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 7 of 18

1. Q121 CBSE 2026
   Define ​the ​following ​terms :

   1. (i)Olfactory ​indicator
   2. (ii)Water ​of ​crystallization
   Show answer

   An olfactory indicator changes smell in acid or base; water of crystallization is the fixed number of water molecules per formula unit.

   (i) An olfactory indicator is a substance whose odour changes in acidic or basic medium, so that a change in smell shows whether a solution is an acid or a base; onion and vanilla essence are examples, as their smell cannot be detected in a basic solution, and clove oil behaves in the same way. (ii) Water of crystallization is the fixed number of water molecules present in one formula unit of a salt, for example the five molecules in blue copper sulphate crystals, ${\text{CuSO}}_4\cdot 5{\text{H}}_2\text{O}$, which turn white on heating when this water is driven off.

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2. Q122 CBSE 2026
   How is a ​universal indicator obtained ? ​How is ​the wide range of pH ​of ​solution tested ​by ​it ?
   Show answer

   A universal indicator is a mixture of several indicators; its colour is matched with a standard chart to read the pH.

   A universal indicator is obtained by mixing several indicators. Because each indicator in the mixture changes colour at a different pH, the universal indicator shows different colours at different concentrations of hydrogen ions in a solution, that is, a different colour for each pH from $0$ to $14$. To test a solution, a drop of it is put on universal indicator paper (or a few drops of the indicator are added to it), and the colour produced is matched with the standard colour chart supplied with the indicator; the pH printed against the matching colour is the pH of the solution.

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3. Q123 CBSE 2026
   Write the preparation of ​Baking ​soda. How does it help ​in ​making ​cakes soft ​and spongy ?
   Show answer

   Carbon dioxide passed into ammonia-saturated sodium chloride solution gives baking soda; on heating it releases carbon dioxide, which makes the cake soft and spongy.

   Preparation: baking soda is sodium hydrogencarbonate, ${\text{NaHCO}}_3$. It is made by passing carbon dioxide through a cold concentrated solution of sodium chloride saturated with ammonia, when the sparingly soluble sodium hydrogencarbonate separates out: $\text{NaCl}+{\text{H}}_2\text{O}+{\text{CO}}_2+{\text{NH}}_3\to {\text{NH}}_4\text{Cl}+{\text{NaHCO}}_3$. Use in baking: baking soda is mixed with a mild edible acid such as tartaric acid to give baking powder, which is added to the cake batter. On heating in the oven the sodium hydrogencarbonate decomposes, $2{\text{NaHCO}}_3\stackrel{\text{Heat}}{\to }{\text{Na}}_2{\text{CO}}_3+{\text{H}}_2\text{O}+{\text{CO}}_2$, and the carbon dioxide given off is trapped in the thick batter. The bubbles raise the batter and, when the cake sets, leave behind the pores that make it soft and spongy. The tartaric acid neutralises the sodium carbonate formed, so the cake does not taste bitter.

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4. Q124 CBSE 2026
   A ​substance ‘X’, commonly used in baking products, reacts with dilute ​HCl to ​produce ​a gas ‘Y’ which is used in one type ​of fire extinguisher. Name ​the substance ‘X’ and gas ‘Y’. ​Write a balanced chemical equation for ​the reaction which takes place.
   Show answer

   X is baking soda, ${\text{NaHCO}}_3$, and Y is carbon dioxide, ${\text{CO}}_2$.

   X is baking soda, sodium hydrogencarbonate (${\text{NaHCO}}_3$), and Y is carbon dioxide (${\text{CO}}_2$). The balanced equation is ${\text{NaHCO}}_3+\text{HCl}⟶\text{NaCl}+{\text{H}}_2\text{O}+{\text{CO}}_2$.

