Grade 7 Chemistry • Official Chapter II: Substances and their structure
Pure Substances and Mixtures
Use observable properties and particle models to decide what a sample contains and how its parts may be separated.
Big idea and learning goals
A sample may contain one chemical substance or several substances physically combined. Appearance alone is not always enough: clear air and clear salt water are mixtures, while a pure compound can contain more than one kind of atom. Chemists combine careful observations, measured properties, and particle models.
By the end of this lesson, you should be able to:
- define pure substance and mixture at the macroscopic and particle levels;
- distinguish homogeneous and heterogeneous mixtures;
- explain why mixture composition can vary while a pure substance has characteristic properties;
- classify particle diagrams without confusing atoms with substances;
- choose an introductory separation method and justify it using a physical-property difference.
1. What does “pure” mean in chemistry?
A pure substance is matter with a fixed chemical identity and a uniform composition throughout the sample. At the particle level, every representative particle belongs to the same substance. A pure sample therefore has characteristic physical properties, such as a definite melting point under stated pressure, that can help identify it.