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Changes Around Us

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Physical & Chemical Changes • Grade 7 Quick Reference

1. Physical Changes

REVERSIBLE? SOMETIMES!

Changes in which only physical properties (like shape, size, state, or color) change, and NO NEW substance is formed!

⚡ Key Characteristics:

  • No alteration to chemical makeup.
  • Easily reversed in many cases (e.g., Ice → Water).
  • Can sometimes be irreversible (e.g., Chopping wood).

Class Examples:

🧊 Melting Ice
🎈 Inflating Balloon
✂️ Paper Origami
🪵 Chopping Veggies

2. Chemical Changes

IRREVERSIBLE

A change in which one or more NEW substances are formed through chemical reactions. Composition changes entirely!

✨ Signs of a Chemical Change:

  • Heat/Light
  • Gas/Bubbles
  • Color Shift
  • New Smell

Classic Reaction

Vinegar + Baking Soda → Carbon Dioxide ↑ + Other Products

*Produces rapid fizzing sound & bubbling!

3. The Lime Water Test

LAB ESSENTIAL

How do we verify if a gas is Carbon Dioxide (CO2)? We bubble it through freshly prepared lime water!

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Observation

Lime water turns milky / cloudy.

Calcium hydroxide [Lime water]
+ Carbon dioxide [CO2]
→ Calcium carbonate [Insoluble Precipitate]
+ Water [H2O]

*The milkiness is due to insoluble Calcium Carbonate particles!

4. Combustion & Fire

Combustion: A chemical reaction in which a substance reacts with oxygen to release heat and light.

Heat Oxygen Fuel 🔥
The Fire Triangle (Must meet all 3!)
Ignition Temp: The minimum temperature required for a substance to catch fire.

5. The Candle Paradox

Does a burning candle undergo a Physical or Chemical change? It undergoes BOTH simultaneously!

Physical Change: Wax melts from solid to liquid, flows down, and solidifies back into solid wax. No new substance created!
Chemical Change: The liquid wax is drawn up the wick, vaporizes, and burns in oxygen to form Carbon Dioxide, water vapor, soot, and light/heat.
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Michael Faraday (19th Century)

Delivered famous lectures on "Chemical History of a Candle", proving candles are the ultimate objects to study physical & chemical processes!

6. Slow & Natural Changes

Rusting of Iron Chemical

A slow chemical change forming a new reddish-brown coating (Iron Oxide).

Iron + Oxygen (Air) + Water → Rust

Weathering

Breaking of rocks due to climate, water & chemicals. Basalt changes chemically into red soil.

Erosion

Pebbles/soil moved by wind or water. 100% physical change that takes thousands of years!

💡 Nature's Spotlight: Bioluminescence

Fireflies produce light without heat through a chemical change inside their bodies. True biological magic!

In a Nutshell Summary

Physical changes change outer appearances (shape, size, state) without new substances. Chemical changes yield entirely new substances through reactions. Nature balances both to continuously shape our world!