Chapter 2 · Acids, Bases and Salts

NCERT Class 10 Science Chapter 2 Exercises

All 15 questions of NCERT Class 10 Science Chapter 2 Exercises (Acids, Bases and Salts), in the book's order, each with its answer and a step-by-step solution.

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

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

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2. Q2
   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)$\text{KCl}$
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   (b) $\text{HCl}$

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3. Q3
   $10$ 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
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   (d) $16$ mL

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4. Q4
   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
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   (c) Antacid

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5. Q5
   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.
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   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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6. Q6
   Compounds such ​as ​alcohols ​and ​glucose ​also contain hydrogen ​but ​are not ​categorised as acids. Describe an Activity to prove it.
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   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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7. Q7
   Why ​does distilled water not ​conduct ​electricity, whereas rain water ​does?
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   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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8. Q8
   Why does ​dry $\text{HCL}$ gas ​not show acidic behaviour ​in ​the absence ​of ​water?
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   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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9. Q9
   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.
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   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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10. Q10
    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?
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    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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11. Q11
    Fresh milk has a pH ​of $6$. How do ​you ​think the pH will change as it ​turns into curd? Explain your ​answer.
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    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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12. Q12
    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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13. Q13
    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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14. Q14
    What ​is a neutralisation ​reaction? ​Give ​two ​examples.
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    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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15. Q15
    Give ​two ​important uses of washing soda ​and ​baking ​soda.
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    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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