The transition between a metal and an insulator. There are several ways in which a metal–insulator transition or the reverse transition can be achieved, including changing the distance between the atoms or the amount of disorder in the system. For example, metallic hydrogen was produced in 1996 by very high compression of hydrogen. Another dramatic example of a metal–insulator transition is given by metal–ammonia solutions. When an alkali metal such as sodium is introduced into liquid ammonia it dissolves to give a bright blue solution. As the concentration of the dissolved sodium increases, there is a transition to a bronze colour, with the liquid becoming a better electrical conductor than liquid mercury. See also Anderson localization.