Water from coffee condensation

Question

Why is it that from a cup of hot coffee, only the water condensates on the inside of the lid? Why aren't there any coffee particles? Does it only have to do with density or is it something else involved?

Answer 1

I agree with Jan. The "coffee molecules", probably mostly alkaloids like caffeine, have larger and heavier structures than water molecules. I would expect most have dipole-dipole intermolecular forces, and (minor) hydrogen bonding may be present for some.

When you heat the solution with thermal energy, when we zoom in to the molecular scale we see this energy us actually kinetic. The molecules move faster, and some at the surface break free of their attractions to their neighbors. Despite water being held together by the strong hydrogen bonding intermolecular force, water molecules will be first and most to gain enough energy to pop out of the liquid solution. As Jan said, there are many more water molecules that coffee solute molecules. Also, the coffee solute's greater weights will make them harder to bring up to speed (though as a general rule of thumb IMFs like H-bonds actually play a more significant role than mass).

So those water molecules that pop out are far from their neighbors, so they constitute what we call a gas or vapor. There is much more water-gas above the solution than coffee-molecule-gas. This water vapor exerts a pressure based on how many molecules there are (and depending on kinetic energy / temperature), and we call this pressure from the gas above the coffee the "vapor pressure of the solution". The vapor pressure of water, despite being low relative to many organic liquids, is far higher than the vapor pressure of the molecules from coffee.

Think about the coffee residue in your pot, or the grounds - do they evaporate? Essentially no. There is essentially no vapor pressure, they are too bound by their IMFs to pop out and become gas (at temperatures a human usually encounters anyway). But water has a high vapor pressure, it is easily evaporated.

So when the gaseous water molecules encounter cooler surfaces, they will transfer some kinetic energy to the surface and transition from energetic gas state to lower-energy liquid state. This is the meaning of "condense". No coffee molecules condense, because none were in the gas phase to begin with (essentially).

Jan had a great point that these concepts are the foundation of the incredibly important process of distillation.

Answer 2

Let’s analyse the sequence of events.

• You have a solution
• part of that solution enters the gas phase
• the part that enters the gas phase moves until
• it finds a surface that it feels comfortable ‘settling’ on.

It should quickly be mentioned that water is the primary component of coffee by far, hands down. Thus, even assuming approximately equal distribution in the gas phase, there should still be a lot more water than anything else. But that alone doesn’t really answer it.

The key quantity you want to be looking at is vapour pressure. The coffee is hot (or at least warm), relatively close to water’s boiling point of $100~\mathrm{^\circ C}$ so the vapour pressure of water will be comparatively large. Most other components of coffee have a boiling point that is much higher and would be solids at room temperature. These two factors combined allow us to allocate them a low vapour pressure even at the temperature of hot coffee. Therefore, the amount of water leaving the coffee is even larger.

The basic principle behind this is distillation; a mixture of multiple compounds that can be separated by their vapour pressures (often correlating with their corresponding boiling points). Typically, liquids will be distilled away far before solids.