Recognizing Physical Changes
The change of water from a liquid to a gas during boiling is a physical change. A physical change occurs when some of the properties of a material change, but the substances in the material remain the same. For example, if yo slowly heat butter in a pan, it changes from a solid to a liquid, but the substances in the butter do not change. Two other examples of physical changes are crumpling a piece of paper and slicing a tomato. Crumpling and slicing are actions that change the size and shape of a material, but not its composition.
Some physical changes can be reversed. Yo can freeze water, melt the ice that forms, and then freeze the water again. Yo can use an iron to remove the wrinkles from a cotton shirt. Yo can braid hair, unbraid the hair, and then braid it again. Some physical changes cannot be reversed. Yo would not expect to reconstruct a whole tomato from tomato slices or to replace the peel on a peeled orange. Figure 15 shows one physical change that can be reversed and one that cannot be reversed.
Figure 15 Braiding hair and cutting hair are examples of physical changes. Braiding is a reversible change. Cutting cannot be reversed.
Section 2.2 Assessment
Reviewing Concepts
List seven examples of physical properties.- Describe three uses of physical properties.
- Name two processes that are used to separate mixtures.
When yo describe a liquid as thick, are yo saying that it has a high or a low viscosity?
Explain why sharpening a pencil is an example of a physical change.
What allows a mixture to be separated by distillation?
Critical Thinking
Designing Experiments How could yo find out whether copper is harder or softer than the plastic used in a plastic cup?
Inferring Why would yo expect the materials used to make pot holders to be poor conductors of heat?
Applying Concepts Silicon dioxide is a solid at room temperature and methanol is a liquid. Which substance has the higher melting point?
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