HiSET Science: Pure Substances and Mixtures

Pure Substances
Pure substances are substances that cannot be further broken down into simpler substances and still retain their characteristics.
Pure substances are categorized as either elements or compounds.
Elements that consist of only one type of atom may be monatomic, diatomic, or polyatomic. For example, helium (He) and copper (Cu) are monatomic elements, and hydrogen (H2) and oxygen (O2) are diatomic elements.
Phosphorus (P4) and sulfur (S8) are polyatomic elements.
Compounds consist of molecules of more than one type of atom. For example, pure water (H2O) is made up of molecules consisting of two atoms of hydrogen bonded to one atom of oxygen, and glucose (C6H12O6) is made up of molecules of six carbon atoms and twelve hydrogen atoms bonded together with six oxygen atoms.

Mixtures
Mixtures can be classified as either homogeneous mixtures or heterogeneous mixtures. The molecules of homogeneous mixtures are distributed uniformly throughout the mixture, but the molecules of heterogeneous mixtures are not distributed uniformly throughout the mixture. Air is an example of a homogeneous mixture, and a pile of sand and rock is an example of a heterogeneous mixture. Solutions are homogeneous mixtures consisting of a solute (the substance that is dissolved) and a solvent (the substance doing the dissolving).

Suspensions
Suspensions are heterogeneous mixtures in which the particle size of the substance suspended is too large to be kept in suspension by Brownian motion. Once left undisturbed, suspensions will settle out to form layers. An example of a suspension is sand stirred into water. Left undisturbed, the sand will fall out of suspension and the water will form a layer on top of the sand.

Mixtures with Compounds
Mixtures are similar to compounds in that they are produced when two or more substances are combined. However, there are some key differences as well. Compounds require a chemical combination of the constituent particles, while mixtures are simply the interspersion of particles. Unlike compounds, mixtures may be separated without a chemical change. A mixture retains the chemical properties of its constitutent particles, while a compound acquires a new set of properties. Given compounds can exist only in specific ratios, while mixtures may be any ratio of the involved substances.