This question can be explained in the light of following concepts. In case of alloy the metals are mixed by melting them. The main point to note is that the proportion is not fixed in case of alloys. But in pure substances the proportion is fixed . And they make same kind of particles .
On the basis of Optical activity the compounds or isomers can be classified into two types:- Those rotate the plane polarised light towards right called the dextro rotatory compounds. Those rotate the plane polarised light towards left are called Laevo rotatory compounds. The equimolar composition of the above two isomers is called racemic mixture.