Caffeine sublimation and melting point

[slft]

Why caffeine has 178 oC as point of sublimation but 237 oC as melting point?

Is it about the pressure? If so, then what is the preesure that makes 237 as melting point instead of sublimation at 178 oC? Cuz the science websites always just said 237 is the melting point, but didn't mention the pressure or any reason why.

 

[chemisttree]

If you keep the caffeine at 178C long enough it will sublime. If you quickly raise the temperature through 178 to 237C you will observe the solid/liquidus transition even though it is slowly subliming at temperatures above 178C. When you finish doing the melting point test and remove your capillary, you will observe the sublimed and recrystallized material some distance above the melted sample.

 

[Blueshift5]

I can provide some insight into the sublimation and melting point of caffeine. Caffeine has a melting point of 237 oC, which means it transitions from a solid to a liquid state at this temperature. However, under certain conditions of pressure and temperature, caffeine can also undergo sublimation, which is the transition from a solid directly to a gas without passing through a liquid state.

The reason why caffeine has a lower sublimation point of 178 oC compared to its melting point is due to the intermolecular forces present in the caffeine molecules. At lower temperatures and pressures, the intermolecular forces are strong enough to hold the caffeine molecules together as a solid. As the temperature increases, these forces weaken, allowing the caffeine molecules to break free and transition into a gas phase through sublimation.

The exact pressure that causes this transition from sublimation to melting at a specific temperature can vary depending on the purity and composition of the caffeine sample. It is also worth noting that the melting and sublimation points of caffeine can be influenced by factors such as impurities, particle size, and the presence of other substances.

In summary, the difference in sublimation and melting points of caffeine is due to the strength of intermolecular forces and can be affected by various factors. It is important to consider these factors when studying the properties of caffeine and other substances.