Traditional solder is a low melting point alloy of lead and tin. (Some countries

Question

Traditional solder is a low melting point alloy of lead and tin. (Some countries have
mandated the future use of lead-free solders, because of concerns relating to the
environmental consequences of lead use and disposal.)
Using appropriate data gleaned from the textbook, explain
(i) why lead has very limited solubility in solid tin (hardly any lead will dissolve in tin at
room temperature);
(ii) why tin is moderately soluble in solid lead (approximately 3.5% of the atoms in lead
can be replaced by tin at room temperature).

Explanation / Answer

A eutectic is an alloy of specific composition which freezes at a single temperature which is lower than the freezing point of the constituent metals (instead of over a temperature range).

The Pb-Sn alloy system is the classic eutectic ( taught in metallurgy.)

Lead is larger than tin. If It is in a simple cubic structure there is not a a lot of room in the spaces so where would the lead go.

Therefore lead has limited solubility in tin.

In the liquid form lead and tin are totally soluable in one another. The formation of a solid solution up to a certain concentration is then dependant on the ability of the crystal lattice of (say) the lead to accomodate tin (or vice versa). The level of lattice strain caused by the mismatch gives an "energetic" limit to the solubility.

Where as tin is moderately soluble in lead due to its small size and melting point

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