Physical Changes vs Chemical Changes
A physical change alters the state, shape or size of a substance without forming a new substance, whereas a chemical change rearranges atoms to form new substances with different properties. In brief: crushing ice is physical, since the ice is still H2O, while electrolysis of water into hydrogen and oxygen is chemical, since two new gases form.
After the first introduction to the two kinds of change, a closer look at the particles explains why a chemical change is not simply a more dramatic physical change. In a chemical change, bonds break and form. This article compares the two types at the particle level, gives several test questions for deciding which type has occurred, describes borderline cases and works through an example with masses.
What happens to particles in each type of change?
In a physical change the particles stay the same, but their arrangement or motion changes. When ice melts, the H2O molecules start to move past each other, but each molecule remains H2O. Dissolving sugar spreads sucrose molecules among water molecules, and the molecules are not altered.
In a chemical change the bonds inside the particles are broken, and atoms are regrouped into new particles. When hydrogen burns, H2 and O2 molecules break apart and the atoms recombine as H2O. The atoms are conserved, but the substances are not.
Physical and chemical changes compared
| Feature | Physical change | Chemical change |
|---|---|---|
| Bonds within particles | Not broken | Broken and re-formed |
| Particles after the change | Same kind | New kinds |
| Atoms | Conserved | Conserved |
| Energy change | Usually small | Usually larger |
| Reversing it | Often easy | Often difficult |
| Written as | H2O(s) → H2O(l) | 2H2 + O2 → 2H2O |
How do you decide which type of change occurred?
- Check the identity. Is the substance at the end the same as at the start? If yes, the change is physical.
- Look for new substances. A gas, a new solid, a permanent color change or a new smell suggests a chemical change.
- Consider energy. Strong heating, cooling or light production without a change of state points to a reaction.
- Try to recover the original. If simple methods such as cooling, evaporation or filtration restore the original substance, the change was physical.
Classified examples
| Change | Type | Evidence |
|---|---|---|
| Butter melting in a pan | Physical | Same fat, different state |
| Butter browning and burning | Chemical | New colors, smell, black residue |
| Salt dissolving in water | Physical | Salt recovered by evaporation |
| Vinegar fizzing on baking soda | Chemical | Carbon dioxide gas forms |
| Bending a wire | Physical | Only the shape changes |
| Leaves changing color in autumn | Chemical | Chlorophyll breaks down, new pigments show |
| Dew forming on grass | Physical | Vapor condenses to liquid water |
What about borderline cases?
The boundary is not always sharp. Dissolving hydrogen chloride gas in water, for example, produces hydrochloric acid by forming ions, which is more than a purely physical change. Dissolving some metals in acid is clearly chemical. Changes such as the melting of a polymer may involve both physical softening and chemical breakdown at higher temperatures. In such cases chemists look at what particles are present before and after.
Worked example: heating copper carbonate
Green copper(II) carbonate powder is heated in a test tube. The gas that escapes is bubbled through limewater, which turns cloudy, and the solid turns black.
- Observations. The green solid becomes black, and a gas is released that turns limewater cloudy. Both indicate a new substance.
- Interpretation. Cloudy limewater is the test for carbon dioxide, and black copper(II) oxide remains: CuCO3 → CuO + CO2.
- Mass check. The molar masses are CuCO3 = 63.5 + 12 + 48 = 123.5 g/mol, CuO = 63.5 + 16 = 79.5 g/mol and CO2 = 44 g/mol. Heating 12.35 g (0.100 mol) of the carbonate gives 7.95 g of CuO and 4.40 g of CO2, and 7.95 + 4.40 = 12.35 g.
- Conclusion. The remaining solid is lighter than the starting solid, because carbon dioxide escaped. The total mass is conserved, and the change is chemical.
How did chemists learn to distinguish them?
In the 1700s, chemists such as Antoine Lavoisier performed experiments with closed vessels and balances, which showed that mass stays the same in chemical changes, and they identified gases such as oxygen and carbon dioxide as products. In 1808, John Dalton published an atomic theory in which chemical change is a rearrangement of atoms, whereas physical change leaves compounds intact. This provided the particle-level picture used today.
Which mistakes are common?
Mistake 1: Believing that atoms are destroyed in chemical changes
The atoms are rearranged, not created or destroyed.
Mistake 2: Saying that a change is chemical because it is hard to reverse
Scrambling an egg is chemical, but shredding paper is hard to reverse and is still physical.
Mistake 3: Using only one clue
Bubbles appear in boiling water (physical) and in vinegar with baking soda (chemical). Use several clues.
Mistake 4: Thinking that physical changes release no energy
Freezing releases energy and melting absorbs energy. The amounts are usually smaller than in many chemical changes.