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The actual melting temperature of gold is 1000° C. However, I noticed when pure gold wire whose diameter is 3 mills is put into the molten blob of solder (60% tin + 40% lead), which is at only 300° C, the gold melts and gets alloyed up with the molten tin.

How does the tin-lead alloy reduce the melting temperature of gold?

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2 Answers 2

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The gold doesn't melt, it dissolves in the molten solder forming another alloy.

You can observe the same thing dissolving ordinary table salt, sodium chloride, in water. Sodium chloride melts at 801 C, but dissolves readily in water.

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    $\begingroup$ And not only that, the solution freezes at a lower temperature than pure water. You also see the same effect with solder, where a lead-tin alloy melts at a lower temperature than either tin or lead. $\endgroup$
    – WhatRoughBeast
    Commented Oct 4, 2016 at 3:09
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    $\begingroup$ But not all alloying reduces the melting temperature right? Why certain alloys having higher melting point compared to the original constituents? $\endgroup$
    – soosai steven
    Commented Oct 4, 2016 at 3:20
  • $\begingroup$ Yes, there are some alloys which have a melting point above any constituent, some alloys will have an intermediate melting point, and some alloys that have a melting point below the melting point for any constituent. You have to study the phase diagram(s) to analyze a particular alloy. $\endgroup$
    – MaxW
    Commented Oct 4, 2016 at 3:30
  • $\begingroup$ Phase diagram... Let me ask Google. $\endgroup$
    – soosai steven
    Commented Oct 4, 2016 at 3:32
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    $\begingroup$ A look at the Au-Pb phase diagram will show a eutectic point at about 15 at.% Au of slightly above 200C (it is actually a eutectic between $AuPb_{3}$ and $Pb$). The highest point on the liquidus line is right over by fcc-Au. Au-Sn similarly has a liquidus with a high point at Au, and several intermediate eutectics, one between $Au_{5}Sn - AuSn$ and one between $AuSn_{2}$ and $Sn$. $\endgroup$
    – Jon Custer
    Commented Oct 4, 2016 at 21:05
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Arbitrarily mixable alloys with two components have a melting temperature that can be considerable lower than the melting temperatures of its constituents. The mixture with the lowest melting temperature is called eutectic. Gold probably forms an alloy with a component of the solder (e.g. Sn). Therefore it "melts" by forming an alloy with a much lower melting temperature.

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  • $\begingroup$ The range of gold alloys of various melting temperatures are used in jewelry making to allow subsequent pieces to be brazed without melting the previously added pieces. $\endgroup$
    – soosai steven
    Commented Oct 4, 2016 at 13:58

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