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5. Q125 CBSE 2026
   Give reasons for the following :

   1. (a)Why do ​acids ​not show acidic ​behaviour ​in the absence of water ?
   2. (b)​Why should curd and sour substances ​not be ​kept in copper vessels ?
   Show answer

   Acids ionise to give ${\text{H}}^{+}$ ions only in water; curd and sour substances contain acids that react with copper to form toxic salts.

   (a) An acid shows acidic behaviour only when it ionises in water to release hydrogen ions (${\text{H}}^{+}$, carried as the hydronium ion ${\text{H}}_3{\text{O}}^{+}$). In the absence of water the acid stays as whole, undissociated molecules, no hydrogen ions are produced, and so it shows none of the acidic properties. (b) Curd and sour substances contain acids. These acids attack the copper surface - dissolving the copper oxide and carbonate layer on it - and form toxic soluble copper salts, which spoil the food and can cause food poisoning.

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6. Q126 CBSE 2026
   Answer the following :

   1. (a)Why does distilled water not conduct electricity, whereas rainwater ​does ?
   2. (b)​While ​diluting an acid, why is it recommended ​that ​the acid should be added ​to water and not water to ​acid ?
   Show answer

   Distilled water has practically no free ions but rainwater has; dilution is strongly exothermic, so acid is added to water.

   (a) Distilled water is very nearly pure water - on distillation the dissolved salts stay behind in the flask and the dissolved gases are driven off by boiling - so it has practically no free ions and cannot carry a current. Rainwater dissolves carbon dioxide from the air as it falls (and sulphur dioxide and oxides of nitrogen where the air is polluted); the carbon dioxide forms carbonic acid, which ionises to give ${\text{H}}^{+}$ and ${\text{HCO}}_3^{-}$ ions, and these free, moving ions carry the current, so rainwater conducts electricity. (b) Diluting a concentrated acid is a strongly exothermic process. If the acid is added slowly, with stirring, to a large volume of water, the heat given out is shared by all that water and the temperature rises only a little. If water were added to the acid, all that heat would be released in a very small volume of liquid; the mixture can boil at once and splash concentrated acid out of the container (which may also crack), causing severe burns.

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7. Q127 CBSE 2026
   Give reasons for the ​following :

   1. (a)Dry ​HCl gas ​does not ​change the ​colour of ​a ​dry blue litmus paper.
   2. (b)Copper sulphate crystals turn white on heating.
   Show answer

   Dry HCl gas has no water, so it forms no ${\text{H}}^{+}$ ions; heating blue ${\text{CuSO}}_4\cdot 5{\text{H}}_2\text{O}$ drives off its water, leaving white ${\text{CuSO}}_4$.

   (a) Dry HCl gas contains no water, so it does not ionise and releases no hydrogen ions; a substance shows acidic behaviour, including its action on litmus, only through the ${\text{H}}^{+}$ ions it produces in water. With no water in the gas and none in the paper, dry blue litmus stays blue. Moist blue litmus paper turns red with the same gas, because the moisture lets it ionise. (b) Blue copper sulphate crystals are hydrated copper sulphate, ${\text{CuSO}}_4\cdot 5{\text{H}}_2\text{O}$, and the five molecules of water of crystallisation are what give them their blue colour. On heating, this water is driven off as vapour and white anhydrous copper sulphate, ${\text{CuSO}}_4$, is left behind: ${\text{CuSO}}_4\cdot 5{\text{H}}_2\text{O}\stackrel{\text{heat}}{\to }{\text{CuSO}}_4+5{\text{H}}_2\text{O}$. Adding water to the white powder turns it blue again.

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8. Q128 CBSE 2026
   (A) Name the acid present in ant sting and give its chemical formula. Give the common method to get ​relief from the pain and ​irritation caused by ant sting. (B) Two ​solutions ​‘X’ and ‘Y’ have ​pH ​values $3.0$ and $9.5$ respectively. Which of these will turn blue litmus to red and which will turn phenolphthalein ​from colourless to pink ?
   Show answer

   Ant-sting acid is methanoic acid, relieved by baking soda; X (pH 3.0) turns blue litmus red; Y (pH 9.5) turns phenolphthalein pink.

