A convex subset of a Banach space is closed if and only if it is weakly closed

For completeness, I will add a proof of this standard result. It relies on the following form of the Hahn-Banach separation theorem: Let $X$ be a real normed space (or generally, locally convex TVS), and suppose that $A\subset X$ is compact and convex, $B\subset X$ is closed and convex, and $A\cap B$ is empty. Then there exists a linear functional $\phi$ such that $\sup_A \phi <\inf_B \phi$.

Apply the above to a closed convex set $B$ and a one-point set $A=\{x\}$ disjoint from it. The functional $\phi$ provides a weakly open set containing $x$ and disjoint from $B$. Thus, $X\setminus B$ is weakly open, and therefore $B$ is weakly closed.

Hint: one direction is easy; the other uses the Hahn-Banach separation theorem.