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 9 of 18

1. Q161 CBSE 2025
   Two statements are given below – one labelled as ​Assertion ​(A) ​and the other ​labelled as Reason (R). Select the ​correct answer ​to this question from the options (a), ​(b), (c) and (d) given below.

   Assertion (A): Sodium chloride and potassium chloride are neutral salts.

   Reason (R): The pH of these salts is 7.0.

   1. (a)Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).
   2. (b)Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of Assertion (A).
   3. (c)Assertion (A) is true, but Reason (R) is false.
   4. (d)Assertion (A) is false, but Reason (R) is true.
   Show answer

   (a) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).

   Step-by-step solution →
2. Q162 CBSE 2025
   Two statements are ​given — one ​labelled Assertion (A) ​and the other ​labelled Reason (R). Select the ​correct answer to this question from the codes (a), (b), (c) ​and ​(d) given below.

   Assertion (A): Concentrated nitric acid is diluted by adding water slowly to acid with constant stirring.

   Reason (R): Concentrated nitric acid is easily soluble in water.

   1. (a)Both, Assertion (A) and Reason (R) are true, and Reason (R) is the correct explanation of Assertion (A).
   2. (b)Both, Assertion (A) and Reason (R) are true, and Reason (R) is not the correct explanation of Assertion (A).
   3. (c)Assertion (A) is true, but Reason (R) is false.
   4. (d)Assertion (A) is false, but Reason (R) is true.
   Show answer

   (d) Assertion (A) is false, but Reason (R) is true.

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3. Q163 CBSE 2025
   (a) ​Write the formula of the ions which

   1. (i)​acids, ​and
   2. (ii)bases generate in ​water ​solutions. (b) Dry HCl ​gas does not change the colour of dry ​litmus paper. ​Why ?
   Show answer

   Acids give ${\text{H}}^{+}$ ions and bases give ${\text{OH}}^{-}$ ions; dry HCl gas does not ionise without water, so litmus does not change.

   (a) (i) Acids give hydrogen ions, ${\text{H}}^{+}$, which exist in water as hydronium ions, ${\text{H}}_3{\text{O}}^{+}$; (ii) bases give hydroxide ions, ${\text{OH}}^{-}$. (b) Dry HCl gas does not ionise, because ionisation needs water; with no water in the gas or on the paper there are no hydrogen ions, and litmus changes colour only in the presence of those ions.

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4. Q164 CBSE 2025
   Zinc granules are ​added to dil. ${\text{H}}_2{\text{SO}}_4$ ​taken ​in a ​test tube. List two changes that are observed in this case. ​Write ​chemical equation for the ​reaction.
   Show answer

   Zinc gives brisk effervescence of hydrogen gas and the test tube becomes warm, as the reaction is exothermic.

   Two changes: (i) brisk effervescence — bubbles of a colourless gas (hydrogen) come off the zinc granules, which slowly dissolve, and the gas burns with a pop; (ii) the test tube becomes warm, because the reaction is exothermic. $\text{Zn}(s)+{\text{H}}_2{\text{SO}}_4(aq)⟶{\text{ZnSO}}_4(aq)+{\text{H}}_2(g)\uparrow$

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5. Q165 CBSE 2025
   Giving ​reason, state ​the advantage ​of ​using baking powder over ​baking ​soda for the preparation of bread ​or ​cakes.
   Show answer

   Baking powder gives soft, spongy bread without the bitter taste, because its acid reacts with ${\text{NaHCO}}_3$ and leaves no sodium carbonate.

   Advantage: with baking powder the bread or cake rises and becomes soft and spongy without the bitter taste that baking soda alone gives. Reason: heated alone, baking soda decomposes as $2{\text{NaHCO}}_3\to {\text{Na}}_2{\text{CO}}_3+{\text{H}}_2\text{O}+{\text{CO}}_2$, and the sodium carbonate left in the food tastes bitter. Baking powder also contains a mild edible acid such as tartaric acid, which reacts with the sodium hydrogencarbonate, ${\text{NaHCO}}_3+{\text{H}}^{+}\to {\text{CO}}_2+{\text{H}}_2\text{O}+\text{sodium salt of the acid}$, so no sodium carbonate is left, and the carbon dioxide makes the bread or cake rise.

