Classifying Matter

Objective: Identify matter, mass, volume, density, and measurement conditions.

Matter has mass and occupies space. A physical property can be observed or measured without changing a substance’s chemical identity, such as density, melting point, or electrical conductivity. An intensive property is independent of sample size under comparable conditions, whereas an extensive property such as mass scales with amount. Density ρ = m/V can help compare samples, but temperature, porosity, and composition affect its measured value. Physical properties alone may not identify a material uniquely because different substances can share similar values. Energy is not matter, and a shadow does not have material mass as a separate object.

Objective: Classify matter by composition and bonding.

A pure substance has a defined composition for a specified chemical species. An element consists of atoms with the same proton number; a compound contains different elements chemically bonded in a fixed stoichiometric relationship, though crystal defects and nonstoichiometry can complicate advanced cases. A mixture combines substances without requiring one fixed chemical formula, and components may often be separated physically. Air is a gas mixture; pure water is a compound; elemental copper is an element. A molecule is a bonded group of atoms, but not all compounds consist of discrete molecules, as ionic solids show. Distinguish a mixture of elements from a new compound formed by reaction.

Objective: Select a separation method and distinguish it from reaction.

Mixture components can sometimes be separated using differences in properties. Filtration can separate an insoluble solid from a fluid; distillation separates based on vaporization and condensation behavior; chromatography exploits different interactions with stationary and mobile phases. Evaporation can recover a dissolved nonvolatile solute from solvent, but conditions and losses matter. These processes rearrange where existing substances are without necessarily changing their chemical identities. Chemical change forms different substances through bond rearrangement; evidence such as gas or color change must be interpreted with controls because physical processes can produce similar appearances. A compound cannot generally be separated into its constituent elements by simple filtration.