Acids, Bases and Salts

1. Acids

Properties and Indicators

  • General Properties: Sour taste, corrosive, conduct electricity in aqueous solution.
  • Chemical Reactions:
    • $\text{Acid} + \text{Metal} \rightarrow \text{Salt} + \text{Hydrogen}$
    • $\text{Acid} + \text{Base} \rightarrow \text{Salt} + \text{Water}$
    • $\text{Acid} + \text{Metal Carbonate} \rightarrow \text{Salt} + \text{Water} + \text{Carbon Dioxide}$
  • Indicator Effects:
    Indicator Acidic Condition
    Litmus Red
    Thymolphthalein Colourless
    Methyl Orange Red

Definitions and Strength

  • Proton Donor: Acids are substances that donate protons ($\text{H}^+$ ions) to other substances.
  • Strong Acids: Completely dissociate in aqueous solution (e.g., $\text{HCl}$, $\text{H}_2\text{SO}_4$).
  • Weak Acids: Partially dissociate in aqueous solution (e.g., $\text{CH}_3\text{COOH}$).

2. Bases and Alkalis

Definitions and Properties

  • Base: A substance (usually a metal oxide or hydroxide) that neutralises an acid.
  • Alkali: A soluble base.
  • Chemical Reactions:
    • $\text{Base} + \text{Acid} \rightarrow \text{Salt} + \text{Water}$
    • $\text{Base} + \text{Ammonium Salt} \rightarrow \text{Salt} + \text{Water} + \text{Ammonia}$
  • Indicator Effects:
    Indicator Alkaline Condition
    Litmus Blue
    Thymolphthalein Blue
    Methyl Orange Yellow

Oxides

  • Basic Oxides: Metal oxides that react with acids (e.g., $\text{CuO}$, $\text{CaO}$).
  • Acidic Oxides: Non-metal oxides that react with bases (e.g., $\text{SO}_2$, $\text{CO}_2$).
  • Amphoteric Oxides: Oxides that react with both acids and bases (e.g., $\text{Al}_2\text{O}_3$, $\text{ZnO}$).

3. pH and Neutralisation

The pH Scale

  • $\text{H}^+$ Concentration: Acids contain $\text{H}^+$ ions; alkalis contain $\text{OH}^-$ ions.
  • Universal Indicator: Used to compare acidity/alkalinity across the pH scale (0-14).
  • pH 7: Neutral.

Neutralisation

  • Ionic Equation: $\text{H}^+(\text{aq}) + \text{OH}^-(\text{aq}) \rightarrow \text{H}_2\text{O}(\text{l})$
  • This reaction occurs when an acid and a base react to form a salt and water.

4. Salts

Solubility Rules

Soluble Insoluble
All $\text{Na}^+, \text{K}^+, \text{NH}_4^+, \text{NO}_3^-$ salts All $\text{CO}_3^{2-}$ except $\text{Na}, \text{K}, \text{NH}_4$
All $\text{Cl}^-$ except $\text{Pb}^{2+}, \text{Ag}^+$ $\text{OH}^-$ except $\text{Na}, \text{K}, \text{NH}_4, \text{Ca}$
All $\text{SO}_4^{2-}$ except $\text{Ba}^{2+}, \text{Ca}^{2+}, \text{Pb}^{2+}$

Preparation of Salts

  • Insoluble Salts: Prepared by precipitation (mixing two soluble salts).
  • Soluble Salts:
    • Titration: Used when both reactants are solutions (Acid + Alkali).
    • Excess Solid Method: Used when one reactant is an insoluble base or metal.
      • $\text{Acid} + \text{Excess Metal/Base/Carbonate} \rightarrow \text{Salt} + \text{H}_2/\text{H}_2\text{O}/(\text{H}_2\text{O} + \text{CO}_2)$.
      • Excess is filtered off, then the solution is evaporated to crystallisation.

Hydration

  • Hydrated Salt: A salt that contains chemically combined water (e.g., $\text{CuSO}_4\cdot 5\text{H}_2\text{O}$).
  • Anhydrous Salt: A salt that contains no water.
  • Water of Crystallisation: The fixed amount of water molecules associated with each formula unit of a salt.

Salt preparation flow chart