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6. Q166 CBSE 2025
   A chemical compound ‘X’ is used to bleach washed clothes ​in laundry as well ​as to make ​drinking water free from germs. Identify ‘X’. How is this compound represented ? Write the method ​of its preparation along with the chemical ​equation for the reaction ​that ​occurs.
   Show answer

   X is bleaching powder, ${\text{CaOCl}}_2$, made by passing chlorine over dry slaked lime.

   X is bleaching powder. It is represented as ${\text{CaOCl}}_2$ (calcium oxychloride), though its actual composition is quite complex. It is prepared by the action of chlorine gas on dry slaked lime: ${\text{Ca(OH)}}_2+{\text{Cl}}_2\to {\text{CaOCl}}_2+{\text{H}}_2\text{O}$.

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7. Q167 CBSE 2025
   1. (a)Write the name and chemical formula of ​a sodium compound which is ​sometimes ​added for faster cooking. How is it ​produced ​from sodium chloride as one of the raw materials ? Give chemical equation for the reaction involved.
   2. (b)The ​compound mentioned in (a) above is also an ingredient of antacids. Why ?
   Show answer

   Sodium hydrogencarbonate, ${\text{NaHCO}}_3$, is made from brine, ammonia and carbon dioxide; being a mild base, it neutralises excess stomach acid.

   (a) The compound is sodium hydrogencarbonate, or baking soda, formula ${\text{NaHCO}}_3$. It is made from brine by saturating the sodium chloride solution with ammonia and then passing carbon dioxide through it: $\text{NaCl}+{\text{H}}_2\text{O}+{\text{CO}}_2+{\text{NH}}_3\to {\text{NH}}_4\text{Cl}+{\text{NaHCO}}_3$; the sparingly soluble sodium hydrogencarbonate separates out. (b) It is an ingredient of antacids because it is a mild, non-corrosive base and therefore neutralises the excess hydrochloric acid in the stomach, raising the pH and relieving the acidity: ${\text{NaHCO}}_3+\text{HCl}\to \text{NaCl}+{\text{H}}_2\text{O}+{\text{CO}}_2$.

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8. Q168 CBSE 2025
   Write ​chemical formula of washing ​soda. ​How is ​it obtained from baking soda ​? List two uses of washing ​soda.
   Show answer

   Washing soda is ${\text{Na}}_2{\text{CO}}_3\cdot 10{\text{H}}_2\text{O}$, made by heating baking soda and recrystallising the sodium carbonate formed.

   Washing soda is sodium carbonate decahydrate, ${\text{Na}}_2{\text{CO}}_3\cdot 10{\text{H}}_2\text{O}$. It is obtained from baking soda in two stages: (i) baking soda is heated, $2{\text{NaHCO}}_3\stackrel{\Delta }{\to }{\text{Na}}_2{\text{CO}}_3+{\text{H}}_2\text{O}+{\text{CO}}_2$; (ii) the sodium carbonate so formed is recrystallised from water, ${\text{Na}}_2{\text{CO}}_3+10{\text{H}}_2\text{O}\to {\text{Na}}_2{\text{CO}}_3\cdot 10{\text{H}}_2\text{O}$. Two uses: it is used in the glass, soap and paper industries, and it is used for removing the permanent hardness of water (it is also a domestic cleaning agent and a raw material for borax).

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9. Q169 CBSE 2025
   What is ​chlor-alkali process ? ​Write chemical equation for the reaction involved in this process. Name the electrode at ​which ​

   1. (i)chlorine gas, ​and
   2. (ii)hydrogen gas ​is given off.
   Show answer

   Brine electrolysis gives sodium hydroxide, chlorine and hydrogen; chlorine is given off at the anode and hydrogen at the cathode.

