What’s the difference between chemical and physical changes and how can you tell them apart? In a nutshell, a chemical change leads to the production of a new substance, while a physical change does not. When a material undergoes a physical change, it may change shapes or forms, but no chemical reactions or new compounds are produced.
Key Takeaways: Chemical and Physical Change Examples
- A chemical change results from a chemical reaction, while a physical change is when matter changes forms but not chemical identity.
- Examples of chemical changes are burning, cooking, rusting, and rotting.
- Examples of physical changes are boiling, melting, freezing, and shredding.
- Many physical changes are reversible, if sufficient energy is supplied. The only way to reverse a chemical change is via another chemical reaction.
Examples of Chemical Changes
When the atoms rearrange themselves to form new chemical bonds, a new compound (product) is formed. A chemical change always involves a chemical reaction. The starting materials and final product are chemically different from one another. Here are some examples of chemical changes:
- Burning wood
- Souring milk
- Mixing acid and base
- Digesting food
- Cooking an egg
- Heating sugar to form caramel
- Baking a cake
- Rusting of iron
Examples of Physical Changes
No new chemical species forms in a physical change. Changing the state of a pure substance between solid, liquid, or gas phase is a physical changes since the identity of the matter does not change. A physical change involves changes in physical properties, but not chemical properties. For example, physical properties change during tempering steel, crystallization, and melting. Here are examples of physical changes:
- Crumpling a sheet of aluminum foil
- Melting an ice cube
- Casting silver in a mold
- Breaking a bottle
- Boiling water
- Evaporating alcohol
- Shredding paper
- Sublimation of dry ice into carbon dioxide vapor
- Carbon changing from graphite into a diamond
How to Tell Whether It’s a Physical or Chemical Change?
Look for an indication that a chemical change occurred. Signs of a chemical change include the following:
- Gas is produced. In liquids, bubbles may form.
- An odor is produced.
- The substance changes color.
- Sound is produced.
- There is a temperature change. The surroundings become either hot or cold.
- Light is produced.
- A precipitate forms.
- The change is difficult or possible to reverse.
A chemical change might not display all of these signs. A physical change likely occurred if you don’t see any of these indications. Be aware a physical change may produce a dramatic change in the appearance of a substance. A physical change can produce every sign of a physical change. This doesn’t mean a chemical reaction occurred. The only way to know for certain whether a change is chemical or physical is a chemical analysis of the starting and ending materials.
It may be hard to tell whether a chemical or physical change occurred in some cases. For example, a physical change occurs when you dissolve sugar in water. The form of the sugar changes, but it remains the same chemically (sucrose molecules). However, when you dissolve the salt in water the salt dissociates into its ions (from NaCl into Na+ and Cl-) so a chemical change occurs. In both cases, a white solid dissolves into a clear liquid and in both cases, you can recover the starting material by removing the water, yet the processes are not the same.
Source
- Atkins, P.W.; Overton, T.; Rourke, J.; Weller, M.; Armstrong, F. (2006). Shriver and Atkins Inorganic Chemistry (4th ed.). Oxford University Press. ISBN 0-19-926463-5.
- Chang, Raymond (1998). Chemistry (6th ed.). Boston: James M. Smith. ISBN 0-07-115221-0.
- Clayden, Jonathan; Greeves, Nick; Warren, Stuart; Wothers, Peter (2001). Organic Chemistry (1st ed.). Oxford University Press. ISBN 978-0-19-850346-0.
- Kean, Sam (2010). The Disappearing Spoon – And Other True Tales From the Periodic Table. Black Swan, London. ISBN 978-0-552-77750-6.
- Zumdahl, Steven S. and Zumdahl, Susan A. (2000). Chemistry (5th Ed.). Houghton Mifflin. ISBN 0-395-98583-8.