Evaporability of Liquids Explained

The evaporability of a liquid refers to how easily it converts into a vapor or gas at a given temperature. This property is directly related to a liquid's volatility. Liquids with higher volatility evaporate more readily.

The primary factor determining a liquid's evaporability is the strength of its intermolecular forces (IMFs). Weaker intermolecular forces require less energy to overcome, allowing molecules to escape into the gaseous phase more easily, thus leading to higher evaporability.

• Stronger IMFs → Lower Evaporability (Higher Boiling Point)
• Weaker IMFs → Higher Evaporability (Lower Boiling Point)

Understanding Intermolecular Forces and Evaporability

Let's analyze the intermolecular forces present in each of the given liquids:

1. Benzene ($\text{C}_6\text{H}_6$):
   • Benzene is a nonpolar molecule due to its symmetrical structure.
   • The only intermolecular forces present are weak London Dispersion Forces (LDFs). These are the weakest type of intermolecular force.
   • Due to weak LDFs, benzene is highly volatile and has high evaporability.
2. Ethyl alcohol ($\text{C}_2\text{H}_5\text{OH}$):
   • Ethyl alcohol has a hydroxyl (-OH) group, which allows it to form hydrogen bonds.
   • It also experiences dipole-dipole interactions and London Dispersion Forces.
   • Hydrogen bonding is a relatively strong intermolecular force, stronger than LDFs alone.
   • Compared to water, ethyl alcohol forms fewer hydrogen bonds per molecule (due to only one -OH group), making its intermolecular forces generally weaker than water's. Therefore, ethyl alcohol is quite volatile but less so than benzene.
3. Water ($\text{H}_2\text{O}$):
   • Water also has -OH groups and is capable of forming strong hydrogen bonds.
   • Each water molecule can form two hydrogen bonds with other water molecules, making the overall network of hydrogen bonding very strong.
   • It also experiences dipole-dipole interactions and London Dispersion Forces.
   • The strong hydrogen bonding in water makes it less volatile than ethyl alcohol and benzene.
4. Mercury (Hg):
   • Mercury is a metal element.
   • It exhibits strong metallic bonding, which is much stronger than typical intermolecular forces like hydrogen bonding or London dispersion forces.
   • Metallic bonding is responsible for its very high boiling point and extremely low evaporability at room temperature. It is considered a non-volatile liquid under normal conditions.

Comparing Evaporability (Decreasing Order)

Based on the strength of intermolecular forces, we can arrange the liquids in decreasing order of their evaporability:

Therefore, the decreasing order of evaporability is:

1. Benzene (2) - Highest evaporability
2. Ethyl alcohol (1)
3. Water (4)
4. Mercury (3) - Lowest evaporability

The correct arrangement in decreasing order of evaporability is 2, 1, 4, 3.