   The chlor-alkali process is the process in which electricity is passed through a concentrated aqueous solution of sodium chloride (brine); the brine decomposes to give sodium hydroxide solution, chlorine gas and hydrogen gas. It is called 'chlor-alkali' because of two of its products — 'chlor' for chlorine and 'alkali' for sodium hydroxide. The reaction is $2\text{NaCl (aq)}+2{\text{H}}_2\text{O (l)}\to 2\text{NaOH (aq)}+{\text{Cl}}_2\text{(g)}+{\text{H}}_2\text{(g)}$. (i) Chlorine gas is given off at the **anode**, the positive electrode. (ii) Hydrogen gas is given off at the **cathode**, the negative electrode (and sodium hydroxide is formed in the solution near the cathode).

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10. Q170 CBSE 2025
    Name the ​three products obtained in chlor-alkali ​process ​mentioning the ​electrode where they are obtained in the ​instrument used. ​Write ​one important use of each product.
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    Hydrogen at the cathode (fuel), chlorine at the anode (water disinfectant) and sodium hydroxide near the cathode (soaps and detergents).

    The three products are (1) hydrogen gas, obtained at the cathode, used as a fuel and for making margarine by hydrogenating vegetable oils; (2) chlorine gas, obtained at the anode, used for disinfecting drinking water and for making bleaching powder, PVC and hydrochloric acid; (3) sodium hydroxide, formed in the solution near the cathode, used in making soaps and detergents and for degreasing metals. The overall reaction is $2\text{NaCl}(aq)+2{\text{H}}_2\text{O}(l)⟶2\text{NaOH}(aq)+{\text{Cl}}_2(g)+{\text{H}}_2(g)$.

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11. Q171 CBSE 2025
    You have two ​solutions ‘X’ and ‘Y’. ‘X’ gives ​red colour ​and ‘Y’ gives blue colour with universal indicator.

    1. (a)In which solution is the hydrogen ion concentration more than the other ? Give reason to justify your answer.
    2. (b)If ‘X’ and ‘Y’ are mixed in proper amounts and the resultant mixture of ‘X’ and ‘Y’, with the universal indicator turns ​green, ​what would be the nature of the resultant mixture ? ​Justify ​your answer.
    Show answer

    X has the higher hydrogen ion concentration (low pH); the green mixture is neutral (pH 7).

    (a) Solution X has the higher hydrogen ion concentration. Red with universal indicator means a low pH, so X is strongly acidic, while blue means that Y is basic, with a pH above 7; the higher the hydrogen ion concentration, the lower the pH. (b) The resultant mixture is neutral. Green with universal indicator corresponds to pH 7: when the acid X and the base Y are mixed in proper amounts they neutralise each other, the hydrogen ions of the acid combining with the hydroxide ions of the base to form water, ${\text{H}}^{+}(\text{aq})+{\text{OH}}^{-}(\text{aq})\to {\text{H}}_2\text{O}(\text{l})$, so neither is left in excess.

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12. Q172 CBSE 2025
    Common ​salt is an important ​raw ​material for various chemicals of daily use. State in brief the method of preparation of

    1. (i)Sodium hydroxide, ​and
    2. (ii)Sodium hydrogen carbonate from ​common salt. Write balanced chemical equations ​of the reactions that ​occur.
    Show answer

    Sodium hydroxide: electrolysis of brine (chlor-alkali process); sodium hydrogencarbonate: ammonia and carbon dioxide passed into brine; both with balanced equations.

