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 16 of 18

1. Q301 CBSE 2020
   List the important products of the Chlor-alkali process. ​Write ​one ​important ​use ​of ​each.
   Show answer

   The chlor-alkali process gives hydrogen (fuel), chlorine (water treatment) and sodium hydroxide (soaps and detergents).

   The chlor-alkali process is the electrolysis of brine: $2\text{NaCl}+2{\text{H}}_2\text{O}\to 2\text{NaOH}+{\text{Cl}}_2+{\text{H}}_2$. Its important products and one use of each: (1) Hydrogen: as a fuel, or in making margarine or ammonia for fertilisers. (2) Chlorine: in water treatment and swimming pools, or in making PVC, disinfectants, CFCs or pesticides. (3) Sodium hydroxide: for de-greasing metals, or in making soaps and detergents, paper or artificial fibres. One use of each is enough; any other correct use also earns credit.

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2. Q302 CBSE 2020
   Give reasons for ​the following :

   1. (i)Only one half ​of ​water molecule is shown in ​the formula of ​Plaster of Paris.
   2. (ii)Sodium hydrogen carbonate is used as an antacid.
   3. (iii)On strong ​heating, blue coloured copper sulphate crystals ​turn white.
   Show answer

   Plaster of Paris has half a water molecule per unit; sodium hydrogen carbonate neutralises stomach acid; heating removes copper sulphate's water of crystallisation.

   (i) Plaster of Paris is calcium sulphate hemihydrate, ${\text{CaSO}}_4\cdot \frac{1}{2}{\text{H}}_2\text{O}$. Two formula units of ${\text{CaSO}}_4$ share one molecule of water of crystallisation, so per formula unit there is half a water molecule. (ii) Sodium hydrogen carbonate is alkaline (a mild base), so it neutralises the excess hydrochloric acid in the stomach and relieves acidity: ${\text{NaHCO}}_3+\text{HCl}⟶\text{NaCl}+{\text{H}}_2\text{O}+{\text{CO}}_2$. (iii) Blue copper sulphate crystals contain water of crystallisation, ${\text{CuSO}}_4\cdot 5{\text{H}}_2\text{O}$. Strong heating removes this water and leaves white anhydrous copper sulphate: ${\text{CuSO}}_4\cdot 5{\text{H}}_2\text{O}\stackrel{\text{heat}}{\to }{\text{CuSO}}_4+5{\text{H}}_2\text{O}$.

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3. Q303 CBSE 2020
   Complete and ​balance ​the ​following chemical equations :

   1. (i)${\text{NaOH}}_{(\text{aq})}+{\text{Zn}}_{(\text{s})}\to$
   2. (ii)${\text{CaCO}}_{3(\text{s})}+{\text{H}}_2{\text{O}}_{(l)}+{\text{CO}}_{2(\text{g})}\to$
   3. (iii)${\text{HCl}}_{(\text{aq})}+{\text{H}}_2{\text{O}}_{(l)}\to$
   Show answer

   The products are sodium zincate with hydrogen, calcium hydrogencarbonate, and hydronium with chloride ions.

   (i) $2{\text{NaOH}}_{(\text{aq})}+{\text{Zn}}_{(\text{s})}\to {\text{Na}}_2{\text{ZnO}}_{2(\text{aq})}+{\text{H}}_{2(\text{g})}$ (sodium zincate and hydrogen gas) (ii) ${\text{CaCO}}_{3(\text{s})}+{\text{H}}_2{\text{O}}_{(l)}+{\text{CO}}_{2(\text{g})}\to {\text{Ca(HCO}}_3{)}_{2(\text{aq})}$ (calcium hydrogencarbonate, soluble in water) (iii) ${\text{HCl}}_{(\text{aq})}+{\text{H}}_2{\text{O}}_{(l)}\to {\text{H}}_3{\text{O}}_{(\text{aq})}^{+}+{\text{Cl}}_{(\text{aq})}^{-}$ (hydronium ion and chloride ion)

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4. Q304 CBSE 2020
   How is washing soda prepared ​from sodium ​carbonate? ​Give its chemical equation. State the type of this salt. Name the type ​of ​hardness of ​water ​which can be ​removed by it?
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   Recrystallising sodium carbonate from water gives washing soda; it is a basic salt and removes permanent hardness of water.

