Acids

Acids are chemical substances that produce hydrogen ions (H⁺) or hydronium ions (H₃O⁺) when dissolved in water. They are characterised by their sour taste, ability to turn blue litmus paper red, and their tendency to react with metals, bases, and carbonates. Acids are among the most important compounds in chemistry and play vital roles in industry, biology, and environmental processes.

Definition and Theories of Acidity

The concept of acids has evolved through several scientific theories:

1. Arrhenius Theory (1884):
   • An acid is a substance that produces hydrogen ions (H⁺) in aqueous solution.
   • Example: HCl (aq) → H⁺ + Cl⁻
   • This theory applies mainly to aqueous solutions.
2. Brønsted–Lowry Theory (1923):
   • An acid is a proton donor, and a base is a proton acceptor.
   • Example: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻
   • This definition is broader and not limited to water-based reactions.
3. Lewis Theory (1923):
   • A Lewis acid is a substance that can accept an electron pair, while a Lewis base donates an electron pair.
   • Example: BF₃ + NH₃ → F₃BNH₃
   • This theory explains acid–base reactions even in non-aqueous systems.

Physical and Chemical Properties of Acids

Physical Properties:

• Have a sour taste (for example, citric acid in lemons and acetic acid in vinegar).
• Corrosive in nature, especially strong mineral acids.
• Conduct electricity in aqueous solution due to ionisation.
• Change the colour of indicators:
  • Turn blue litmus red.
  • Turn methyl orange red.
  • Turn phenolphthalein colourless.

Chemical Properties:

1. Reaction with Metals:Acids react with active metals such as zinc and magnesium to produce hydrogen gas.Zn + 2HCl → ZnCl₂ + H₂
2. Reaction with Bases (Neutralisation):Acids react with bases to form salt and water.HCl + NaOH → NaCl + H₂O
3. Reaction with Carbonates and Bicarbonates:Produces carbon dioxide, water, and a salt.2HCl + Na₂CO₃ → 2NaCl + CO₂ + H₂O
4. Reaction with Metal Oxides:Forms a salt and water, showing that metal oxides are basic in nature.2HCl + CuO → CuCl₂ + H₂O
5. Ionisation in Water:Acids ionise in water to produce hydrogen ions.HNO₃ → H⁺ + NO₃⁻

Classification of Acids

Acids can be classified on several bases:
1. Based on Source:

• Mineral (Inorganic) Acids: Derived from minerals. Examples: Hydrochloric acid (HCl), Sulphuric acid (H₂SO₄), Nitric acid (HNO₃).
• Organic Acids: Found in living organisms. Examples: Acetic acid (CH₃COOH), Citric acid, Lactic acid.

2. Based on Strength:

• Strong Acids: Completely ionise in water. Examples: HCl, HNO₃, H₂SO₄.
• Weak Acids: Partially ionise in water. Examples: CH₃COOH, H₂CO₃, HCN.

3. Based on Concentration:

• Concentrated Acids: Contain a large amount of acid and little water.
• Dilute Acids: Contain a small amount of acid dissolved in a large amount of water.

4. Based on Basicity (Number of Ionisable Hydrogen Atoms):

• Monobasic Acids: Produce one H⁺ ion per molecule (for example, HCl, HNO₃).
• Dibasic Acids: Produce two H⁺ ions per molecule (for example, H₂SO₄, H₂CO₃).
• Tribasic Acids: Produce three H⁺ ions per molecule (for example, H₃PO₄).

5. Based on Volatility:

• Volatile Acids: Easily vapourise (for example, HCl, HNO₃).
• Non-volatile Acids: Do not easily vapourise (for example, H₂SO₄).

Common Acids and Their Uses

pH and Acidity

The pH scale measures the strength of acids and bases, ranging from 0 to 14:

• pH < 7: Acidic solution
• pH = 7: Neutral (pure water)
• pH > 7: Basic (alkaline) solution

The lower the pH value, the stronger the acid. For example, the pH of HCl (a strong acid) is around 1, while that of acetic acid (a weak acid) is around 3–4.

Environmental and Biological Importance

• In Industry: Acids are used in the production of fertilisers, detergents, plastics, and explosives.
• In Biology: The human stomach secretes hydrochloric acid (HCl) to aid digestion and kill pathogens.
• In Environment: Acid rain, caused by sulphuric and nitric acids formed from industrial emissions, leads to soil degradation, corrosion of buildings, and damage to aquatic life.

Safety and Handling

Acids, particularly strong mineral acids, are highly corrosive and must be handled with care:

• Use protective gloves and eyewear.
• Always add acid to water, never the reverse, to avoid violent reactions.
• Store acids in non-reactive containers away from heat.

Natural Occurrence

Acids occur naturally in various forms:

• Citric acid in lemons and oranges.
• Tartaric acid in grapes.
• Lactic acid in sour milk and yoghurt.
• Formic acid in ant and bee stings.