What is the NaK alloy, and why does it remain liquid at low temperatures?

NaK is a commercial alloy composed of sodium and potassium, typically containing approximately 78 percent potassium. It is widely used as a heat-transfer fluid and as an organic reactant.1

NaK remains liquid at low temperatures because it forms a eutectic system. A eutectic is a mixture of substances that melts at a temperature lower than that of its individual components. While pure sodium has a melting point of 97.72 °C and pure potassium melts at 63.38 °C, the NaK eutectic alloy has a significantly lower melting point of −10 °C (14 °F). This phenomenon occurs because sodium and potassium atoms have dissimilar atomic volumes, which prevents them from forming a complete solid solution and instead results in a eutectic mixture with a depressed melting point.123

Would you like to learn more about other low-melting alloys formed by alkali metals, such as those involving rubidium or cesium?