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 14 of 18

1. Q261 CBSE 2023
   These ​consists of ​two ​statements ​– ​Assertion (A) and Reason (R). Answer this question selecting ​the appropriate option given below ​:

   Assertion (A): It is advised that while diluting an acid one should add water to acid and not acid to water keeping the solution continuously stirred.

   Reason (R): The process of dissolving an acid into water is highly exothermic.

   1. (a)Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of (A).
   2. (b)Both Assertion (A) and Reason (R) are true but Reason (R) is not the correct explanation of (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.

   Step-by-step solution →
2. Q262 CBSE 2023
   1. (i)A compound 'X' which ​is ​prepared from gypsum ​has the property of hardening when ​mixed with proper quantity of water. Identify 'X' and write its chemical ​formula.
   2. (ii)​State the difference in chemical ​composition between baking soda and baking ​powder.
   Show answer

   X is Plaster of Paris, ${\text{CaSO}}_4\cdot \frac{1}{2}{\text{H}}_2\text{O}$; baking soda is one compound, baking powder a mixture with a mild edible acid.

   (i) 'X' is Plaster of Paris, i.e. calcium sulphate hemihydrate, formula ${\text{CaSO}}_4\cdot \frac{1}{2}{\text{H}}_2\text{O}$. It is obtained by heating gypsum, ${\text{CaSO}}_4\cdot 2{\text{H}}_2\text{O}$, to $373$ K. (ii) Baking soda is a single compound, sodium hydrogen carbonate (${\text{NaHCO}}_3$), whereas baking powder is a mixture of sodium hydrogen carbonate and a mild edible acid such as tartaric acid.

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3. Q263 CBSE 2023
   A student ​took a small amount of copper oxide in ​a ​conical flask and added dilute hydrochloric acid to it with constant stirring. He observed a change in colour of ​the ​solution.

   1. (i)Write the name of the ​compound formed and its colour.
   2. (ii)Write a balanced ​chemical equation for the reaction involved.
   Show answer

   Copper(II) chloride, ${\text{CuCl}}_2$, is formed and the solution is blue-green.

   (i) The compound formed is copper(II) chloride (cupric chloride), ${\text{CuCl}}_2$, and the solution turns blue-green. (ii) $\text{CuO}+2\text{HCl}\to {\text{CuCl}}_2+{\text{H}}_2\text{O}$

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4. Q264 CBSE 2023
   On ​heating 'X' at 373 K, ​it ​loses water molecules and becomes 'Y'. 'Y' is a substance ​which doctors use for supporting fractured ​bones in the ​right position.

   1. (i)Identify ​'X' and ​'Y'.
   2. (ii)How can 'X' be reobtained from 'Y' ?
   Show answer

   X is gypsum, ${\text{CaSO}}_4\cdot 2{\text{H}}_2\text{O}$; Y is Plaster of Paris; adding water to Y gives X back.

   (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, ${\text{CaSO}}_4\cdot \frac{1}{2}{\text{H}}_2\text{O}$. (ii) X is reobtained by adding water to Y: ${\text{CaSO}}_4\cdot \frac{1}{2}{\text{H}}_2\text{O}+\frac{3}{2}{\text{H}}_2\text{O}⟶{\text{CaSO}}_4\cdot 2{\text{H}}_2\text{O}$, and the mixture sets into a hard mass of gypsum.

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5. Q265 CBSE 2023
   Write balanced ​chemical equation for ​the reaction that occurs when :

   1. (i)​blue ​coloured ​copper sulphate crystals are heated ​and
   2. (ii)Sodium ​hydrogen carbonate ​is heated during cooking.
   Show answer

   Heating copper sulphate crystals gives anhydrous ${\text{CuSO}}_4$ and water; heating sodium hydrogen carbonate gives ${\text{Na}}_2{\text{CO}}_3$, water and ${\text{CO}}_2$.

