Rational Number Space is Separable

Theorem
Let $\Q$ be the set of all rational numbers.

Let $d: \Q \times \Q \to \R$ be the Euclidean metric on $\Q$.

Then $\left({\Q, d}\right)$ is separable.

Proof
From Rational Numbers are Countably Infinite, $\Q$ is itself countable.

The result follows from Topological Space is Everywhere Dense relative to Itself.