MCQ Detailed View

Alcohols are organic compounds that contain a hydroxyl group (-OH) attached to a carbon atom. Their solubility in water depends on the balance between their hydrophilic (water-loving) and hydrophobic (water-repelling) parts. The hydroxyl group is highly polar and can form hydrogen bonds with water molecules, which increases solubility. On the other hand, the hydrocarbon chain (alkyl group) is nonpolar and decreases solubility as its length increases.

Lower molecular weight alcohols, such as methanol (CH₃OH), ethanol (C₂H₅OH), and propanol (C₃H₇OH), have small hydrocarbon chains. The strong hydrogen bonding between their hydroxyl group and water molecules dominates, making them highly soluble in water. For example:

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  Methanol and ethanol are completely miscible with water in all proportions.

  
  


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  Propanol is also very soluble due to its short carbon chain.

  
  

As the molecular weight of alcohols increases (e.g., butanol, pentanol, hexanol), the nonpolar hydrocarbon part becomes larger compared to the polar -OH group. This reduces the overall polarity of the molecule and makes it less soluble in water, as the hydrophobic effect overcomes hydrogen bonding.

Option 1 is therefore correct because lower molecular weight alcohols readily dissolve in water due to their strong polar interactions. The other options are incorrect:

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  Insoluble in water: False, small alcohols mix well with water.

  
  


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  Soluble in water on heating: Not necessary for low-weight alcohols as they are already soluble at room temperature.

  
  


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  Insoluble in all solvents: Incorrect, alcohols can dissolve in both water and many organic solvents.

  
  

This concept is fundamental in Chemistry, explaining the relationship between molecular structure, polarity, hydrogen bonding, and solubility of organic compounds.