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 1 of 11

1. Q1 NCERT · Exercises
   A ​solution ​turns ​red ​litmus blue, its pH ​is likely to ​be

   1. (a)$1$
   2. (b)$4$
   3. (c)$5$
   4. (d)$10$
   Show answer

   (d) $10$

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2. Q2 NCERT · Intext Questions (Page 18)
   You have ​been provided with three ​test tubes. One of them ​contains distilled water and ​the other two contain an acidic solution and a basic solution, respectively. If you are given only red litmus ​paper, how ​will you identify the contents ​of each test tube?
   Show answer

   Red litmus turning blue shows the base; that blue strip turns red in the acid and stays blue in distilled water.

   Red litmus paper turns blue only in a basic solution; an acidic solution and distilled water both leave it red, so red litmus alone cannot separate those two. Step 1: Dip a strip of red litmus paper into each of the three test tubes (a fresh strip for each). The tube in which the paper turns blue contains the basic solution. Step 2: The strip that has turned blue is now blue litmus paper. Cut it in two and touch it to the two remaining solutions. The solution that turns this blue paper red is the acidic solution, and the solution that leaves it blue is the distilled water. (One test in Step 2 is enough: whichever tube reacts, the third tube is the other one.)

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3. Q3 NCERT · Exercises
   A solution reacts ​with ​crushed egg-shells to ​give ​a gas ​that ​turns lime-water ​milky. The solution contains

   1. (a)$\text{NaCl}$
   2. (b)$\text{HCl}$
   3. (c)$\text{LiCl}$
   4. (d)$\text{KCl}$
   Show answer

   (b) $\text{HCl}$

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4. Q4 NCERT · Exercises
   $10$ mL of a solution of $\text{NaOH}$ ​is found ​to be completely neutralised by $8$ mL ​of a given solution of $\text{HCl}$. If we take $20$ mL of the same solution of $\text{NaOH}$, ​the amount $\text{HCl}$ solution (the same solution ​as before) required ​to neutralise it will be

   1. (a)$4$ mL
   2. (b)$8$ mL
   3. (c)$12$ mL
   4. (d)$16$ mL
   Show answer

   (d) $16$ mL

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5. Q5 NCERT · Exercises
   Which ​one ​of ​the following ​types of ​medicines ​is used for ​treating indigestion?

   1. (a)Antibiotic
   2. (b)Analgesic
   3. (c)Antacid
   4. (d)Antiseptic
   Show answer

   (c) Antacid

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6. Q6 NCERT · Exercises
   Write word ​equations and then ​balanced equations for the reaction taking place when –

   1. (a)dilute sulphuric acid reacts with zinc granules.
   2. (b)dilute ​hydrochloric acid reacts with magnesium ​ribbon.
   3. (c)dilute sulphuric acid ​reacts with aluminium powder. ​
   4. (d)​dilute hydrochloric acid reacts with iron filings.
   Show answer

   Each metal displaces hydrogen from the dilute acid, giving ${\text{ZnSO}}_4$, ${\text{MgCl}}_2$, ${\text{Al}}_2({\text{SO}}_4{)}_3$ and ${\text{FeCl}}_2$ respectively.

   (a) $\text{Zinc}+\text{Dilute sulphuric acid}⟶\text{Zinc sulphate}+\text{Hydrogen}$; balanced: $\text{Zn}(s)+{\text{H}}_2{\text{SO}}_4(aq)⟶{\text{ZnSO}}_4(aq)+{\text{H}}_2(g)$. (b) $\text{Magnesium}+\text{Dilute hydrochloric acid}⟶\text{Magnesium chloride}+\text{Hydrogen}$; balanced: $\text{Mg}(s)+2\text{HCl}(aq)⟶{\text{MgCl}}_2(aq)+{\text{H}}_2(g)$. (c) $\text{Aluminium}+\text{Dilute sulphuric acid}⟶\text{Aluminium sulphate}+\text{Hydrogen}$; balanced: $2\text{Al}(s)+3{\text{H}}_2{\text{SO}}_4(aq)⟶{\text{Al}}_2({\text{SO}}_4{)}_3(aq)+3{\text{H}}_2(g)$. (d) $\text{Iron}+\text{Dilute hydrochloric acid}⟶\text{Iron(II) chloride}+\text{Hydrogen}$; balanced: $\text{Fe}(s)+2\text{HCl}(aq)⟶{\text{FeCl}}_2(aq)+{\text{H}}_2(g)$.

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7. Q7 NCERT · Exercises
   Compounds such ​as ​alcohols ​and ​glucose ​also contain hydrogen ​but ​are not ​categorised as acids. Describe an Activity to prove it.
   Show answer

   Glucose and alcohol solutions do not light the bulb in the circuit that glows with acid, as they release no ${\text{H}}^{+}$ ions.

