Chapter 2 · Acids, Bases and Salts

NCERT Exemplar Class 10 Science Chapter 2 Short Answer Questions

All 12 questions of NCERT Exemplar Class 10 Science Chapter 2 Short Answer Questions (Acids, Bases and Salts), in the book's order, each with its answer and a step-by-step solution.

1. Q31
   Match the acids ​given ​in ​Column ​(A) with their correct source ​given ​in ​Column (B)
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   (a)-(iv), (b)-(iii), (c)-(ii), (d)-(i). That is, lactic acid comes from curd, acetic acid from vinegar, citric acid from lemon and oxalic acid from tomato.

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2. Q32
   Match the important chemicals given in ​Column (A) ​with ​the ​chemical formulae given ​in Column ​(B)
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   (a)-(ii), (b)-(iii), (c)-(iv), (d)-(i). That is, plaster of Paris is ${\text{CaSO}}_4\cdot \frac{1}{2}{\text{H}}_2\text{O}$, gypsum is ${\text{CaSO}}_4\cdot 2{\text{H}}_2\text{O}$, bleaching powder is ${\text{Ca(ClO)}}_2$ and slaked lime is ${\text{Ca(OH)}}_2$.

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3. Q33
   What will be the action of the ​following substances on ​litmus ​paper? ​Dry $\text{HCl}$ gas, ​Moistened ${\text{NH}}_3$ gas, Lemon juice, Carbonated soft ​drink, ​Curd, Soap solution.
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   Dry $\text{HCl}$ has no effect; moist ${\text{NH}}_3$ and soap turn red litmus blue; lemon juice, soft drink and curd turn blue litmus red.

   Dry $\text{HCl}$ gas: no action on dry litmus paper, because it cannot ionise without water. Moistened ${\text{NH}}_3$ gas: basic, turns red litmus blue. Lemon juice: acidic, turns blue litmus red. Carbonated soft drink: acidic, turns blue litmus red. Curd: acidic, turns blue litmus red. Soap solution: basic, turns red litmus blue.

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4. Q34
   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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   Ant stings contain methanoic (formic) acid, $\text{HCOOH}$; rubbing with baking soda or calamine solution neutralises it.

   The acid present in an ant sting is methanoic acid (formic acid), formula $\text{HCOOH}$. Relief is obtained by rubbing the affected area with a mild base — baking soda, that is sodium hydrogencarbonate (${\text{NaHCO}}_3$), or moist calamine solution — which neutralises the acid: $\text{HCOOH}+{\text{NaHCO}}_3\to \text{HCOONa}+{\text{H}}_2\text{O}+{\text{CO}}_2$.

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5. Q35
   What ​happens ​when ​nitric ​acid is added ​to egg shell?
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   Brisk effervescence of carbon dioxide occurs, forming calcium nitrate and water, and the calcium carbonate shell slowly dissolves.

   Egg shell is made of calcium carbonate. Nitric acid reacts with it to give calcium nitrate, water and carbon dioxide, so brisk effervescence is seen and the shell slowly dissolves: ${\text{CaCO}}_3(s)+2{\text{HNO}}_3(aq)\to {\text{Ca(NO}}_3{\text{)}}_2(aq)+{\text{H}}_2\text{O}(l)+{\text{CO}}_2(g)$. The gas evolved turns lime water milky, which confirms it is carbon dioxide.

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6. Q36
   A student prepared solutions of

   1. (i)an acid and
   2. (ii)a base in two separate beakers. ​She ​forgot to label the solutions and litmus paper is ​not ​available in the laboratory. Since both the solutions ​are colourless, how will she distinguish between the two?
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   Use another indicator (turmeric, china rose, phenolphthalein) or add sodium hydrogencarbonate: only the acid gives effervescence of carbon dioxide.

   She can use any other indicator, or a chemical test. (i) Indicators: turmeric paper stays yellow in the acid and turns red in the base; china rose extract turns dark pink in the acid and green in the base; phenolphthalein stays colourless in the acid and turns pink in the base. (ii) Chemical test: add a pinch of sodium hydrogencarbonate to a little of each solution — the acid gives brisk effervescence of carbon dioxide, ${\text{NaHCO}}_3+\text{HCl}\to \text{NaCl}+{\text{H}}_2\text{O}+{\text{CO}}_2$, while the base gives no gas. The gas turns lime water milky, confirming the beaker it came from holds the acid.

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7. Q37
   How ​would ​you distinguish between ​baking powder and ​washing ​soda by heating?
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   Heat both and pass the gas through lime water: baking powder gives carbon dioxide that turns it milky; washing soda does not.

   Heat a little of each solid in a separate dry test tube and pass the gas evolved through lime water. Baking powder contains sodium hydrogencarbonate, which decomposes on heating to give carbon dioxide: $2{\text{NaHCO}}_3\stackrel{\Delta }{\to }{\text{Na}}_2{\text{CO}}_3+{\text{H}}_2\text{O}+{\text{CO}}_2$, so the lime water turns milky. Washing soda, ${\text{Na}}_2{\text{CO}}_3\cdot 10{\text{H}}_2\text{O}$, only loses its water of crystallisation on heating: ${\text{Na}}_2{\text{CO}}_3\cdot 10{\text{H}}_2\text{O}\stackrel{\Delta }{\to }{\text{Na}}_2{\text{CO}}_3+10{\text{H}}_2\text{O}$, and gives no carbon dioxide, so the lime water stays clear. The solid that turns lime water milky is baking powder.

