Which of the following will not have a uniform chemical composition?

A mixture is a material, which consists of two or more pure substances (elements or compounds) which are not chemically combined but physically mixed in any proportion. The mixture shows the properties of all its constituents and it can be separated into its constituents by using physical method. Air is a homogeneous mixture of many gases like nitrogen (78%), oxygen (21%), carbon dioxide, inert gases and water vapours. Copper suphate, sodium chloride and water are compounds as they are made up of two or more elements chemically combined in a fixed ratio by mass. Therefore, they have definite compositions.