Question

3.1. Carefully explain the following observations:

(i) Silver dissolves poorly in copper at 100°C.

(ii) Copper dissolves poorly in silver at 100°C.

(iii) The amount of copper (in atom per cent) that can dissolve in silver at 100°C is greater than the amount of silver (in atom per cent) that can dissolve in copper at 100°C.

Answer 1

The miscibility of a pair of liquids refers to the degree to which they mix spontaneously. A pair of partially miscible liquids mixes under some conditions but not at others. The phase diagram for a pair of partially miscible liquids (Molten) Cu and Ag illustrates their behavior as a function of temperature and overall composition. Two phases are present (i.e., two layers are observed in the container).

One phase is mostly substance Cu with some Ag dissolved in it.

The other phase is mostly substance Ag with some Cu dissolved in it.

Inside the two phase regions, the compositions of the two phases are determined by drawing horizontal lines (for the composition of the "Ag-rich" phase) and the other line for the "Cu-rich" phase).

These are called "tie lines" or "levers". The lever rule is used to calculate the relative amounts of the two phases, The compositions of the two phases are numerically at the top. Outside the curve, Cu and Ag mix spontaneously to form one layer. The highest temperature at which two phases form is called the upper consolute temperature

The lever rule is used to determine weight percentages of each phase of a binary equilibrium phase diagram. It is used to determine the percent weight of liquid and solid phases for a given binary composition and temperature that is between the liquidus and solid..

A binary alloy contains two components chemically recognizable species Silver and copper.

Solubility Limit of a component in a phase is the maximum amount of the component that can be dissolved in it.The same concepts apply to solid phases: Cu and Ni are mutually soluble in any amount (unlimited solid solubility),

In the phase diagram of copper silver system the points governed are-

1) 3 single phase regions ( Liquid, α, β)

2) Limited solubility: α: mostly Cu β: mostly Ag

3) TE : No liquid below TE

3) CE: Min. melting TE composition

4) Eutectic transition

L ( C _E) α ( C_{α E}) + β ( C_{β E} )