When most solids are heated, they melt into liquids. This behaviour makes sense in terms of entropy, or disorder: a liquid state is usually more disordered than a solid state, and higher temperatures mean that the particles within a material vibrate randomly with more energy. One exception, however, is helium-3. This isotope of helium solidifies on heating, because in its solid state, fluctuations in its atoms’ nuclear spin (or internal “rotation”) give it a higher entropy than its liquid counterpart. This phenomenon is known as the Pomeranchuk effect after the theoretical physicist Isaak Pomeranchuk, who predicted it in 1950.
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