   (i) ${\text{CuSO}}_4\cdot 5{\text{H}}_2\text{O}\stackrel{\Delta }{\to }{\text{CuSO}}_4+5{\text{H}}_2\text{O}$ (blue crystals turn white). (ii) $2{\text{NaHCO}}_3\stackrel{\Delta }{\to }{\text{Na}}_2{\text{CO}}_3+{\text{H}}_2\text{O}+{\text{CO}}_2$

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6. Q266 CBSE 2023
   The industrial ​process used for the manufacture of caustic soda involves electrolysis of an ​aqueous solution of compound 'X'. ​In this process, two gases 'Y' and 'Z' are liberated. ​'Y' is liberated at cathode ​and 'Z', which is liberated at ​anode, ​on treatment with dry slaked lime forms a compound 'B'. Name X, Y, Z and ​B.
   Show answer

   X is sodium chloride, Y is hydrogen, Z is chlorine and B is bleaching powder.

   X is sodium chloride (common salt), used as its aqueous solution called brine. Y is hydrogen, ${\text{H}}_2$, liberated at the cathode. Z is chlorine, ${\text{Cl}}_2$, liberated at the anode. B is bleaching powder, calcium oxychloride ${\text{CaOCl}}_2$.

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7. Q267 CBSE 2023
   Two solutions M and N give Red and Blue colour respectively with a universal indicator.

   1. (i)In ​which solution will the hydrogen ion concentration be ​more ​? Justify ​your answer.
   2. (ii)If both M and N solutions are mixed and ​the resultant ​mixture is ​tested with a universal indicator, it turns green. What is the nature of the salt formed ? Justify your answer.
   Show answer

   Solution M has more hydrogen ions (red, low pH); on mixing, the green mixture (pH $7$) shows the salt formed is neutral.

   (i) M. Red with universal indicator means a pH of about $2$, and since $\text{pH}=-{\log }_{10}[{\text{H}}^{+}]$, the lower the pH the larger the hydrogen ion concentration. N is blue, pH about $10$, so its hydrogen ion concentration is far smaller. (ii) The salt formed is neutral. The mixture turns green, which is pH $7$, so the acid M and the base N have neutralised each other completely; since M is a strong acid, a neutral product means N is a strong base, and the salt of a strong acid and a strong base, such as $\text{NaCl}$ from $\text{HCl}$ and $\text{NaOH}$, is neutral.

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8. Q268 CBSE 2023
   A knife which is used to cut a fruit ​was immediately dipped into water ​containing drops of blue litmus solution. If the colour ​of ​the ​solution ​is changed to red, what inference can be drawn about the nature of the ​fruit ​and why ?
   Show answer

   The fruit is acidic: its juice, mixed with the water, turned blue litmus red, and acids turn blue litmus red.

   Inference: the fruit is acidic in nature; its juice contains an acid (citrus fruits such as lemon and orange, for example, contain citric acid). Reason: acids turn blue litmus red. The juice left on the knife mixed with the water, and since the blue litmus solution turned red, the juice must contain an acid. Water alone is neutral and does not change the colour of litmus.

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9. Q269 CBSE 2023
   A chemical ​compound X is used in glass, soap ​and ​paper industries. On treatment with ethanoic ​acid, it forms salt, water and carbon dioxide.

   1. (a)Identify X.
   2. (b)​How is this compound obtained from brine ? Write the equations ​involved.
   3. (c)​State the number of molecules of water ​of crystallization present in compound X.
   Show answer

   X is washing soda, ${\text{Na}}_2{\text{CO}}_3\cdot 10{\text{H}}_2\text{O}$; it is made from brine via sodium hydrogencarbonate and has 10 molecules of water of crystallisation.

