Physical and Chemical Changes

In chemistry, changes can be sorted into two types: physical changes and chemical changes. Knowing the difference helps you predict what happens to materials in real life, from melting ice to rusting iron.

Physical changes

• What happens: The substance stays the same, but its form changes.
• Particles: The same particles are present; only their arrangement or spacing changes.
• Common examples: melting ice, boiling water, dissolving sugar in water, breaking glass, cutting paper.
• Reversibility: Often reversible (ice can refreeze), but not always (a shattered glass is still glass).

Chemical changes

• What happens: New substances form with different properties.
• Particles: Atoms rearrange and new bonds form.
• Common clues: color change, gas made (bubbles), a solid forms from solutions (precipitate), temperature change, light produced, a new smell.
• Examples: rusting iron, burning magnesium, baking a cake, vinegar reacting with baking soda.

How to tell the difference

• Ask: Is a new substance made? If yes, it is chemical.
• Change of state (solid ⇄ liquid ⇄ gas) is physical.
• Dissolving is usually physical: the solute spreads out between water particles without forming a new substance.
• Reversibility is a hint, not a rule. Some physical changes are hard to reverse; some chemical changes can be reversed only by another chemical reaction.

Particle view

Physical change: particles of the same substance move closer or further apart, or get rearranged, but do not change identity. Chemical change: atoms are rearranged to make new substances with new particle combinations.

Energy changes

Chemical reactions often involve larger energy changes. In an exothermic reaction, thermal energy is transferred to the surroundings, so the temperature of the surroundings increases. Some processes are endothermic and take in energy, making the surroundings cooler.