Burning of a candle is which type of change?

Answer: Both Physical and Chemical. Melting of wax is **Physical**, but burning of the wick and wax into CO2 is **Chemical**. Thus, it is both.

Which chemical is used to prevent rusting in iron pipes (Galvanization)?

Answer: Zinc. **Zinc** is more reactive than iron and acts as a sacrificial layer to prevent iron from rusting.

Antacids are used to treat acidity in the stomach. Chemically they are:

Answer: Weak Bases. They are **Weak Bases** (like Milk of Magnesia) that neutralize the excess HCl acid in our stomach.

Which of the following acid is found in curd and sour milk?

Answer: Lactic Acid. Lactic acid is produced by bacteria during the fermentation of lactose in milk.

What is the color of phenolphthalein in a basic solution?

Answer: Pink. Phenolphthalein is a synthetic indicator that turns pink in basic solutions and remains colorless in acidic ones.

Home/Rajasthan 4th Grade Exam/everyday-science/

Chemical Reactions & Acids/Bases

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Expert Answer & Key Takeaways

Mastering Chemical Reactions & Acids/Bases is essential for high-fidelity technical architecture and senior engineering roles in 2026.

Chemistry Decoder: The Magic of Change

Phase 1: The Foundation (Physical vs Chemical)

Not all changes are permanent. If you can "Undo" it, it's usually physical. If it's "Forever", it's usually chemical.

Feature	Physical Change	Chemical Change
Definition	Only form/state changes.	New substance is formed.
Reversibility	Reversible (usually).	Irreversible (usually).
Examples	Melting ice, Cutting paper.	Rusting iron, Curdling milk.

Phase 2: The Core Logic (Acids & Bases)

1. The pH Scale (Strength Meter)

Think of the pH scale (0 to 14) as a balance. 7 is The Neutral Center (Water).

Acids (0 to 6): Sour taste, turns Blue Litmus Red. (Ex: Lemon, Stomach acid).
Bases (8 to 14): Bitter taste, soapy feel, turns Red Litmus Blue. (Ex: Soap, Baking soda).

2. Types of Reactions

Type	Concept	Example
Exothermic	Releases Heat (Gets hot).	Burning wood, Respiration.
Endothermic	Absorbs Heat (Gets cold).	Photosynthesis, Melting ice.
Neutralization	Acid + Base $
ightarrow$ Salt + Water.	Antacid for acidity.

Phase 3: The Exam Edge (Redox & Corrosion)

1. Oxidation & Reduction (Redox)

Oxidation: Gaining Oxygen or Losing Electrons.
Reduction: Losing Oxygen or Gaining Electrons.

2. Everyday Corrosion

Rusting of Iron: Needs both Air (Oxygen) and Moisture. It is a chemical change.
Galvanization: Coating iron with Zinc to prevent rusting.
Rancidity: When oil/fats in food get oxidized and smell bad. (Nitrogen gas in chips packets prevents this).

3. REET Specific: Indicators & pH

Natural Indicators: Turmeric (Turns Red in Base), Litmus (extracted from Lichen).
Synthetic Indicators: Phenolphthalein (Pink in Base, Colorless in Acid), Methyl Orange.
pH Scale: 0-6 (Acidic), 7 (Neutral), 8-14 (Basic/Alkaline).

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