   (a) X is washing soda, sodium carbonate decahydrate, ${\text{Na}}_2{\text{CO}}_3\cdot 10{\text{H}}_2\text{O}$. With ethanoic acid: ${\text{Na}}_2{\text{CO}}_3+2{\text{CH}}_3\text{COOH}\to 2{\text{CH}}_3\text{COONa}+{\text{H}}_2\text{O}+{\text{CO}}_2$. (b) Ammonia and carbon dioxide are passed through brine to form sodium hydrogencarbonate: $\text{NaCl}+{\text{H}}_2\text{O}+{\text{CO}}_2+{\text{NH}}_3\to {\text{NH}}_4\text{Cl}+{\text{NaHCO}}_3$. On heating, this gives sodium carbonate: $2{\text{NaHCO}}_3\to {\text{Na}}_2{\text{CO}}_3+{\text{H}}_2\text{O}+{\text{CO}}_2$. Recrystallisation of sodium carbonate gives washing soda: ${\text{Na}}_2{\text{CO}}_3+10{\text{H}}_2\text{O}\to {\text{Na}}_2{\text{CO}}_3\cdot 10{\text{H}}_2\text{O}$. (c) 10 molecules of water of crystallisation.

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10. Q270 CBSE 2023
    Consider the following salts :

    1. (i)$\text{YCl}$
    2. (ii)${\text{NH}}_4\text{X}$
    3. (iii)${\text{ZCO}}_3$ (a) What would be the ​pH of ​the salt solution if ​in $\text{YCl}$, Y is sodium ? Give reason for your answer. (b) If in salt ${\text{NH}}_4\text{X}$, X is nitrate, then its solution will give ​what colour with universal indicator ? Why ? (c) What would be ​the change in ​colour in blue litmus solution if ${\text{ZCO}}_3$ is ​added to it and Z is potassium ?
    Show answer

    Sodium chloride solution has pH 7, ammonium nitrate gives orange with universal indicator, and potassium carbonate leaves blue litmus blue.

    (a) pH 7. With Y = sodium the salt is sodium chloride, $\text{NaCl}$, the salt of a strong acid ($\text{HCl}$) and a strong base ($\text{NaOH}$), so its solution is neutral. (b) Orange. With X = nitrate the salt is ammonium nitrate, ${\text{NH}}_4{\text{NO}}_3$, the salt of a strong acid (${\text{HNO}}_3$) and a weak base (${\text{NH}}_4\text{OH}$), so its solution is acidic, with a pH below 7 (about 5 when dilute); universal indicator therefore shows an acidic colour, orange, not green. (c) No change: the blue litmus stays blue. With Z = potassium the salt is potassium carbonate, ${\text{K}}_2{\text{CO}}_3$, the salt of a weak acid (${\text{H}}_2{\text{CO}}_3$) and a strong base ($\text{KOH}$), so its solution is basic, and a basic solution does not turn blue litmus red (it would turn red litmus blue).

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11. Q271 CBSE 2023
    (a) Write a balanced equation ​to show the reaction that occurs when a piece of aluminium is dipped in a ​dilute solution of

    1. (i)sulphuric acid ​and ​
    2. (ii)sodium hydroxide. (b) Write the ​colour of the solution formed ​when copper oxide is treated ​with ​hydrochloric acid. Give reason for this observation.
    Show answer

    Aluminium gives aluminium sulphate or sodium aluminate with hydrogen; copper oxide with HCl gives a blue-green solution of copper chloride.

    (a) (i) $2\text{Al}(s)+3{\text{H}}_2{\text{SO}}_4(aq)\to {\text{Al}}_2({\text{SO}}_4{)}_3(aq)+3{\text{H}}_2(g)$: aluminium sulphate and hydrogen gas are formed. (ii) $2\text{Al}(s)+2\text{NaOH}(aq)+2{\text{H}}_2\text{O}(l)\to 2{\text{NaAlO}}_2(aq)+3{\text{H}}_2(g)$: sodium aluminate and hydrogen gas are formed. (b) The solution turns blue-green, and the black copper oxide dissolves. Reason: copper oxide is a metal oxide, so it reacts with hydrochloric acid to form a salt, copper(II) chloride, and water, $\text{CuO}+2\text{HCl}\to {\text{CuCl}}_2+{\text{H}}_2\text{O}$; the blue-green colour is due to the copper(II) chloride formed.

