Chapter 2 · Acids, Bases and Salts

NCERT Class 10 Science Chapter 2 Intext Questions (Page 25)

All 6 questions of NCERT Class 10 Science Chapter 2 Intext Questions (Page 25) — Acids, Bases and Salts — in the book's order, each with its answer and a step-by-step solution.

1. Q1
   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?
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   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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2. Q2
   Why ​does an aqueous ​solution of an acid ​conduct ​electricity?
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   An acid ionises in water to give free, mobile ions, and their movement towards the electrodes carries the current.

   An acid dissolved in water ionises to give hydrogen ions, ${\text{H}}^{+}(aq)$ (hydronium ions, ${\text{H}}_3{\text{O}}^{+}$), and the corresponding negative ions — for example $\text{HCl}+{\text{H}}_2\text{O}\to {\text{H}}_3{\text{O}}^{+}+{\text{Cl}}^{-}$. These ions are charged and free to move; when the electrodes are connected, the positive ions move to the negative electrode and the negative ions to the positive electrode, and this movement of ions carries the electric current. So an aqueous solution of an acid conducts electricity (the bulb glows in Activity 2.8).

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3. Q3
   Why does ​dry $\text{HCl}$ ​gas not ​change the ​colour ​of the dry ​litmus paper?
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   Dry $\text{HCl}$ gas cannot ionise without water, so it gives no ${\text{H}}^{+}$ ions and dry litmus does not change colour.

   Litmus changes colour in response to hydrogen ions, ${\text{H}}^{+}(aq)$, and it is these ions that give $\text{HCl}$ its acidic character. $\text{HCl}$ produces hydrogen ions only when it dissolves in water: $\text{HCl}+{\text{H}}_2\text{O}\to {\text{H}}_3{\text{O}}^{+}+{\text{Cl}}^{-}$. Dry $\text{HCl}$ gas and dry litmus paper contain no water, so the $\text{HCl}$ molecules cannot ionise, no ${\text{H}}^{+}(aq)$ ions are formed, and the litmus does not change colour. With moist litmus the water present dissolves the gas, ions form, and the paper turns red (Activity 2.9).

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4. Q4
   While diluting an acid, ​why ​is ​it ​recommended that the acid should be ​added to water ​and ​not water ​to the acid?
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   Dissolving acid in water is highly exothermic; a large volume of water absorbs the heat, whereas water added to acid boils and splashes.

   Dissolving a concentrated acid in water is a highly exothermic process — a large amount of heat is released. When the acid is added slowly to a large volume of water with constant stirring, the heat is absorbed by the large mass of water and the temperature rises only slightly. If water is added to the concentrated acid instead, the small amount of water added receives all the heat at once; it boils, the mixture may splash out and cause serious acid burns, and the glass container may crack because of excessive local heating. Hence the acid must always be added to water, slowly and with stirring, and never water to the acid.

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5. Q5
   How ​is the concentration of ​hydronium ions (${\text{H}}_3{\text{O}}^{+}$) ​affected ​when a solution of ​an acid ​is ​diluted?
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   Dilution lowers the concentration of hydronium ions, because the same number of ions is spread through a larger volume.

   On dilution, water is added to the acid solution. The number of hydronium ions stays the same but the volume of the solution increases, so the same ions are spread through a larger volume: the concentration of hydronium ions (${\text{H}}_3{\text{O}}^{+}$) per unit volume decreases. Doubling the volume with water, for example, halves the concentration.

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6. Q6
   How is the concentration ​of ​hydroxide ions (${\text{OH}}^{-}$) ​affected when excess base ​is dissolved in ​a solution ​of sodium ​hydroxide?
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   The ${\text{OH}}^{-}$ concentration increases, because more hydroxide ions are added to practically the same volume, making the solution more strongly basic.

   Sodium hydroxide gives hydroxide ions in water: $\text{NaOH}(s)\stackrel{{\text{H}}_2\text{O}}{\to }{\text{Na}}^{+}(aq)+{\text{OH}}^{-}(aq)$. When excess base is dissolved in the solution, more ${\text{OH}}^{-}$ ions are added to practically the same volume of solution, so the number of hydroxide ions per unit volume — the concentration of ${\text{OH}}^{-}$ ions — increases, and the solution becomes more strongly basic (its pH rises).

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