There are definitely pure races. Since purity is actually very easy to achieve.

For example one negroid or mongoloid in a family, if further mixing was only with Whites, will remove the coloured blood after about 3 generations. And vice versa. It's called breeding out a gene (outbreeding).

The original pure races, as described by scientists/anthrologists, are the caucasian, negroid, and mongoloid race (there are others but they are less numerous).

I don't know why you're being downvoted of course pure races(Caucasian, Negroid, Mongoloid) exist however it is impossible for people to be purely lets say as an example Anglo due to Europeans have little bits of DNA from other parts of Europe.