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12. Q272 CBSE 2023
    (a) Suggest ​one remedial measure each to counteract the change in pH in human beings in following cases :

    1. (i)Production of too ​much acid in ​stomach during indigestion
    2. (ii)Stung by a honey bee ​/ nettle leaves (b) ​Fresh milk has ​a pH of 6. When it changes into curd will its pH increase or decrease ​? Why ?
    Show answer

    An antacid neutralises stomach acid; baking soda or a dock leaf soothes stings; the pH of milk decreases as lactic acid forms in curd.

    (a)(i) Take an antacid - a mild base such as milk of magnesia (magnesium hydroxide) or baking soda - which neutralises the excess hydrochloric acid in the stomach and brings the pH back towards normal. (a)(ii) A honey bee injects an acid, so rubbing a mild base such as baking soda (sodium hydrogen carbonate) on the stung area neutralises it and gives relief. A nettle leaf injects methanoic acid, and rubbing the area with the leaf of a dock plant, which contains a base, relieves it. (b) The pH will **decrease** (from about $6$ to about $4$). When milk turns into curd, bacteria convert the lactose in it into lactic acid; the milk therefore becomes more acidic, and a more acidic liquid has a lower pH.

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13. Q273 CBSE 2023
    1. (i)Suggest a safe procedure of diluting a ​strong concentrated ​acid.
    2. (ii)Name ​the salt formed when ​sulphuric ​acid is added to sodium ​hydroxide and write its pH.
    3. (iii)Dry HCl gas does ​not change the colour of dry blue litmus paper. Why ?
    Show answer

    Add acid to water slowly, stirring; sodium sulphate has pH 7; dry HCl has no water to ionise in, so dry litmus stays blue.

    (i) Take the water first and add the concentrated acid to it slowly, in a thin stream down the side of the vessel or along a glass rod, stirring constantly; never add water to the concentrated acid. The dilution is highly exothermic, and adding water to the acid releases so much heat in so small a volume that the mixture spits out and can cause burns or crack the container. (ii) The salt formed is sodium sulphate, ${\text{Na}}_2{\text{SO}}_4$, by $2\text{NaOH}+{\text{H}}_2{\text{SO}}_4\to {\text{Na}}_2{\text{SO}}_4+2{\text{H}}_2\text{O}$. Being the salt of a strong acid and a strong base it is neutral, so its pH is $7$. (iii) Blue litmus is turned red by hydrogen ions, and hydrogen chloride releases hydrogen ions only when it dissolves in water. With the gas dry and the litmus dry there is no water, so no ionisation takes place, no ${\text{H}}^{+}$ ions are produced and the colour does not change.

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14. Q274 CBSE 2023
    1. (a)Sometimes ​the pH of our mouth gets lower ​than 5.5. Why ​?
    2. (b)A basic ​salt 'X' is obtained ​by heating baking soda ​followed ​by crystallisation. Identify 'X' and state its two industrial uses.
    3. (c)Why do copper ​sulphate crystals turn white on heating ?
    Show answer

    Bacterial acids lower mouth pH below 5.5; X is washing soda; copper sulphate turns white as heating drives out its water of crystallisation.

    (a) Bacteria in the mouth degrade the sugar and food particles left after eating, and the acids they produce lower the pH below $5.5$; below that value the enamel, which is calcium phosphate, starts to be corroded. (b) X is washing soda, sodium carbonate decahydrate, ${\text{Na}}_2{\text{CO}}_3\cdot 10{\text{H}}_2\text{O}$, formed by $2{\text{NaHCO}}_3\stackrel{\Delta }{\to }{\text{Na}}_2{\text{CO}}_3+{\text{H}}_2\text{O}+{\text{CO}}_2$ followed by crystallisation. Two industrial uses: in the glass, soap and paper industries, and in the manufacture of borax and other sodium compounds. (c) Blue copper sulphate crystals are ${\text{CuSO}}_4\cdot 5{\text{H}}_2\text{O}$ and the blue colour is due to the water of crystallisation. On heating, the five water molecules are driven off, leaving white anhydrous ${\text{CuSO}}_4$.

