In-text Questions — Acids, Bases and Ions in Water — Acids, Bases and Salts

1. 12 marksNCERT Cl-10 Science, In-text Qs after §2.2.1, Q1

   Why do HCl, HNO₃, etc., show acidic characters in aqueous solutions while solutions of compounds like alcohol and glucose do not show acidic character?

   Hint. Both kinds of molecules contain hydrogen — the difference is what each does with that hydrogen once dissolved in water.

   Step 1 — Recall what actually makes a solution acidic. Acidic behaviour comes from the presence of H⁺ (hydronium, H₃O⁺) ions in solution — not merely from a molecule containing hydrogen atoms.

   Step 2 — Compare the two kinds of compounds. HCl and HNO₃ ionise in water, releasing free H⁺ ions, which is why they conduct electricity and show acidic properties. Alcohol and glucose do contain hydrogen atoms, but these atoms are held in a way that does not ionise in water — no free H⁺ ions are released.

   Step 3 — Confirm with the chapter's own activity. Activity 2.8 shows exactly this: the bulb glows for HCl and H₂SO₄ solutions, but not for glucose or alcohol solutions, since only the acids are releasing ions into solution.

   ✦ Answer: Acidic character depends on producing H⁺(aq) ions in solution, which HCl and HNO₃ do on ionising, but alcohol and glucose do not — their hydrogen atoms don't separate off as H⁺ ions, so they show no acidic behaviour.

   Where students slip. Saying alcohol and glucose 'have no hydrogen to give' — they do contain hydrogen atoms; the point is that those atoms don't ionise into free H⁺ in solution.

2. 22 marksNCERT Cl-10 Science, In-text Qs after §2.2.1, Q2

   Why does an aqueous solution of an acid conduct electricity?

   Hint. Electric current in a solution needs something to physically carry the charge from one electrode to the other.

   Step 1 — Recall what carries current through a solution. Electric current through any solution is carried by the movement of ions, not by electrons moving through a wire the way they do in a metal.

   Step 2 — Apply this to an acid. An acid ionises in water to produce H⁺ (as H₃O⁺) ions and the corresponding negative ion (e.g. Cl⁻ from HCl). These free-moving ions carry the current through the solution, which is why the bulb glows in Activity 2.8 when the acid solution is tested.

   ✦ Answer: An aqueous acid solution conducts electricity because the acid ionises in water, producing H⁺(aq) and an anion, and it is the movement of these free ions through the solution that carries the current.

   Where students slip. Saying 'because acids are corrosive' or similarly vague reasoning — conduction specifically requires charged particles (ions) free to move, which is the actual mechanism here.

3. 32 marksNCERT Cl-10 Science, In-text Qs after §2.2.1, Q3

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

   Hint. Look at the equation the chapter gives for how H⁺ ions actually appear from HCl.

   Step 1 — Recall how HCl produces H⁺ ions. The chapter's own equation, HCl + H₂O → H₃O⁺ + Cl⁻, shows that H⁺ ions are only released from HCl in the presence of water — the H⁺ ion cannot separate from the Cl⁻ without water's help.

   Step 2 — Apply this to the dry case. Dry HCl gas has no water, so this ionisation cannot happen — no H⁺ ions are produced. Since dry litmus paper is also without moisture, there is no ionisation and therefore no acidic behaviour to show, so the colour does not change.

   ✦ Answer: Dry HCl gas doesn't ionise into H⁺ and Cl⁻ ions without water present (HCl + H₂O → H₃O⁺ + Cl⁻), so with no H⁺ ions formed and no moisture on the dry litmus paper either, no acidic behaviour — and no colour change — occurs.

   Where students slip. Saying HCl 'isn't an acid until dissolved' — HCl is still the same compound; the precise point is that it needs water to ionise and release H⁺, which is what acidic behaviour actually depends on.

4. 42 marksNCERT Cl-10 Science, In-text Qs after §2.2.1, Q4

   While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?

   Hint. Recall which direction the heat problem gets worse — adding a small amount of something to a large amount, or the reverse.

   Step 1 — Recall why this matters at all. Dissolving a concentrated acid in water is highly exothermic, releasing a large amount of heat.

   Step 2 — Compare the two orders of mixing. If water is added to a concentrated acid, the small amount of water added gets heated intensely and suddenly, which can make it boil and splash the acid out violently, causing burns, and the sudden local heating can even crack the glass container.

   Step 3 — Why the recommended order avoids this. Adding acid slowly to a large volume of water instead lets the heat spread out through the much larger volume of water, keeping the temperature rise gradual and controlled.

   ✦ Answer: Acid should be added to water, not the reverse, because mixing is highly exothermic — adding water to concentrated acid can make the small amount of water added boil and splash acid out violently, causing burns, whereas adding acid to a large volume of water lets the heat spread out safely.

   Where students slip. Saying only 'it's safer' without explaining the actual mechanism (localised boiling and splashing) — the question specifically wants the reasoning, not just the rule.

5. 51 markNCERT Cl-10 Science, In-text Qs after §2.2.1, Q5

   How is the concentration of hydronium ions (H₃O⁺) affected when a solution of an acid is diluted?

   Hint. Dilution adds more of one thing and keeps the amount of everything else the same.

   Step 1 — Recall what dilution does. Diluting a solution means adding more water while keeping the amount of dissolved acid the same, which spreads the same number of ions over a larger volume.

   Step 2 — Apply this to hydronium ions. Since the same number of H₃O⁺ ions is now spread through more water, the concentration of H₃O⁺ ions per unit volume decreases.

   ✦ Answer: The concentration of hydronium ions (H₃O⁺) decreases when an acid solution is diluted.

   Where students slip. Confusing 'diluting' with 'adding more acid' — dilution specifically means adding water, which lowers ion concentration, the opposite of adding more acid.

6. 61 markNCERT Cl-10 Science, In-text Qs after §2.2.1, Q6

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

   Hint. This time, unlike the previous question, more of the actual solute is being added, not more water.

   Step 1 — Recognise the difference from dilution. Here, more base is being dissolved into the solution — this adds more solute, unlike dilution, which adds more water.

   Step 2 — Apply this to hydroxide ions. Dissolving additional base releases more OH⁻ ions into the same solution, so the concentration of hydroxide ions increases.

   ✦ Answer: The concentration of hydroxide ions (OH⁻) increases when excess base is dissolved into a sodium hydroxide solution.

   Where students slip. Answering as if this were the same as the previous dilution question — this one adds more base (increasing OH⁻), while the previous one added water (decreasing H₃O⁺); the two questions are opposites in structure.

Solutions written by the tuition.in editorial team and checked against the NCERT Class 10 Science textbook, Reprint 2026-27 (jesc102.pdf) — five in-text question sets (19 questions total) plus one end-of-chapter Exercise (15 questions, not 20 as some older manifests claim). Bleaching powder is written as Ca(ClO)₂ in the current text, matching the chapter's own in-text question.. Questions are referenced from the NCERT textbook for identification.