[mana]

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?

[Borek]

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.

[mjc123]

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).

[mana]

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