= Solution
At $100\,{}^\circ\mathrm C$, the stable sequence is \b[$(\mathrm{Bi})$, $(\mathrm{Bi})+(\beta\mathrm{Sn})$, $(\beta\mathrm{Sn})$]. The stable two-solid <common tangent> lies below the <liquid> curve. There is no stable <liquid> interval, although a metastable extrapolated <liquid> <Gibbs free energy> can still be drawn.
\Image[/past-exam-of-the-materials-science-course-of-the-university-of-cambridge/2019/paper-1-gibbs-tangents.png]
{title=Qualitative Gibbs free-energy envelopes above, at and below a binary eutectic}
{description=Original illustrative curves with constructed tangencies. Free-energy units and curvatures are arbitrary, not measured thermodynamic data for bismuth-tin.}
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