[lightarrow (OP)]

Pure Sn melts at 232 °C

Pure Pb melts at 327.5 °C

Their eutectic alloy (61.9% Sn & 38.1% Pb) melts at 183 °C.

Question: if I mix 61.9 grams of pure Sn fine powder and 38.1 grams of pure Pb fine powder and I slowly increase the temperature, at which temperature the mix will start melting?

At 183 °C?

Or at the lowest melting point compound of the two (that is Sn) = 232 °C ?

It's a real conundrum for me.

[Batroost]

I think the two metal powders would melt at their respective individual melting points - until they are both liquid you have a 'mixture' not an alloy. Once they have both melted (i.e. above 327 degrees) then you can form the eutectic - so I'm guessing the alloy wouldn't solidify until you were below 183 degrees.

[daveshorts]

You would probably not have to heat it above 232 as the solid Pb would dissolve in the liquid Sn.

[Bored chemist]

Once you got to 232 the Sn would melt. It would then dissolve the Pb. It's quite possible that it would do so exothermicly and thus the temperature woud rise, even without external heating. It might be an interesting experiment.

[lightarrow (OP)]

Once you got to 232 the Sn would melt. It would then dissolve the Pb. It's quite possible that it would do so exothermicly and thus the temperature woud rise, even without external heating. It might be an interesting experiment.

Ok, however, now please consider this: it's possible that such a reaction, if it happens, starts before Sn melts and, however, at 183°C both metal's atoms can diffuse into each other at a significant rate; so an initial small amount of liquid eutectic can form, which, then, just being liquid, can ease the process.

What do you think?

[eric l]

Once you got to 232 the Sn would melt. It would then dissolve the Pb. It's quite possible that it would do so exothermicly and thus the temperature woud rise, even without external heating. It might be an interesting experiment.

I can confirm this.  I saw a demonstration of DSC (differental scanning calorimetry) once on the components of the solder used for electronic circuitry - probably the same as this eutectic alloy.  It showed a clear peak for the melting of Sn (endothermic), but followed by a long zone of endothermic process but no distinct peak for the melting of Pb.  
With cooling there was a relatively narrow exothermic peak at lower temperature.