ${\text{H}}^{+}(aq)$ ions, the less acidic the solution and the higher its pH. When the ${\text{H}}^{+}$ and ${\text{OH}}^{-}$ concentrations are equal the solution is neutral (pH $7$), and when ${\text{OH}}^{-}$ ions are in excess it is basic (pH above $7$).

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8. Q28 NCERT · Intext Questions (Page 28)
   Do basic solutions ​also ​have ${\text{H}}^{+}(aq)$ ions? If ​yes, ​then why ​are ​these basic?
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   Yes; basic solutions contain some ${\text{H}}^{+}$ from water, but they are basic because ${\text{OH}}^{-}$ ions far outnumber ${\text{H}}^{+}$ ions.

   Yes. Basic solutions also contain ${\text{H}}^{+}(aq)$ ions, because water itself ionises to a very small extent: ${\text{H}}_2\text{O}(l)\rightleftharpoons {\text{H}}^{+}(aq)+{\text{OH}}^{-}(aq)$. They are basic because the concentration of ${\text{OH}}^{-}(aq)$ ions in them is much greater than the concentration of ${\text{H}}^{+}(aq)$ ions; this excess of hydroxide ions gives the solution its basic nature (pH above $7$).

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9. Q29 NCERT · Intext Questions (Page 28)
   Under what soil ​condition do you ​think a farmer would ​treat the soil of his ​fields with quick ​lime ​(calcium oxide) or slaked ​lime (calcium hydroxide) or ​chalk (calcium carbonate)?
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   A farmer treats acidic soil (pH below $7$) with quick lime, slaked lime or chalk, which neutralise the excess acid.

   When the soil is acidic — its pH is below $7$, too low for the healthy growth of plants. Quick lime ($\text{CaO}$) and slaked lime (${\text{Ca(OH)}}_2$) are bases and chalk (${\text{CaCO}}_3$) is a basic salt; each neutralises the excess acid in the soil and raises its pH towards the range the crops need.

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10. Q30 NCERT · Intext Questions (Page 33)
    What ​is ​the common ​name ​of ​the ​compound ${\text{Ca(ClO)}}_2$?
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    The common name of ${\text{Ca(ClO)}}_2$ is bleaching powder.

    Bleaching powder. ${\text{Ca(ClO)}}_2$ is calcium oxychloride (calcium hypochlorite), the compound obtained by the action of chlorine on dry slaked lime, ${\text{Ca(OH)}}_2$, and known commonly as bleaching powder.

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11. Q31 NCERT · Intext Questions (Page 33)
    Name the ​substance which ​on ​treatment ​with ​chlorine yields bleaching powder.
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    Dry slaked lime, calcium hydroxide, which gives bleaching powder on treatment with chlorine.

    Dry slaked lime, that is calcium hydroxide, ${\text{Ca(OH)}}_2$. On treatment with chlorine it yields bleaching powder: $2{\text{Ca(OH)}}_2+2{\text{Cl}}_2\to {\text{Ca(ClO)}}_2+{\text{CaCl}}_2+2{\text{H}}_2\text{O}$.

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12. Q32 NCERT · Intext Questions (Page 33)
    Name the ​sodium compound which ​is ​used ​for ​softening hard ​water.
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    Washing soda (sodium carbonate), which removes hardness by precipitating calcium and magnesium salts as insoluble carbonates.

    Washing soda — sodium carbonate, ${\text{Na}}_2{\text{CO}}_3\cdot 10{\text{H}}_2\text{O}$. It removes the permanent hardness of water by precipitating the dissolved calcium and magnesium salts as insoluble carbonates, for example ${\text{CaCl}}_2(aq)+{\text{Na}}_2{\text{CO}}_3(aq)\to {\text{CaCO}}_3(s)+2\text{NaCl}(aq)$.

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13. Q33 NCERT · Intext Questions (Page 33)
    What ​will happen if a solution ​of sodium hydrocarbonate ​is heated? Give the ​equation of ​the ​reaction involved.
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    Sodium hydrogencarbonate decomposes on heating, giving carbon dioxide, water and sodium carbonate: $2{\text{NaHCO}}_3\to {\text{Na}}_2{\text{CO}}_3+{\text{H}}_2\text{O}+{\text{CO}}_2$.

