Lithium be cut with a knife. When cut, it possesses a silvery-white color that quickly changes to gray as it oxidizes to lithium oxide. While it has one of the lowest melting points among all metals (180 °C), it has the highest melting and boiling points of the alkali metals. Lithium has a very low density (0.534 g/cm), comparable with pine wood. It is the least dense of all elements that are solids at room temperature; the next lightest solid element (potassium, at 0.862 g/cm) is more than 60% denser. Furthermore, apart from helium and hydrogen, it is less dense than
Dense set close to it (see Diophantine approximation). Formally, a subset "A" of a topological space "X" is dense in X if for any point "x" in "X", any neighborhood of "x" contains at least one point from "A" (i.e., "A" has non-empty intersection with every non-empty open subset of "X"). Equivalently, "A" is dense in "X" if and only if the only closed subset of "X" containing "A" is "X" itself. This can also be expressed by saying that the closure of "A" is "X", or that the interior of the complement of "A" is empty. The density of a topological