Caffeine sublimation and melting point

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SUMMARY

Caffeine exhibits a sublimation point of 178°C and a melting point of 237°C due to the effects of pressure on phase transitions. The melting point is defined under standard atmospheric pressure, while sublimation occurs at lower temperatures when the pressure is not sufficient to maintain the solid phase. When caffeine is heated from 178°C to 237°C, a solid-to-liquid transition occurs despite ongoing sublimation, leading to the observation of recrystallized material above the melted sample.

PREREQUISITES
  • Understanding of phase transitions in chemistry
  • Knowledge of sublimation and melting point concepts
  • Familiarity with the effects of pressure on physical states
  • Basic laboratory techniques for melting point determination
NEXT STEPS
  • Research the impact of pressure on phase diagrams
  • Study the thermodynamic principles behind sublimation and melting
  • Learn about the experimental methods for determining melting points
  • Explore the properties of caffeine and its behavior under varying temperatures and pressures
USEFUL FOR

Chemistry students, researchers in material science, and professionals involved in pharmaceutical development will benefit from this discussion on caffeine's phase behavior.

slft
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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.
 
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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.
 

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