    (i) Sodium hydroxide — chlor-alkali process: electricity is passed through a concentrated solution of sodium chloride (brine). Chlorine is liberated at the anode, hydrogen at the cathode, and sodium hydroxide is formed in the solution: $2\text{NaCl}(aq)+2{\text{H}}_2\text{O}(l)\to 2\text{NaOH}(aq)+{\text{Cl}}_2(g)+{\text{H}}_2(g)$. (ii) Sodium hydrogencarbonate — brine is saturated with ammonia and carbon dioxide is then bubbled through it; the sparingly soluble sodium hydrogencarbonate separates out and is filtered off: $\text{NaCl}+{\text{H}}_2\text{O}+{\text{CO}}_2+{\text{NH}}_3\to {\text{NH}}_4\text{Cl}+{\text{NaHCO}}_3$.

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13. Q173 CBSE 2025
    State the chemical property in ​each case ​on which the following uses ​of baking ​soda are ​based upon :

    1. (i)as an anti-acids
    2. (ii)as a constituent ​in making baking powder
    3. (iii)in soda-acid ​fire-extinguishers
    Show answer

    Baking soda is a mild base that neutralises stomach acid, and reacts with acid to give carbon dioxide, which raises dough and smothers fires.

    (i) It is a mild base, so it neutralises the excess hydrochloric acid in the stomach: ${\text{NaHCO}}_3+\text{HCl}⟶\text{NaCl}+{\text{H}}_2\text{O}+{\text{CO}}_2$. (ii) It reacts with the mild edible acid of the baking powder, and also decomposes on heating, to liberate carbon dioxide, whose bubbles make the dough rise. (iii) With sulphuric acid it liberates a large volume of carbon dioxide at once, and this gas, being heavier than air and non-supporter of combustion, smothers the fire.

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14. Q174 CBSE 2025
    In an experiment a student dipped pH papers in four different solutions A, ​B, C and D and reported ​his observation as given ​below : (i) In which one of ​these solutions is ​the concentration of

    1. (1)hydrogen / hydronium ions;
    2. (2)hydroxyl ions maximum ? (ii) Give one example each of the ​two solutions identified in ​(i) above. (iii) What ​would be the pH of the resultant mixture obtained when these two solutions are mixed together in equal proportions ? Justify your answer.

    Colour of pH paper in the four solutions

    Solution	A	B	C	D
    Colour of pH paper	Light green	Blue	Green	Red

    Show answer

    Solution D has most hydrogen ions and B most hydroxyl ions (e.g. HCl and NaOH); mixed to neutralise each other, they give pH 7.

    (i) (1) Solution D (red, lowest pH) has the maximum concentration of hydrogen / hydronium ions. (2) Solution B (blue, basic) has the maximum concentration of hydroxyl ions. (ii) D: dilute hydrochloric acid, $\text{HCl}$. B: sodium hydroxide solution, $\text{NaOH}$. (iii) pH 7 — the mixture is neutral. Taken in amounts that exactly neutralise each other, the acid and the base react to form a salt and water, $\text{HCl}+\text{NaOH}\to \text{NaCl}+{\text{H}}_2\text{O}$: the hydrogen ions and hydroxyl ions combine to form water, and sodium chloride, the salt of a strong acid and a strong base, is neutral.

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15. Q175 CBSE 2025
    Design an experimental ​set-up to demonstrate that “Alcohol and ​glucose contain hydrogen but ​are not categorised as ​acids”. ​Also give the reason to justify this fact.
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    The bulb glows with dilute hydrochloric acid but not with glucose or alcohol, as they give no ${\text{H}}^{+}$ ions in water.

    Set-up: fix two iron nails about $2$ cm apart in a rubber cork, stand the cork on a beaker so that the nails dip into the liquid, and connect the nails through a $6$ V battery, a torch bulb and a switch. Fill the beaker first with dilute hydrochloric acid — the bulb glows. Wash the beaker and the nails and repeat with a glucose solution and then with alcohol — the bulb does not glow in either case. Reason: a solution conducts only if it contains free ions, so the dark bulb shows that glucose and alcohol give no hydrogen ions in water. A substance is an acid because it produces ${\text{H}}^{+}$ ions in aqueous solution, not merely because its molecule contains hydrogen; in glucose and alcohol the hydrogen is held by covalent bonds (largely in —OH groups) that do not ionise in water. Hence they contain hydrogen but are not categorised as acids.

