🧪 Acids, Bases & Salts – Full Breakdown

1) How Do We Define Acids & Bases?

a) Arrhenius Concept (ICSE main definition)

Acid: A substance that gives H⁺ ions in aqueous solution.
Base: A substance that gives OH⁻ ions in aqueous solution.

HCl → H⁺ + Cl⁻ ✅
NaOH → Na⁺ + OH⁻ ✅

Limitation: Only works for aqueous solutions. What about reactions without water? E.g., NH₃(g) + HCl(g) → NH₄Cl(s) — this is an acid-base reaction but there's no water. Arrhenius can't explain this.

b) Bronsted-Lowry Concept (out of syllabus but you should know)

Acid: Proton (H⁺) donor.
Base: Proton (H⁺) acceptor.

HCl + H₂O → H₃O⁺ + Cl⁻
HCl donates H⁺ (acid), H₂O accepts H⁺ (base).

This explains why NH₃ is a base: NH₃ + H⁺ → NH₄⁺ (NH₃ accepts a proton).

2) Types of Acids

Based on Strength

• Strong acids: Completely dissociate in water. HCl, H₂SO₄, HNO₃, HBr, HI, HClO₄
• Weak acids: Partially dissociate. CH₃COOH (acetic acid), H₂CO₃ (carbonic), H₃PO₄ (phosphoric), H₂SO₃ (sulphurous), oxalic acid, citric acid, tartaric acid

Based on Basicity (number of H⁺ ions an acid gives)

Based on Origin

• Mineral acids: HCl, H₂SO₄, HNO₃ (from minerals, strong)
• Organic acids: CH₃COOH, citric acid, lactic acid (from plants/animals, weak)

Concentrated vs Dilute

• Concentrated: Large amount of acid dissolved in small amount of water (high concentration of H⁺)
• Dilute: Small amount of acid mixed with large amount of water (low concentration of H⁺)

This is NOT the same as strong vs weak! You can have a concentrated weak acid (concentrated vinegar = still CH₃COOH, partially dissociated) or a dilute strong acid (dilute HCl = completely dissociated but few H⁺ overall). Strength = degree of dissociation. Concentration = amount dissolved.

3) Types of Bases

Alkalis (soluble bases)

All alkalis are bases, but NOT all bases are alkalis. Only soluble bases are called alkalis.

• Soluble bases (alkalis): NaOH, KOH, Ca(OH)₂, NH₄OH, Ba(OH)₂
• Insoluble bases: Cu(OH)₂, Fe(OH)₃, Al(OH)₃, Zn(OH)₂

Based on Strength

• Strong bases: NaOH, KOH, Ca(OH)₂, Ba(OH)₂ (completely dissociate)
• Weak bases: NH₄OH, Al(OH)₃, Cu(OH)₂ (partially dissociate)

Acidity of a Base

The number of OH⁻ ions a base gives. NaOH → 1 OH⁻ (acidity = 1), Ca(OH)₂ → 2 OH⁻ (acidity = 2).

4) pH Scale

pH = -log[H⁺] (you don't need to calculate log in ICSE, just understand the concept).

• pH 0-6: Acidic (more H⁺, lower pH)
• pH 7: Neutral (pure water)
• pH 8-14: Alkaline (more OH⁻, higher pH)

Indicators

Importance of pH in Everyday Life

• Stomach: pH ~1.5 (HCl helps digestion). Antacids (Mg(OH)₂, Al(OH)₃) neutralise excess acid.
• Soil: Plants grow best at pH 6.5-7. If too acidic → add lime (CaO). If too alkaline → add organic matter (manure).
• Toothpaste: Slightly alkaline to neutralise acids produced by bacteria in mouth (prevents tooth decay).
• Bee sting: Acidic (formic acid) → apply baking soda (NaHCO₃, base) to neutralise.
• Wasp sting: Alkaline → apply vinegar (CH₃COOH, acid) to neutralise.

5) Chemical Properties of Acids — All Reactions

6) Chemical Properties of Bases (Alkalis) — All Reactions

7) Salts — Classification & Preparation

Types of Salts

Methods of Salt Preparation

8) Important Salts — Properties, Preparation & Uses

a) Sodium Chloride (NaCl) — Common Salt

Source: Sea water, rock salt
Preparation: Neutralisation (HCl + NaOH), or evaporation of sea water
Uses: Cooking, preserving food, making NaOH (chlor-alkali process), making Cl₂ and H₂

b) Washing Soda (Na₂CO₃·10H₂O)

Preparation (Solvay process): NaCl + NH₃ + CO₂ + H₂O → NaHCO₃↓ + NH₄Cl. Then heat NaHCO₃ → Na₂CO₃ + CO₂ + H₂O. Crystallise with 10H₂O.

Properties: Efflorescent — loses water of crystallisation on exposure to air and becomes Na₂CO₃ powder (this is why washing soda crystals become white powder over time).

