What is the difference between a physical change and a chemical change?

Question

Accepted Answer

A physical change alters a substance's form or appearance without changing its chemical identity, while a chemical change transforms a substance into one or more new substances with different chemical properties. A physical change alters the form, appearance, or state of a substance without changing its chemical composition or identity. Examples include melting ice (H₂O remains H₂O), dissolving sugar in water, cutting paper, and changing the shape of clay. A chemical change (chemical reaction) transforms one or more substances into entirely new substances with different chemical properties. Examples include burning wood (cellulose becomes CO₂ and H₂O), rusting iron (Fe becomes Fe₂O₃), cooking an egg (proteins denature and reform), and digesting food. Chemical changes involve the breaking and forming of chemical bonds.

The claim that there is no difference ignores the fundamental distinction between changing form and changing chemical identity. The permanence assertion is often reversed in reality; many chemical changes (like burning) are practically irreversible, while physical changes (like freezing water) are easily reversed. Both types of changes occur in all states of matter, not restricted to specific phases.

Understanding the distinction between physical and chemical changes is important for the DSST Principles of Physical Science I exam. Students should be able to classify common transformations as physical or chemical, know the indicators of chemical change (color change, gas production, temperature change, precipitate formation, new odor), and understand that physical changes are generally more easily reversible than chemical changes. This distinction is fundamental to understanding matter and its transformations.

DSST Principles of Physical Science I: Question of the Day

Get a free DSST Principles of Physical Science I question every day

Ready for the full DSST Principles of Physical Science I? Earn 3 college credits.