What temperature would water freeze instantly?
There is not really a magical temperature. To get water to freeze instantly, you would have to find some way to remove the requisite heat content of that water instantly. Heat is lost by either or a combination of radiation, conduction or convection to areas of less heat content. The quicker that transfer, the quicker freezing will occur. Thus a thin sheet of water on a sub-freezing surface will freeze quicker than a deeper amount of water. So there is not a definite answer, as several other factors must be considered.