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15. Q275 CBSE 2023
    You ​have three test tubes containing three colourless liquids, say conc. sulphuric acid, ​conc. solution of sodium hydroxide and conc. solution of sodium sulphate. You also have strips of pH paper.

    1. (a)How would identify these three liquids using pH paper strips ?
    2. (b)Which two ​liquids, when mixed together ​in ​proper ​quantities can form ​a neutral salt solution ?
    3. (c)Write the chemical equation for the reaction that will occur between the ​two liquids mixed in (b).
    Show answer

    pH paper: red for sulphuric acid, dark blue for sodium hydroxide, green for sodium sulphate; the acid with the base gives a neutral salt.

    (a) Put one drop of each liquid on a separate strip of pH paper and match the colour against the standard colour chart. The liquid that turns the paper red (pH about 1) is concentrated sulphuric acid; the liquid that turns it dark blue or violet (pH about 14) is concentrated sodium hydroxide solution; the liquid that turns it green (pH about 7) is concentrated sodium sulphate solution. (b) Sulphuric acid and sodium hydroxide solution. (c) $2\text{NaOH}(aq)+{\text{H}}_2{\text{SO}}_4(aq)⟶{\text{Na}}_2{\text{SO}}_4(aq)+2{\text{H}}_2\text{O}(l)$

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16. Q276 CBSE 2023
    A substance ​‘X’ is used as a building material and is insoluble in water. ​When it reacts with dil. HCl, it produces a gas which turns lime ​water milky.

    1. (i)Write ​the chemical name ​and formula of ‘X’.
    2. (ii)Write chemical ​equations for the chemical ​reactions involved in the above statements.
    Show answer

    X is calcium carbonate, ${\text{CaCO}}_3$; it gives carbon dioxide with dilute HCl, and this gas turns lime water milky.

    (i) ‘X’ is calcium carbonate, ${\text{CaCO}}_3$, met as limestone or marble. (ii) With dilute hydrochloric acid: ${\text{CaCO}}_3(s)+2\text{HCl}(aq)⟶{\text{CaCl}}_2(aq)+{\text{H}}_2\text{O}(l)+{\text{CO}}_2(g)$. The gas turning lime water milky: ${\text{CO}}_2(g)+{\text{Ca(OH)}}_2(aq)⟶{\text{CaCO}}_3(s)+{\text{H}}_2\text{O}(l)$.

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17. Q277 CBSE 2023
    1. (i)Name one antacid. ​State how it helps to relieve indigestion ​in the stomach.
    2. (ii)A student wants to dilute a strong ​acid such ​as sulphuric acid. For this purpose, why should the acid be added ​to water and not ​water to the acid?
    Show answer

    Milk of magnesia neutralises excess stomach acid; acid goes into water so the heat spreads through a large volume and the process stays safe.

    (i) Milk of magnesia, which is magnesium hydroxide ${\text{Mg(OH)}}_2$, is an antacid. It is a mild base, and it neutralises the excess hydrochloric acid secreted in the stomach, raising the pH towards normal and relieving the pain and burning of indigestion: ${\text{Mg(OH)}}_2+2\text{HCl}\to {\text{MgCl}}_2+2{\text{H}}_2\text{O}$. (ii) The dilution of concentrated sulphuric acid is highly exothermic. If water is added to the acid, the small amount of water added has to absorb all the heat, so it is heated violently, may boil and splash the concentrated acid out of the container, causing burns, and the container may crack. Adding the acid slowly to a large volume of water, with constant stirring, lets the heat spread through the whole of the water, so the temperature rise is small and the process is safe.

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18. Q278 CBSE 2023
    1. (i)What property do acids and bases have in common ​? Explain it with ​an example.
    2. (ii)A compound which is prepared from ​gypsum has the property of hardening when mixed ​with water. Identify the compound and write its formula. How is ​this compound prepared ​? ​Describe it in the form of a ​chemical equation only.
    Show answer

    Acids and bases both give ions in water and conduct electricity; the compound is plaster of Paris, made by heating gypsum at 373 K.

