Question 20
Define (i) salt (ii) normal salt (iii) acid salt – with relevant examples and equations.
(i) Salt — A salt is a compound formed by partial or complete replacement of the replaceable hydrogen ions of an acid by a metallic ion or ammonium ion [basic radical].
NaOH + H2SO4 ⟶ NaHSO4 + H2O [Partial replacement]
2NaOH + H2SO4 ⟶ Na2SO4 + 2H2O [Complete replacement]
(ii) Normal salt — The salt formed by complete replacement of the replaceable hydrogen ion of an acid molecule by a basic radical [metallic or ammonium ion].
For example,
2NaOH + H2SO4 ⟶ Na2SO4 + 2H2O
2NaOH + H2SO3 ⟶ Na2SO3 + 2H2O
[Both H ions in sulphuric and sulphurous acid are replaced by metallic radical — sodium. ]
(3) Acid salt — The salt formed by partial replacement of the replaceable hydrogen ion of an acid molecule by a basic radical [metallic or ammonium ion]. For example,
NaOH + H2SO4 ⟶ NaHSO4 + H2O
NaOH + H2SO3 ⟶ NaHSO3 + H2O
[Only one H ion in sulphuric and sulphurous acid is replaced by metallic radical — sodium. ]
Chapter Overview: Acids, Bases and Salts
This chapter covers the properties, definitions, and reactions of acids, bases, and salts. According to the Arrhenius theory, acids produce H+ ions in aqueous solution and bases produce OH− ions. The Bronsted-Lowry theory defines acids as proton donors and bases as proton acceptors. The pH scale (0-14) measures the concentration of H+ ions: pH < 7 is acidic, pH = 7 is neutral, pH > 7 is basic. Students must understand the properties of acids (sour taste, turn blue litmus red, react with metals to produce H2) and bases (bitter taste, soapy feel, turn red litmus blue). Neutralisation is the reaction between an acid and a base to form salt and water. Salts are classified as normal, acidic, or basic salts. Understanding the preparation of salts by neutralisation, direct combination, and double decomposition is essential. The chapter also covers indicators (litmus, phenolphthalein, methyl orange) and their colour changes. Strong acids (HCl, H2SO4, HNO3) are fully ionised; weak acids (CH3COOH, H2CO3) are partially ionised.
Key Definitions & Reactions
| Concept | Details |
|---|---|
| Arrhenius Acid | Substance that produces H+ (H3O+) ions in aqueous solution |
| Arrhenius Base | Substance that produces OH− ions in aqueous solution |
| pH Scale | Measures H+ ion concentration; pH = −log[H+]; range 0 to 14 |
| Neutralisation | Acid + Base → Salt + Water (e.g., HCl + NaOH → NaCl + H2O) |
| Basicity of an Acid | Number of replaceable H+ ions (HCl = monobasic, H2SO4 = dibasic, H3PO4 = tribasic) |
| Acidity of a Base | Number of replaceable OH− ions (NaOH = monoacidic, Ca(OH)2 = diacidic) |
| Normal Salt | All H+ ions replaced by metal ions (e.g., Na2SO4) |
| Acid Salt | Partial replacement of H+ (e.g., NaHSO4) |
Must-Know Concepts
- Acid + Metal → Salt + H2↑ (e.g., 2HCl + Zn → ZnCl2 + H2↑)
- Acid + Metal Oxide → Salt + Water (e.g., 2HCl + CuO → CuCl2 + H2O)
- Acid + Carbonate → Salt + Water + CO2↑ (e.g., 2HCl + CaCO3 → CaCl2 + H2O + CO2↑)
- Strong acids are fully dissociated; weak acids are partially dissociated
- Dilute acids are those with more water; concentrated acids have less water
- Indicators: Litmus (red/blue), Phenolphthalein (colourless/pink), Methyl orange (red/yellow)
- Water of crystallisation: CuSO4·5H2O (blue vitriol), Na2CO3·10H2O (washing soda)
Strong vs Weak Acids
| Property | Strong Acid | Weak Acid |
|---|---|---|
| Ionisation | Complete | Partial |
| pH | Very low (0-2) | Moderately low (3-5) |
| Conductivity | Good conductor | Poor conductor |
| Examples | HCl, H2SO4, HNO3 | CH3COOH, H2CO3 |
Important Diagrams to Practice
- pH scale with common substances marked (lemon juice, vinegar, blood, soap solution)
- Laboratory preparation of HCl gas from NaCl and conc. H2SO4
- Action of dilute acid on metal, carbonate, and base (with gas collection)
Common Mistakes
- Confusing "dilute" with "weak" and "concentrated" with "strong" - these are different concepts
- Writing unbalanced equations for neutralisation reactions
- Forgetting that dry HCl gas does not turn blue litmus red (water is needed for ionisation)
- Mixing up indicator colours: phenolphthalein is colourless in acid (not white)
- Not writing water of crystallisation in salt formulae when asked
Scoring Tips
- Always balance chemical equations - marks are deducted for unbalanced equations
- When asked about "role of water", explain that water helps in ionisation of acids/bases
- Memorise indicator colour changes in tabular form for quick recall
- For salt preparation questions, state the method AND write the balanced equation
Frequently Asked Questions
Why does dry HCl gas not show acidic properties?
Dry HCl does not contain water, so it cannot ionise to produce H+ ions. Acidic properties are due to H+ ions, which form only in the presence of water. Hence, dry HCl does not turn dry blue litmus red.
What is the difference between strong/weak and concentrated/dilute?
Strong/weak refers to the degree of ionisation. A strong acid is fully ionised (HCl), a weak acid is partially ionised (CH3COOH). Concentrated/dilute refers to the amount of acid dissolved in water. A dilute strong acid is still fully ionised.
Why is neutralisation an exothermic reaction?
During neutralisation, H+ ions from the acid combine with OH− ions from the base to form water (H+ + OH− → H2O). The formation of the covalent bond in water releases energy, making the reaction exothermic.