NaK is highly reactive with water (like its constituent elements) and may catch fire when exposed to air, so must be handled with special precautions.

The eutectic mixture consists of 77% potassium and 23% sodium by mass (NaK-77), and it is a liquid from −12.6 to 785 °C (9.3 to 1,445.0 °F), and has a density of 0.866 g/cm3 at 21 °C (70 °F) and 0.855 g/cm3 at 100 °C (212 °F), making it less dense than water.

NaK has a very high surface tension, which makes large amounts of it pull into a bun-like shape.

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.

[8] Sodium–potassium alloy polymerizes dimethyldichlorosilane into polysilanes with a Si-Si backbone and methyl radicals, primarily dodecamethylcyclohexasilane.

[clarification needed] NaK has been used as the coolant in experimental fast neutron nuclear reactors.

Use of lead or pure sodium, the other materials used in practical reactors, would require continual heating to maintain the coolant as a liquid.

The first nuclear reactor in space,[10][11] the United States' experimental SNAP-10A satellite, used NaK as coolant.

The NaK was circulated through the core and thermoelectric converters by a liquid metal direct current conduction-type pump.

[15][16] In addition to the wide liquid temperature range, NaK has a very low vapor pressure, which is important in the vacuum of space.

An unintended consequence of the usage as a coolant on orbiting satellites has been the creation of additional space debris.

Eutectic NaK (NaK-77, an alloy of 77% potassium and 23% sodium by mass) can be used as a hydraulic fluid in high-temperature and high-radiation environments, for temperature ranges of −12 to 760 °C (10 to 1,400 °F).

[21] NaK may also be used to transmit forces inside high temperature pressure transducers as an alternative to mercury.