What this chapter is about
Every day, things around us change. Ice melts into water. A matchstick burns. Milk turns into curd. Some changes are easy to reverse, while others are not. This chapter helps you understand the difference between physical changes and chemical changes.
In Class 7, you learn to observe changes more carefully. You begin to notice that some changes only affect how a substance looks or feels. Other changes create entirely new substances with new properties. Understanding this difference is a key step in learning chemistry.
After studying this chapter, you should be able to look at any change and tell whether it is physical or chemical. You should also understand which changes can be undone and which cannot.
Key ideas
- A physical change is a change in the form, size, shape or state of a substance. The substance remains the same. No new substance is formed.
- A chemical change is a change where one or more new substances are formed. The original substance cannot be easily recovered.
- Physical changes are usually reversible. For example, water can freeze into ice and ice can melt back into water.
- Chemical changes are usually irreversible. For example, once paper burns, you cannot get the paper back.
- Signs of a chemical change include: release of heat or light, change in colour, formation of gas bubbles, or formation of a solid from liquids.
- A change of state (solid to liquid to gas) is a physical change because no new substance is created.
- Rusting of iron is a chemical change. Iron combines with oxygen and moisture to form iron oxide, which is a new substance.
- Dissolving sugar in water is a physical change. The sugar is still sugar; it has just spread into the water.
Formulas and facts to remember
1. Physical change: Only appearance or state changes; substance stays the same.
2. Chemical change: A new substance with new properties is formed.
3. Reversible change: A change that can be undone easily. Example: melting of wax.
4. Irreversible change: A change that cannot be undone easily. Example: cooking an egg.
5. Rusting: Iron + Oxygen + Water → Iron oxide (rust). This is a chemical change.
6. Burning: When something burns, it combines with oxygen. New substances like ash and gases form.
7. Curdling of milk: Adding lemon juice or curd to milk causes a chemical change. The milk becomes curd.
8. Cutting or tearing: Cutting paper or cloth is a physical change because no new substance forms.
Worked examples
Example 1: Melting ice
Reema takes an ice cube from the freezer. After 10 minutes at room temperature, it becomes water.
Question: Is this a physical or chemical change?
Solution:
- Step 1: Check if a new substance formed. The ice was frozen water. After melting, it is liquid water.
- Step 2: Water remained water. Only the state changed from solid to liquid.
- Step 3: This change can be reversed by putting the water back in the freezer.
- Answer: This is a physical change.
Example 2: Burning a candle
Arjun lights a candle. The wax melts, and the wick burns with a flame. Some wax drips down.
Question: Are there physical and chemical changes here?
Solution:
- Step 1: The wax near the flame melts. Melting wax is a physical change (solid to liquid).
- Step 2: The wick burns. Burning produces light, heat, carbon dioxide and water vapour. These are new substances.
- Step 3: Burning the wick is a chemical change.
- Answer: Both physical change (wax melting) and chemical change (wick burning) happen together.
Example 3: Iron nail left in water
Priya leaves an iron nail in a cup of water for two weeks. The nail turns reddish-brown.
Question: What type of change is this?
Solution:
- Step 1: The reddish-brown layer is rust (iron oxide).
- Step 2: Rust is a new substance. It is different from iron.
- Step 3: You cannot turn rust back into iron easily.
- Answer: This is a chemical change.
Common mistakes
- Thinking melting is a chemical change → Melting only changes state; the substance stays the same, so it is physical.
- Believing all irreversible changes are chemical → Some physical changes are also hard to reverse, like breaking a glass, but no new substance forms.
- Confusing dissolving with a chemical reaction → Dissolving salt or sugar in water is physical; the salt or sugar can be recovered by evaporation.
- Thinking burning and melting are the same → Burning creates new substances; melting only changes state.
- Assuming colour change always means chemical change → Sometimes colour changes are physical, like mixing paints, but often colour change does signal a chemical change.
Quick revision
- Physical change: same substance, different form or state.
- Chemical change: new substance formed with new properties.
- Melting, freezing, dissolving and tearing are physical changes.
- Burning, rusting and curdling are chemical changes.
- Look for heat, light, gas bubbles or colour change to spot chemical changes.
- Reversible usually means physical; irreversible usually means chemical.