MCQ Detailed View

The boiling point of a substance is the temperature at which its vapor pressure equals the external atmospheric pressure, causing the liquid to convert into vapor. For ethyl alcohol (ethanol, C₂H₅OH), the boiling point is approximately 78°C at standard atmospheric pressure (1 atm).

The reason ethanol has a boiling point higher than many other small organic molecules is due to hydrogen bonding. Ethanol contains a hydroxyl group (-OH), where the highly electronegative oxygen atom forms strong intermolecular attractions with hydrogen atoms of neighboring ethanol molecules. Breaking these hydrogen bonds requires significant energy, which raises the boiling point.

For comparison:

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  Methanol (CH₃OH): Boiling point ~65°C

  
  


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  Ethanol (C₂H₅OH): Boiling point ~78°C

  
  


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  Propanol (C₃H₇OH): Boiling point ~97°C
  This trend shows that as the alcohol chain length increases, van der Waals forces also increase, leading to higher boiling points.

  
  

In contrast, compounds of similar molecular mass without hydrogen bonding, such as diethyl ether (C₂H₅–O–C₂H₅), boil at only about 34°C, highlighting how hydrogen bonding strongly influences boiling behavior.

The boiling point of ethanol also has practical importance:

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  Alcohol distillation: Ethanol is separated from water and other substances by distillation, which relies on its boiling point.

  
  


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  Fuel applications: Ethanol is widely used as a biofuel and fuel additive; its relatively low boiling point helps in easy vaporization inside engines.

  
  


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  Medical and laboratory use: Ethanol is commonly used as a solvent and disinfectant, where its volatility plays a key role.

  
  

Thus, knowing the boiling point of ethanol (78°C) is fundamental in physical chemistry, industrial processes, and laboratory techniques.