5. Glycerol is more viscous than ethanol because:
a. It can make more H bonds per molecule than ethanol
b. Due to its tendency to form polymers
c. Its molecular weight is greater than ethanol
d. It has more carbon atoms than ethanol

Glycerol exhibits higher viscosity than ethanol primarily due to its ability to form more hydrogen bonds per molecule. This intermolecular force significantly strengthens attractions between glycerol molecules compared to ethanol.

Correct Answer

a. It can make more H bonds per molecule than ethanol.

Option Analysis

Glycerol (C₃H₈O₃) has three hydroxyl (-OH) groups, enabling up to three hydrogen bonds per molecule, which creates extensive networks resisting flow. Ethanol (C₂H₅OH) forms only one hydrogen bond per molecule, leading to weaker intermolecular forces and lower viscosity.

Option b is incorrect; glycerol does not polymerize under normal conditions but relies on hydrogen bonding, not covalent polymerization. Option c overlooks that molecular weight contributes to viscosity but plays a secondary role here—glycerol’s higher weight (92 g/mol vs. ethanol’s 46 g/mol) matters less than hydrogen bonding strength.

Option d fails as carbon atoms (3 in glycerol vs. 2 in ethanol) influence size minimally; hydrogen bonding dominates viscosity differences.

Glycerol proves more viscous than ethanol in key CSIR NET Life Sciences questions, highlighting hydrogen bonding’s role in molecular interactions. This comparison reveals how structural differences dictate physical properties like flow resistance.

Hydrogen Bonding Dominance

Glycerol’s three -OH groups allow multiple intermolecular hydrogen bonds, forming a robust network that impedes molecular movement. Ethanol’s single -OH limits bonding, resulting in freer flow and lower viscosity.

Why Other Factors Fall Short

• Greater molecular weight aids viscosity but cannot override hydrogen bonding effects.

• No polymerization tendency exists in glycerol at standard conditions.

• Extra carbon atoms add bulk minimally without enhancing key forces.

Exam Relevance

For CSIR NET aspirants, recognize viscosity as intermolecular force-driven, prioritizing H-bond count over mass or atoms. Practice similar questions on polyols like ethylene glycol, which rank between ethanol and glycerol.