Exercise 26, § 5

Let $k$ be a field, $A$ an integral $k$-algebra of finite type. Now prove that $\mathrm{Lis}(A)$ is dense if and only if $A$ is a separable $k$-algebra; it amounts to the same thing to suppose that the field of fractions of $A$ is a separable extension of $k$ (A, V, p. 115).