Acids, Bases, and Salts

1. Introduction & Definitions

Acid: A substance which in aqueous solution tastes sour, turns blue litmus red, and releases H⁺ ions.

Base: A substance which in aqueous solution tastes bitter, feels soapy, turns red litmus blue, and releases OH⁻ ions.

Alkali: A base that is soluble in water (e.g., NaOH, KOH).

Salt: An ionic compound formed by the neutralization of an acid and a base.

2. Properties of Acids & Bases

Properties of Acids

Sour taste
Blue litmus → Red
Conduct electricity (aqueous)
React with metals → H₂ gas
React with carbonates → CO₂ gas

Properties of Bases

Bitter taste and soapy touch
Red litmus → Blue
Conduct electricity (aqueous)
React with non-metallic oxides → Salt + Water
React with acids → Neutralization

3. Chemical Reactions

A. Acid + Metal

Acid + Metal → Salt + Hydrogen gas
Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g) ↑

B. Acid + Metal Carbonates/Bicarbonates

2HCl + Na₂CO₃ → 2NaCl + H₂O + CO₂ ↑

HCl + NaHCO₃ → NaCl + H₂O + CO₂ ↑

C. Neutralization Reaction

Acid + Base → Salt + Water (Exothermic)
HCl + NaOH → NaCl + H₂O

4. pH Scale

The pH scale measures the acidity or basicity of a solution from 0 to 14.

0 (Acidic) <---------- 7 (Neutral) ----------> 14 (Basic)

Substance	pH Value	Nature
Gastric juice	~1.2	Highly acidic
Lemon juice	~2.2	Acidic
Pure water	7.0	Neutral
Blood	7.4	Slightly basic
Milk of Magnesia	10.5	Basic

Importance of pH:

Tooth Decay: Starts if mouth pH is below 5.5.
Stomach: Antacids (bases) neutralize excess HCl.
Plants: Require specific soil pH for healthy growth.

5. Important Salts

Common Name	Chemical Formula	Preparation	Key Uses
Caustic Soda	NaOH	Chlor-alkali process	Soaps, paper, degreasing metals
Bleaching Powder	CaOCl₂	Cl₂ + Dry slaked lime	Disinfectant, bleaching textiles
Baking Soda	NaHCO₃	Solvay process	Baking, antacid, fire extinguishers
Washing Soda	Na₂CO₃·10H₂O	Recrystallization of Na₂CO₃	Removing permanent hardness of water
Plaster of Paris	CaSO₄·½H₂O	Heating Gypsum at 373 K	Fractured bones, toys, decoration

Water of Crystallization

The fixed number of water molecules present in one formula unit of a salt. For example, Blue Vitriol (CuSO₄·5H₂O) contains 5 molecules of water of crystallization.

Exam Tip: Be careful with Plaster of Paris preparation. If gypsum is heated above 373 K, it forms "Dead Burnt Plaster" (CaSO₄), which loses its setting property.