Question

52. Solder is a mixture of tin and lead. The melting point of tin is 232 ◦C and

that of lead is 327 ◦C. The melting point for solder (depending on the mixing

fractions) generally lies in the range 180 ◦C to 200 ◦C. Why is the melting

point of the mixture lower than that of either ingredient?

Answer 1

An alloy is nothing but an intimate mixture between metals (as primary components). In this context, there are two (pertinent) different kinds of alloys : (i) isomorphous alloys or solutions, and (ii) eutectic alloys.

Solder is a eutectic alloy, not an isomorphous solution. A eutectic is formed between metals of very different latice parameters and crystal structures (eg : Sn-Pb or Ag-Cu). A eutectic alloy is called a two-phase alloy because if you look at it under a microscope, you can distinguish two different types of regions each with its own composition and crystal structure. Each of these phases is in fact, just an isomorphous alloy. So here's what happens when I try to alloy Pb with Sn : I start with pure Sn and slowly add Pb to it. At very low concentrations of Pb, the Pb atoms actually occupy the positions of the Sn atoms in the Sn lattice. But Pb atoms being larger than Sn atoms, this will strain the lattice locally. Beyond a certain concentration, it becomes infeasible to substitute the bulky Pb atoms in the spaces meant for Sn. Instead, it becomes easier to make a new phase which is predominantly Pb, with a small portion of the Sn atoms substituting for the Pb. So, a eutectic alloy consists of two distincet phases - each phase rich in one of the components. But each of these phases is now strained because of the distortion resulting from different radii. This strain causes a lowering of the melting temperatures.