What is the difference between a chemical and a physical change?

Physical changes: same substance, new look

In a physical change, a substance’s appearance changes, but its particles stay the same. Cutting paper, crushing a can, stretching a rubber band and breaking a glass are all physical changes. The pieces are smaller or a different shape, but they are still paper, metal, rubber and glass.

Changes of state are physical changes too. When ice melts, it becomes liquid water, and when water boils, it becomes steam, but every molecule is still H₂O; the molecules just move faster and spread farther apart. Dry ice, which is solid carbon dioxide, turns straight into a gas at −78.5 °C. This is called sublimation, and the gas is still CO₂.

Dissolving sugar or salt in water is usually treated as a physical change, because you can get the solid back by letting the water evaporate.

Chemical changes: something new forms

In a chemical change, the bonds between atoms in the starting substances break, and the atoms join up in new combinations. The result is one or more new substances with new properties. The atoms themselves are not destroyed; they are only rearranged.

Burning wood, rusting iron, cooking an egg, baking a cake, souring milk and digesting food are all chemical changes. When iron rusts, gray metal and oxygen gas become a flaky reddish-brown solid: 4Fe + 3O₂ → 2Fe₂O₃. When natural gas burns on a stove, methane and oxygen become carbon dioxide and water: CH₄ + 2O₂ → CO₂ + 2H₂O.

Signs of a chemical change

You cannot see atoms rearranging, but you can look for clues. The five most common signs are a color change that does not come from simply mixing colors, a gas being produced (bubbles or fizzing), a solid forming when two liquids are mixed (called a precipitate), a change in temperature, and light being given off. A new smell can be a clue as well.

Mixing baking soda and vinegar shows two signs at once: it fizzes because carbon dioxide gas forms, and the mixture gets slightly colder. The equation is NaHCO₃ + CH₃COOH → CH₃COONa + H₂O + CO₂. A burning candle gives off heat and light, and a sliced apple turns brown as it reacts with oxygen in the air.

Clues can be misleading

The signs are hints, not proof. Boiling water makes bubbles, but the bubbles are only water vapor, so boiling is a physical change. Mixing blue and yellow paint gives green, but the paints are only mixed, not turned into a new substance. A glowing light bulb gives off light and heat, yet the bulb does not change into anything new.

To decide for sure, ask whether a new substance with a different chemical formula has formed. Whether a change is easy to reverse is a useful hint too: melted ice freezes again, but a burned match cannot become a match again. Not every physical change is easy to undo, though, since a broken plate stays broken.

Side-by-side comparisons

Comparing pairs makes the difference clear. Melting butter is physical, but burning toast is chemical. Boiling water is physical, but passing electricity through water to split it into hydrogen and oxygen gases is chemical: 2H₂O → 2H₂ + O₂. Tearing a sheet of paper is physical, but setting it on fire is chemical.

Chemists also talk about physical and chemical properties. Physical properties, such as color, density and melting point, can be observed without changing the substance. Chemical properties, such as whether something burns or reacts with an acid, only show up when a chemical change happens.