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 17 of 18

1. Q321 CBSE 2019
   List ​the changes that ​are observed when ​dil. $\text{HCl}$ is added ​to a small amount of copper ​oxide ​in a beaker. Write balanced chemical equation ​for the reaction.
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   Black copper oxide dissolves and the solution turns blue-green as copper(II) chloride forms, with no gas given off.

   Changes observed: the black copper oxide dissolves on stirring, and the colourless solution becomes blue-green, because copper(II) chloride is formed. No gas is given off. Balanced equation: $\text{CuO(s)}+2\text{HCl(aq)}⟶{\text{CuCl}}_2\text{(aq)}+{\text{H}}_2\text{O(l)}$

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2. Q322 CBSE 2019
   A student prepared ​solutions of

   1. (i)an acid and
   2. (ii)​a base in two separate beakers but forgot ​to label the solutions and litmus paper is not available in the laboratory. Since both the ​solutions ​are colourless, how will he distinguish between the ​two using (a) phenolphthalein and (b) methyl orange ?
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   Phenolphthalein turns pink in the base and stays colourless in the acid; methyl orange turns red in the acid and yellow in the base.

   (a) Add a few drops of phenolphthalein to a sample of each solution: the base turns pink, while the acid remains colourless. (b) Add a few drops of methyl orange to a fresh sample of each solution: the acid turns red, while the base turns yellow.

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3. Q323 CBSE 2019
   Identify the acid and base ​which form sodium hydrogen ​carbonate. Write chemical equation in support of your answer. State whether this compound is ​acidic, basic ​or neutral. Also write its pH ​value.
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   Sodium hydrogen carbonate forms from carbonic acid and sodium hydroxide; it is basic, with pH above 7 (about 8.3).

   Acid: carbonic acid, ${\text{H}}_2{\text{CO}}_3$ (a weak acid). Base: sodium hydroxide, $\text{NaOH}$ (a strong base). Equation: $\text{NaOH}+{\text{H}}_2{\text{CO}}_3⟶{\text{NaHCO}}_3+{\text{H}}_2\text{O}$ Nature: sodium hydrogen carbonate is basic, because it is the salt of a strong base and a weak acid. pH: above $7$; its solution has a pH of about $8.3$ (in the range $7$ to $10$).

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4. Q324 CBSE 2019
   State the effect of concentration ​of ${\text{H}}^{+}(\text{aq})$ ions on the ​nature ​of the solution. ​Do basic solutions also ​have ${\text{H}}^{+}(\text{aq})$ ions ? If yes, then why are ​these ​basic ​?
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   More ${\text{H}}^{+}$ ions make a solution more acidic; basic solutions still contain ${\text{H}}^{+}$ ions but have more ${\text{OH}}^{-}$ ions.

   Effect of the concentration of ${\text{H}}^{+}(\text{aq})$ ions: It decides the nature of the solution. The greater the concentration of ${\text{H}}^{+}(\text{aq})$ ions, the more acidic the solution and the lower its pH. As the concentration of ${\text{H}}^{+}(\text{aq})$ ions decreases, the solution becomes less acidic, then neutral (pH $7$) and then basic (pH above $7$). Do basic solutions have ${\text{H}}^{+}(\text{aq})$ ions? Yes. Basic solutions also contain ${\text{H}}^{+}(\text{aq})$ ions, since water itself always gives some ${\text{H}}^{+}(\text{aq})$ and ${\text{OH}}^{-}(\text{aq})$ ions. Why they are basic: In a basic solution the concentration of ${\text{OH}}^{-}(\text{aq})$ ions is greater than the concentration of ${\text{H}}^{+}(\text{aq})$ ions.

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5. Q325 CBSE 2019
   Identify the acid and the base ​from which sodium ​chloride is obtained. Which ​type of ​salt ​is it? When ​is it called rock salt? ​How is ​rock salt formed?
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   Sodium chloride comes from hydrochloric acid and sodium hydroxide, is a neutral salt, and forms rock salt beds when seas dry up.

   Sodium chloride is obtained from hydrochloric acid (HCl, the acid) and sodium hydroxide (NaOH, the base): $\text{HCl}+\text{NaOH}⟶\text{NaCl}+{\text{H}}_2\text{O}$. It is a neutral salt (from a strong acid and a strong base; pH $7$). Solid sodium chloride found in deposits as large crystals, often brown because of impurities, is called rock salt. Beds of rock salt were formed when seas of bygone ages dried up.