   Take two iron nails about $1$ cm apart in a rubber cork, fit the cork into a beaker, and connect the nails through a bulb and a switch to a $6$ V battery. Pour in dilute hydrochloric acid so that it covers the nails and switch on: the bulb glows, because the acid ionises to give free ${\text{H}}^{+}$ and ${\text{Cl}}^{-}$ ions that carry the current. Now wash and dry the beaker and nails and repeat with glucose solution, and then with alcohol: in both cases the bulb does not glow. Alcohol and glucose dissolve as whole molecules and release no ${\text{H}}^{+}$ ions, so their solutions do not conduct. Since acidic behaviour comes from free hydrogen ions in solution and not from hydrogen atoms in a formula, alcohols and glucose are not classified as acids even though their molecules contain hydrogen.

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8. Q8 NCERT · Exercises
   Why ​does distilled water not ​conduct ​electricity, whereas rain water ​does?
   Show answer

   Distilled water has practically no free ions, whereas rain water contains ions from dissolved carbon dioxide (carbonic acid), so only rain water conducts.

   A liquid conducts electricity only when it contains ions that are free to move. Distilled water is almost pure ${\text{H}}_2\text{O}$ — distillation leaves the dissolved salts behind and drives off the dissolved gases — and water itself ionises only to a minute extent, about $10^{-7}$ mol per litre, so there are practically no charge carriers and no current passes. Rain water, on the other hand, dissolves carbon dioxide and other gases from the air as it falls; the carbon dioxide forms carbonic acid, ${\text{CO}}_2+{\text{H}}_2\text{O}\rightleftharpoons {\text{H}}_2{\text{CO}}_3$, which ionises to give ${\text{H}}^{+}$ and ${\text{HCO}}_3^{-}$ ions. These free, mobile ions carry the charge, so rain water conducts electricity.

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9. Q9 NCERT · Exercises
   Why does ​dry $\text{HCL}$ gas ​not show acidic behaviour ​in ​the absence ​of ​water?
   Show answer

   Dry $\text{HCl}$ has no ${\text{H}}^{+}$ ions; only in water does it ionise to give the hydrogen ions that cause acidity.

   Acidic behaviour is the behaviour of hydrogen ions in solution, not of hydrogen atoms in a molecule. Dry hydrogen chloride gas consists of covalent $\text{H}-\text{Cl}$ molecules in which the hydrogen is bonded to the chlorine, so no ${\text{H}}^{+}$ ions are present and none of the acid tests responds — dry blue litmus held in the gas, for instance, stays blue. When water is added, the polar water molecules pull each molecule apart, $\text{HCl}(g)+{\text{H}}_2\text{O}(l)⟶{\text{H}}_3{\text{O}}^{+}(aq)+{\text{Cl}}^{-}(aq)$, and the hydrogen ions so produced give the solution all its acidic properties. Water is therefore a reactant in the production of the acidic species, not merely the medium it sits in.

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10. Q10 NCERT · Exercises
    Five ​solutions A, B, C, D and ​E when tested with universal indicator showed pH as $4$, $1$, $11$, $7$ ​and $9$, respectively. Which solution is

    1. (a)neutral? ​
    2. (b)​strongly alkaline?
    3. (c)strongly acidic?
    4. (d)weakly acidic?
    5. (e)weakly alkaline? Arrange the ​pH in increasing ​order of hydrogen-ion concentration.
    Show answer

    Neutral D, strongly alkaline C, strongly acidic B, weakly acidic A, weakly alkaline E; increasing $[{\text{H}}^{+}]$: C, E, D, A, B.

    (a) D, whose pH is $7$; (b) C, whose pH is $11$; (c) B, whose pH is $1$; (d) A, whose pH is $4$; (e) E, whose pH is $9$. Arranged in increasing order of hydrogen-ion concentration the solutions run C, E, D, A, B — that is, the pH values taken in the order $11$, $9$, $7$, $4$, $1$, because $[{\text{H}}^{+}]$ rises as the pH falls: $10^{-11}<10^{-9}<10^{-7}<10^{-4}<10^{-1}{\text{mol L}}^{-1}$.

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11. Q11 NCERT · Exercises
    Equal lengths of magnesium ribbons are taken in ​test tubes A and B. Hydrochloric acid ($\text{HCl}$) is added ​to test tube ​A, while acetic acid (${\text{CH}}_3\text{COOH}$) ​is added ​to test tube B. ​Amount and concentration taken for both the acids are same. ​In which test tube will the fizzing occur more vigorously and why?
    Show answer

    Tube A fizzes more vigorously, because $\text{HCl}$ is a strong acid that gives a higher hydrogen-ion concentration than weak ${\text{CH}}_3\text{COOH}$.

