Changes Around Us: Physical and Chemical
1. Types of Changes
Physical Changes:
- Change in appearance (shape, size, state) but no new substance formed.
- Examples: Melting ice, folding paper, boiling water, inflating a balloon.
Chemical Changes:
- New substances formed through a chemical reaction.
- Examples: Burning wood, rusting iron, mixing vinegar and baking soda, curdling milk.
2. Characteristics
- Physical Changes: No new substance, often reversible (e.g., melting ice can refreeze).
- Chemical Changes: New substances formed, usually irreversible (e.g., burnt wood becomes ash).
3. Key Examples
Physical Changes:
- Melting ice cubes, chopping vegetables, drying wet clothes.
Chemical Changes:
- Rusting (iron → iron oxide).
- Combustion (burning magnesium → magnesium oxide + heat + light).
- Lime water turning milky (carbon dioxide + lime water → calcium carbonate).
4. Combustion
- A chemical reaction with oxygen, producing heat/light.
- Needs: Fuel, Oxygen, Ignition Temperature (minimum temperature to start burning).
- Example: Burning a candle (involves both physical changes like melting wax and chemical changes like burning vapor).
5. Reversible vs. Irreversible Changes
- Reversible: Can get original substance back (e.g., melting ice, boiling water).
- Irreversible: Cannot get original substance back (e.g., burning wood, making popcorn).
6. Desirable vs. Undesirable Changes
- Desirable: Useful changes (e.g., cooking food, ripening fruits).
- Undesirable: Harmful changes (e.g., rusting, food decay).
- Note: Some changes (e.g., decomposition) can be both depending on context.
7. Natural Changes
- Weathering: Physical (breaking rocks) and chemical (iron in rocks forming rust) changes form soil.
- Erosion: Physical change where wind/water moves soil, sand, or rocks.
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