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6. Q326 CBSE 2019
   1. (a)While diluting an acid, ​why ​is it recommended that ​the ​acid should be added to water and ​not ​water to the acid ?
   2. (b)Dry hydrogen chloride ​gas does not change ​the colour of dry litmus paper. Why ?
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   Acid is added to water because dilution is highly exothermic; dry HCl gives no ${\text{H}}^{+}$ ions without water, so dry litmus is unchanged.

   (a) Dissolving an acid in water is a highly exothermic process. When the acid is added slowly to water with constant stirring, the heat released is absorbed by the large amount of water, so the mixture stays safe. If water is added to the concentrated acid, the heat boils the small amount of water: the mixture may splash out and cause burns, and the container may crack from the excessive local heating. (b) Hydrogen chloride forms hydrogen ions (${\text{H}}^{+}$, present as ${\text{H}}_3{\text{O}}^{+}$) only in the presence of water. Dry HCl gas and dry litmus paper contain no water, so no ${\text{H}}^{+}$ ions are formed and the litmus does not change colour.

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7. Q327 CBSE 2019
   1. (a)What ​does pH ​scale ​measure ?
   2. (b)Write its range.
   3. (c)State ​the ​significance of highest and lowest values of pH ​scale.
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   The pH scale measures hydrogen ion concentration, from 0 to 14; pH 0 is the strongest acid and pH 14 the strongest base.

   (a) The pH scale measures the hydrogen ion concentration in a solution, that is, how acidic or basic the solution is. (b) Its range is from $0$ to $14$ (with $7$ as neutral). (c) The lowest value, pH $0$, means the highest hydrogen ion concentration: a very strongly acidic solution. The highest value, pH $14$, means the lowest hydrogen ion concentration (the highest hydroxide ion concentration): a very strongly basic (alkaline) solution.

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8. Q328 CBSE 2019
   How ​is sodium hydroxide manufactured in industries ? Name the process. In ​this process a ​gas X is ​formed as ​by-product. This gas reacts with lime water to give a compound Y, which is used as a bleaching agent in the chemical industry. Identify X and ​Y and write ​the chemical equation of the reactions involved.
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   Electrolysis of brine (chlor-alkali process) gives sodium hydroxide; the by-product X is chlorine, which forms bleaching powder Y with slaked lime.

   Sodium hydroxide is manufactured by passing electricity through brine (a concentrated aqueous solution of sodium chloride). The process is called the chlor-alkali process. $2\text{NaCl}(aq)+2{\text{H}}_2\text{O}(l)⟶2\text{NaOH}(aq)+{\text{Cl}}_2(g)+{\text{H}}_2(g)$ X is chlorine gas, ${\text{Cl}}_2$, given off at the anode. With calcium hydroxide (slaked lime, as in lime water) it forms Y, bleaching powder, ${\text{CaOCl}}_2$: ${\text{Ca(OH)}}_2+{\text{Cl}}_2⟶{\text{CaOCl}}_2+{\text{H}}_2\text{O}$

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9. Q329 CBSE 2019
   1. (a)​Why is electrolysis of ​brine called ‘Chlor-alkali process’ ​? Write the chemical equation involved ​in this ​process.
   2. (b)A few crystals ​of hydrated copper ​sulphate are heated in a dry test-tube. Enlist ​any two observations.
   Show answer

   Brine electrolysis gives chlorine and an alkali, NaOH; heated hydrated copper sulphate turns white and gives water droplets.

   (a) The electrolysis of brine gives chlorine ("chlor") and sodium hydroxide, an alkali ("alkali"), so it is called the chlor-alkali process: $2\text{NaCl}(aq)+2{\text{H}}_2\text{O}(l)⟶2\text{NaOH}(aq)+{\text{Cl}}_2(g)+{\text{H}}_2(g)$ (b) (i) The blue crystals turn white, as anhydrous copper sulphate forms. (ii) Droplets of water appear on the cooler inner walls of the test tube. (${\text{CuSO}}_4\cdot 5{\text{H}}_2\text{O}\stackrel{\text{heat}}{\to }{\text{CuSO}}_4+5{\text{H}}_2\text{O}$)

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10. Q330 CBSE 2019
    Write the chemical formula of washing ​soda. How can it be obtained ​from baking soda ? ​List two industries in which ​washing ​soda ​is used for other ​purposes than washing ​clothes.
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    Washing soda is ${\text{Na}}_2{\text{CO}}_3\cdot 10{\text{H}}_2\text{O}$, made by heating baking soda and recrystallising the product; used in the glass and soap industries.