   Washing soda is prepared by recrystallisation of sodium carbonate: sodium carbonate is dissolved in water and crystallised, and each formula unit takes up ten molecules of water of crystallisation. ${\text{Na}}_2{\text{CO}}_3+10{\text{H}}_2\text{O}\to {\text{Na}}_2{\text{CO}}_3\cdot 10{\text{H}}_2\text{O}$ It is a basic salt (the salt of the strong base sodium hydroxide and the weak acid carbonic acid). It removes the permanent hardness of water.

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5. Q305 CBSE 2020
   1. (i)Draw a labelled diagram to show the preparation ​of hydrogen ​chloride gas in laboratory.
   2. (ii)Test the gas evolved first with dry ​and then with wet litmus paper. In which of the ​two cases, does ​the litmus paper show ​change in colour ?
   3. (iii)State the reason of exhibiting ​acidic character by dry $\text{HCl}$ gas / $\text{HCl}$ solution.
   Show answer

   HCl is made from NaCl and concentrated sulphuric acid; only wet litmus turns red, since HCl forms ${\text{H}}^{+}$ ions only in water.

   (i) Labelled diagram: solid sodium chloride and concentrated sulphuric acid in a test tube closed by a cork; a delivery tube passes through the cork and carries the hydrogen chloride gas out, through a guard tube of anhydrous calcium chloride that dries it. $\text{NaCl}(s)+{\text{H}}_2{\text{SO}}_4(\text{conc.})⟶{\text{NaHSO}}_4+\text{HCl}(g)$ (ii) The colour changes only in the second case: wet blue litmus paper turns red, while dry litmus paper shows no change. (iii) Dry HCl gas shows no acidic character, because without water it does not form ions. In water (HCl solution) it gives ${\text{H}}^{+}$ ions, present as hydronium ions, ${\text{H}}_3{\text{O}}^{+}$: $\text{HCl}+{\text{H}}_2\text{O}⟶{\text{H}}_3{\text{O}}^{+}+{\text{Cl}}^{-}$. These ions give HCl solution its acidic character.

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6. Q306 CBSE 2020
   During electrolysis of brine, a gas ​‘G’ ​is liberated at anode. When this ​gas ‘G’ is passed through slaked lime, a ​compound ‘C’ is formed, which is used for disinfecting drinking water.

   1. (i)​Write formula of ‘G’ ​and ‘C’.
   2. (ii)State ​the chemical equation involved.
   3. (iii)What is common name of compound ‘C’ ? Give its ​chemical name.
   Show answer

   G is chlorine, ${\text{Cl}}_2$, and C is bleaching powder (calcium oxychloride), ${\text{CaOCl}}_2$, formed from chlorine and slaked lime.

   (i) G = ${\text{Cl}}_2$ (chlorine gas); C = ${\text{CaOCl}}_2$. (ii) ${\text{Ca(OH)}}_2+{\text{Cl}}_2\to {\text{CaOCl}}_2+{\text{H}}_2\text{O}$ (iii) Common name: bleaching powder; chemical name: calcium oxychloride.

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7. Q307 CBSE 2020
   A chemical compound ​‘X’ is used ​in the soap and glass industry. It is prepared from ​brine.

   1. (a)Write ​the chemical name, common name and chemical formula of ‘X’.
   2. (b)Write the equation involved ​in its preparation.
   3. (c)What happens when it ​is treated with water ​containing Ca or ​Mg salts ?
   Show answer

   X is washing soda, ${\text{Na}}_2{\text{CO}}_3\cdot 10{\text{H}}_2\text{O}$, made from brine; it removes permanent hardness by precipitating calcium and magnesium carbonates.

