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Chapter 2 · Question 5

What do all acids and all bases have in common? Explain the role of water in making a substance show acidic or basic behaviour, giving examples of HCl and NaOH.

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Answer

Direct Answer

All acids produce hydrogen ions (H+\text{H}^+) in aqueous solutions, which are responsible for their acidic character. All bases produce hydroxide ions (OH−\text{OH}^-) in aqueous solutions, which are responsible for their basic character. Water plays a crucial role: dry HCl gas does not change the colour of dry blue litmus paper, but moist litmus turns red — because H+\text{H}^+ ions separate from HCl only in the presence of water. In water, HCl dissociates as: HCl+H2O→H3O++Cl−\text{HCl} + \text{H}_2\text{O} \rightarrow \text{H}_3\text{O}^+ + \text{Cl}^-. The H+\text{H}^+ ion does not exist alone; it combines with water to form the hydronium ion (H3O+\text{H}_3\text{O}^+). Similarly, NaOH dissolves in water to produce ions: NaOH(s)→H2ONa+(aq)+OH−(aq)\text{NaOH(s)} \xrightarrow{\text{H}_2\text{O}} \text{Na}^+\text{(aq)} + \text{OH}^-\text{(aq)}.

Simple Explanation

Every acid releases H+\text{H}^+ ions when dissolved in water, and every base releases OH−\text{OH}^- ions. Water is the key — without water, acids and bases cannot show their characteristic behaviour. Dry HCl gas sitting in a tube will not turn dry litmus paper red, but if the litmus is even slightly moist, it instantly turns red. That is because water pulls the HCl molecule apart into H+\text{H}^+ and Cl−\text{Cl}^- ions. The H+\text{H}^+ ions don't float alone — they attach to water molecules to form hydronium ions (H3O+\text{H}_3\text{O}^+). Similarly, solid NaOH is just a white pellet — only when water dissolves it do the Na+\text{Na}^+ and OH−\text{OH}^- ions become free.

Exam-Ready Structure

The commonality of all acids and bases is explained through the ions they produce in water. 1. What all acids have in common: All acids produce hydrogen ions, H+(aq)\text{H}^+\text{(aq)}, when dissolved in water. These ions are responsible for the characteristic properties of acids. Acids contain H+\text{H}^+ as the cation along with anions such as Cl−\text{Cl}^- in HCl, NO3−\text{NO}_3^- in HNO₃, SO42−\text{SO}_4^{2-} in H₂SO₄, and CH3COO−\text{CH}_3\text{COO}^- in CH₃COOH. 2. What all bases have in common: All bases produce hydroxide ions, OH−(aq)\text{OH}^-\text{(aq)}, when dissolved in water. These ions are responsible for basic properties. Bases that dissolve in water are specifically called alkalis. 3. Role of water — the HCl experiment (Activity 2.9): (a) Solid NaCl is taken in a dry test tube and concentrated H2SO4\text{H}_2\text{SO}_4 is added. HCl gas is evolved and tested with dry blue litmus paper — no colour change. (b) The same gas is then tested with moist blue litmus paper — the paper turns red. (c) Inference: Dry HCl gas does not release H+\text{H}^+ ions. Only in the presence of water do HCl molecules ionise to release hydrogen ions: HCl+H2O→H3O++Cl−\text{HCl} + \text{H}_2\text{O} \rightarrow \text{H}_3\text{O}^+ + \text{Cl}^-. (d) The H+\text{H}^+ ion does not exist independently in water; it combines with a water molecule to form the hydronium ion (H3O+\text{H}_3\text{O}^+). Thus, acidic behaviour is due to H3O+\text{H}_3\text{O}^+ or H+(aq)\text{H}^+\text{(aq)} ions. 4. Similarly, NaOH dissociates in water: NaOH(s)→H2ONa+(aq)+OH−(aq)\text{NaOH(s)} \xrightarrow{\text{H}_2\text{O}} \text{Na}^+\text{(aq)} + \text{OH}^-\text{(aq)}. 5. Non-acids: Compounds like glucose and alcohol also contain hydrogen but do not produce H+\text{H}^+ ions in water, so they do not show acidic character.

Key Points

  • All acids produce H+(aq)\text{H}^+\text{(aq)} ions in water; all bases produce OH−(aq)\text{OH}^-\text{(aq)} ions in water
  • Dry HCl gas does not turn dry blue litmus red — water is needed to release H+\text{H}^+ ions
  • HCl+H2O→H3O++Cl−\text{HCl} + \text{H}_2\text{O} \rightarrow \text{H}_3\text{O}^+ + \text{Cl}^- (hydronium ion formation)
  • H+\text{H}^+ ions do not exist independently in water; they combine with H2O\text{H}_2\text{O} to form H3O+\text{H}_3\text{O}^+
  • Glucose and alcohol contain hydrogen but do not produce ions in water — therefore not acidic
  • Bases that dissolve in water are called alkalis (e.g., NaOH, KOH); not all bases are alkalis

Common Mistakes

  • Thinking all hydrogen-containing compounds are acids — only those that produce H+\text{H}^+ ions in water are acids
  • Writing H+\text{H}^+ instead of H+(aq)\text{H}^+\text{(aq)} or H3O+\text{H}_3\text{O}^+ for aqueous solutions — free protons do not exist in water

Related Questions

Q4

What is a neutralisation reaction? Explain with the help of a simple activity involving sodium hydroxide and hydrochloric acid. Give the balanced chemical equation.

Q6

Why should an acid always be added to water and not water to the acid while diluting? What happens to the concentration of H3O+\text{H}_3\text{O}^+ ions when an acid is diluted?

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