    Formula of washing soda: ${\text{Na}}_2{\text{CO}}_3\cdot 10{\text{H}}_2\text{O}$ (sodium carbonate decahydrate). From baking soda: heating baking soda gives 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 this sodium carbonate from water gives washing soda, ${\text{Na}}_2{\text{CO}}_3+10{\text{H}}_2\text{O}⟶{\text{Na}}_2{\text{CO}}_3\cdot 10{\text{H}}_2\text{O}$. Industries (any two): glass industry, soap industry, paper industry (also the manufacture of sodium compounds such as borax).

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11. Q331 CBSE 2019
    Salt ‘P’, commonly ​used in bakery products, on heating gets converted into another salt ‘Q’ ​which ​itself is used for ​the removal of hardness of water and a ​gas ‘R’ is ​evolved. The gas ‘R’ when passed ​through freshly prepared lime water turns milky. Identify ‘P’, ‘Q’ and ‘R’, giving ​chemical equation for the justification of your answer.
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    P is sodium hydrogencarbonate, Q is sodium carbonate and R is carbon dioxide, which turns lime water milky.

    P: Sodium hydrogencarbonate (baking soda), ${\text{NaHCO}}_3$. Q: Sodium carbonate, ${\text{Na}}_2{\text{CO}}_3$ (as washing soda, ${\text{Na}}_2{\text{CO}}_3\cdot 10{\text{H}}_2\text{O}$, it removes the permanent hardness of water). R: Carbon dioxide, ${\text{CO}}_2$. On heating P: $2{\text{NaHCO}}_3\stackrel{\text{heat}}{\to }{\text{Na}}_2{\text{CO}}_3+{\text{H}}_2\text{O}+{\text{CO}}_2$ R turns lime water milky: ${\text{Ca(OH)}}_2+{\text{CO}}_2\to {\text{CaCO}}_3+{\text{H}}_2\text{O}$ (calcium carbonate is a white, insoluble solid).

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12. Q332 CBSE 2019
    A white powder is used by doctors to support ​fractured ​bones.

    1. (a)Write the name and chemical formula of the powder.
    2. (b)​How is this powder prepared ?
    3. (c)​When this white powder is mixed with water, a hard solid mass is ​obtained. Write a balanced chemical equation for the ​change.
    4. (d)​Give ​one more use of this white powder.
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    Plaster of Paris, ${\text{CaSO}}_4\cdot \frac{1}{2}{\text{H}}_2\text{O}$, is made by heating gypsum at 373 K and sets to gypsum with water.

    (a) Plaster of Paris; its chemical formula is ${\text{CaSO}}_4\cdot \frac{1}{2}{\text{H}}_2\text{O}$ (calcium sulphate hemihydrate). (b) It is prepared by heating gypsum, ${\text{CaSO}}_4\cdot 2{\text{H}}_2\text{O}$, at $373$ K: ${\text{CaSO}}_4\cdot 2{\text{H}}_2\text{O}\stackrel{373\text{K}}{\to }{\text{CaSO}}_4\cdot \frac{1}{2}{\text{H}}_2\text{O}+1\frac{1}{2}{\text{H}}_2\text{O}$. (c) ${\text{CaSO}}_4\cdot \frac{1}{2}{\text{H}}_2\text{O}+1\frac{1}{2}{\text{H}}_2\text{O}\to {\text{CaSO}}_4\cdot 2{\text{H}}_2\text{O}$ (gypsum, the hard solid mass). (d) It is used for making toys, decorative materials and statues, and for making surfaces smooth (any one).

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13. Q333 CBSE 2019
    A ​white powder is added while baking cakes to make it soft and spongy. ​Name ​its main ingredients. Explain the function of each ​ingredient. Write ​the chemical reaction ​taking place when the powder ​is heated during baking.
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    Baking powder: baking soda releases carbon dioxide to make the cake soft and spongy; tartaric acid neutralises the bitter sodium carbonate formed.