   (A) The acid present in an ant sting is methanoic acid (formic acid), $\text{HCOOH}$. Relief is obtained by rubbing the stung area with a mild base such as moist baking soda (sodium hydrogencarbonate, ${\text{NaHCO}}_3$), which neutralises the acid; calamine solution, containing zinc carbonate, may be used in the same way. (B) ‘X’, of pH $3.0$, is acidic and will turn blue litmus red. ‘Y’, of pH $9.5$, is basic and will turn phenolphthalein from colourless to pink.

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9. Q129 CBSE 2026
   Explain ​chlor-alkali process with ​chemical ​equation. Name the products formed ​at anode and ​cathode.
   Show answer

   Brine electrolysis gives chlorine at the anode, hydrogen at the cathode and sodium hydroxide near the cathode.

   The chlor-alkali process is the electrolysis of brine, a concentrated aqueous solution of sodium chloride. The electric current decomposes it into sodium hydroxide, chlorine and hydrogen; the name comes from chlorine ("chlor") and sodium hydroxide ("alkali"). $2\text{NaCl}(aq)+2{\text{H}}_2\text{O}(l)⟶2\text{NaOH}(aq)+{\text{Cl}}_2(g)+{\text{H}}_2(g)$ Anode: chlorine gas, ${\text{Cl}}_2$. Cathode: hydrogen gas, ${\text{H}}_2$; sodium hydroxide solution is formed near the cathode.

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10. Q130 CBSE 2026
    Give reasons :

    1. (i)For dilution of ​an ​acid, acid is added into water and not water into acid.
    2. (ii)Dry $\text{HCl}$ gas does not turn blue ​litmus red whereas dilute hydrochloric ​acid ​does.
    3. (iii)​Blue colour of Copper(II) ​sulphate crystals turns white ​on heating.
    Show answer

    Mixing acid with water is highly exothermic; ${\text{H}}^{+}$ ions form only in water; heating removes the water of crystallisation that makes copper sulphate blue.

    (i) Dissolving an acid in water is highly exothermic. If water is added to a concentrated acid, the small amount of water is heated so much that the mixture can splash out and cause burns, and the container may crack from local overheating; adding the acid slowly to a large amount of water, with stirring, lets the water absorb the heat safely. (ii) Hydrogen ions, which turn blue litmus red, are produced only in the presence of water: $\text{HCl}+{\text{H}}_2\text{O}⟶{\text{H}}_3{\text{O}}^{+}+{\text{Cl}}^{-}$. Dry $\text{HCl}$ gas contains no ${\text{H}}^{+}$ ions, but dilute hydrochloric acid does. (iii) The blue colour is due to the water of crystallisation in ${\text{CuSO}}_4\cdot 5{\text{H}}_2\text{O}$. On heating this water is lost and white anhydrous copper sulphate remains: ${\text{CuSO}}_4\cdot 5{\text{H}}_2\text{O}\stackrel{\text{heat}}{\to }{\text{CuSO}}_4+5{\text{H}}_2\text{O}$.

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11. Q131 CBSE 2026
    1. (a)Give the chemical ​name ​and formula of Plaster of Paris.
    2. (b)Write the chemical equation of its ​preparation.
    3. (c)​Give any ​two ​uses of ​it.
    Show answer

    Plaster of Paris is calcium sulphate hemihydrate, made by heating gypsum at 373 K; it supports fractured bones and is used for toys.

    (a) Plaster of Paris is calcium sulphate hemihydrate (calcium sulphate half water), ${\text{CaSO}}_4\cdot \frac{1}{2}{\text{H}}_2\text{O}$. (b) It is prepared by heating gypsum at $373$ K: ${\text{CaSO}}_4\cdot 2{\text{H}}_2\text{O}\stackrel{373\text{K}}{\to }{\text{CaSO}}_4\cdot \frac{1}{2}{\text{H}}_2\text{O}+1\frac{1}{2}{\text{H}}_2\text{O}$. (c) Any two uses: doctors use it as plaster for supporting fractured bones in the right position; it is used for making toys, decorative materials and for making surfaces smooth.

