There are many ways that matter can be changed.
For example, a blacksmith could take a piece of metal and hammer it into a different shape, melt it into a liquid, etch it with acid, or leave it to rust. All these actions will change the metal, but some will just change the appearance of the metal, while others change its chemical composition. Learn more about these two types of changes by reading the tabs.
A physical change is a change that alters the physical properties of a material without changing the composition of the substances in the material. Physical changes affect the form or appearance of a material, but the chemical composition of the substances in the material stay the same. It is important to note that no new substances are formed because of a physical change.
Folding, cutting, ripping, or crumpling paper all change the overall size, shape, and appearance of the paper, but the composition remains unchanged. No new substances have been created; just smaller, differently shaped pieces of the same material.
Boiling water changes the state of matter of the water, but chemically it still consists of the same H2O molecules. Their speed and spacing has changed, but no new substance has been created.
A chemical change is a change that occurs when a substance reacts and forms one or more new substances. Another name for a chemical change is a chemical reaction. New substances with different compositions and properties are formed as the result of a chemical change.
When iron (Fe) metal rusts (oxidizes), it reacts with the oxygen in the air to create an entirely new substance: iron oxide (Fe2O3 – also called iron (III) oxide or ferric oxide). The new substance has a different chemical composition and properties and can be identified by a different chemical formula.
When wood burns in a campfire, a chemical change is occurring. The fire will produce carbon dioxide and water vapor gases, while the wood is changed into ash. The resulting materials are completely new substances with different chemical compositions and properties.
This table highlights several pieces of evidence that you can look for to help identify that a chemical change has taken place.
| Evidence | Example |
|---|---|
| Color change | Toasting a piece of bread makes it get darker. |
| Temperature change | Mixing vinegar and baking soda together makes it feel cold. |
| A new gas is produced | The yeast in bread dough produces carbon dioxide bubbles, which make the bread rise. |
| Light or smell is produced | A campfire burns bright and creates a smoky odor. |
| A solid precipitate* is formed | Dissolved minerals come together in the body to form an insoluble substance, causing a kidney stone. |
*A precipitate is a solid that is formed when two solutions of dissolved compounds are mixed and form a product that is insoluble (it cannot dissolve, so it comes out of the solution).
For Example
There are many examples of physical and chemical changes, a few of which are summarized below.
Physical Changes
- Cutting/ripping/tearing
- Crumpling/bending/folding
- Phase changes
- Making a mixture
Chemical Changes
- Baking
- Rusting/oxidizing
- Burning
- Decomposition
- Reacting with acid
Let's Practice
How well can you differentiate between physical and chemical changes? Find out by completing this activity. Select the correct answer to each question.
Which One?
1. Which statement describes a physical change?
frying an egg
slicing bread
baking a cake
Physical changes alter the appearance, but not the chemical composition of the substances in the material. The processes of frying an egg and baking a cake change the chemical makeup of the substances in the foods and creates new substances.
Physical changes alter the appearance, but not the chemical composition of the substances in the material. The processes of frying an egg and baking a cake change the chemical makeup of the substances in the foods and creates new substances.
Physical changes alter the appearance, but not the chemical composition of the substances in the material. The processes of frying an egg and baking a cake change the chemical makeup of the substances in the foods and creates new substances.
2. Which statement describes a chemical change?
bleaching your hair
cutting your hair
brushing your hair
Chemical changes alter the chemical composition of the substances in a material. Cutting or brushing your hair just changes its appearance, without changing its chemical make-up. Because bleaching causes hair to change color, bleaching is likely to be a chemical change.
Chemical changes alter the chemical composition of the substances in a material. Cutting or brushing your hair just changes its appearance, without changing its chemical make-up. Because bleaching causes hair to change color, bleaching is likely to be a chemical change.
Chemical changes alter the chemical composition of the substances in a material. Cutting or brushing your hair just changes its appearance, without changing its chemical make-up. Because bleaching causes hair to change color, bleaching is likely to be a chemical change.
3. Which statement describes a physical change?
an apple rotting
mixing trail mix
digesting food
Physical changes alter the appearance, but not the chemical composition of a material. Recall that mixtures (both heterogeneous and homogeneous) are created by physically mixing different components together (no new substances are formed).
Physical changes alter the appearance, but not the chemical composition of a material. Recall that mixtures (both heterogeneous and homogeneous) are created by physically mixing different components together (no new substances are formed).
Physical changes alter the appearance, but not the chemical composition of a material. Recall that mixtures (both heterogeneous and homogeneous) are created by physically mixing different components together (no new substances are formed).
4. Which statement describes a chemical change?
water vapor condensing into a liquid
hydrogen peroxide decomposing into water and oxygen
water freezing into solid ice cubes
Chemical changes alter the chemical composition of a substance and create something new. In a phase change the molecules change speed and spacing from each other, but chemically remain unchanged. Hydrogen peroxide (H2O2) decomposes to produce water (H2O) and oxygen (O2). Because new substances are formed, decomposition is a chemical change.
Chemical changes alter the chemical composition of a substance and create something new. In a phase change the molecules change speed and spacing from each other, but chemically remain unchanged. Hydrogen peroxide (H2O2) decomposes to produce water (H2O) and oxygen (O2). Because new substances are formed, decomposition is a chemical change.
Chemical changes alter the chemical composition of a substance and create something new. In a phase change the molecules change speed and spacing from each other, but chemically remain unchanged. Hydrogen peroxide (H2O2) decomposes to produce water (H2O) and oxygen (O2). Because new substances are formed, decomposition is a chemical change.
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