Why the activity of solid is considered as one?

Question

Why the activity of solid is considered as one?

Answer 1

That is a convention!

It is like saying that under the standard conditions water solidifies at 0C and vaporises at 100C.

If the solid is pure in its phase a=1

If the solid is pure a=1

If the gaz is pure a=1

If the liquid is the solvent a=1

For diluted ideal solution a=1 when the solute’s concentration is 1M and if the solute behaves as if it was infinitely diluted meaning it does not interacts with its neighbours (that is fictional of course).