    (i) Both acids and bases produce ions when dissolved in water, an acid giving ${\text{H}}^{+}(aq)$ ions and a base giving ${\text{OH}}^{-}(aq)$ ions, so the aqueous solutions of both conduct electricity. Example: dilute hydrochloric acid, $\text{HCl}(aq)\to {\text{H}}^{+}(aq)+{\text{Cl}}^{-}(aq)$, and sodium hydroxide solution, $\text{NaOH}(aq)\to {\text{Na}}^{+}(aq)+{\text{OH}}^{-}(aq)$, each make a bulb glow when placed in a circuit. (They also share other properties, such as changing the colour of indicators.) (ii) The compound is plaster of Paris, calcium sulphate hemihydrate, ${\text{CaSO}}_4\cdot \frac{1}{2}{\text{H}}_2\text{O}$. It is prepared by heating gypsum at $373$ K: ${\text{CaSO}}_4\cdot 2{\text{H}}_2\text{O}\stackrel{373\text{K}}{\to }{\text{CaSO}}_4\cdot \frac{1}{2}{\text{H}}_2\text{O}+1\frac{1}{2}{\text{H}}_2\text{O}$.

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19. Q279 CBSE 2023
    A metal ‘M’ ​on ​reacting with dilute ​acid liberates a gas ‘G’. ​The same metal also liberates gas ‘G’ when reacts with ​a base.

    1. (i)Write the name of ​gas ‘G’.
    2. (ii)How will you test the presence of ​this gas?
    3. (iii)Write chemical equations for the reactions of the metal with (1) an acid and (2) a base.
    Show answer

    G is hydrogen, which burns with a pop sound; with zinc as M, acid and base both give hydrogen.

    (i) The gas ‘G’ is hydrogen. (ii) Bring a burning candle or splint near the mouth of the tube holding the gas: it burns with a pop sound, which is the test for hydrogen. (iii) Taking the metal ‘M’ as zinc — (1) with an acid: $\text{Zn}(s)+2\text{HCl}(aq)⟶{\text{ZnCl}}_2(aq)+{\text{H}}_2(g)$; (2) with a base: $\text{Zn}(s)+2\text{NaOH}(aq)⟶{\text{Na}}_2{\text{ZnO}}_2(aq)+{\text{H}}_2(g)$, the salt being sodium zincate.

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20. Q280 CBSE 2023
    A student dropped a few ​pieces ​of white marble in ​dilute hydrochloric acid contained in a test tube. The gas ​evolved was then passed through freshly prepared lime water.

    1. (i)Name the gas evolved.
    2. (ii)State the change the student would ​observe in lime water.
    3. (iii)What change would be observed if excess ​of this gas is passed through lime water? Give reason for your answer in ​(ii) and (iii) given above.
    Show answer

    The gas is carbon dioxide; lime water turns milky (insoluble calcium carbonate), then clear with excess gas (soluble calcium hydrogencarbonate).

    (i) The gas evolved is carbon dioxide, ${\text{CO}}_2$, from ${\text{CaCO}}_3+2\text{HCl}\to {\text{CaCl}}_2+{\text{H}}_2\text{O}+{\text{CO}}_2$. (ii) The lime water turns milky. Reason: carbon dioxide reacts with calcium hydroxide to form calcium carbonate, ${\text{Ca(OH)}}_2+{\text{CO}}_2\to {\text{CaCO}}_3\downarrow +{\text{H}}_2\text{O}$, and this calcium carbonate is insoluble in water, so it stays suspended and makes the liquid look milky. (iii) On passing excess carbon dioxide the milkiness disappears and the solution becomes clear again. Reason: the insoluble calcium carbonate reacts with the excess gas and water to form calcium hydrogencarbonate, ${\text{CaCO}}_3+{\text{CO}}_2+{\text{H}}_2\text{O}\to {\text{Ca(HCO}}_3{)}_2$, which is soluble in water.

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