    The powder is baking powder. Its main ingredients are sodium hydrogen carbonate (baking soda, ${\text{NaHCO}}_3$) and a mild edible acid, tartaric acid. Functions: sodium hydrogen carbonate gives off carbon dioxide gas on heating, and the gas bubbles make the cake rise, soft and spongy; tartaric acid neutralises the sodium carbonate formed, which would otherwise give the cake a bitter taste. Reaction on heating: $2{\text{NaHCO}}_3\stackrel{\text{heat}}{\to }{\text{Na}}_2{\text{CO}}_3+{\text{H}}_2\text{O}+{\text{CO}}_2$.

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14. Q334 CBSE 2019
    In an industrial process used for the manufacture of sodium ​hydroxide, a gas ‘A’ is formed as a by-product. ​The ​gas ‘A’ reacts with lime water to give a ​compound ‘B’ which is used as a bleaching agent in the ​chemical industry. Identify ​‘A’ ​and ‘B’. Also give the chemical equations of the reactions involved.
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    A is chlorine gas, ${\text{Cl}}_2$, and B is bleaching powder, ${\text{CaOCl}}_2$, formed when chlorine reacts with calcium hydroxide.

    A is chlorine gas, ${\text{Cl}}_2$, and B is bleaching powder, ${\text{CaOCl}}_2$ (calcium oxychloride). Chlor-alkali process (electrolysis of brine): $2\text{NaCl}(aq)+2{\text{H}}_2\text{O}(l)⟶2\text{NaOH}(aq)+{\text{Cl}}_2(g)+{\text{H}}_2(g)$ Formation of bleaching powder: ${\text{Ca(OH)}}_2+{\text{Cl}}_2⟶{\text{CaOCl}}_2+{\text{H}}_2\text{O}$

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15. Q335 CBSE 2019
    A dry ​pellet of a common ​base B, when kept in open absorbs moisture and turns ​sticky. The compound B is also a by-product of chlor-alkali process. Identify B. State the ​type of reaction that occurs ​when B is ​treated with an acidic oxide, say sulphur ​dioxide. Also write ​chemical equation for the reaction involved.
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    B is sodium hydroxide; its reaction with sulphur dioxide is neutralisation: $2\text{NaOH}+{\text{SO}}_2\to {\text{Na}}_2{\text{SO}}_3+{\text{H}}_2\text{O}$.

    B is sodium hydroxide, NaOH. Its dry pellets absorb moisture from the air and turn sticky (it is deliquescent), and it is produced in the chlor-alkali process, the electrolysis of brine: $2\text{NaCl}(\text{aq})+2{\text{H}}_2\text{O}(\text{l})\to 2\text{NaOH}(\text{aq})+{\text{Cl}}_2(\text{g})+{\text{H}}_2(\text{g})$. When B is treated with an acidic oxide such as sulphur dioxide, a neutralisation reaction takes place: a base reacts with an acidic oxide to give a salt and water. $2\text{NaOH}+{\text{SO}}_2\to {\text{Na}}_2{\text{SO}}_3+{\text{H}}_2\text{O}$ (sodium sulphite and water).

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16. Q336 CBSE 2019
    Write the main difference between an acid and ​a base. ​With the help ​of ​suitable ​examples explain the term neutralization ​and the ​formation of -

    1. (i)acidic,
    2. (ii)basic and
    3. (iii)​neutral salts.
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    An acid gives ${\text{H}}^{+}$ and a base gives ${\text{OH}}^{-}$; neutralization gives salt and water; examples: ${\text{NH}}_4\text{Cl}$ acidic, ${\text{Na}}_2{\text{CO}}_3$ basic, ${\text{KNO}}_3$ neutral.

