2.6 Classifying Matter

Notes

• A pure substance has a fixed composition. An element has one kind of atom (oxygen, gold). A compound has two or more elements chemically combined in a fixed ratio (water, salt).
• A mixture is two or more substances physically combined, each keeping its properties.
• A homogeneous mixture (solution) looks the same throughout, like salt water. A heterogeneous mixture has visibly different parts, like salad or sand and water.
• Mixtures can be separated by physical methods such as filtering, evaporation and distillation. Compounds need a chemical change.