Physical changes involve states of matter and energy. No new substance is created during a physical change, although the matter takes a different form. The size, shape, and color of matter may change. Physical changes occur when substances are mixed but don't chemically react.
How to Identify a Physical Change
One way to identify a physical change is that such a change may be reversible, especially a phase change. For example, if you freeze water into an ice cube, you can melt it into the water again. Ask yourself:
- Is the change reversible? Not all physical changes are easy to reverse.
- Was there a color change (with exceptions), bubble formation, or formation of a precipitate? These are all signs of a chemical change, not a physical change.
- Is the chemical identity of the end product the same as it was before the change? If the answer is yes, it's a physical change. If the answer is no, it's a chemical change.
Examples of Physical Changes
Remember, the appearance of matter changes in a physical change, but its chemical identity remains the same.
- Crushing a can
- Melting an ice cube
- Boiling water
- Mixing sand and water
- Breaking a glass
- Dissolving sugar and water
- Shredding paper
- Chopping wood
- Mixing red and green marbles
- Sublimation of dry ice
- Crumpling a paper bag
- Melting solid sulfur into liquid sulfur. This is an interesting example since the state change does cause a color change, even though the chemical composition is the same before and after the change. Several nonmetals, such as oxygen and radon, change color as they change phase.
- Chopping an apple
- Mixing salt and sand
- Filling a candy bowl with different candies
- Vaporizing liquid nitrogen
- Mixing flour, salt, and sugar
- Mixing water and oil
Indications of a Chemical Change
Sometimes the easiest way to identify a physical change is to rule out the possibility of a chemical change. There may be several indications that a chemical reaction has occurred. Note: It's possible for a substance to change color or temperature during a physical change.
- Evolving bubbles or releasing gas
- Absorbing or releasing heat
- Changing color
- Releasing an odor
- Inability to reverse the change
- Precipitation of a solid from a liquid solution
- Formation of a new chemical species. This is the best and surest indicator. A change in the chemical properties of the sample may indicate a chemical change (e.g., flammability, oxidation state).