   (a) Chemical name: sodium carbonate decahydrate; common name: washing soda; chemical formula: ${\text{Na}}_2{\text{CO}}_3\cdot 10{\text{H}}_2\text{O}$. (b) $\text{NaCl}+{\text{H}}_2\text{O}+{\text{CO}}_2+{\text{NH}}_3⟶{\text{NH}}_4\text{Cl}+{\text{NaHCO}}_3$; then $2{\text{NaHCO}}_3\stackrel{\text{heat}}{\to }{\text{Na}}_2{\text{CO}}_3+{\text{H}}_2\text{O}+{\text{CO}}_2$; then ${\text{Na}}_2{\text{CO}}_3+10{\text{H}}_2\text{O}⟶{\text{Na}}_2{\text{CO}}_3\cdot 10{\text{H}}_2\text{O}$. (c) It removes the permanent hardness of the water: the calcium and magnesium salts are converted into insoluble calcium carbonate and magnesium carbonate, which separate out, for example ${\text{CaCl}}_2+{\text{Na}}_2{\text{CO}}_3⟶{\text{CaCO}}_3+2\text{NaCl}$.

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8. Q308 CBSE 2020
   1. (a)What ​is ‘Water of ​crystalisation’ ?
   2. (b)With ​the help of equations, state what happens when (i) baking ​soda is heated ​during cooking ? (ii) gypsum is ​heated at 373 K ?
   Show answer

   Water of crystallisation is the fixed number of water molecules per formula unit; baking soda gives sodium carbonate, gypsum gives Plaster of Paris.

   (a) Water of crystallisation is the fixed number of water molecules present in one formula unit of a salt, for example the five water molecules in each formula unit of blue copper sulphate crystals, ${\text{CuSO}}_4\cdot 5{\text{H}}_2\text{O}$. (b) (i) When baking soda is heated, it decomposes into sodium carbonate, water and carbon dioxide; the carbon dioxide makes the cake or bread rise and become soft and spongy: $2{\text{NaHCO}}_3\stackrel{\text{heat}}{\to }{\text{Na}}_2{\text{CO}}_3+{\text{H}}_2\text{O}+{\text{CO}}_2$ (ii) When gypsum is heated at $373$ K, it loses one and a half of its two water molecules per formula unit and becomes Plaster of Paris (calcium sulphate hemihydrate): ${\text{CaSO}}_4\cdot 2{\text{H}}_2\text{O}\stackrel{373\text{K}}{\to }{\text{CaSO}}_4\cdot \frac{1}{2}{\text{H}}_2\text{O}+1\frac{1}{2}{\text{H}}_2\text{O}$

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9. Q309 CBSE 2020
   With ​the help of labelled diagram, show ​an experimental ​setup for the reaction of Magnesium with dilute sulphuric ​acid. Give equation ​of the reaction ​involved.
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   Magnesium and dilute sulphuric acid give hydrogen, which fills soap bubbles and burns with a pop sound near a flame.

   Set-up (labelled diagram): a few pieces of magnesium ribbon are taken in a test tube held in a clamp on a stand, and dilute sulphuric acid is added. The mouth of the test tube is closed with a cork carrying a delivery tube, whose inner end is above the acid and whose outer end dips into soap solution in a trough. The gas formed passes through the delivery tube and fills soap bubbles; when a burning candle is brought near a gas-filled bubble, the gas burns with a pop sound, which shows that it is hydrogen. Labels: stand, clamp, test tube, magnesium ribbon, dilute sulphuric acid, cork, delivery tube, trough, soap solution, bubbles filled with hydrogen gas, burning candle. Equation: $\text{Mg}(s)+{\text{H}}_2{\text{SO}}_4(aq)⟶{\text{MgSO}}_4(aq)+{\text{H}}_2(g)\uparrow$

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10. Q310 CBSE 2020
    A cloth ​strip dipped in onion juice is used for ​testing a liquid ‘X’. The liquid ​‘X’ changes its odour. Which type ​of an indicator is ​onion juice ? The liquid ‘X’ turns blue litmus red. List ​the observations the ​liquid ‘X’ will show on reacting with the following :

    1. (a)Zinc granules
    2. (b)Solid sodium carbonate Write the chemical equations for the reactions involved.
    Show answer

    Onion juice is an olfactory indicator and X is an acid; with zinc it gives hydrogen, with sodium carbonate carbon dioxide.