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8. Q38
   Salt A commonly ​used ​in bakery products on ​heating gets converted ​into another salt B which itself is used for ​removal of hardness of water and a gas C ​is evolved. The gas ​C when passed through lime water, turns it milky. Identify A, B ​and C.
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   A is baking soda (${\text{NaHCO}}_3$), B is sodium carbonate (${\text{Na}}_2{\text{CO}}_3$), and C is carbon dioxide (${\text{CO}}_2$).

   A is baking soda, sodium hydrogencarbonate (${\text{NaHCO}}_3$). B is sodium carbonate (${\text{Na}}_2{\text{CO}}_3$), sold as washing soda, which removes the hardness of water. C is carbon dioxide (${\text{CO}}_2$). The reaction is $2{\text{NaHCO}}_3\stackrel{\Delta }{\to }{\text{Na}}_2{\text{CO}}_3+{\text{H}}_2\text{O}+{\text{CO}}_2$.

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9. Q39
   In one of the industrial processes used ​for manufacture of sodium hydroxide, a gas X is formed as by product. The gas ​X reacts ​with lime water to give a ​compound Y which is used as a bleaching ​agent in chemical industry. Identify ​X ​and Y giving the chemical ​equation of the reactions involved.
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   X is chlorine (${\text{Cl}}_2$) from the chlor-alkali process; Y is bleaching powder (${\text{CaOCl}}_2$), formed when chlorine reacts with slaked lime.

   The process is the chlor-alkali process, the electrolysis of brine: $2\text{NaCl}(aq)+2{\text{H}}_2\text{O}(l)\to 2\text{NaOH}(aq)+{\text{Cl}}_2(g)+{\text{H}}_2(g)$. X is chlorine, ${\text{Cl}}_2$, liberated at the anode. Passed over slaked lime it gives Y, bleaching powder (calcium oxychloride), ${\text{CaOCl}}_2$: ${\text{Ca(OH)}}_2+{\text{Cl}}_2\to {\text{CaOCl}}_2+{\text{H}}_2\text{O}$.

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10. Q40
    Fill ​in the ​missing ​data in the following ​table

    Salt obtained from — the printed dash marks each blank to be filled

    Name of the salt	Formula	Salt obtained from: Base	Salt obtained from: Acid
    (i) Ammonium chloride	${\text{NH}}_4\text{Cl}$	${\text{NH}}_4\text{OH}$	—
    (ii) Copper sulphate	—	—	${\text{H}}_2{\text{SO}}_4$
    (iii) Sodium chloride	$\text{NaCl}$	$\text{NaOH}$	—
    (iv) Magnesium nitrate	${\text{Mg(NO}}_3{\text{)}}_2$	—	${\text{HNO}}_3$
    (v) Potassium sulphate	${\text{K}}_2{\text{SO}}_4$	—	—
    (vi) Calcium nitrate	${\text{Ca(NO}}_3{\text{)}}_2$	${\text{Ca(OH)}}_2$	—

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    Row by row the missing entries are $\text{HCl}$; ${\text{CuSO}}_4$ and ${\text{Cu(OH)}}_2$; $\text{HCl}$; ${\text{Mg(OH)}}_2$; $\text{KOH}$ and ${\text{H}}_2{\text{SO}}_4$; ${\text{HNO}}_3$.

    (i) Acid: $\text{HCl}$. (ii) Formula: ${\text{CuSO}}_4$; Base: ${\text{Cu(OH)}}_2$. (iii) Acid: $\text{HCl}$. (iv) Base: ${\text{Mg(OH)}}_2$. (v) Base: $\text{KOH}$; Acid: ${\text{H}}_2{\text{SO}}_4$. (vi) Acid: ${\text{HNO}}_3$.

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11. Q41
    What are strong ​and weak acids? ​In the ​following list of ​acids, ​separate strong acids from weak acids. Hydrochloric acid, citric acid, ​acetic acid, nitric ​acid, ​formic acid, sulphuric acid.
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    Strong acids ionise completely (hydrochloric, nitric, sulphuric); weak acids ionise partially (citric, acetic, formic).

    A strong acid is one that ionises completely in aqueous solution and therefore gives a high concentration of hydrogen ions; a weak acid ionises only partially and gives a low concentration of hydrogen ions. Strong acids in the list: hydrochloric acid, nitric acid and sulphuric acid. Weak acids in the list: citric acid, acetic acid and formic acid.

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12. Q42
    When zinc metal ​is treated with a dilute solution of a ​strong acid, a ​gas is evolved, which is utilised in the hydrogenation of ​oil. Name the gas evolved. Write ​the chemical equation of the ​reaction involved and also ​write a test to detect ​the gas formed.
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    The gas is hydrogen: $\text{Zn}+{\text{H}}_2{\text{SO}}_4\to {\text{ZnSO}}_4+{\text{H}}_2$; it burns with a pop sound near a flame.

    The gas is hydrogen, ${\text{H}}_2$. The reaction is $\text{Zn}(s)+{\text{H}}_2{\text{SO}}_4(aq)\to {\text{ZnSO}}_4(aq)+{\text{H}}_2(g)$ with dilute sulphuric acid, or $\text{Zn}(s)+2\text{HCl}(aq)\to {\text{ZnCl}}_2(aq)+{\text{H}}_2(g)$ with dilute hydrochloric acid. Test: bring a burning candle or splinter near the mouth of the tube — the gas burns with a pop sound, which confirms hydrogen.

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