Chapter 2 · ICSE Class 8 Chemistry (Selina Concise)

Physical and Chemical Changes

Deepen your understanding of physical and chemical changes, and learn to identify signs of chemical reactions.

🔄 Chapter 2

Wax melting as a physical change compared with iron rusting as a chemical change Wax melting from solid to liquid is a physical change with no new substance formed, while iron rusting in the presence of air and moisture is a chemical change that forms a new substance, rust 🕯️→💧 🔩→🟤 Physical Change wax melting Chemical Change iron rusting
2.1

Signs of a Chemical Change

Signs that a chemical change has occurred include color change, gas formation (bubbles), temperature change, and formation of a precipitate.

1

What sign indicates a chemical change when vinegar is added to baking soda?

Gas formation (bubbles of carbon dioxide)

2

What sign indicates a chemical change when iron rusts?

A color change (from grey metal to reddish-brown rust)

2.2

Reversible and Irreversible Changes

Physical changes are usually reversible (like melting/freezing). Chemical changes are usually irreversible, since new substances form.

1

Can burnt wood be turned back into wood?

No, since burning is a chemical change that permanently forms new substances (ash, gases)

2

Can melted wax be turned back into solid wax?

Yes, since melting is a physical change that's reversible by cooling

Practice

Exercises

1. Name one sign of a chemical change.

2. Is boiling water a physical or chemical change?

3. Is baking a cake a physical or chemical change?

4. Why are chemical changes usually irreversible?

5. Give an example of a reversible physical change.

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