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12. Q132 CBSE 2026
    During electrolysis of brine solution, a gas (A) is liberated at the anode. When the gas (A) is ​passed through slaked lime, ​a ​compound (B) is formed which is used for disinfecting drinking water.

    1. (i)Write the names of ​(A) ​and (B).
    2. (ii)Write the chemical equation involved.
    3. (iii)Write one use ​of compound (B) ​other than disinfecting ​water.
    Show answer

    A is chlorine and B is bleaching powder, ${\text{CaOCl}}_2$; B is also used to bleach cotton and linen in the textile industry.

    (i) (A) is chlorine, ${\text{Cl}}_2$, the gas liberated at the anode during the electrolysis of brine; (B) is bleaching powder, calcium oxychloride, ${\text{CaOCl}}_2$. (ii) ${\text{Ca(OH)}}_2+{\text{Cl}}_2⟶{\text{CaOCl}}_2+{\text{H}}_2\text{O}$ (iii) Bleaching powder is used for bleaching cotton and linen in the textile industry. It is also used for bleaching wood pulp in paper factories and as an oxidising agent in many chemical industries.

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13. Q133 CBSE 2026
    Write ​the ​preparation of ​following compounds with balanced chemical ​equation :

    1. (i)​Baking soda
    2. (ii)Bleaching ​powder
    3. (iii)Plaster ​of Paris
    Show answer

    Baking soda from brine, ammonia and carbon dioxide; bleaching powder from chlorine and dry slaked lime; Plaster of Paris by heating gypsum at 373 K.

    (i) Baking soda (${\text{NaHCO}}_3$) is prepared by passing carbon dioxide and ammonia into brine: $\text{NaCl}+{\text{H}}_2\text{O}+{\text{CO}}_2+{\text{NH}}_3\to {\text{NH}}_4\text{Cl}+{\text{NaHCO}}_3$. (ii) Bleaching powder (${\text{CaOCl}}_2$) is prepared by the action of chlorine on dry slaked lime: ${\text{Ca(OH)}}_2+{\text{Cl}}_2\to {\text{CaOCl}}_2+{\text{H}}_2\text{O}$. (iii) Plaster of Paris 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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14. Q134 CBSE 2026
    Write ​the preparation of the following ​compounds ​with balanced chemical equation ​:

    1. (i)$\text{NaOH}$ ​by chlor-alkali process
    2. (ii)Baking ​Soda
    3. (iii)Plaster of Paris
    Show answer

    Electrolysis of brine gives sodium hydroxide; brine with ammonia and carbon dioxide gives baking soda; heating gypsum at 373 K gives Plaster of Paris.

    (i) Electricity is passed through brine (aqueous sodium chloride); chlorine forms at the anode, hydrogen at the cathode and sodium hydroxide solution near the cathode: $2\text{NaCl(aq)}+2{\text{H}}_2\text{O(l)}⟶2\text{NaOH(aq)}+{\text{Cl}}_2\text{(g)}+{\text{H}}_2\text{(g)}$. (ii) Ammonia and carbon dioxide are passed through brine, and sodium hydrogencarbonate (baking soda) separates out: $\text{NaCl}+{\text{H}}_2\text{O}+{\text{CO}}_2+{\text{NH}}_3⟶{\text{NH}}_4\text{Cl}+{\text{NaHCO}}_3$. (iii) Gypsum is heated at $373$ K and loses part of its water of crystallisation: ${\text{CaSO}}_4\cdot 2{\text{H}}_2\text{O}\stackrel{373\text{K}}{\to }{\text{CaSO}}_4\cdot \frac{1}{2}{\text{H}}_2\text{O}$ (Plaster of Paris) $+1\frac{1}{2}{\text{H}}_2\text{O}$.

