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Why does rate of evaporation of solution decreases on adding a non volatile component?

Also what happens if we add a volatile solute?

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The rate of evaporation decreases because the activity of the solvent in the liquid phase decreases, while the activity in the vapor phase (or solid, for that matter) stays the same. This is also the reason for the increase in boiling point and the decrease in melting point of the solvent.

If we add a volatile solute, it will evaporate too, thus changing the solvent activity in the vapor phase as well. The resulting behavior can't be characterized universally; you'll have to consult the binary phase diagram of boiling for the particular system, and these can be pretty diverse.

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