Chapter 2 – Acids, Bases & Salts: Comprehensive Study Notes

Acids

• Physical traits
– Sour taste.
– Release $H^+$ ions when dissolved in water (e.g.
$HCl \xrightarrow{H_2O} H^+ + Cl^-$).
– Turn blue litmus red.
– Corrode metals and react vigorously with bases.

• Classification by origin
– Natural (obtained directly from biological sources)
▸ Oxalic (tomato, spinach).
▸ Tartaric (tamarind).
▸ Citric (citrus fruits).
▸ Lactic (curd).
▸ Formic (ant sting, nettle).
– Synthetic (laboratory-made)
▸ $H<em>2SO</em>4$ (sulphuric).
▸ $HNO_3$ (nitric).

• Strength terminology
– Concentrated: high solute proportion.
– Dilute: low solute proportion.
– While diluting, always add acid dropwise into water with stirring; reaction is highly exothermic.

• Hydronium concept
– Isolated $H^+$ does not exist; in water it forms $H<em>3O^+$. $H^+ + H</em>2O \rightarrow H_3O^+$.

Bases / Alkalis

• Physical traits
– Bitter taste.
– Soapy touch.
– Produce $OH^-$ ions in aqueous medium (e.g.
$NaOH \xrightarrow{H_2O} Na^+ + OH^-$).
– Turn red litmus blue.

• Difference: Bases insoluble in water remain just ‘bases’; water-soluble ones are ‘alkalis’. Examples: $NaOH, KOH, Ca(OH)_2$.

• Electrical conduction
– Both acids and bases conduct electricity because of free $H^+$ / $OH^-$ ions.

Indicators

• Purpose: Identify acidic/basic nature through colour or odour changes.

• Natural indicators
– Litmus (purple)
▸ Acid → red, Base → blue.
– Turmeric (yellow)
▸ Acid → no change, Base → red/brown.
– Red-cabbage juice (purple)
▸ Acid → reddish, Base → greenish.

• Olfactory (smell) indicators
– Onion, vanilla, clove: lose characteristic smell in basic solutions but retain in acids.

• Synthetic indicators
– Phenolphthalein: colourless in acid, pink in base.
– Methyl orange: red in acid, yellow/orange in base.

• Indicator summary table

1. Red litmus: Acid (red, no change) | Base (blue).

2. Blue litmus: Acid (red) | Base (blue, no change).

3. Turmeric: Acid (yellow) | Base (reddish).

4. Phenolphthalein: Acid (colourless) | Base (pink).

5. Methyl orange: Acid (red) | Base (yellow).

6. Olfactory (onion/vanilla): Acid (normal smell) | Base (smell lost).

Chemical properties of Acids

1. Reaction with metals
   $\text{Acid} + \text{Metal} \rightarrow \text{Salt} + H<em>2$ ↑ Examples: • $H</em>2SO<em>4 + Zn \rightarrow ZnSO</em>4 + H<em>2$ ↑. • $6HCl + 2Fe \rightarrow 2FeCl</em>3 + 3H<em>2$ ↑. – Test: liberated $H</em>2$ burns with a ‘pop’ sound.

2. Reaction with metal carbonates / hydrogen-carbonates
   $\text{Acid} + MCO<em>3 / MHCO</em>3 \rightarrow \text{Salt} + CO<em>2 + H</em>2O$
   Examples:
   • $2HCl + Na<em>2CO</em>3 \rightarrow 2NaCl + CO<em>2 + H</em>2O$.
   • $HCl + NaHCO<em>3 \rightarrow NaCl + CO</em>2 + H<em>2O$. – $CO</em>2$ test: turns lime water $Ca(OH)<em>2$ milky due to $CaCO</em>3$ ppt. Excess $CO<em>2$ forms soluble $Ca(HCO</em>3)_2$, clearing the solution.

3. Reaction with bases (Neutralisation)
   $\text{Acid} + \text{Base} \rightarrow \text{Salt} + H<em>2O$ Ex: $HCl + NaOH \rightarrow NaCl + H</em>2O$.

4. Reaction with metallic oxides (basic in nature)
   $\text{Acid} + \text{Metal Oxide} \rightarrow \text{Salt} + H<em>2O$ Example: $2HCl + CuO \rightarrow CuCl</em>2 + H_2O$.

Chemical properties of Bases

• Metals: Most bases do not react with metals, but strong base $2NaOH$ reacts with Zn:
$2NaOH + Zn \rightarrow Na<em>2ZnO</em>2 + H_2$ ↑ (sodium zincate).

• Reaction with non-metallic oxides (acidic in nature)
$\text{Base} + CO<em>2 \rightarrow \text{Salt} + H</em>2O$
Example: $2NaOH + CO<em>2 \rightarrow Na</em>2CO<em>3 + H</em>2O$.

pH Concept

• Definition: $pH = -\log[H^+]$ (power of hydrogen).
• Scale: 0 (strong acid) → 7 (neutral) → 14 (strong base).
• Example calculation:
Given $[H^+] = 1\times10^{-3}\ \text{mol L}^{-1}$
$pH = -\log(1\times10^{-3}) = 3$ (acidic).

• Typical values
– Gastric juice: 1–2 (dark red on universal).
– Lemon juice/vinegar: ~2–3.
– Pure water: 7.
– Blood: 7.4.
– Soap solution: 8–10.
– Sodium hydroxide: 13–14.

