Definition and Characteristics of Physical Changes
A physical change is a change in the physical properties of a substance without altering its chemical composition. The substance remains chemically the same before and after the change.
Common physical changes include changes of state such as melting, freezing, boiling, condensation, and sublimation. For example, ice melting to water is a physical change because both ice and water are H₂O molecules.
Physical changes may also involve changes in shape, size, or dissolving a substance in a solvent, such as salt dissolving in water. The process is usually reversible by physical means, e.g., evaporating the water to recover the salt.
- No new substances are formed.
- Chemical formula remains unchanged.
- Often reversible.
- Examples: melting ice, boiling water, dissolving salt in water.
Definition and Characteristics of Chemical Changes
A chemical change involves a transformation that alters the chemical composition of the original substance(s), producing one or more new substances with different chemical properties.
Chemical changes are often accompanied by observable signs such as colour change, gas evolution (bubbling), formation of a precipitate, or energy changes (heat, light).
Examples include combustion of wood, rusting of iron, and digestion of food. For instance, rusting of iron involves iron reacting with oxygen and water to form iron oxide, a new substance with different properties.
- New substances with different chemical formulas are formed.
- Chemical bonds are broken and new bonds formed.
- Usually irreversible by simple physical means.
- Examples: burning wood, rusting iron, baking a cake.
Distinguishing Physical and Chemical Changes
To distinguish between physical and chemical changes, consider whether the chemical identity of the substance changes and if new substances are produced.
Physical changes do not produce new substances and are often reversible. Chemical changes produce new substances and are usually irreversible.
Observations such as colour change, gas production, or precipitate formation strongly suggest a chemical change.
- Check if chemical composition changes.
- Look for signs: colour change, gas, precipitate, energy change.
- Test reversibility: physical changes often reversible, chemical usually not.
Examples and Balanced Equations
Example of a physical change: melting ice to water: H₂O(s) → H₂O(l) (no chemical change).
Example of a chemical change: combustion of methane: CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g).
In the combustion reaction, methane and oxygen react to form carbon dioxide and water, new substances with different properties.
- Physical change: H₂O(s) → H₂O(l) (melting ice)
- Chemical change: CH₄ + 2O₂ → CO₂ + 2H₂O (combustion)
Worked Example: Identifying the Type of Change
A student heats copper carbonate, which decomposes to copper oxide and carbon dioxide gas. The reaction is: CuCO₃(s) → CuO(s) + CO₂(g).
Observations: green solid changes to black solid and gas bubbles form.
Explanation: The original substance (copper carbonate) breaks down into new substances (copper oxide and carbon dioxide), indicating a chemical change.
- Observation: colour change and gas evolution.
- New substances formed: CuO and CO₂.
- Conclusion: chemical change due to new substances and gas production.
At-a-glance summary
Comparison table
| Aspect | Physical Change | Chemical Change |
|---|---|---|
| Definition | Change in physical properties without altering chemical composition | Change resulting in formation of new substances with different chemical composition |
| Reversibility | Usually reversible by physical means | Usually irreversible by physical means |
| Examples | Melting ice, boiling water, dissolving salt | Burning wood, rusting iron, baking bread |
| Observations | Change of state, shape, size | Colour change, gas evolution, precipitate formation, energy change |
| Chemical Formula | Remains the same | Changes due to new substances formed |
Active recall
Quick check
1. Is melting ice a physical or chemical change?
Physical change, because the chemical composition of water (H₂O) remains the same.
2. What evidence suggests a chemical change has occurred?
Formation of new substances indicated by colour change, gas evolution, precipitate formation, or energy change.
3. Write a balanced equation for the combustion of methane.
CH₄ + 2O₂ → CO₂ + 2H₂O
4. Can physical changes be reversed?
Yes, physical changes are usually reversible by physical means.