What is a pure substance that is present before a reaction but not after.

They are like other investigations? say there was a murder u have to do blood test and finger prints which run through a lab. 

It is a mixture. simply put, a compound must be pure, that is, of only one chemical structure and composition. a mixture is made of different chemical substances (i.e. elements, molecules, compounds) that are not chemically bonded to each other, but instead physically grouped together, as in the case of gasoline (a homogeneous solution).

D=m/v =57.9/3 = 19.3 g/cm^3