= Solution
A useful schematic is the <upper critical solution temperature> form of a <binodal>: a dome in temperature versus <chromatin> <concentration>. Below the dome, an intermediate overall <concentration> separates into a dilute phase and a chromatin-rich phase; above it the liquid is homogeneous. At a fixed temperature, the two intersections of a horizontal tie line give the compositions of the coexisting phases, not the <concentrations> of two different initial samples.
\Image[/past-exam-of-the-genetics-course-of-the-university-of-cambridge/2024/paper-1-chromatin-phase-diagram.png]
{title=Schematic upper-critical-solution-temperature model for <chromatin> demixing and weaker attraction at lower salt}
The diagram is a \b[model, not measured phase-boundary data]. The stated observation at one salt <concentration> does not by itself establish whether heating suppresses or promotes demixing. A <lower critical solution temperature> system would instead demix on heating, and would need the corresponding inverted geometry. The following answers make the usual enthalpically favourable-demixing assumption explicit.
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