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15. Q135 CBSE 2026
    1. (a)​Name the acid present in ant’s sting.
    2. (b)Give reason : (i) ​While diluting ​an ​acid, it is recommended that ​the acid should be added to water. (ii) ​Baking soda ​is used as an antacid.
    Show answer

    Ant stings contain methanoic acid; acid is added to water because dilution is exothermic; baking soda, a mild base, neutralises stomach acid.

    (a) Methanoic acid (formic acid), $\text{HCOOH}$. (b)(i) The dilution of an acid is a highly exothermic process. If water is added to a concentrated acid, the small amount of water becomes so hot that it boils, the mixture splashes out and can cause burns, and the container may crack from the local heating. Adding the acid slowly to a large volume of water, with constant stirring, lets the water absorb the heat safely. (ii) Baking soda (sodium hydrogencarbonate) is a mild base. It neutralises the excess hydrochloric acid in the stomach and gives relief from acidity: ${\text{NaHCO}}_3+\text{HCl}⟶\text{NaCl}+{\text{H}}_2\text{O}+{\text{CO}}_2$.

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16. Q136 CBSE 2026
    Define ​the ​following ​terms with an example ​:

    1. (i)Olfactory ​indicators
    2. (ii)​Water of crystallisation
    3. (iii)Basic ​salt
    Show answer

    Olfactory indicators change smell (onion); water of crystallisation is fixed water in the crystal; a basic salt comes from a strong base and weak acid.

    (i) An olfactory indicator is a substance whose smell changes, or is lost, in acidic and basic solutions, so it tells an acid from a base by odour instead of by colour. Example: onion - a cloth strip carrying the onion smell keeps it with dilute $\text{HCl}$ but loses it with dilute $\text{NaOH}$ (vanilla essence and clove oil behave the same way). (ii) Water of crystallisation is the fixed number of water molecules chemically combined with one formula unit of a salt in its crystal. Example: copper sulphate crystals, ${\text{CuSO}}_4\cdot 5{\text{H}}_2\text{O}$ (blue vitriol), which carry five such molecules and turn white on heating. (iii) A basic salt is the salt of a strong base and a weak acid; its aqueous solution has pH greater than $7$ and turns red litmus blue. Example: sodium carbonate, ${\text{Na}}_2{\text{CO}}_3$ (pH about $11.6$), formed from $\text{NaOH}$ and the weak acid ${\text{H}}_2{\text{CO}}_3$.

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17. Q137 CBSE 2026
    The pH plays ​an important ​role in our ​daily life. Strength of acids ​and bases depends on the number of ${\mathrm{H}}^{+}$ ions and $\mathrm{O}{\mathrm{H}}^{-}$ ions produced by these respectively. pH scale helps ​in predicting the strength of acids and bases by measuring hydrogen ion ​concentration ​in a solution. Answer the following ​:

    1. (a)What type of indicator is used to predict how strong an acid or base is ?[1]
    2. (b)Three solutions A, B and C showed pH as 3, 5 and 7. Which solution has the highest concentration of ${\mathrm{H}}^{+}$ ions ?[1]
    3. (c)What are strong acids and weak acids ?[2]
    4. (c)OR : (i) Under what condition is rainwater called acid rain ? (ii) In the following acids, identify weak acids : Nitric acid, hydrochloric acid, acetic acid, sulphuric acid, formic acid.[2]
    Show answer

    A universal indicator shows strength; solution A (pH 3) has the highest ${\mathrm{H}}^{+}$; strong acids ionise completely, weak acids partially.