    On heating, the sodium hydrogencarbonate (baking soda, ${\text{NaHCO}}_3$; printed as 'sodium hydrocarbonate') decomposes: carbon dioxide gas is given off with effervescence, water is formed and sodium carbonate is left in the solution. Equation: $2{\text{NaHCO}}_3\stackrel{\text{heat}}{\to }{\text{Na}}_2{\text{CO}}_3+{\text{H}}_2\text{O}+{\text{CO}}_2$. It is a thermal decomposition reaction; the gas turns lime water milky.

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14. Q34 NCERT · Intext Questions (Page 33)
    Write ​an ​equation ​to show the ​reaction ​between ​Plaster of Paris ​and water.
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    Plaster of Paris takes up water and sets to gypsum: ${\text{CaSO}}_4\cdot \frac{1}{2}{\text{H}}_2\text{O}+1\frac{1}{2}{\text{H}}_2\text{O}\to {\text{CaSO}}_4\cdot 2{\text{H}}_2\text{O}$.

    ${\text{CaSO}}_4\cdot \frac{1}{2}{\text{H}}_2\text{O}+1\frac{1}{2}{\text{H}}_2\text{O}\to {\text{CaSO}}_4\cdot 2{\text{H}}_2\text{O}$ — Plaster of Paris (calcium sulphate hemihydrate) takes up $1\frac{1}{2}$ molecules of water per ${\text{CaSO}}_4$ and sets to gypsum (calcium sulphate dihydrate), a hard solid mass. In whole numbers: $2({\text{CaSO}}_4\cdot \frac{1}{2}{\text{H}}_2\text{O})+3{\text{H}}_2\text{O}\to 2({\text{CaSO}}_4\cdot 2{\text{H}}_2\text{O})$.

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15. Q35 Exemplar · Multiple Choice Questions
    What happens ​when a ​solution of an acid is mixed with a solution of a ​base in a test tube?

    1. (i)The temperature of the solution increases
    2. (ii)The temperature ​of the ​solution ​decreases
    3. (iii)The temperature of the solution remains ​the same
    4. (iv)Salt formation takes place

    1. (a)(i) only
    2. (b)(i) and (iii)
    3. (c)(ii) and (iii)
    4. (d)(i) and (iv)
    Show answer

    (d) (i) and (iv)

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16. Q36 Exemplar · Multiple Choice Questions
    An ​aqueous ​solution turns red litmus ​solution blue. Excess addition of ​which of the following ​solution ​would ​reverse the change?

    1. (a)Baking powder
    2. (b)Lime
    3. (c)Ammonium hydroxide solution
    4. (d)Hydrochloric acid
    Show answer

    (d) Hydrochloric acid

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17. Q37 Exemplar · Multiple Choice Questions
    During the ​preparation of hydrogen chloride gas on ​a humid day, the gas ​is usually passed through the ​guard tube containing ​calcium ​chloride. The role of calcium ​chloride ​taken in the guard tube is to

    1. (a)absorb the evolved gas
    2. (b)moisten the gas
    3. (c)absorb moisture from the gas
    4. (d)absorb ${\text{Cl}}^{-}$ ions from the evolved gas
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    (c) absorb moisture from the gas

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18. Q38 Exemplar · Multiple Choice Questions
    Which of ​the following salts does ​not ​contain ​water of ​crystallisation?

    1. (a)Blue vitriol
    2. (b)Baking soda
    3. (c)Washing soda
    4. (d)Gypsum
    Show answer

    (b) Baking soda

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19. Q39 Exemplar · Multiple Choice Questions
    Sodium carbonate ​is a ​basic salt ​because it ​is a salt ​of

    1. (a)strong acid and strong base
    2. (b)weak acid and weak base
    3. (c)strong acid and weak base
    4. (d)weak acid and strong base
    Show answer

    (d) weak acid and strong base

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20. Q40 Exemplar · Multiple Choice Questions
    Calcium ​phosphate is ​present in tooth ​enamel. ​Its ​nature ​is

    1. (a)basic
    2. (b)acidic
    3. (c)neutral
    4. (d)amphoteric
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    (a) basic

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