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16. Q176 CBSE 2025
    Write chemical equations to show what ​happens when an acid ​reacts with ​a

    1. (i)metal
    2. (ii)​base and ​
    3. (iii)carbonate Write the ​name of the main product formed in ​each case.
    Show answer

    An acid with a metal, base or carbonate forms a salt: zinc sulphate or sodium chloride, with hydrogen, water, or water and carbon dioxide.

    (i) $\text{Zn}+{\text{H}}_2{\text{SO}}_4⟶{\text{ZnSO}}_4+{\text{H}}_2\uparrow$; main product zinc sulphate, a salt, together with hydrogen gas. (ii) $\text{NaOH}+\text{HCl}⟶\text{NaCl}+{\text{H}}_2\text{O}$; main product sodium chloride, a salt, together with water. (iii) ${\text{Na}}_2{\text{CO}}_3+2\text{HCl}⟶2\text{NaCl}+{\text{H}}_2\text{O}+{\text{CO}}_2\uparrow$; main product sodium chloride, a salt, together with water and carbon dioxide.

    Step-by-step solution →
17. Q177 CBSE 2025
    1. (i)Name the ​gas liberated ​when an acid reacts with a metal. How is this gas ​tested ?
    2. (ii)Write the chemical equation ​for the reaction of zinc ​metal with (1) hydrochloric acid, and (2) sodium hydroxide Name the compound of zinc obtained in each case.
    Show answer

    Hydrogen burns with a pop; zinc with hydrochloric acid gives zinc chloride, and with sodium hydroxide gives sodium zincate, each with hydrogen.

    (i) Hydrogen gas. It is tested by bringing a burning matchstick or candle near the gas: it burns with a pop sound. (ii) (1) $\text{Zn}(\text{s})+2\text{HCl}(\text{aq})\to {\text{ZnCl}}_2(\text{aq})+{\text{H}}_2(\text{g})$ — the compound of zinc is zinc chloride, ${\text{ZnCl}}_2$. (2) $2\text{NaOH}(\text{aq})+\text{Zn}(\text{s})\to {\text{Na}}_2{\text{ZnO}}_2(\text{s})+{\text{H}}_2(\text{g})$ — the compound of zinc is sodium zincate, ${\text{Na}}_2{\text{ZnO}}_2$.

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18. Q178 CBSE 2025
    Name and state in brief the process used to obtain sodium hydroxide from brine. In this process, along ​with the main product (i.e., sodium hydroxide), two gases A and ​B are also produced at the two electrodes. ​Name A and B specifying the ​name of their respective ​electrodes. One of these gases, when reacting with dry calcium hydroxide, produces a compound C which is used as an oxidising agent in many chemical industries as ​well as to make drinking water free from germs. Name C ​and write the chemical equation for the reaction in its formation.
    Show answer

    Chlor-alkali process; A is chlorine (anode), B is hydrogen (cathode); C is bleaching powder, ${\text{CaOCl}}_2$, made from chlorine and ${\text{Ca(OH)}}_2$.

    The process is the chlor-alkali process, the electrolysis of brine. When electricity is passed through a concentrated aqueous solution of sodium chloride, the brine decomposes to give sodium hydroxide in solution together with two gases: $2\text{NaCl(aq)}+2{\text{H}}_2\text{O(l)}\to 2\text{NaOH(aq)}+{\text{Cl}}_2\text{(g)}+{\text{H}}_2\text{(g)}$. Gas A is chlorine, liberated at the anode, the positive electrode, where chloride ions are oxidised; gas B is hydrogen, liberated at the cathode, the negative electrode, where water is reduced and the sodium hydroxide is formed. The gas that reacts with dry calcium hydroxide is chlorine, and compound C is bleaching powder, calcium oxychloride, ${\text{CaOCl}}_2$, which is used as an oxidising agent in many chemical industries and for making drinking water free from germs. Its formation is ${\text{Ca(OH)}}_2+{\text{Cl}}_2\to {\text{CaOCl}}_2+{\text{H}}_2\text{O}$.

