Physical Changes in Chemistry

A physical change is a type of change in which the form of matter is altered but one substance is not transformed into another. The size or shape of matter may be changed, but no chemical reaction occurs.

Physical changes are usually reversible. Note that whether a process is reversible or not is not truly a criterion for being a physical change. For example, smashing a rock or shredding paper are physical changes that cannot be undone.

Contrast this with a chemical change, in which chemical bonds are broken or formed so that the starting and ending materials are chemically different. Most chemical changes are irreversible. On the other hand, melting water into ice (and other phase changes) can be reversed.

Physical Change Examples

Examples of physical changes include:

• Crumpling a sheet or paper (a good example of a reversible physical change)
• Breaking a pane of glass (the chemical composition of the glass remains the same)
• Freezing water into ice (the chemical formula is not changed)
• Chopping vegetables (cutting separates molecules, but does not alter them)
• Dissolving sugar in water (sugar mixes with water, but the molecules are not changed and may be recovered by boiling off the water)
• Tempering steel (hammering the steel does not change its composition, but does alter its properties, including hardness and flexibility)

Categories of Physical Changes

It's not always easy to tell chemical and physical changes apart. Here are some types of physical changes that may help:

• Phase Changes - Altering the temperature and/or pressure can change the phase of a material, yet its composition is unchanged,
• Magnetism - If you hold a magnet up to iron, you'll temporarily magnetize it. This is a physical change because it's not permanent and no chemical reaction occurs.
• Mixtures - Mixing together materials where one is not soluble in the other is a physical change. Note the properties of a mixture may be different from its components. For example, if you mix together sand and water, you can pack the sand into a shape. Yet, you can separate the components of the mixture by allowing them to settle or by using a sieve.
• Crystallization - Crystallizing a solid does not produce a new molecule, even though the crystal will have different properties from other solids. Turning graphite into a diamond doesn't produce a chemical reaction.
• Alloys - Mixing together two or more metals is a physical change that is not reversible. The reason alloying is not a chemical change is that the components retain their original identities.
• Solutions - Solutions are tricky because it may be hard to tell whether or not a chemical reaction has occurred when you mix together the materials. Usually, if there is no color change, temperature change, precipitate formation, or gas production, the solution is a physical change. Otherwise, a chemical reaction has occurred and a chemical change is indicated.