Acids, Bases and Salts

Why This Chapter Matters

7-10 marks in board exams every year. pH, properties, reactions, and important salts (baking soda, washing soda, bleaching powder, plaster of Paris) are all tested.

Prerequisites

•Chapter 1: Chemical reactions and types

•Indicators: litmus, phenolphthalein

•Basic atomic structure

Core Concepts

1. Acids

Substances producing H+ ions in aqueous solution.

•Taste sour | Turn blue litmus RED | pH < 7

•React with metals: Zn + H2SO4 -> ZnSO4 + H2

•React with carbonates: Na2CO3 + H2SO4 -> Na2SO4 + H2O + CO2

Strong acids: HCl, H2SO4, HNO3

Weak acids: CH3COOH (acetic acid), H2CO3 (carbonic acid)

2. Bases and Alkalis

Substances producing OH- ions in aqueous solution.

Alkali = soluble base (NaOH, KOH, Ca(OH)2)

All alkalis are bases; not all bases are alkalis.

•Taste bitter, soapy feel | Turn red litmus BLUE | pH > 7

3. pH Scale

Measures H+ ion concentration.

pH 0-7 = acidic | pH 7 = neutral | pH 7-14 = basic

Key pH values: HCl (1), vinegar (3), blood (7.4), baking soda (8.3), NaOH (14)

Tooth decay starts when pH < 5.5 (enamel CaCO3 dissolves).

Human blood: 7.35-7.45 (slightly basic).

4. Neutralisation

Acid + Base -> Salt + Water (exothermic)

HCl + NaOH -> NaCl + H2O

H2SO4 + 2NaOH -> Na2SO4 + 2H2O

5. Important Salts

Common Salt (NaCl):

Source: sea water, rock salt

Chlor-alkali process: NaCl + H2O --(electricity)--> NaOH + Cl2 + H2

Baking Soda (NaHCO3):

On heating: 2NaHCO3 -> Na2CO3 + H2O + CO2 (CO2 makes baked goods rise)

With acid: NaHCO3 + HCl -> NaCl + H2O + CO2

Uses: baking, antacid, fire extinguisher

Washing Soda (Na2CO3.10H2O):

Basic in nature. Uses: cleaning agent, softening hard water, glass/soap/paper industry.

Bleaching Powder (CaOCl2):

Made by: Ca(OH)2 + Cl2 -> CaOCl2 + H2O

Released Cl2 disinfects water and bleaches fabric.

Plaster of Paris (CaSO4.1/2H2O):

From gypsum: CaSO4.2H2O --(373K)--> CaSO4.1/2H2O + 3/2 H2O

Sets hard when water added (forms gypsum again).

Store in moisture-proof container.

Uses: casts for fractures, moulds, chalk.

Solved Examples

Q1: What happens when excess CO2 passed into Ca(OH)2?

First: Ca(OH)2 + CO2 -> CaCO3 (milky) + H2O

Excess CO2: CaCO3 + CO2 + H2O -> Ca(HCO3)2 (soluble, milkiness disappears)

Q2: pH = 4. What happens when diluted?

pH 4 = acidic. Dilution decreases H+ concentration -> pH increases towards 7.

PYQs

2023: pH of pure water? What if HCl dissolved?

Pure water pH = 7. Adding HCl increases H+ ions -> pH decreases below 7.

2022: Formula of baking soda. Why is it an antacid?

NaHCO3. Neutralises excess HCl in stomach: NaHCO3 + HCl -> NaCl + H2O + CO2

2021: Why does tooth decay start when pH < 5.5?

Acid produced by bacteria corrodes tooth enamel (CaCO3):

CaCO3 + 2HCl -> CaCl2 + H2O + CO2

2020: Formula and preparation of bleaching powder.

CaOCl2. Ca(OH)2 + Cl2 -> CaOCl2 + H2O (dry slaked lime + chlorine at 313K)

MCQ Practice

Q1. pH of neutral solution: (A) 0 (B) 7 (C) 14 (D) Depends on solvent

Answer: B

Q2. Acid + Metal Carbonate gives:

(A) Salt + water (B) Salt + CO2 (C) Salt + H2O + CO2 (D) Oxide + H2O

Answer: C

Q3 (Hard). Excess CO2 into NaOH gives:

(A) Na2CO3 (B) NaHCO3 (C) Na2O (D) NaOH

Answer: B (NaOH + CO2 -> NaHCO3 when excess CO2)

Revision Notes

ACID: H+ ions, sour, red litmus, pH < 7
BASE: OH- ions, bitter, blue litmus, pH > 7
NEUTRALISATION: Acid + Base -> Salt + Water (exothermic)

pH SCALE: 0 (strongest acid) to 14 (strongest base), 7 = neutral

IMPORTANT SALTS:
NaHCO3 (baking soda): antacid, baking
Na2CO3.10H2O (washing soda): cleaning
CaOCl2 (bleaching powder): disinfection, bleaching
CaSO4.1/2H2O (Plaster of Paris): casts, moulds
Gypsum --(heat)--> POP --(water)--> Gypsum (sets hard)

Common mistakes: Confusing alkali with base. Baking powder is NOT same as baking soda.