fusion
Summary
Fusion is melting, the solid-to-liquid phase change, which absorbs heat (ΔHfus > 0) at a constant temperature. The testable point is that heat added at the melting point breaks intermolecular attractions instead of raising temperature. (In nuclear chemistry, fusion means light nuclei combining.)
Detail
Meaning: Fusion is the change from solid to liquid. The reverse is freezing. At the melting point, absorbed heat overcomes the attractions holding the solid together, so potential energy rises while average kinetic energy, and therefore temperature, stays constant. Fusion is endothermic (ΔHfus > 0) and entropy increases. Freezing releases the same quantity of heat. In nuclear chemistry and physics, fusion instead means light nuclei joining into a heavier nucleus and releasing energy from the mass defect.
Equation: q = nΔHfus, or q = mLf, where q is heat absorbed (J), n is amount (mol), ΔHfus is molar enthalpy of fusion (J/mol), m is mass (kg), and Lf is specific latent heat of fusion (J/kg).
On the MCAT: A heating curve shows a flat plateau at the melting point. You may add up heat for several segments, using q = mcΔT within a single phase and q = nΔHfus on the plateau. A longer plateau or larger ΔHfus suggests stronger intermolecular forces. Phase diagrams place fusion along the solid-liquid line. Nuclear questions contrast fusion with fission.
Common confusion: Students expect temperature to climb while a solid melts. It holds steady while solid and liquid coexist, and it rises again only once the sample is fully liquid. Also, ΔHfus is usually much smaller than ΔHvap for the same substance, because melting loosens the attractions in a substance far less than vaporizing it separates them.
Sources
- Chemistry: The Central Science
- Chemistry: Atoms First
- General Chemistry: Principles and Modern Applications
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.