    Onion juice is an olfactory indicator (a substance whose odour changes in an acidic or a basic medium; the smell of onion is lost in a basic medium, while an acid leaves it unchanged). Since X turns blue litmus red, X is an acid (for example dilute $\text{HCl}$). (a) With zinc granules, bubbles of a colourless, odourless gas are evolved; the gas is hydrogen and burns with a pop sound: $\text{Zn}(s)+2\text{HCl}(aq)⟶{\text{ZnCl}}_2(aq)+{\text{H}}_2(g)$. (b) With solid sodium carbonate there is brisk effervescence; a colourless, odourless gas, carbon dioxide, is evolved and turns lime water milky: ${\text{Na}}_2{\text{CO}}_3(s)+2\text{HCl}(aq)⟶2\text{NaCl}(aq)+{\text{H}}_2\text{O}(l)+{\text{CO}}_2(g)$, and ${\text{Ca(OH)}}_2(aq)+{\text{CO}}_2(g)⟶{\text{CaCO}}_3(s)+{\text{H}}_2\text{O}(l)$.

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11. Q311 CBSE 2020
    (a) ​Name the gas liberated when ​an acid reacts with a metal. How is ​the presence ​of this gas tested ? (b) What happens when zinc reacts with

    1. (a)$\text{HCl}$, and
    2. (b)$\text{NaOH}$ ? Write the chemical ​equation and ​name of the salt produced in each case.
    3. (c)Identify the acid and ​base for ammonium chloride salt. Write the nature ​(acidic/basic/neutral) and pH value of this salt.
    Show answer

    Hydrogen (pop test); zinc forms zinc chloride with $\text{HCl}$ and sodium zincate with $\text{NaOH}$; ammonium chloride is acidic, pH below 7.

    (a) Hydrogen gas (${\text{H}}_2$) is liberated. Test: when a burning candle or matchstick is brought near the gas (for example, near a soap bubble filled with it), the gas burns with a pop sound. (b) With $\text{HCl}$: zinc reacts to form zinc chloride and hydrogen gas, $\text{Zn}+2\text{HCl}⟶{\text{ZnCl}}_2+{\text{H}}_2\uparrow$; the salt is zinc chloride. With $\text{NaOH}$ (on warming): zinc reacts to form sodium zincate and hydrogen gas, $\text{Zn}+2\text{NaOH}⟶{\text{Na}}_2{\text{ZnO}}_2+{\text{H}}_2\uparrow$; the salt is sodium zincate. (c) Acid: hydrochloric acid ($\text{HCl}$); base: ammonium hydroxide (${\text{NH}}_4\text{OH}$). Ammonium chloride is the salt of a strong acid and a weak base, so it is acidic in nature and its pH is less than $7$.

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12. Q312 CBSE 2020
    Define water ​of ​crystallisation. ​Give the chemical formula for two compounds ​as examples. How can it ​be proved ​that ​the water of crystallisation ​makes a difference in the state and colour of the compounds ?
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    Water of crystallisation is the fixed number of water molecules in a formula unit; heating blue ${\text{CuSO}}_4\cdot 5{\text{H}}_2\text{O}$ turns it into white powder.

    Water of crystallisation is the fixed number of water molecules present in one formula unit of a salt. Examples: copper sulphate pentahydrate, ${\text{CuSO}}_4\cdot 5{\text{H}}_2\text{O}$, and washing soda, ${\text{Na}}_2{\text{CO}}_3\cdot 10{\text{H}}_2\text{O}$ (gypsum, ${\text{CaSO}}_4\cdot 2{\text{H}}_2\text{O}$, is another). Proof: heat a few crystals of hydrated copper sulphate in a dry boiling tube. Water droplets appear on the cooler part of the tube. Colour: the blue copper sulphate turns white. State: the blue crystals change into a white powder (anhydrous copper sulphate), ${\text{CuSO}}_4\cdot 5{\text{H}}_2\text{O}\stackrel{\text{heat}}{\to }{\text{CuSO}}_4+5{\text{H}}_2\text{O}$. On adding a few drops of water to the cooled powder the blue colour reappears, so the difference is due to the water of crystallisation.

