Chemical and physical changes are two ways matter can change. In a chemical change, a chemical reaction occurs and a new substance is produced. In a physical change, matter changes forms, state, appearance or size but does not change its chemical identity. So, the difference between chemical and physical changes is that a chemical change alters the chemical composition of matter, while a physical change does not
A chemical change produces a new substance, while a physical change alters the form of matter but not its chemical identity.
Key Takeaways: Chemical and Physical Changes
- A chemical change produces one or more new substances.
- A physical change changes the form or state of matter without changing its chemical identity.
- Signs such as gas formation, a precipitate, light, or an unexpected temperature or color change can provide evidence of a chemical reaction.
- These signs are clues, not absolute proof that a chemical change occurred.
- Reversibility is not a reliable way to distinguish chemical and physical changes.
- Phase changes, such as melting, freezing, and boiling, are physical changes.
- Chemical and physical changes sometimes occur together.
Chemical Change vs Physical Change
| Feature | Chemical Change | Physical Change |
|---|---|---|
| Chemical identity | Changes | Stays the same |
| New substance formed? | Yes | No |
| Chemical bonds | Bonds break and/or form as atoms rearrange | Chemical identity remains unchanged |
| Common examples | Burning, rusting, cooking | Melting, cutting, crushing |
| Reversible? | Sometimes | Sometimes |
| Evidence | May include gas, precipitate, light, odor, or temperature/color change | Often involves a change in state, shape, size, or appearance |
Practice Identifying Chemical and Physical Changes
Download and print this worksheet for practicing identifying chemical and physical changes. The worksheet and answer key are PDF files, or you can right-click, save, and print the PNG image.
Chemical Changes
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.
The defining feature of a chemical change is the formation of a new substance, not whether the change is difficult to reverse or whether it produces a dramatic visible effect.
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 wood
- Mixing baking soda and vinegar
- Decomposing hydrogen peroxide
Physical Changes
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, condensation, 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 water
- Boiling water
- Sublimation of dry ice into carbon dioxide gas
- Shredding paper
- Crushing a can
- Breaking glass
- 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
- Light
- Color change
- Bubbling
- Odor
- Sound
- Formation of a precipitate
If none of these signs are present, it’s a good bet a physical change occurred.
However, none of these observations by itself proves that a chemical change occurred. Physical changes produce some of the same effects. For example, boiling water produces bubbles, but boiling is a physical change. The strongest evidence is showing that substances with new chemical compositions formed.
Are Physical Changes Reversible?
Reversibility is not a reliable test for distinguishing chemical and physical changes.
Some physical changes are easy to reverse. Water freezes into ice and melts back into liquid water. Other physical changes are difficult or impractical to reverse. Shredding a sheet of paper does not change the chemical identity of the paper, but putting the original sheet back together is nearly impossible.
Chemical changes can also be reversible. A reverse chemical reaction can convert products back into reactants under suitable conditions.
Instead of asking whether a change is reversible, ask the more important question: Did the chemical identity of the substance change?
Tricky Examples
| Change | Chemical or Physical? | Why? |
|---|---|---|
| Melting ice | Physical | H₂O remains H₂O |
| Boiling water | Physical | H₂O remains H₂O |
| Dissolving sugar in water | Physical | Sugar molecules remain chemically unchanged |
| Dissolving salt in water | Physical | The ions separate and become hydrated, but no new substance is produced; the salt can be recovered |
| Rusting iron | Chemical | Iron reacts to form iron oxides |
| Burning wood | Chemical | Combustion forms new substances |
| Cooking an egg | Chemical | Heat causes chemical changes in proteins |
| Cutting paper | Physical | Composition remains essentially unchanged |
| Burning a candle | Both | Wax melts and vaporizes physically while wax vapor burns chemically |
Can a Change Be Both Chemical and Physical?
Chemical and physical changes can occur at the same time.
A burning candle is a familiar example. The solid wax melts into liquid wax and then vaporizes. Melting and vaporization are physical changes because the wax retains its chemical identity. Meanwhile, wax vapor reacts with oxygen in a combustion reaction, producing new substances such as carbon dioxide and water. Combustion is a chemical change.
Cooking can also involve both types of change. For example, heating food may melt fats or evaporate water while chemical reactions simultaneously produce new compounds.
Learn More
Explore chemical and physical changes in greater detail and learn how they relate to chemical and physical properties of matter:
- Examples of Physical Changes
- Examples of Chemical Properties
- Is Dissolving Salt a Chemical or Physical Change?
- Examples of Physical Properties
Common Misconceptions
“All physical changes are reversible.”
No. Reversibility does not determine whether a change is physical or chemical. Breaking glass and shredding paper are physical changes even though restoring the original objects is difficult.
“Bubbles always mean a chemical reaction occurred.”
No. Gas production can indicate a chemical reaction, but boiling also produces bubbles and is a physical change.
“A color change always means a chemical change.”
No. A color change can provide evidence of a reaction, but physical processes such as mixing pigments can also change color.
“Dissolving is always a chemical change.”
No. Many dissolution processes, including dissolving sugar or salt in water, are classified as physical changes because the substances retain their chemical identities.
“Chemical changes are permanent.”
Not necessarily. Some chemical reactions are reversible. What defines a chemical change is the formation of substances with different chemical compositions.
Frequently Asked Questions
What is the main difference between a chemical and physical change?
A chemical change produces one or more new substances, while a physical change changes the form or state of matter without changing its chemical identity.
Is melting a chemical or physical change?
Melting is a physical change. For example, ice and liquid water are both H₂O.
Is boiling water a chemical change?
No. Boiling water is a physical change because liquid H₂O becomes gaseous H₂O without forming a new substance.
Is dissolving a chemical or physical change?
It depends on the substances involved, but many familiar examples of dissolving are physical changes. Dissolving sugar or salt in water does not produce a new substance and is generally classified as a physical change.
Is burning a chemical or physical change?
Burning is a chemical change because combustion produces new substances. However, physical changes can occur at the same time. In a burning candle, for example, wax melts and vaporizes before the vapor undergoes combustion.
References
- 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.; Zumdahl, Susan A. (2000). Chemistry (5th ed.). Houghton Mifflin. ISBN 0-395-98583-8.