Ch-2: Acids, Bases and Salts 🧪
1. Introduction & Indicators (सूचक)
- Acids: Sour in taste (खट्टे), turn blue litmus red.
- Bases: Bitter in taste (कड़वे), turn red litmus blue.
- Neutralization: Acids and bases nullify each other’s effect.
2. Types of Indicators
- Natural: Litmus (from Lichen), Turmeric (हल्दी), Red Cabbage.
- Synthetic: Methyl Orange, Phenolphthalein.
- Olfactory Indicators (गंधीय सूचक): Substances whose odour changes in acidic/basic media.
Ex: Onion (प्याज), Vanilla, Clove oil (लौंग का तेल).
3. Reaction with Metals
Acid + Metal → Salt + Hydrogen gas ↑
- धातु अम्ल से हाइड्रोजन विस्थापित करती है।
- Test for $H_2$: Brings a burning candle $\implies$ Pop Sound!
Fig: Testing Hydrogen gas
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4. Reaction with Carbonates
Metal Carbonate + Acid → Salt + $CO_2$ + $H_2O$
- धातु कार्बोनेट अम्ल के साथ $CO_2$ गैस उत्पन्न करते हैं।
- Lime Water Test: Carbon dioxide turns lime water milky.
- Equation: $Ca(OH)_2 + CO_2 \to CaCO_3 \downarrow$ (White ppt) $+ H_2O$.
- If excess $CO_2$ is passed, milkiness disappears as $Ca(HCO_3)_2$ is soluble.
5. Neutralization Reaction (उदासीनीकरण)
Base + Acid → Salt + Water
- Ex: $NaOH + HCl \to NaCl + H_2O$.
- अम्ल एवं क्षारक की अभिक्रिया से लवण तथा जल प्राप्त होते हैं।
6. Reaction with Metallic/Non-Metallic Oxides
- Metallic Oxides are Basic in nature (e.g., $CuO$).
$\to$ Metal Oxide + Acid $\to$ Salt + Water. - Non-Metallic Oxides are Acidic in nature (e.g., $CO_2$).
$\to$ Non-metal Oxide + Base $\to$ Salt + Water.
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7. What do all Acids & Bases have in common?
- All Acids generate $H^+(aq)$ ions. These ions are responsible for acidic properties.
- All Bases generate $OH^-(aq)$ ions in water.
- Water is essential: Dry $HCl$ gas does not change litmus color!
8. Dilution (तनुकरण)
- Mixing acid/base with water decreases ion concentration per unit volume.
- WARNING! Always add Acid to Water slowly with stirring. Never add water to concentrated acid (Exothermic explosion risk!).
9. The pH Scale
- ‘p’ in pH stands for potenz (German for power).
- Range: 0 (Highly Acidic) to 14 (Highly Basic).
- $pH = 7$ is Neutral (Pure water/Blood).
pH Spectrum Color Guide
0 (Acidic)7 (Neutral)14 (Alkaline)
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10. Importance of pH in Daily Life
- Survival: Our body works in pH 7.0 – 7.8.
Acid Rain: pH < 5.6. Harmful for aquatic life. - Soil: Plants need specific pH for healthy growth. Farmers add lime (Base) to acidic soil.
- Digestion: Stomach produces $HCl$. Too much acid causes indigestion.
Remedy: Antacids (Milk of Magnesia – $Mg(OH)_2$). - Tooth Decay: Starts when mouth pH < 5.5.
Toothpaste is basic to neutralize acids produced by bacteria. - Self Defence: Bee-stings inject Methanoic Acid. Baking soda gives relief.
Nettle leaves sting due to acid; Dock plant (basic) is the remedy.
11. Naturally Occurring Acids
- • Vinegar: Acetic Acid
- • Orange/Lemon: Citric Acid
- • Tamarind: Tartaric Acid
- • Tomato: Oxalic Acid
- • Curd: Lactic Acid
- • Ant sting: Methanoic Acid
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12. Chemicals from Common Salt ($NaCl$)
- 1. Sodium Hydroxide ($NaOH$): Produced by Chlor-alkali process.
$2NaCl + 2H_2O \to 2NaOH + Cl_2 \uparrow + H_2 \uparrow$. - 2. Bleaching Powder ($CaOCl_2$):
$Ca(OH)_2 + Cl_2 \to CaOCl_2 + H_2O$. Used for disinfecting water. - 3. Baking Soda ($NaHCO_3$): Used in pakoras, fire extinguishers.
On heating: $NaHCO_3 \to Na_2CO_3 + H_2O + CO_2$. - 4. Washing Soda ($Na_2CO_3 \cdot 10H_2O$): Recrystallization of Sodium Carbonate. Used in glass/soap industries.
13. Water of Crystallization
- Fixed number of water molecules in one formula unit.
Ex: Blue Copper Sulphate ($CuSO_4 \cdot 5H_2O$). - Plaster of Paris (P.O.P): $CaSO_4 \cdot \frac{1}{2}H_2O$.
Made by heating Gypsum ($CaSO_4 \cdot 2H_2O$) at 373 K. - P.O.P becomes hard Gypsum when mixed with water. टूटी हड्डियों को जोड़ने के लिए उपयोग किया जाता है।
Exam Tip: pH of strong acid + strong base salts is always 7 (Neutral). Strong acid + Weak base = Acidic salt (pH < 7).
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