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13. Q313 CBSE 2020
    1. (a)Identify the acid and the base whose combination forms the common salt. Write ​the formula and ​chemical ​name of this salt. Name the source ​from ​which it is obtained.
    2. (b)What is rock salt ? Mention its colour and the reason it is due to.
    3. (c)​What happens when an electric current is passed through ​brine ? Write the chemical equation for it.
    Show answer

    Common salt $\text{NaCl}$ forms from $\text{HCl}$ and $\text{NaOH}$; rock salt is often brown due to impurities; brine electrolysis gives $\text{NaOH}$, ${\text{Cl}}_2$ and ${\text{H}}_2$.

    (a) Common salt is formed by the combination of hydrochloric acid ($\text{HCl}$) and sodium hydroxide ($\text{NaOH}$): $\text{HCl}+\text{NaOH}⟶\text{NaCl}+{\text{H}}_2\text{O}$. Formula: $\text{NaCl}$; chemical name: sodium chloride. Source: sea water (it is also mined as rock salt). (b) Rock salt is common salt (sodium chloride) found as deposits of solid salt in the form of large crystals, formed when seas of bygone ages dried up; it is mined like coal. Its colour is often brown, and the colour is due to impurities. (c) The brine decomposes: sodium hydroxide is formed, chlorine gas is given off at the anode and hydrogen gas at the cathode (the chlor-alkali process). $2\text{NaCl}(aq)+2{\text{H}}_2\text{O}(l)⟶2\text{NaOH}(aq)+{\text{Cl}}_2(g)+{\text{H}}_2(g)$

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14. Q314 CBSE 2019
    A metal compound X ​reacts with dilute hydrochloric ​acid to produce ​brisk effervescence. The gas evolved forms a white precipitate ​when passed through lime water. ​Write balanced chemical equations involved in ​the above mentioned ​chemical ​reactions.
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    X is a metal carbonate such as ${\text{CaCO}}_3$; it gives ${\text{CO}}_2$, which turns lime water milky.

    The gas turns lime water milky, so it is carbon dioxide, and X is a metal carbonate, for example calcium carbonate: ${\text{CaCO}}_3(s)+2\text{HCl}(aq)⟶{\text{CaCl}}_2(aq)+{\text{H}}_2\text{O}(l)+{\text{CO}}_2(g)$ ${\text{Ca(OH)}}_2(aq)+{\text{CO}}_2(g)⟶{\text{CaCO}}_3(s)+{\text{H}}_2\text{O}(l)$ (white precipitate) Any metal carbonate or hydrogencarbonate is accepted for X; with sodium carbonate the first equation is ${\text{Na}}_2{\text{CO}}_3+2\text{HCl}⟶2\text{NaCl}+{\text{H}}_2\text{O}+{\text{CO}}_2$.

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15. Q315 CBSE 2019
    What is ​water of ​crystallisation ? Name ​and give ​the formula ​of a salt which ​contains ​water of crystallisation.
    Show answer

    Fixed number of water molecules in one formula unit of a crystalline salt; example: copper sulphate pentahydrate, ${\text{CuSO}}_4\cdot 5{\text{H}}_2\text{O}$.

    Water of crystallisation is the fixed number of water molecules present in one formula unit of a salt in its crystalline form. Example: copper sulphate pentahydrate (blue vitriol), ${\text{CuSO}}_4\cdot 5{\text{H}}_2\text{O}$, which has five molecules of water of crystallisation in each formula unit. (Washing soda, ${\text{Na}}_2{\text{CO}}_3\cdot 10{\text{H}}_2\text{O}$, and gypsum, ${\text{CaSO}}_4\cdot 2{\text{H}}_2\text{O}$, are also correct examples.)

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16. Q316 CBSE 2019
    Why is sodium hydrogen carbonate an ​alkaline ​salt ​? ​List its ​two ​important ​uses.
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    Sodium hydrogen carbonate, a salt of a strong base and a weak acid, is alkaline; used in baking powder and as an antacid.

