Melting point

[Suyogya]

Chemistry help

How to define the melting point of a mixture, which is composed of substances having different- different melting points?

[John Cuthber]

It's practically impossible.

It depends too strongly on the nature of the materials you mix.

For example, with two simple metals, lead and tin, you gate a phase diagram that looks like this.

https://www.chegg.com/homework-help/questions-and-answers/using-lead-tin-phase-diagram-figure-63-determine-liquid-solid-phase-compositions-nominal-c-q9962295

 

[Suyogya]

5 minutes ago, John Cuthber said:

It's practically impossible.

It depends too strongly on the nature of the materials you mix.

For example, with two simple metals, lead and tin, you gate a phase diagram that looks like this.

https://www.chegg.com/homework-help/questions-and-answers/using-lead-tin-phase-diagram-figure-63-determine-liquid-solid-phase-compositions-nominal-c-q9962295

 

Then, mixtures should have no melting point. But they have, how?

[studiot]

First you need to understand what is meant by melting point.

A true solid is a pure substance with a definite chemical composition and a definite (crystalline) structure.

Such a substance has a definite single temperature melting point.

A substance which does not have a definite chemical composition and/or no crystalline structure will exhibit a melting temperature range.

 

It is common in junior school to compare the cooling curves or softening/melting curves for a wax or resin and a pure liquid/solid such as /water ice to plot this difference.

 

Have you done this experiment?

[studiot]

I thought you wanted an answer to this.

John Cuthber's answer included nearly the full complexity.

Mine started more simply, leading to the all important difference between melting of a pure substance and a mixture.

The all important difference is that for a pure substance, the chemical composition does not change on melting, but for a mixture it does change.

Obviously I can stop wasting my time.