Chemical and physical changes both are changes in the structure of matter. In a chemical change, a chemical reaction occurs and a new substance is produced. In a physical change, matter changes forms but does not change its chemical identity.
A chemical change involves a chemical reaction to produce a new product. It is a change at the molecular level of matter. Chemical bonds between atoms break and then form to connect different atoms.
Examples of Chemical Changes
In a chemical change, new product forms as atoms rearrange themselves. Chemical bonds are broken and reform to make new molecules. Examples of chemical changes include:
- Souring milk
- Digesting food
- Cooking an egg
- Baking a cake
- Rusting iron
- Mixing an acid and a base
- Burning a candle
- Mixing baking soda and vinegar
A physical change is a change in matter that alters its form but not its chemical identity. The size or shape of matter often changes, but there is no chemical reaction. Phase changes are physical changes. These include melting, boiling, vaporization, freezing, sublimation and deposition. Breaking, crumpling, or molding matter also results in a physical change. Many physical changes are reversible.
Examples of Physical Changes
Examples of physical changes include:
- Melting an ice cube
- Freezing an egg
- Boiling water
- Sublimation of dry ice into carbon dioxide gas
- Shredding paper
- Crushing a can
- Breaking a bottle
- Chopping vegetables
- Mixing sand and salt
- Making sugar crystals
- Dissolving sugar in water (the sugar mixes with the water, but can be recovered by evaporation or boiling)
How to Tell Chemical and Physical Changes Apart
The key to distinguishing between chemical and physical changes is determining whether there is a new substance that wasn’t there before. If you see signs of a chemical reaction, it’s probably a chemical change. Signs of a reaction include:
- Temperature change
- Color change
- Formation of a precipitate
If none of these signs are present, it’s a good bet a physical change occurred.
Some people use reversibility as a test for chemical and physical changes. The premise is that a physical change can be undone, while a chemical change can only be reversed by another chemical reaction. This is not a great test because there are too many exceptions. While you can melt and freeze an ice cube (a physical change), it’s much harder to reassemble shredded paper (another physical change).
Most physical changes can be reversed if energy is added. Chemical changes can only be reversed via another chemical reaction.