Physical Properties
NaK containing 40% to 90% potassium by weight is liquid at room temperature. The eutectic mixture consists of 77% potassium and 23% sodium, is liquid from −12.6 to 785°C, and has a density of 866 kg/m³ at 21°C and 855 kg/m³ at 100°C, making it less dense than water. It is highly reactive with water and is stored under dry nitrogen (or, even better, dry argon) for safety reasons.
When stored in air, it forms a yellow potassium superoxide coating and may ignite. This superoxide reacts explosively with organics. NaK is not dense enough to sink in most hydrocarbons, but will sink in lighter mineral oil. It is unsafe to store in this manner if the superoxide has formed. A large explosion took place at the Oak Ridge Y-12 facility on December 8, 1999, when NaK cleaned up after an accidental spill and inappropriately treated with mineral oil was scratched with a metal tool. It also attacks Teflon.
NaK has a very high surface tension, which makes large amounts of it pull into a bun-like shape. Its specific heat capacity is 982 J/(kg.K), which is roughly one-fourth of that for water but heat capacity is higher over possible heat difference and flow speed due to higher thermal conductivity.
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NaK alloy is liquid at room temperature.
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