Question

What is so special about water at `4^@ C`?

Answer 1

Because at `1` `atm`, this is the `"densest"` state that water exhibits, i.e. with respect to solid, or liquid (or of course gas).

Explanation:

And given this density, `"ice"` will float in water. That the liquid phase is denser than the solid phase is an highly unusual phenomenon, that is shared by few other materials.

If ice were denser than liquid water, ice bergs would probably sink, and the polar ice caps would possibly extend a lot farther around the globe.