Matter and Its Properties
Classification
Matter is either a pure substance or a mixture. Pure substances have fixed composition: elements contain one kind of atom; compounds contain two or more elements chemically bonded in fixed ratios. Mixtures are physical combinations: homogeneous mixtures (solutions) have uniform composition, while heterogeneous mixtures have visibly different regions. Separation exploits physical differences: filtration for insoluble solids, distillation for boiling-point differences, chromatography for differing attractions, and evaporation for a dissolved solid.
Properties and changes
Physical properties are observed without changing identity (density, melting point, conductivity); intensive properties do not depend on amount, extensive properties do. Chemical properties describe reactivity. A physical change alters state or form but not composition; a chemical change produces new substances, often with color change, gas, precipitate, or heat. The law of definite proportions fixes a compound's mass ratio; the law of multiple proportions applies to different compounds of the same elements.
Worked example
Classify brass, carbon dioxide, oxygen, and salt water. Brass is a homogeneous mixture (alloy of Cu and Zn). Carbon dioxide is a compound (C and O chemically combined). Oxygen is an element (O2). Salt water is a homogeneous mixture (solution). A magnet separating iron filings from sulfur is a physical separation; heating iron and sulfur to form FeS is a chemical change.
Traps
Calling a homogeneous mixture a compound because it looks uniform; treating an alloy as a compound; using chemical change for dissolving salt, which is usually physical; confusing intensive and extensive properties; and forgetting that a compound's components can be separated only by chemical means.