Chemistry — Acids, Bases, Salts, pH Scale, and Indicators

Acids, bases, and salts form a core chemistry topic in Kerala PSC exams. Questions test pH values, indicator colors, everyday applications, and neutralization reactions. Expect 2-3 questions per paper.

Definitions

Concept	Arrhenius Definition	Bronsted-Lowry Definition
Acid	Produces H+ ions in water	Proton (H+) donor
Base	Produces OH- ions in water	Proton (H+) acceptor
Amphoteric	Can act as both acid and base	Water (H2O), Aluminium hydroxide, Zinc oxide

Properties of Acids and Bases

Property	Acids	Bases
Taste	Sour	Bitter
Touch	—	Soapy/slippery
Litmus paper	Turns blue litmus red	Turns red litmus blue
pH range	0 to below 7	Above 7 to 14
Electrical conductivity	Conduct electricity (electrolytes)	Conduct electricity (electrolytes)
React with metals	Produce hydrogen gas	Some react with metals (e.g., NaOH + Zn)
React with carbonates	Produce CO2 gas	No reaction

The pH Scale

pH Value	Nature	Examples
0	Strongest acid	—
1	Very strong acid	Battery acid (H2SO4)
2	Strong acid	Gastric acid (HCl in stomach), Lemon juice
3	Moderately acidic	Vinegar
4	Acidic	Tomato juice
5	Weakly acidic	Black coffee, Banana
6	Slightly acidic	Milk (6.5), Urine (6.0), Saliva
7	Neutral	Pure water
7.4	Slightly basic	Human blood (7.35-7.45)
8	Weakly basic	Sea water, Baking soda solution
9	Mildly basic	Borax solution
10	Basic	Milk of Magnesia
11	Moderately basic	Ammonia solution
12	Strong base	Soapy water
13	Very strong base	Bleach (NaClO)
14	Strongest base	NaOH solution (concentrated)

Key fact: pH stands for “potential of Hydrogen.” It was introduced by S.P.L. Sorensen in 1909.

Common Acids

Acid	Formula	Found In / Use
Hydrochloric acid	HCl	Gastric juice (stomach acid); cleaning agent
Sulfuric acid	H2SO4	”King of chemicals”; car batteries; fertilizer production
Nitric acid	HNO3	Fertilizers; explosives (TNT, RDX)
Acetic acid	CH3COOH	Vinegar (4-8% solution)
Citric acid	C6H8O7	Citrus fruits (lemon, orange)
Carbonic acid	H2CO3	Soda water, carbonated drinks
Phosphoric acid	H3PO4	Cola drinks; fertilizers
Oxalic acid	C2H2O4	Tomatoes, spinach; rust removal
Tartaric acid	C4H6O6	Grapes, tamarind; baking powder
Formic acid	HCOOH	Ant stings, bee stings; named from Latin “formica” (ant)
Lactic acid	C3H6O3	Sour milk, curd; muscle fatigue
Ascorbic acid	C6H8O6	Vitamin C (amla, citrus)

Common Bases

Base	Formula	Use
Sodium hydroxide	NaOH	Caustic soda; soap making; drain cleaner
Potassium hydroxide	KOH	Caustic potash; soft soap; alkaline batteries
Calcium hydroxide	Ca(OH)2	Slaked lime; whitewashing; water treatment
Magnesium hydroxide	Mg(OH)2	Milk of Magnesia (antacid)
Ammonium hydroxide	NH4OH	Cleaning agent; smelling salts
Aluminium hydroxide	Al(OH)3	Antacid (e.g., Gelusil)
Calcium oxide	CaO	Quicklime; used in cement

Indicators

Indicator	In Acid	In Base	Neutral
Litmus	Red	Blue	Purple
Phenolphthalein	Colorless	Pink/Magenta	Colorless
Methyl orange	Red/Pink	Yellow	Orange
Turmeric (haldi)	Yellow (no change)	Reddish-brown	Yellow
China rose (Hibiscus)	Dark pink/Magenta	Green	—
Universal indicator	Red-Orange-Yellow	Blue-Violet	Green

Olfactory indicators: Onion (loses smell in base), Vanilla (loses smell in base), Clove oil (loses smell in base).