    The fizzing occurs more vigorously in test tube A, the one containing hydrochloric acid. $\text{HCl}$ is a strong acid and ionises almost completely in water, whereas ${\text{CH}}_3\text{COOH}$ is a weak acid and ionises only partially. At the same concentration tube A therefore holds a much higher concentration of hydrogen ions, and since the hydrogen gas is produced by those ions reacting with the magnesium, the bubbles come off faster in A: $\text{Mg}+2\text{HCl}\to {\text{MgCl}}_2+{\text{H}}_2$. Both tubes finally give the same volume of hydrogen — only the rate differs.

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12. Q12 NCERT · Exercises
    Fresh milk has a pH ​of $6$. How do ​you ​think the pH will change as it ​turns into curd? Explain your ​answer.
    Show answer

    The pH falls: lactic acid from bacterial fermentation raises the hydrogen-ion concentration, so curd has a lower pH than milk.

    The pH will **fall** — it drops below $6$, set curd commonly lying around pH $4$ to $5$. As the milk sets, the Lactobacillus bacteria in it ferment the lactose (milk sugar) into lactic acid. That acid releases hydrogen ions into the milk, so $[{\text{H}}^{+}]$ rises; and because pH runs opposite to hydrogen-ion concentration, a rise in $[{\text{H}}^{+}]$ shows up as a fall in pH. This is exactly why curd tastes sour while fresh milk does not.

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13. Q13 NCERT · Exercises
    A ​milkman adds a very small amount ​of baking soda to fresh ​milk.

    1. (a)Why does he shift ​the pH of the fresh milk from $6$ to slightly alkaline?
    2. (b)​Why ​does this milk take a long time to set ​as curd?
    Show answer

    Baking soda makes milk slightly alkaline, so it keeps longer; the lactic acid must first neutralise the soda, so curd sets slowly.

    (a) Baking soda, ${\text{NaHCO}}_3$, is a mild base. Added to the milk it neutralises the small amount of lactic acid already present and lifts the pH from $6$ to a little above $7$. Milk that is slightly alkaline does not sour readily, so it stays fresh for much longer. (b) Curd sets only when enough lactic acid has been produced to bring the milk back down to the acidic pH at which its protein coagulates. In the treated milk the lactic acid must first be used up neutralising the added baking soda, and only the acid formed after that begins to lower the pH. A much larger quantity of acid therefore has to be produced, so the milk takes a long time to set as curd.

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14. Q14 NCERT · Exercises
    Plaster ​of ​Paris should ​be stored ​in a moisture-proof container. Explain ​why?
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    Moisture makes plaster of Paris set into hard gypsum, which cannot be moulded again, so it must be kept in a moisture-proof container.

    Plaster of Paris is calcium sulphate hemihydrate, ${\text{CaSO}}_4\cdot \frac{1}{2}{\text{H}}_2\text{O}$. On coming into contact with moisture it reacts with water and sets into a hard solid mass of gypsum, ${\text{CaSO}}_4\cdot 2{\text{H}}_2\text{O}$: $${\text{CaSO}}_4\cdot \frac{1}{2}{\text{H}}_2\text{O}+1\frac{1}{2}{\text{H}}_2\text{O}\to {\text{CaSO}}_4\cdot 2{\text{H}}_2\text{O}$$ Once it has set it can no longer be made into a paste, so it becomes useless for casts, moulds and plastering. Even the moisture present in damp air is enough to start this reaction, which is why Plaster of Paris must be stored in a moisture-proof container.

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15. Q15 NCERT · Exercises
    What ​is a neutralisation ​reaction? ​Give ​two ​examples.
    Show answer

    An acid reacts with a base to give a salt and water, as in $\text{NaOH}+\text{HCl}\to \text{NaCl}+{\text{H}}_2\text{O}$.

    A neutralisation reaction is a reaction in which an acid reacts with a base to give a salt and water, the acid and the base cancelling each other's characteristic properties. Its ionic core is ${\text{H}}^{+}(aq)+{\text{OH}}^{-}(aq)\to {\text{H}}_2\text{O}(l)$, and the general pattern is $\text{Acid}+\text{Base}\to \text{Salt}+\text{Water}$. Two examples: sodium hydroxide with hydrochloric acid, $\text{NaOH}+\text{HCl}\to \text{NaCl}+{\text{H}}_2\text{O}$; and calcium hydroxide with sulphuric acid, ${\text{Ca(OH)}}_2+{\text{H}}_2{\text{SO}}_4\to {\text{CaSO}}_4+2{\text{H}}_2\text{O}$.

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16. Q16 NCERT · Exercises
    Give ​two ​important uses of washing soda ​and ​baking ​soda.
    Show answer

    Washing soda: glass, soap and paper industries and removing permanent hardness; baking soda: baking powder for soft cakes, and as an antacid.