    (a) A universal indicator - a mixture of several indicators, used as a solution or as pH paper. It gives a different colour at each pH, so its colour reports how strong the acid or base is, which litmus or phenolphthalein cannot do. (b) Solution A. Since $[{\mathrm{H}}^{+}]=10^{-\mathrm{pH}}$, the concentrations are $10^{-3}$ mol/L in A, $10^{-5}$ mol/L in B and $10^{-7}$ mol/L in C, so the smallest pH, 3, carries the highest hydrogen ion concentration. (c) A strong acid ionises completely in water and so gives a large number of ${\mathrm{H}}^{+}$ ions (for example $\mathrm{HCl}$, ${\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4$, $\mathrm{HN}{\mathrm{O}}_3$); a weak acid ionises only partially and so gives far fewer ${\mathrm{H}}^{+}$ ions (for example $\mathrm{C}{\mathrm{H}}_3\mathrm{COOH}$, ${\mathrm{H}}_2\mathrm{C}{\mathrm{O}}_3$). OR (c) (i) Rain water is called acid rain when its pH is less than 5.6. (ii) The weak acids in the list are acetic acid and formic acid; nitric, hydrochloric and sulphuric acids are strong.

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18. Q138 CBSE 2026
    Read the following passage and answer the questions that follow : In the ​given diagram, ​when electricity is passed through an aqueous solution of a common salt, a substance ‘Z’ is produced, along with the ​evolution ​of gases ‘X’ and ‘Y’. When a burning matchstick is brought near the gas ‘Y’, it burns with a pop sound, whereas ‘X’ is used ​for disinfecting drinking water. ​When ‘X’ is passed through slaked lime, an insoluble ​substance ‘A’ is produced.

    1. (a)Write the names of the gases ‘X’ and ‘Y’.[1]
    2. (b)Write the balanced chemical equation for the formation of substance ‘Z’.[1]
    3. (c)(i)What happens : (I) If a drop of red litmus solution is added to the aqueous solution of substance ‘Z’ ? (II) If ${\text{CO}}_2$ gas is passed through ammoniacal solution of aqueous NaCl ?[2]
    4. (c)(ii)OR : Identify the substance ‘A’ and write the balanced chemical equation of the reaction which takes place in its preparation.[2]
    Show answer

    X is chlorine, Y hydrogen, Z sodium hydroxide; red litmus turns blue; ${\text{CO}}_2$ with ammoniacal brine gives baking soda; A is bleaching powder.

    (a) ‘X’ is chlorine gas, ${\text{Cl}}_2$, given off at the anode; ‘Y’ is hydrogen gas, ${\text{H}}_2$, given off at the cathode. (b) ‘Z’ is sodium hydroxide, formed in the electrolysis of brine (chlor-alkali process): $2\text{NaCl}(aq)+2{\text{H}}_2\text{O}(l)⟶2\text{NaOH}(aq)+{\text{Cl}}_2(g)+{\text{H}}_2(g)$ (c)(i) (I) The red litmus turns blue, because sodium hydroxide is a base (an alkali). (II) Sodium hydrogencarbonate (baking soda) is formed and, being sparingly soluble, separates out as a white solid, together with ammonium chloride: $\text{NaCl}+{\text{H}}_2\text{O}+{\text{CO}}_2+{\text{NH}}_3⟶{\text{NH}}_4\text{Cl}+{\text{NaHCO}}_3$ OR (c)(ii) ‘A’ is bleaching powder (calcium oxychloride, ${\text{CaOCl}}_2$), made by passing chlorine over dry slaked lime: ${\text{Ca(OH)}}_2+{\text{Cl}}_2⟶{\text{CaOCl}}_2+{\text{H}}_2\text{O}$

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19. Q139 CBSE 2026
    Read the ​following ​passage and answer the questions that follow : Salts are formed by reactions of acids and bases. Strong acids react with strong bases to form neutral salts. Weak acids ​react with strong bases to form basic salts. Acidic salts ​are formed by strong acids and weak bases. ${\text{Na}}_2{\text{CO}}_3$ and ${\text{NaHCO}}_3$ are salts of strong base ($\text{NaOH}$) and weak ​acid (${\text{H}}_2{\text{CO}}_3$). Bleaching powder is prepared ​when slaked lime reacts with dry chlorine gas. Crystalline salts are hydrated ​and lose water on heating. Gypsum on heating at 373 K gives Plaster of Paris.

