🧪 Chapter 2: Acids, Bases and Salts
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1. Understanding Acids and Bases
🍋 What are Acids?
- They are sour in taste.
- They turn blue litmus red.
- They release H⁺ ions (or Hydronium ions, H₃O⁺) when dissolved in water.
- Examples: Hydrochloric acid (HCl), Sulphuric acid (H₂SO₄), Acetic acid (CH₃COOH).
🧼 What are Bases?
- They are bitter in taste and soapy/slippery to touch.
- They turn red litmus blue.
- They release OH⁻ ions (Hydroxide ions) when dissolved in water.
- Examples: Sodium Hydroxide (NaOH), Calcium Hydroxide (Ca(OH)₂).
A base that is highly soluble in water is called an alkali. (Note: All alkalis are bases, but not all bases are alkalis).
2. Indicators (The Color Changers)
Indicators are substances that change their color or smell in different types of substances.
| Indicator Type | Name | Color in Neutral | Color in Acid | Color in Base |
|---|---|---|---|---|
| Natural | Litmus | Purple | Red | Blue |
| Natural | Turmeric | Yellow | Yellow | Reddish-brown |
| Synthetic | Phenolphthalein | Colorless | Colorless | Pink |
| Synthetic | Methyl Orange | Orange | Red | Yellow |
👃 Olfactory Indicators
These change their smell in acidic or basic media (e.g., Onion, Vanilla extract, Clove oil).
In a base, they lose their smell. In an acid, their smell remains intact.
3. Chemical Properties of Acids & Bases
A. Reaction with Metals
Acids react with active metals to give salt and hydrogen gas.
Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)↑
B. Reaction with Metal Carbonates & Bicarbonates
Acids react with metal carbonates/bicarbonates to form salt, carbon dioxide, and water.
🧪 VVIP Concept (Lime Water Test)
When the evolved CO₂ is passed through lime water (Calcium hydroxide), it turns milky due to the formation of a white precipitate of Calcium Carbonate.
Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s)↓ + H₂O(l)
If CO₂ is passed in excess, the milkiness disappears because soluble Calcium Bicarbonate is formed.
C. Neutralization Reaction
When an acid and a base react, they nullify each other's effect to form salt and water.
HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
D. Reaction of Metallic & Non-Metallic Oxides
- Metallic Oxides: Basic in nature. They react with acids to give salt and water (just like neutralization).
- Non-Metallic Oxides: Acidic in nature. They react with bases to give salt and water.
4. The pH Scale
A scale for measuring hydrogen ion concentration in a solution. 'p' stands for 'potenz' (power) in German.
- 0 to 6.9: Acidic (Lower the value, stronger the acid)
- 7.0: Neutral (Pure water, normal salt)
- 7.1 to 14: Basic (Higher the value, stronger the base)
Importance of pH in Everyday Life
- Digestive System: Our stomach produces HCl (approx pH 1.2) which helps in digestion. Excess acid causes acidity. We use Antacids (mild bases like Milk of Magnesia) to neutralize it.
- Tooth Decay: Starts when the pH of the mouth falls below 5.5. Bacteria produce acid by degrading sugar. The acid corrodes the tooth enamel (made of calcium hydroxyapatite, the hardest substance in the body).
- Animal & Plant Self-Defense: Bee stings inject methanoic acid causing severe pain. Applying a mild base like baking soda gives relief. Nettle leaves also have stinging hair injecting methanoic acid.
⚠️ Common Mistake: Concentrated vs Strong
Students often confuse Concentrated with Strong.
- A Strong Acid completely breaks apart into H⁺ ions in water (like HCl).
- A Concentrated Acid just means there is very little water mixed into it. You can have a concentrated weak acid!
5. Important Chemical Compounds (The Family of Salts)
1. Sodium Hydroxide (Caustic Soda) - NaOH
Preparation (Chlor-alkali process): When electricity is passed through an aqueous solution of Sodium Chloride (called Brine), it decomposes to form NaOH, Cl₂, and H₂.
Uses: Degreasing metals, making soaps and detergents, paper making.
2. Bleaching Powder - CaOCl₂
Preparation: Produced by the action of chlorine on dry slaked lime.
Uses: Bleaching cotton in textiles, disinfecting drinking water to make it germ-free, used as an oxidizing agent in chemical industries.
3. Baking Soda (Sodium Bicarbonate) - NaHCO₃
Preparation: Solvay process.
Uses: Antacid ingredient, used in fire extinguishers.
Baking soda is bitter. Baking powder is a mix of baking soda and a mild edible acid (like tartaric acid). The acid neutralizes the bitter Na₂CO₃ formed when baking.
4. Washing Soda - Na₂CO₃·10H₂O
Preparation: Recrystallization of sodium carbonate gives washing soda.
Uses: Glass, soap, and paper industries. Used for removing permanent hardness of water.
5. Plaster of Paris (POP) - CaSO₄·½H₂O
Preparation: On heating Gypsum (CaSO₄·2H₂O) at 373K (100°C), it loses water molecules to become Calcium Sulphate Hemihydrate (POP).
Uses: Doctors use it for supporting fractured bones in the right position. Making toys, decoration materials, and smoothing surfaces.
🎁 BONUS: Master Revision Hacks
💧 Water of Crystallization
It is the fixed number of water molecules present in one formula unit of a salt.
Blue Vitriol: Copper Sulphate (CuSO₄·5H₂O). It is blue, but when heated, it loses the 5 water molecules and turns into a white powder.
⭐ Top 3 Most Repeated Board Questions
Answer: Because HCl only produces H⁺ ions in the presence of water. No water = no ions = no color change!
Answer: The dilution process is highly exothermic. If water is added to acid, the sudden heat generated may cause the mixture to splash out and cause burns, or break the glass container. Always add acid drop-by-drop to water while stirring.
Answer: Be ready to explain the formation of NaOH, and clearly state where the gases go (Cl₂ at anode, H₂ at cathode).