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NCERT Class 10 Science Acids, Bases and Salts: In-text questions (page 25) Solutions

Step-by-step working for every question of In-text questions (page 25) in NCERT's Class 10 Science book, chapter “Acids, Bases and Salts”, written by Shishya. Each question is in our own short words; open the NCERT chapter for the exact wording and figures.

1. In-text (page 25) Q1

   Why do HCl, HNO₃ and similar compounds show acidic character in water, while alcohol and glucose solutions do not?

   1. HCl, HNO₃ and similar compounds are acids that ionise in water to release hydrogen ions (H⁺).
   2. These free H⁺ ions are responsible for the acidic character of the solution.
   3. Alcohol and glucose are molecular compounds that do not ionise in water.
   4. Since they do not release H⁺ ions, their solutions do not show acidic character.
   5. Thus, the presence of free H⁺ ions in water determines whether a substance shows acidic character.

   Answer: HCl and HNO₃ release H⁺ ions in water, showing acidic character, while alcohol and glucose do not ionise and hence do not show acidic character.

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2. In-text (page 25) Q2

   Why does an aqueous solution of an acid conduct electricity?

   1. Acids are compounds that contain hydrogen atoms.
   2. When an acid dissolves in water, it reacts with the water molecules.
   3. This reaction causes the acid molecules to break apart into charged particles called ions.
   4. Specifically, the acid releases hydrogen ions (H+) and negative ions (anions).
   5. These ions are free to move around in the solution.
   6. The movement of these charged ions allows the solution to carry an electric current.
   7. Therefore, the aqueous solution conducts electricity due to the presence of mobile ions.

   Answer: An aqueous solution of an acid conducts electricity because it contains free-moving ions (H+ and anions) that carry the electric current.

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3. In-text (page 25) Q3

   Why does dry HCl gas not change the colour of dry litmus paper?

   1. HCl is a covalent compound that exists as discrete molecules in the gaseous state.
      why?

      In a covalent bond two atoms share a pair of electrons so that each gets a full outer shell. Carbon forms such bonds because gaining or losing four electrons would need too much energy.

   2. In dry gas form, the HCl molecules do not dissociate into ions.
   3. Acids show acidic properties only when they release hydrogen ions (H⁺) in a solution.
   4. Since there is no water, the dry HCl gas cannot produce H⁺ ions.
   5. Without H⁺ ions, the gas cannot react with the litmus paper to change its colour.
   6. Therefore, dry HCl gas does not change the colour of dry litmus paper.

   Answer: Dry HCl gas does not change the colour of dry litmus paper because it does not produce H⁺ ions in the absence of water.

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4. In-text (page 25) Q4

   While diluting an acid, why should the acid be added to water and not water to the acid?

   1. Diluting an acid is a highly exothermic process that releases a large amount of heat energy.
   2. Water has a high specific heat capacity, meaning it can absorb a lot of heat without a significant rise in temperature.
   3. If water is added to acid, the small volume of water at the bottom gets superheated instantly and boils violently.
   4. This violent boiling causes the concentrated acid to splash out of the container, which is dangerous and corrosive.
   5. Adding acid slowly to water allows the heat to be absorbed by the large volume of water, keeping the temperature rise controlled.
   6. Therefore, acid must always be added to water to prevent splashing and ensure safety.

   Answer: Acid should be added to water because the dilution is exothermic; adding water to acid causes the small amount of water to boil and splash the acid, whereas adding acid to water allows the heat to be safely absorbed by the larger volume of water.

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5. In-text (page 25) Q5

   How does diluting an acid solution affect the concentration of hydronium ions (H₃O⁺)?

   1. Diluting an acid solution means adding water to it.
   2. Adding water increases the total volume of the solution.
   3. The number of hydronium ions (H₃O⁺) remains the same, but they are now spread over a larger volume.
   4. Therefore, the number of hydronium ions per unit volume decreases.
   5. This results in a lower concentration of hydronium ions in the solution.

   Answer: Diluting an acid decreases the concentration of hydronium ions (H₃O⁺) per unit volume.

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6. In-text (page 25) Q6

   How is the concentration of hydroxide ions (OH⁻) affected when excess base is dissolved in sodium hydroxide solution?

   1. Sodium hydroxide is a strong base that dissociates completely in water to release hydroxide ions.
   2. When an excess of another base is added to the sodium hydroxide solution, it also dissociates to release more hydroxide ions.
   3. The total number of hydroxide ions in the solution increases due to the addition of the extra base.
   4. Since the volume of the solution does not increase significantly compared to the amount of ions added, the concentration of hydroxide ions rises.

   Answer: The concentration of hydroxide ions increases.

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