    Main difference: An acid gives hydrogen ions, ${\text{H}}^{+}(aq)$, in aqueous solution, whereas a base gives hydroxide ions, ${\text{OH}}^{-}(aq)$. Neutralization: The reaction of an acid with a base to form a salt and water is called neutralization; the ${\text{H}}^{+}$ ions of the acid combine with the ${\text{OH}}^{-}$ ions of the base to form water. Example: $\text{NaOH}(aq)+\text{HCl}(aq)⟶\text{NaCl}(aq)+{\text{H}}_2\text{O}(l)$ (i) Acidic salt (strong acid + weak base; solution pH below 7): ${\text{NH}}_4\text{OH}+\text{HCl}⟶{\text{NH}}_4\text{Cl}+{\text{H}}_2\text{O}$ (ii) Basic salt (weak acid + strong base; solution pH above 7): $2\text{NaOH}+{\text{H}}_2{\text{CO}}_3⟶{\text{Na}}_2{\text{CO}}_3+2{\text{H}}_2\text{O}$ (iii) Neutral salt (strong acid + strong base; solution pH 7): $\text{KOH}+{\text{HNO}}_3⟶{\text{KNO}}_3+{\text{H}}_2\text{O}$ Any other correct examples earn credit.

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17. Q337 CBSE 2019
    Blue litmus solution ​is ​added to ​two test tubes A and B containing dilute HCl ​and NaOH solution respectively. ​In which test tube a colour change will be ​observed? State the ​colour ​change and give its reason.
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    Test tube A: blue litmus turns red because HCl is an acid; NaOH is a base, so B stays blue.

    The colour change is observed in test tube A (dilute HCl): the blue litmus solution turns red. Reason: HCl is an acid (it gives ${\text{H}}^{+}$ ions in water), and acids turn blue litmus red. NaOH in test tube B is a base, which does not change the colour of blue litmus, so B stays blue.

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18. Q338 CBSE 2019
    A teacher ​provided acetic acid, water, lemon juice, aqueous solution of sodium hydrogen carbonate and sodium hydroxide to students ​in ​the school laboratory ​to determine the pH values of these substances ​using pH papers. One of the students ​reported the pH values of the given substances ​as 3, 12, 4, 8 and 14 respectively. Which one of these values is not correct ? Write its correct value ​stating the reason.
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    The pH of water is wrong: the correct value is $7$, as water is neutral.

    The value $12$ reported for water is not correct. Correct value: $7$. Reason: water is neutral - neither acidic nor basic - because it contains equal amounts of ${\text{H}}^{+}$ and ${\text{OH}}^{-}$ ions; a pH of $12$ would mean a strongly basic solution. (The other values fit: acetic acid $3$ and lemon juice $4$ are acidic, sodium hydrogen carbonate solution $8$ is mildly basic and sodium hydroxide $14$ is strongly basic.)

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19. Q339 CBSE 2019
    On adding a few ​drops of universal indicator in three ​colourless solutions X, Y and Z taken separately in three test tubes, ​a student observed ​the changes in colour ​as green in X, red in Y and blue in ​Z.

    1. (a)Arrange X, Y and Z in increasing order of their pH values.
    2. (b)Which one of the three, X, Y and Z, will change the colour of phenolphthalein ? ​Why ?
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    Increasing pH order is Y, X, Z; Z changes phenolphthalein because it is basic.

    (a) Y (red: strongly acidic, pH about $1$ to $2$) < X (green: neutral, pH $7$) < Z (blue: basic, pH about $9$ to $10$). The increasing order of pH is Y, X, Z. (b) Z. Phenolphthalein is colourless in acidic and neutral solutions and turns pink only in a basic solution. Z is basic (pH more than $7$), so it turns phenolphthalein pink, while X and Y leave it colourless.

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20. Q340 CBSE 2019
    What is ​observed ​when 2 mL of ​dilute hydrochloric acid is ​added to ​1 g of sodium carbonate taken ​in a clean and dry test tube? Write chemical equation for ​the reaction involved.
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    Brisk effervescence of carbon dioxide gas; ${\text{Na}}_2{\text{CO}}_3+2\text{HCl}⟶2\text{NaCl}+{\text{H}}_2\text{O}+{\text{CO}}_2$.

    Observation: brisk effervescence; bubbles of a colourless, odourless gas, carbon dioxide, are given off and the sodium carbonate dissolves (the gas turns lime water milky). Equation: ${\text{Na}}_2{\text{CO}}_3(s)+2\text{HCl}(aq)⟶2\text{NaCl}(aq)+{\text{H}}_2\text{O}(l)+{\text{CO}}_2(g)$

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