Uses: Washing (cleaning agent), glass making, removing permanent hardness of water.

c) Baking Soda (NaHCO₃)

Preparation: NaCl + NH₃ + CO₂ + H₂O → NaHCO₃↓ + NH₄Cl (the first step of Solvay process)
Property: On heating → Na₂CO₃ + CO₂ + H₂O. The CO₂ released makes cakes/breads rise.
Uses: Baking, antacid (neutralises stomach acid), fire extinguisher (reacts with H₂SO₄ to produce CO₂ foam).

d) Bleaching Powder (CaOCl₂ — Calcium Oxychloride)

Preparation: Ca(OH)₂ (slaked lime) + Cl₂ → CaOCl₂ + H₂O
Property: In presence of moisture, releases Cl₂ which is a bleaching agent (oxidises coloured substances). Bleaching is permanent — Cl₂ destroys the colour molecule, doesn't just cover it.
Uses: Bleaching cotton/linen, disinfecting water, making chloroform.
Storage: Keep in airtight container — reacts with CO₂ and moisture from air.

e) Plaster of Paris (CaSO₄·½H₂O — Calcium Sulphate Hemihydrate)

Preparation: Gypsum (CaSO₄·2H₂O) heated at 373-393 K → CaSO₄·½H₂O + 1½H₂O

Setting of Plaster of Paris: When mixed with water, it rehydrates to form gypsum: CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O (hard solid). This is an exothermic reaction and volume expands slightly (why it fills moulds perfectly).

Uses: Plastering fractured bones, making casts/moulds, dentistry, decorative work.

f) Blue Vitriol (CuSO₄·5H₂O — Copper Sulphate Pentahydrate)

Blue crystals. On heating → loses water → becomes white (anhydrous CuSO₄). Add water back → turns blue again. This is a test for water.
Uses: Fungicide, electroplating, battery electrolyte.

g) Green Vitriol (FeSO₄·7H₂O — Ferrous Sulphate Heptahydrate)

Green crystals. On heating → decomposes to Fe₂O₃ (red-brown) + SO₂ + SO₃. The SO₂ has a characteristic choking smell.
Uses: Making ink, water treatment (coagulant), medicine (iron deficiency).

9) Hygroscopic, Deliquescent & Efflorescent Substances

10) Analytical Chemistry — Action of NaOH & NH₄OH on Metal Salt Solutions

This is where you identify cations by the colour of precipitate formed. ICSE loves this in 2-3 mark questions.

Action of NaOH

Action of NH₄OH

11) Top Board-Style Questions

Q1: Why should you add acid to water and not water to acid while diluting?

Answer: Dilution of acid is highly exothermic. If water is added to concentrated acid, the heat generated can cause the water to boil instantly, leading to splashing of concentrated acid which can cause severe burns. Adding acid to water slowly ensures the heat is dissipated by the large volume of water.

Q2: Why is HNO₃ not used to prepare H₂ by reacting with metals?

Answer: HNO₃ is a strong oxidising agent. It oxidises the H₂ produced to H₂O and itself gets reduced to NO, NO₂, etc. So H₂ gas is not liberated. (Exception: with Mg and Mn, H₂ is produced because these metals are strong reducing agents.)

Q3: Distinguish between NaHCO₃ and Na₂CO₃ using a chemical test.

Answer: Heat both. NaHCO₃ decomposes to give CO₂ gas (turns limewater milky). Na₂CO₃ does not decompose on heating (it's stable). Alternatively, add MgSO₄ solution — Na₂CO₃ gives white precipitate (MgCO₃), NaHCO₃ gives no precipitate.

Q4: What happens when Plaster of Paris is heated above 393 K?

Answer: It forms dead burnt plaster (anhydrous CaSO₄), which does NOT set with water. This is because it has completely lost all water molecules and cannot rehydrate to form the interlocking crystal structure.

Q5: Why does anhydrous CuSO₄ turn blue when water is added?

Answer: Anhydrous CuSO₄ is white. When it combines with water, it forms CuSO₄·5H₂O (blue vitriol). Water molecules coordinate with Cu²⁺ ion, causing a colour change due to d-orbital splitting. This is used as a test for water (white → blue).

Q6: A white precipitate formed when NaOH is added to a salt solution. The precipitate dissolves in excess NaOH but not in excess NH₄OH. Identify the cation.

Answer: Al³⁺ (aluminium). The white precipitate is Al(OH)₃. It dissolves in excess NaOH (forms NaAlO₂) but does not dissolve in excess NH₄OH. If it dissolved in both, it would be Zn²⁺.

Q7: What is the difference between a strong acid and a concentrated acid?

Answer: Strong acid = completely dissociates in water (e.g., HCl → H⁺ + Cl⁻ 100%). Concentrated acid = large amount of acid in small water (high molarity). A dilute strong acid (dilute HCl) is still a strong acid — it dissociates completely but has fewer H⁺ per litre. A concentrated weak acid (conc. CH₃COOH) is still weak — most molecules remain undissociated.