information:classification, linguistic:voice_change, quality:correction
Overview

The enthalpy of fusion is almost always a positive quantity; helium is the only known exception. Helium-3 has a negative enthalpy of fusion at temperatures below 0.3 K. Helium-4 also has a very slightly negative enthalpy of fusion below 0.77 K (−272.380 °C). This means that, at appropriate constant pressures, these substances freeze with the addition of heat. In the case of He, this pressure range is between 24.992 and 25.00 atm (2,533 kPa).