Physical and chemical changes- CIE iGCSE Chemistry Notes - New Syllabus
Physical and chemical changes for iGCSE Chemistry Notes
Core Syllabus
- Identify physical and chemical changes, and describe the differences between them
Physical and Chemical Changes
Physical and Chemical Changes
In Chemistry, substances can undergo changes that are classified into two main categories: physical changes and chemical changes. Understanding the differences between these types of changes is essential for distinguishing between changes in form and changes in composition.
Physical Change
- A physical change is a change in which the form or appearance of a substance changes, but its chemical composition remains the same.
- No new substances are formed during a physical change.
- It often involves a change in state (such as solid ↔ liquid ↔ gas), size, shape, or texture.
- Most physical changes are reversible by simple physical methods (e.g., heating, cooling, dissolving, filtering).
- There is usually little or no energy change.
- Examples of physical changes:
- Melting of ice to form water
- Boiling of water to form steam
- Dissolving salt or sugar in water
- Crushing a can or breaking glass
- Condensation of water vapour to form liquid water
Chemical Change
- A chemical change is a change in which a substance reacts to form one or more new substances with different chemical properties.
- This type of change involves the breaking and making of chemical bonds.
- Chemical changes are usually not easily reversible.
- Energy is often absorbed or released in the form of heat, light, or sound during the reaction.
- Signs of chemical change include:
- Change in colour
- Production of gas (bubbling or fizzing)
- Formation of a precipitate (a solid from two solutions)
- Change in temperature or light emission
- New smell or odour
- Examples of chemical changes:
- Burning of wood or paper (combustion)
- Rusting of iron to form iron(III) oxide
- Cooking of food (e.g., frying an egg)
- Electrolysis of water into hydrogen and oxygen
- Neutralisation of acid with alkali
Key Differences Between Physical and Chemical Changes:
| Aspect | Physical Change | Chemical Change |
|---|---|---|
| New substance formed? | No | Yes |
| Reversibility | Usually reversible | Usually not reversible |
| Energy change involved? | Usually minimal | Often significant (heat, light, etc.) |
| Change in chemical composition? | No | Yes |
| Examples | Melting, boiling, dissolving, breaking | Burning, rusting, cooking, rotting |
Example
During an experiment, a student heats blue copper(II) sulfate crystals until a white powder is formed and water droplets appear on the test tube wall. Is this a physical or chemical change?
▶️Answer/Explanation
This is a physical change. The blue copper(II) sulfate (\( \text{CuSO}_4 \cdot 5\text{H}_2\text{O} \)) loses water of crystallization when heated, forming anhydrous white \( \text{CuSO}_4 \). No new substance is formed at the molecular level — only the water is removed. This change is reversible by adding water.
Example
Iron filings are mixed with sulfur powder and the mixture is heated strongly. A black solid is formed that is not magnetic. What kind of change is this?
▶️Answer/Explanation
This is a chemical change. Heating the iron and sulfur causes a chemical reaction: \( \text{Fe} + \text{S} \rightarrow \text{FeS} \). A new substance, iron(II) sulfide, is formed with different properties — it’s not magnetic and cannot be separated into iron and sulfur again. Heat is required, and the change is irreversible by physical means.
Example
Carbon dioxide is passed through limewater and a white precipitate forms. Identify the type of change and explain your reasoning.
▶️Answer/Explanation
This is a chemical change. Carbon dioxide reacts with calcium hydroxide in limewater to form calcium carbonate, a white precipitate:
\( \text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3 + \text{H}_2\text{O} \)
A new insoluble product is formed, which confirms a chemical change. The reaction is not reversible by simple physical means.
Example
When ethanol evaporates at room temperature, does this involve a physical or chemical change?
▶️Answer/Explanation
This is a physical change. Ethanol changes from liquid to gas, but its molecular structure stays the same. The process involves no chemical reaction and is completely reversible by condensation. This is a classic example of a change in state.
