Acids, Bases & Salts

Acid: a substance that releases H⁺ (hydrogen) ions in water. Sour taste. Turns blue litmus red.

Base: releases OH⁻ (hydroxide) ions in water. Bitter taste, soapy feel. Turns red litmus blue. A water-soluble base is called an alkali.

Salt: formed when an acid reacts with a base — neutralization. Acid + Base → Salt + Water. Example: HCl + NaOH → NaCl + H₂O.

Common acids and their occurrence:

• HCl (hydrochloric acid): stomach digestive juice.
• H₂SO₄ (sulphuric acid): car batteries, industrial use.
• HNO₃ (nitric acid): industrial.
• CH₃COOH (acetic acid): vinegar.
• Citric acid: lemons, oranges.
• Lactic acid: curd, sour milk; produced in muscles during exercise.
• Tartaric acid: tamarind.
• Formic acid: ant stings, bee stings.
• Oxalic acid: spinach, tomatoes.

Common bases:

• NaOH (sodium hydroxide / caustic soda): soap-making, drain cleaner.
• KOH (potassium hydroxide): soft soaps.
• Ca(OH)₂ (calcium hydroxide / slaked lime): whitewashing.
• Mg(OH)₂ (milk of magnesia): antacid.
• NH₄OH (ammonium hydroxide): glass cleaner.

pH scale: measures acidity/basicity. Ranges from 0 to 14.

• pH < 7: acidic. Stronger acid → lower pH. Stomach acid pH ~1–2.
• pH = 7: neutral. Pure water.
• pH > 7: basic (alkaline). Stronger base → higher pH.
• Blood pH: ~7.4 (slightly basic). Death possible if outside 6.8–7.8.

Indicators:

• Litmus: blue → red in acid; red → blue in base.
• Phenolphthalein: colourless in acid, pink in base.
• Methyl orange: red in acid, yellow in base.
• Universal indicator: gradient of colours from red (acid) through green (neutral) to violet (strong base).

Salts:

• NaCl (common salt): preservation, seasoning, electrolyte.
• NaHCO₃ (baking soda / sodium bicarbonate): cooking, antacid, fire extinguisher.
• Na₂CO₃ (washing soda): laundry, glass-making.
• CaSO₄·½H₂O (plaster of Paris): casts, statues.
• CuSO₄·5H₂O (blue vitriol): blue colour due to water of crystallization.

Example 1 — Antacid action:
Indigestion is caused by excess HCl in stomach. An antacid like milk of magnesia (Mg(OH)₂) neutralizes it.
Reaction: 2HCl + Mg(OH)₂ → MgCl₂ + 2H₂O.

Example 2 — Acid rain:
SO₂ and NO_x from factories react with rainwater to form H₂SO₃ and HNO₃ → acidic rain. Damages buildings (especially marble) and harms plants/aquatic life.
Reaction: SO₂ + H₂O → H₂SO₃ (sulphurous acid).

Example 3 — Bee sting:
Bee sting is acidic (formic acid). Apply a mild base like baking soda (NaHCO₃) for relief.
Wasp sting is alkaline → apply vinegar (acetic acid).

Example 4 — Bread/cake rising:
Baking soda + tartaric acid + heat → CO₂ gas + salt + water. The CO₂ makes the dough rise.

Example 5 — Industrial use:

• Tooth decay: bacteria produce acid from sugar → enamel dissolves below pH 5.5. Toothpaste is basic → neutralizes.
• Soil acidity: farmers add slaked lime (Ca(OH)₂) to neutralize acidic soil.

pH values to remember:

Plaster of Paris is made by heating gypsum (CaSO₄·2H₂O) at 100°C; loses 3/4 of water. Adding water reverses, hardening into a stone-like form — used for casts.

Salt hydrates:

• Blue vitriol: CuSO₄·5H₂O — turns white on heating (loses water).
• Gypsum: CaSO₄·2H₂O — used in cement and plaster.
• Washing soda: Na₂CO₃·10H₂O — efflorescent (loses water in air).

Common trap: not all sour substances are acids in the chemical sense; pH < 7 is the precise test. Conversely, not all bitter foods are basic (e.g. coffee is acidic).