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To put it short, why is the cooling curve for a solution slanted during the phase change? The pure solvent plateaus at a fixed temperature, but for a solution, the phase change occurs over a range of temperatures. What explains this phenomenon?

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  • $\begingroup$ Well, it would be kinda surprising in freezing/melting of mixture had sharp peaks, even for pure compounds they can be quite broad, or even variable. $\endgroup$ – Mithoron Mar 13 '18 at 0:16
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During the phase change, the composition of the solution phase is changing from that of the original solution as the solidifying component is removed. This doesn't happen for a pure liquid. In a simple system, the slant continues until both components begin to solidify simultaneously - the eutectic point. The temperature then flattens out because the composition of the liquid phase doesn't change. It stays flat until no liquid remains. This can be complicated if there are other solid phases or formation of new chemical components.

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