Importance of pH in daily life

• Human body functions optimally at $7.0\text{–}7.8$; deviation disturbs metabolism.
• Acid rain (pH < 5.6) harms aquatic life & monuments.
• Soil pH controls plant growth; acidic soils treated with lime $CaO$, basic soils with organic matter.
• Indigestion: excess $HCl$ neutralised by antacids (e.g.
baking soda $NaHCO<em>3$, milk of magnesia $Mg(OH)</em>2$).
• Tooth decay when mouth pH < 5.5; toothpaste (basic) neutralises acid.
• Self-defence
– Nettle leaves / ant stings inject methanoic acid; relief by rubbing dock plant (basic) or baking-soda paste.
– Wasp stings are alkaline; relieve with mild acid (vinegar).

Electrolysis of Brine (Chlor-Alkali Process)

• Brine = aqueous $NaCl$.
• Electrolysis products
– Cathode: $2H<em>2O + 2e^- \rightarrow H</em>2 + 2OH^-$.
– Anode: $2Cl^- \rightarrow Cl_2 + 2e^-$.
– Solution left: $Na^+ + OH^-$ → $NaOH$ (caustic soda).

• Overall: brine → $Cl<em>2$ (gas), $H</em>2$ (gas), $NaOH$ (aq).

• Uses
– $Cl<em>2$: PVC, disinfectants, CFCs, pesticides, bleaching. – $H</em>2$: fuel, margarine, ammonia synthesis.
– $NaOH$: soaps/detergents, paper, rayon, water treatment.

Salts & their Nature

• Formation: $\text{Acid} + \text{Base} \rightarrow \text{Salt} + H_2O$.
• Classification (relative strength of parents)

1. Strong acid + strong base → Neutral salt (pH ≈ 7), e.g.
   $NaCl, KNO_3$.

2. Weak acid + strong base → Basic salt (pH > 7), e.g.
   $Na<em>2CO</em>3, NaHCO_3$.

3. Strong acid + weak base → Acidic salt (pH < 7), e.g.
   $CuSO<em>4, (NH</em>4)<em>2SO</em>4$.

4. Weak acid + weak base → Near-neutral but context dependent.

Important Commercial Salts

1. Bleaching Powder ($CaOCl_2$)

• Preparation: $Cl<em>2 + Ca(OH)</em>2 \rightarrow CaOCl<em>2 + H</em>2O$.
• Uses:
– Bleaching cotton, linen, wood pulp.
– Disinfecting drinking water.
– Oxidising agent in chemical industry.
– Laundry bleaching.

2. Baking Soda (Sodium hydrogen carbonate, $NaHCO_3$)

• Lab synthesis (Solvay): $NaCl + NH<em>3 + CO</em>2 + H<em>2O \rightarrow NaHCO</em>3\downarrow + NH<em>4Cl$. • Mild, non-corrosive basic salt; decomposes on heating: $2NaHCO</em>3 \xrightarrow{\Delta} Na<em>2CO</em>3 + H<em>2O + CO</em>2$ ↑.
• Baking powder = $NaHCO<em>3 +$ mild edible acid; $CO</em>2$ released makes cake/bread fluffy.
• Also used in antacids and soda-acid fire extinguishers.

3. Washing Soda (Sodium carbonate decahydrate, $Na<em>2CO</em>3 \cdot 10H_2O$)

• Obtained by heating $NaHCO<em>3$ → $Na</em>2CO_3$ then recrystallising with water.
• Uses:
– Domestic cleaning agent.
– Softens hard water.
– Glass, soap, paper industries.
– Precursor for other sodium compounds (borax etc.).

4. Plaster of Paris (POP, $CaSO<em>4 \cdot \tfrac12 H</em>2O$)

• Prepared by heating gypsum:
$CaSO<em>4 \cdot 2H</em>2O \xrightarrow{373\,K} CaSO<em>4 \cdot \tfrac12 H</em>2O + \tfrac32 H_2O$.
• Reacts with water to revert to gypsum, forming hard mass; used for casts, dentistry, ornaments, wall smoothening.

5. Water of Crystallisation

• Fixed number of water molecules in a crystalline salt; imparts colour and shape.
– Examples:
▸ $CuSO<em>4 \cdot 5H</em>2O$ (blue),
▸ $FeSO<em>4 \cdot 7H</em>2O$ (green),
▸ $Na<em>2CO</em>3 \cdot 10H<em>2O$ (washing soda), ▸ $CaSO</em>4 \cdot 2H_2O$ (gypsum).

Laboratory Activities & Observations

• Activity 2.1: Testing various samples with red/blue litmus, phenolphthalein, methyl orange—confirmed acid/base nature via expected colour changes.
• Activity 2.8/2.9: Electrical conductivity—bulb glows with $HCl, H<em>2SO</em>4$ (ionise), but not with glucose or alcohol (no ions).
• Litmus experiment with dry vs. moist paper: Dry litmus does not change colour in $HCl$ gas; moisture required for ionisation.
• ‘Fish can’t survive in hot water’ remark: indicates temperature/pH sensitivity of aquatic life.

These bullet-pointed notes encapsulate every concept, reaction, equation, example, test, and real-life implication mentioned across the transcript, providing a comprehensive study resource on Chapter 2 “Acids, Bases & Salts”.