Question

A solid normally absorbs heat as it melts, but there is a known exception at temperatures close to absolute zero. Find out which solid it is and give a physical explanation for it.

Short answer

Answer: Helium-3 is the solid that behaves exceptionally as it melts near absolute zero temperatures. Its unique behavior is due to the isotopic nature of Helium-3 and quantum mechanical effects it experiences at ultra-low temperatures, such as Bose-Einstein condensation and superfluidity. These effects are influenced by the distinct properties of helium - small atomic mass, weak attractive forces (van der Waals forces), and limited abundance of the isotope on Earth.

Step-by-step solution

Identify the solid

After researching, we find that the exceptional solid is Helium-3 (³He). Helium-3 is a rare isotope of the chemical element helium and is known to exhibit unique properties at low temperatures, including its behavior as it melts.

Understand the behavior of Helium-3

Near the absolute zero temperature, Helium-3 exhibits superfluidity. Superfluidity is a state of matter in which the substance behaves like a fluid without viscosity, allowing it to flow without resistance along surfaces. In the case of Helium-3, it can even flow counter-gravity. This behavior is caused by the quantum mechanical effect known as Bose-Einstein condensation, where atoms at ultra-low temperatures form a single quantum state.

Explain the absorption of heat during the melting of Helium-3

Near absolute zero, Helium-3 exists in both its solid and superfluid states. In contrast to most substances, which absorb heat during melting, Helium-3 absorbs heat to turn itself from a solid into the superfluid phase. This process is endothermic, meaning it requires heat energy to break the bonds between atoms. In the superfluid phase, Helium-3 is able to absorb heat, minimize entropy, and even cause the solid Helium-3 to convert into a superfluid upon contact, in a process called latent heat of fusion. This unique property allows Helium-3 to transfer heat more efficiently and have lower entropy values than other solids.

Physical explanation of the exceptional behavior of Helium-3

The exceptional behavior in Helium-3 melting occurs mainly due to its isotopic nature and the quantum mechanical effects it experiences at ultra-low temperatures. The distinct properties of helium, which include a very small atomic mass and weak attractive forces between atoms (van der Waals forces), make it prone to quantum mechanical effects, such as Bose-Einstein condensation and superfluidity. Furthermore, the scarcity of Helium-3 and its low abundance contribute to its unique behavior, as the supply of this isotope on Earth is limited.