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19. Q179 CBSE 2025
    1. (i)On adding dilute HCl to a test tube ​containing sodium carbonate, ​an effervescence ​is observed. ​Explain the observation giving chemical equation for the reaction. How is the ​gas released tested in a school laboratory ?
    2. (ii)What ​happens when excess carbon ​dioxide passes through lime water ? Write chemical equations to explain your ​answer.
    Show answer

    Carbon dioxide is released; it turns lime water milky, and excess carbon dioxide clears it by forming soluble calcium hydrogencarbonate.

    (i) Sodium carbonate, a metal carbonate, reacts with dilute hydrochloric acid to form sodium chloride, water and carbon dioxide gas; the bubbles of carbon dioxide escaping from the liquid are the effervescence: ${\text{Na}}_2{\text{CO}}_3+2\text{HCl}\to 2\text{NaCl}+{\text{H}}_2\text{O}+{\text{CO}}_2$. Test: the gas is passed through lime water (calcium hydroxide solution), which turns milky because insoluble white calcium carbonate is formed: ${\text{Ca(OH)}}_2+{\text{CO}}_2\to {\text{CaCO}}_3+{\text{H}}_2\text{O}$. (ii) When carbon dioxide is passed through lime water it first turns milky, as above. On passing excess carbon dioxide the milkiness disappears, because the calcium carbonate reacts with water and more carbon dioxide to form calcium hydrogencarbonate, which is soluble in water: ${\text{CaCO}}_3+{\text{H}}_2\text{O}+{\text{CO}}_2\to {\text{Ca(HCO}}_3{)}_2$.

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20. Q180 CBSE 2025
    1. (i)A small ​amount of ​copper oxide is taken in a beaker ​and dilute HCl is added to it slowly. Name the compound formed and state the colour of the solution formed. ​Write the ​balanced chemical equation for the reaction that occurs.
    2. (ii)A compound ‘X’ on heating ​at 373 K, loses water molecules and becomes ‘Y’. Substance ‘Y’ is used by doctors for supporting fractured bones in the right position. I. Identify ‘X’ and ‘Y’. II. How can ‘X’ be obtained ​from ‘Y’ ​?
    Show answer

    Copper oxide gives blue-green ${\text{CuCl}}_2$ solution; X is gypsum, Y is Plaster of Paris, and adding water to Y gives X back.

    (i) The compound formed is copper(II) chloride, ${\text{CuCl}}_2$, with water. The copper oxide dissolves and the solution becomes blue-green: $\text{CuO}+2\text{HCl}\to {\text{CuCl}}_2+{\text{H}}_2\text{O}$. (ii) I. X is gypsum, calcium sulphate dihydrate, ${\text{CaSO}}_4\cdot 2{\text{H}}_2\text{O}$; Y is Plaster of Paris, calcium sulphate hemihydrate, ${\text{CaSO}}_4\cdot \frac{1}{2}{\text{H}}_2\text{O}$: ${\text{CaSO}}_4\cdot 2{\text{H}}_2\text{O}\stackrel{373\text{K}}{\to }{\text{CaSO}}_4\cdot \frac{1}{2}{\text{H}}_2\text{O}+1\frac{1}{2}{\text{H}}_2\text{O}$. II. X is obtained from Y by mixing Plaster of Paris with water; it changes back into gypsum, setting as a hard solid mass: ${\text{CaSO}}_4\cdot \frac{1}{2}{\text{H}}_2\text{O}+1\frac{1}{2}{\text{H}}_2\text{O}\to {\text{CaSO}}_4\cdot 2{\text{H}}_2\text{O}$.

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