    1. (a)What is the formula of POP (Plaster of Paris) ? What is its chemical name ?[1]
    2. (b)Name the compound of sodium which is used for softening hard water. What is its chemical formula ?[1]
    3. (c)(i)(I) What is water of crystallization ? (II) Write the preparation of bleaching powder.[2]
    4. (c)(ii)OR : (I) Name an indicator used by visually impaired children. (II) Give one use of bleaching powder.[2]
    Show answer

    POP is ${\text{CaSO}}_4\cdot \frac{1}{2}{\text{H}}_2\text{O}$; washing soda softens hard water; dry slaked lime and chlorine give bleaching powder.

    (a) Plaster of Paris is ${\text{CaSO}}_4\cdot \frac{1}{2}{\text{H}}_2\text{O}$; its chemical name is calcium sulphate hemihydrate (calcium sulphate half-hydrate). (b) Washing soda, that is sodium carbonate decahydrate; its formula is ${\text{Na}}_2{\text{CO}}_3\cdot 10{\text{H}}_2\text{O}$. (c)(i)(I) Water of crystallization is the fixed number of water molecules present in one formula unit of a salt, held in its crystal — for example ${\text{CuSO}}_4\cdot 5{\text{H}}_2\text{O}$ holds five such molecules. (II) Bleaching powder is prepared by the action of dry chlorine on dry slaked lime: ${\text{Ca(OH)}}_2+{\text{Cl}}_2⟶{\text{CaOCl}}_2+{\text{H}}_2\text{O}$. OR (c)(ii)(I) Vanilla essence, an olfactory indicator (onion or clove oil is equally acceptable), since it is identified by smell and not by colour. (II) Bleaching powder is used for disinfecting drinking water (also for bleaching cotton and linen, for bleaching wood pulp in the paper industry, and as an oxidising agent).

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20. Q140 CBSE 2026
    Read the following passage and answer the questions that follow : Various substances can be classified as ​acids, bases and salts. Based on their strength, acids and bases are further classified as strong and ​weak. We can find this by using universal indicators, which shows different colours at different concentrations ​of hydrogen ions in a solution. A pH ​scale has been developed, which ​indicates acidic or basic ​nature of a solution, from 0 (very acidic) to ​14 ​(very alkaline).

    1. (i)Define universal indicator.[1]
    2. (ii)The pH of solution ‘A’ is 6 and the pH of solution ‘B’ is 10. Which solution has more hydrogen ion concentration ?[1]
    3. (iii)(a)What is neutralisation reaction ? Give one example.[2]
    4. (iii)(b)OR : Four solutions A, B, C and D when tested with universal indicator showed pH as 5, 1, 9 and 13 respectively. Which solution is : (I) strongly alkaline, and (II) strongly acidic ?[2]
    Show answer

    Solution A (pH 6) has more hydrogen ions; a universal indicator is a mixture showing different colours; neutralisation gives salt and water.

    (i) A universal indicator is a mixture of several indicators that shows different colours at different concentrations of hydrogen ions in a solution; matching its colour against a chart gives the pH, and so shows how strongly acidic or basic the solution is. (ii) Solution ‘A’ (pH 6) has more hydrogen-ion concentration, because the lower the pH, the higher the hydrogen-ion concentration. (iii)(a) A neutralisation reaction is the reaction between an acid and a base to give a salt and water, for example $\text{NaOH}+\text{HCl}⟶\text{NaCl}+{\text{H}}_2\text{O}$. OR : (iii)(b) (I) D (pH 13) is strongly alkaline; (II) B (pH 1) is strongly acidic.

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