Pure substances versus mixtures

A pure substance has a definite, fixed chemical composition, and as a result it always shows the same set of properties no matter where the sample originally came from — distilled water and a pure metal are both examples. A mixture, by contrast, has no fixed composition and therefore no single set of definite properties; paint (a blend of oils, pigment and additives) and concrete (a blend of sand, cement and water) are examples of mixtures. Pure substances split further into elements and compounds. An element cannot be broken down into anything simpler by ordinary chemical means, and elements themselves are further grouped as metals, non-metals, or metalloids. Metals are lustrous (shiny), good conductors of heat and electricity, ductile (can be drawn into wire) and malleable (can be hammered into thin sheets) — gold, silver, copper and iron are typical examples, and mercury is unusual among metals in being a liquid at room temperature. Non-metals lack lustre (with exceptions like diamond and iodine), are poor conductors (except graphite), and are brittle rather than ductile or malleable — iodine, nitrogen and carbon are examples. Metalloids, such as arsenic, silicon and germanium, show properties that sit in between those of metals and non-metals. A compound, unlike an element, CAN be broken down into simpler substances by ordinary chemical changes, because it consists of two or more elements combined together in a fixed proportion. A mixture, in contrast, contains two or more substances present in NO fixed proportion, and its components can usually be separated again by purely physical methods. Mixtures themselves are further divided into homog …