Chemical Forums
Chemistry Forums for Students => Physical Chemistry Forum => Topic started by: mana on September 03, 2018, 01:42:55 AM
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hi
in the eutectic mixture graph, why at first B melts sooner than A (from left to right in the graph), I know at first because B is less than A probably it must melt sooner than A, but when the melting point of B is higher than A why it happens?
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I believe you misunderstand the plot, but please elaborate. What you wrote so far is ambiguous.
When you have a mixture of a given composition and you heat it up, you move on the vertical line, none of the components of the mixture melts first - at some point the mixture starts to melt, so in a way they both melt together.
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Neither A nor B melts first - what melts first is a mixture of the eutectic composition. For example, if you take a mixture of composition 80% A + 20% B, which has excess A compared to the eutectic composition, as you heat it (along a vertical line on the graph), when you get to the eutectic melting temperature, a mixture of the eutectic composition melts, until all the B has melted (so you can get no more eutectic mixture). Then you have liquid eutectic and solid A. As the temperature rises, A melts until you cross the solid-liquid equilibrium line , when all the A has melted and you have liquid of the original composition.
What makes the difference is which side of the eutectic composition your starting mixture is - not which component has the higher melting point (as a pure substance).
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For example, if you take a mixture of composition 80% A + 20% B, which has excess A compared to the eutectic composition, as you heat it (along a vertical line on the graph), when you get to the eutectic melting temperature, a mixture of the eutectic composition melts, until all the B has melted (so you can get no more eutectic mixture). Then you have liquid eutectic and solid A. As the temperature rises, A melts until you cross the solid-liquid equilibrium line , when all the A has melted and you have liquid of the original composition.
What makes the difference is which side of the eutectic composition your starting mixture is - not which component has the higher melting point (as a pure substance).
first of all, sorry for my poor English :-[, and thanks I think I have got the answer, you mean because A is excess in 80% A and 20% B, in this position I have a melted eutectic mixture and the excess amount of A which remains solid by increasing the heat solid A melts too