Neutralization Reaction

Acid + Base = Salt + Water

Reaction	Type of Salt
Strong acid + Strong base	Neutral salt (pH = 7)
Strong acid + Weak base	Acidic salt (pH below 7)
Weak acid + Strong base	Basic salt (pH above 7)
Weak acid + Weak base	Depends on relative strength

Examples

Reaction	Salt Formed
HCl + NaOH = NaCl + H2O	Sodium chloride (neutral, pH 7)
HCl + NH4OH = NH4Cl + H2O	Ammonium chloride (acidic)
CH3COOH + NaOH = CH3COONa + H2O	Sodium acetate (basic)
H2SO4 + Ca(OH)2 = CaSO4 + 2H2O	Calcium sulfate (Plaster of Paris precursor)

Types of Salts

Type	Formation	pH	Examples
Normal salt	Complete neutralization	Varies	NaCl, Na2SO4, KNO3
Acidic salt	Partial neutralization of polybasic acid	Below 7	NaHSO4, NaHCO3
Basic salt	Partial neutralization of polyacidic base	Above 7	Pb(OH)Cl, Bi(OH)2Cl
Double salt	Two simple salts crystallized together	—	Potash alum: K2SO4.Al2(SO4)3.24H2O
Complex salt	Contains complex ion	—	K4[Fe(CN)6] (Potassium ferrocyanide)

Important Salts for PSC

Salt	Common Name	Use
NaCl	Common salt / Table salt	Food; raw material for NaOH, Cl2, Na2CO3
NaHCO3	Baking soda (Sodium bicarbonate)	Baking; antacid; fire extinguisher
Na2CO3.10H2O	Washing soda	Cleaning; softening hard water
CaSO4.1/2H2O	Plaster of Paris	Casts, moulds, sculptures
CaSO4.2H2O	Gypsum	Cement additive; plaster
NaOH	Caustic soda	Soap, paper, textiles
Ca(OCl)2	Bleaching powder	Water purification; bleaching
KNO3	Saltpetre	Gunpowder, fertilizer
CaCO3	Limestone / Marble / Chalk	Construction, cement

Buffer Solutions

Feature	Details
Definition	A solution that resists change in pH when small amounts of acid or base are added
Acidic buffer	Weak acid + its salt with strong base (e.g., CH3COOH + CH3COONa)
Basic buffer	Weak base + its salt with strong acid (e.g., NH4OH + NH4Cl)
Human blood	Acts as a buffer; maintained at pH 7.35-7.45 by carbonate buffer system
Importance	Essential in biological systems, industrial processes, laboratory work

Everyday Applications

Application	Chemistry
Antacid tablets	Mg(OH)2 or Al(OH)3 neutralizes excess HCl in stomach
Bee sting treatment	Apply baking soda (NaHCO3) — bee venom is acidic
Wasp sting treatment	Apply vinegar (acetic acid) — wasp venom is basic
Tooth decay	Bacteria produce acids (pH below 5.5 decays enamel); toothpaste is basic
Soil treatment	Acidic soil treated with quicklime (CaO); basic soil treated with organic matter
Factory waste treatment	Neutralization before discharge into water bodies

PSC Quick Recall

Question	Answer
pH of pure water	7 (neutral)
pH of human blood	7.35-7.45
King of chemicals	Sulfuric acid (H2SO4)
Acid in ant sting	Formic acid
Acid in vinegar	Acetic acid
Acid in lemon	Citric acid
Acid in curd/sour milk	Lactic acid
Baking soda chemical name	Sodium bicarbonate (NaHCO3)
Milk of Magnesia is	Mg(OH)2 (antacid)
Phenolphthalein in base turns	Pink
Litmus is obtained from	Lichens
pH term was coined by	S.P.L. Sorensen (1909)
Plaster of Paris formula	CaSO4.1/2H2O
Bleaching powder formula	Ca(OCl)2