Science Module: Acids, Bases, and Salts

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General Science: Acids, Bases, and Salts

📅 Topic: General Science (Chemistry)

This chapter, "Acids, Bases, and Salts," is a goldmine for competitive exams. The questions here are practical: "What acid is in a bee sting?", "Why do we use baking soda in cakes?", "What is the chemical formula of Plaster of Paris?". We will move from the theoretical definitions to the chemicals of daily life.

1. Indicators: The Chemical Detectives

How do we know if something is acidic or basic without tasting it? We use Indicators.

A. Natural Indicators

Litmus: Extracted from Lichen (a plant).
- Acid: Turns Blue Litmus → Red. (Remember: Acid = Danger = Red).
- Base: Turns Red Litmus → Blue. (Remember: Base = Blue).
Turmeric:
- Acid: No change (Yellow).
- Base: Turns Reddish-Brown (Think of soap on a curry stain).

B. Synthetic Indicators (Exam Favorites)

Memorize this logic, it is often asked in matching questions:

Phenolphthalein: Colourless in Acid | Pink in Base.
Methyl Orange: Red/Pink in Acid | Yellow in Base.

C. Olfactory Indicators

Substances whose smell changes (e.g., Onion, Vanilla). Base destroys the smell; Acid does not.

2. Chemical Properties (The Reactions)

A. Reaction with Metals

Acid + Metal → Salt + Hydrogen Gas.
Test: The evolved Hydrogen gas burns with a "Pop" sound.
Example: Zinc + Sulphuric Acid → Zinc Sulphate + H₂.

B. Reaction with Metal Carbonates

Acid + Carbonate → Salt + Water + Carbon Dioxide (CO₂).
Test: The evolved CO₂ gas turns Lime Water Milky.
Application: Used in Fire Extinguishers.

C. Neutralization

Acid + Base → Salt + Water + Heat.
Real Life: Taking an Antacid (Base) to cure Indigestion (Stomach Acid).

        ⭐ The Nature of Oxides (Crucial Concept):

        Metal Oxides are BASIC: Example: Copper Oxide (CuO). Reacts with acid.
Non-Metal Oxides are ACIDIC: Example: CO2, SO2. React with base.

        Note: This is why industrial smoke (Non-metal oxides) causes Acid Rain.

3. The pH Scale (Power of Hydrogen)

Scale: 0 to 14. (Neutral = 7, Acid < 7, Base > 7).

Everyday Importance (High Yield Section)

Stomach: Produces Hydrochloric Acid (HCl). Excess is treated with Antacids (like Magnesium Hydroxide / Milk of Magnesia).
Tooth Decay: Starts when mouth pH drops below 5.5. Bacteria produce acid from sugar. Toothpaste is Basic.
Bee/Ant Sting: Injects Methanoic Acid (Formic Acid). Remedy: Baking Soda (Base).
Nettle Leaves: Inject Methanoic acid. Remedy: The Dock plant (basic) grows nearby.
Acid Rain: When rain water pH is less than 5.6.

4. Chemicals from Common Salt (The "Salt" Family)

This is the most important section for factual questions. Memorize the Common Name, Chemical Name, and Formula.

A. Sodium Hydroxide (Caustic Soda)

Formula: NaOH
Process: Chlor-Alkali Process (Electrolysis of Brine).
Uses: Soaps, detergents, paper making.

B. Bleaching Powder

Chemical Name: Calcium Oxychloride.
Formula: CaOCl₂
Uses: Bleaching cotton, disinfecting drinking water (germ killer).

C. Baking Soda

Chemical Name: Sodium Hydrogen Carbonate.
Formula: NaHCO₃
Action: On heating, it releases CO₂, which makes cakes rise.

D. Washing Soda

Chemical Name: Sodium Carbonate Decahydrate.
Formula: Na₂CO₃·10H₂O
Uses: Glass, soap, paper. Removing permanent hardness of water.

E. Plaster of Paris (POP)

Chemical Name: Calcium Sulphate Hemihydrate.
Formula: CaSO₄·1/2H₂O
Preparation: Heating Gypsum at 373K. (Used for plastering fractures, toys).

5. Water of Crystallization

Are crystals really dry? No. Many salts contain fixed water molecules.
Copper Sulphate (CuSO₄·5H₂O): It is Blue. If heated, it loses water and turns White. Add water, and it turns Blue again.

6. Mentor’s Final Drill (Exam-Ready Questions)

Q: What is the White Precipitate formed when CO₂ is passed through Lime Water?
A: Calcium Carbonate (CaCO₃).

Q: Which acid is present in Tamarind?
A: Tartaric Acid. (Others: Tomato = Oxalic, Curd = Lactic, Lemon = Citric).

Q: Why does distilled water NOT conduct electricity, but rain water does?
A: Distilled water is pure (no ions). Rain water dissolves gases (CO₂, SO₂) forming acids that release ions.

Q: Difference between Baking Soda and Baking Powder?
A: Baking Soda is pure NaHCO₃. Baking Powder is a mixture of Baking Soda + a mild edible acid (like Tartaric Acid).

Q: Which gas is liberated at the Anode during the Chlor-Alkali process?
A: Chlorine Gas.

Action Plan: Create a cheat sheet for Section 4. Confusing "Carbonate" vs "Hydrogen Carbonate" or "Hemihydrate" vs "Dihydrate" is where students lose marks. Precision is key!

Science Notes: Acids, Bases, and Salts