Which of the following materials fall in the category of a “pure substance”?
(a) Ice
(b) Milk
(c) Iron
(d) Hydrochloric acid
(e) Calcium oxide
(f) Mercury
(g) Back
(h) Wood
(i) Air.

An element is a pure substance, which is made up of only one kind of atoms. It cannot be further reduced to simpler substances by any physical or chemical methods. An atom is the smallest unit of an element. Elements are classified into metals, non-metals, metalloids and noble gases. Example: Carbon

A compound is a pure substance that consists of two or more elements combined chemically in a fixed proportion by weight and can be broken down into elements by chemical methods only. Example: Sodium chloride (NaCl)

Mixtures consist of two or more different kinds of particles or two or more pure substances mixed together but not chemically bound.

Example: The sugar solution is a mixture because it can be separated into its constituents by physical processes (like evaporation). It shows the properties of both sugar and water.]