# Liquid-solid phase composition: Binary phase diagrams

In binary phase diagram, We draw a tie line to know the composition of the liquid phase or the solid phase. Where the tie line is just a horizontal line.

The question is why doesn't the liquid or solid phase composition change as you change the general composition of the alloy? Why does the horizontal line give the liquid/solid composition for any general composition? Shouldn't the matter behave differently as you change the general composition?

Borek
Mentor
I am not sure I understand your question. Horizontal line is just a line of constant temperature. No matter what the composition is, you can heat the mixture to any temperature. Then, from the diagram, you can read what to expect - just a solid mixture, just a liquid mixture, or some combination of both.

I am not sure I understand your question. Horizontal line is just a line of constant temperature. No matter what the composition is, you can heat the mixture to any temperature. Then, from the diagram, you can read what to expect - just a solid mixture, just a liquid mixture, or some combination of both.
My bad, Say you have a alloy made of 40% A and 60% B and another alloy 30% A and 70% B, if you heat both to the same temperature where you would have two phases ( Solid and liquid), Using tie line, You will find that the liquid or solid phase has the same composition regardless of the general composition of the alloy. Like shouldn't each alloy have a different cooling composition?

It might seem a dumb question but I dont really understand why the tie line would give the composition of the liquid or solid phase for any general composition at some temperature.

Example - alloy of ice and salt. The composition of liquid depends on the temperature and thus solubility of salt in water. Not on general composition of alloy - only the amount of solid and liquid varies.

Borek
Mentor
Vertical line is where the mixture has a given composition (defined by the x coordinate). When there are two phases present they can have different compositions as long as their mixture has the correct ratio of both alloyed substances.

Say you have a jar of water solution of a salt that has solubility of 100 g of salt per 100 g of water at 100°C, but is almost insoluble at 0°C. You start with a 50/50 mixture at 100°C and you cool it down. Salt drops out from the solution and you have two phases - pure salt, and solution that has less and less dissolved salt. But the content of the jar is still the same 50/50.