    Sodium hydrogen carbonate (${\text{NaHCO}}_3$) is the salt of a strong base, sodium hydroxide ($\text{NaOH}$), and a weak acid, carbonic acid (${\text{H}}_2{\text{CO}}_3$). The strong base outweighs the weak acid, so its solution in water is basic, with pH above $7$ (about $8.3$): it is an alkaline salt. Uses (any two): (i) In baking powder, to make cakes and bread soft and spongy, because it gives off carbon dioxide; (ii) As an antacid, to neutralise excess acid in the stomach; (iii) In soda-acid fire extinguishers.

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17. Q317 CBSE 2019
    Out of $\text{HCl}$ and ${\text{CH}}_3\text{COOH}$, which one is ​a weak ​acid ​and ​why ? Explain with ​the ​help ​of an ​example.
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    Acetic acid, ${\text{CH}}_3\text{COOH}$, is the weak acid because it ionises only partly in water.

    Acetic acid (${\text{CH}}_3\text{COOH}$) is the weak acid. It ionises only partly in water, so its solution contains a much lower concentration of hydronium ions (${\text{H}}_3{\text{O}}^{+}$) than hydrochloric acid of the same concentration, which ionises completely. Example: zinc granules react vigorously with dilute hydrochloric acid, giving brisk effervescence of hydrogen, but only slowly with dilute acetic acid of the same concentration. Other correct examples also earn credit, such as universal indicator showing a lower pH for hydrochloric acid than for acetic acid of the same concentration.

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18. Q318 CBSE 2019
    What ​is ​brine ​? ​What happens when an electric current ​is passed through ​it ? Write chemical ​equation for it.
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    Brine is concentrated aqueous sodium chloride; electrolysis gives sodium hydroxide, chlorine and hydrogen: $2\text{NaCl}+2{\text{H}}_2\text{O}⟶2\text{NaOH}+{\text{Cl}}_2+{\text{H}}_2$.

    Brine is a concentrated aqueous solution of sodium chloride (common salt), $\text{NaCl}$. When an electric current is passed through brine, it decomposes to form sodium hydroxide, chlorine gas and hydrogen gas: chlorine is given off at the anode, hydrogen at the cathode, and sodium hydroxide solution is formed near the cathode. This is the chlor-alkali process. $2\text{NaCl(aq)}+2{\text{H}}_2\text{O(l)}⟶2\text{NaOH(aq)}+{\text{Cl}}_2\text{(g)}+{\text{H}}_2\text{(g)}$

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19. Q319 CBSE 2019
    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.
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    Methanoic (formic) acid, $\text{HCOOH}$; relief by rubbing a mild base such as baking soda, which neutralises the acid.

    The acid present in an ant sting is methanoic acid (formic acid), $\text{HCOOH}$. To get relief, the stung area is rubbed with a mild base such as baking soda (sodium hydrogencarbonate) solution, which neutralises the acid.

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20. Q320 CBSE 2019
    “Sodium hydrogen carbonate is ​a ​basic salt.” Justify this ​statement. ​How ​is it ​converted into washing soda ?
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    Sodium hydrogen carbonate is a basic salt (strong base, weak acid); heating it gives sodium carbonate, which recrystallises to washing soda.

    Sodium hydrogen carbonate is the salt of a strong base, sodium hydroxide ($\text{NaOH}$), and a weak acid, carbonic acid (${\text{H}}_2{\text{CO}}_3$). Such a salt gives a basic solution (pH about 8; it turns red litmus blue), so ${\text{NaHCO}}_3$ is a basic salt. Conversion into washing soda: on heating, sodium hydrogen carbonate decomposes into sodium carbonate, $2{\text{NaHCO}}_3\stackrel{\text{heat}}{\to }{\text{Na}}_2{\text{CO}}_3+{\text{H}}_2\text{O}+{\text{CO}}_2$; recrystallising the sodium carbonate from water gives washing soda, ${\text{Na}}_2{\text{CO}}_3+10{\text{H}}_2\text{O}⟶{\text{Na}}_2{\text{CO}}_3\cdot 10{\text{H}}_2\text{O}$.

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