A change is any alteration in the appearance, form, state, or properties of a substance. Some changes only affect the physical properties of a substance, while others produce new substances with different properties.
To identify the type of change, ask yourself:
"Has only the shape, size, or state changed, or has a new substance been formed?"
This is the simplest and most important way to distinguish between physical and chemical changes.
Physical Change:
A physical change is a change in which only the physical properties of a substance—such as its shape, size, or state—change.
Key Characteristics:
- Only the shape, size, or state changes.
- No new substance is formed.
- The chemical composition remains unchanged.
- The original substance retains its identity.
- Many physical changes are reversible, although some may be irreversible (e.g., breaking a glass).
Everyday Examples
| Physical Change | Why? |
|---|---|
| Cutting vegetables | Only the size and shape change. |
| Folding paper | Only the shape changes. |
| Breaking glass | Only the size and shape change. |
| Melting ice | Only the state changes (solid \(\to\) liquid). |
| Freezing water | Only the state changes (liquid \(\to\) solid). |
Chemical Change:
A chemical change is a change in which one or more new substances are formed. The new substance has different properties from the original substance.
Key Characteristics:
- A new substance is formed.
- The chemical composition changes.
- The new substance has different properties.
- Most chemical changes are irreversible.
- Chemical changes often involve:
- Heat
- Light
- Gas formation
- Colour change
- Formation of a solid (precipitate)
Everyday Examples:
| Chemical Change | Why? |
|---|---|
| Rusting of iron | Iron changes to rust, which is a new substance |
| Burning paper | Ash, smoke, and gases are formed. |
| Burning wood | Ash and gases are produced. |
| Cooking vegetables | New substances with different taste and smell are formed. |
| Milk turning into curd | A new substance (curd) is formed. |
How to Identify the Type of Change?
Use these two simple questions:
Step 1:
Is a new substance formed?
- Yes \(\to\) Chemical Change
- No \(\to\) Go to Step 2
Step 2:
Has only the shape, size, or state changed?
- Yes → Physical Change
Remember: The formation of a new substance is the most reliable way to identify a chemical change.