    Washing soda, ${\text{Na}}_2{\text{CO}}_3\cdot 10{\text{H}}_2\text{O}$ — two uses: it is used in the glass, soap and paper industries and in making other sodium compounds such as borax; and it is used for removing the permanent hardness of water, its carbonate ion precipitating the dissolved calcium and magnesium salts as insoluble carbonates. Baking soda, ${\text{NaHCO}}_3$ — two uses: it is the active ingredient of baking powder (baking soda mixed with a mild edible acid such as tartaric acid), which releases carbon dioxide on heating and makes cakes and bread soft and spongy; and it is used as an antacid, its mild basicity neutralising excess acid in the stomach and relieving indigestion.

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17. Q17 NCERT · Intext Questions (Page 22)
    Why should curd and ​sour substances not be ​kept ​in ​brass and copper ​vessels?
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    Acids in curd and sour foods react with the vessel's copper and zinc, forming poisonous salts that spoil the food.

    Curd and sour substances contain acids (lactic acid in curd; citric, tartaric or acetic acid in sour foods). Brass (copper and zinc) and copper are metals, and acids react with metals to form a salt and hydrogen gas: Acid + Metal $\to$ Salt + Hydrogen. The copper and zinc salts so formed dissolve into the food; they are poisonous, spoil the taste and colour of the food and make it unfit for eating. So curd and sour substances should not be kept in brass or copper vessels.

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18. Q18 NCERT · Intext Questions (Page 22)
    Which ​gas ​is usually liberated when an ​acid reacts with a metal? Illustrate ​with ​an example. How will you test ​for the presence ​of this gas?
    Show answer

    Hydrogen gas is liberated, e.g. when zinc reacts with dilute sulphuric acid; it burns with a pop sound near a flame.

    The gas usually liberated is hydrogen (${\text{H}}_2$). Example: zinc reacts with dilute sulphuric acid to give zinc sulphate and hydrogen, $\text{Zn}(s)+{\text{H}}_2{\text{SO}}_4(aq)\to {\text{ZnSO}}_4(aq)+{\text{H}}_2(g)$ (Activity 2.3). Test: pass the gas through soap solution so that it fills soap bubbles (or collect it in a test tube) and bring a burning candle or matchstick near it — hydrogen burns with a 'pop' sound, $2{\text{H}}_2+{\text{O}}_2\to 2{\text{H}}_2\text{O}$.

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19. Q19 NCERT · Intext Questions (Page 22)
    Metal ​compound A reacts with dilute hydrochloric acid to produce effervescence. The gas evolved ​extinguishes ​a ​burning candle. Write ​a balanced chemical ​equation for the reaction ​if one of the compounds formed is calcium chloride.
    Show answer

    A is calcium carbonate; the gas is carbon dioxide, and ${\text{CaCO}}_3+2\text{HCl}\to {\text{CaCl}}_2+{\text{H}}_2\text{O}+{\text{CO}}_2$.

    The gas is carbon dioxide (it extinguishes a burning candle), so A is a metal carbonate; since calcium chloride is formed, A is calcium carbonate, ${\text{CaCO}}_3$ (limestone, chalk or marble). Balanced equation: ${\text{CaCO}}_3(s)+2\text{HCl}(aq)\to {\text{CaCl}}_2(aq)+{\text{H}}_2\text{O}(l)+{\text{CO}}_2(g)$.

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20. Q20 NCERT · Intext Questions (Page 25)
    Why do $\text{HCl}$, ${\text{HNO}}_3$, etc., show acidic ​characters in ​aqueous ​solutions while ​solutions ​of compounds like alcohol ​and glucose do ​not show ​acidic character?
    Show answer

    Acids ionise in water to give ${\text{H}}^{+}$ (hydronium) ions; alcohol and glucose do not ionise, so they release no ${\text{H}}^{+}$.

    The acidic character of a solution is due to hydrogen ions, ${\text{H}}^{+}(aq)$ (hydronium ions, ${\text{H}}_3{\text{O}}^{+}$). Acids such as $\text{HCl}$ and ${\text{HNO}}_3$ ionise in water to give these ions: $\text{HCl}+{\text{H}}_2\text{O}\to {\text{H}}_3{\text{O}}^{+}+{\text{Cl}}^{-}$; ${\text{HNO}}_3+{\text{H}}_2\text{O}\to {\text{H}}_3{\text{O}}^{+}+{\text{NO}}_3^{-}$. Alcohol (${\text{C}}_2{\text{H}}_5\text{OH}$) and glucose (${\text{C}}_6{\text{H}}_{12}{\text{O}}_6$) also contain hydrogen, but they do not ionise in water — their hydrogen is not released as ${\text{H}}^{+}$ — so their solutions contain no hydrogen ions and show no acidic character. Activity 2.8 confirms this: the bulb glows in the acid solutions, which contain ions, but not in glucose or alcohol solutions.

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