Question

An iron needle is floating on water. When some amount of detergent is added to water, the iron needle
1. remains unaffected
2. sinks
3. stands upright
4. starts moving on the water surface

Answer 1

Correct Answer - Option 2 : sinks

The correct answer is option 2) i.e. sinks

CONCEPT:

• Surface tension: Surface tension is the tendency of water molecules to adhere to each other so as to form the minimum surface area possible.
  • Surface tension is a measure of the cohesive forces between liquid molecules present at the surface.​
  • Molecules at the surface of a liquid are partly exposed to the surrounding. These molecules experience fewer cohesive interactions and have more energy to escape to the surrounding.
• Therefore, energy is required to break the intermolecular interaction and bring molecules from the bulk of the liquid to the surface. The stronger the intermolecular interaction, the greater is the surface tension.

​EXPLANATION:

• Due to surface tension, the free surface of the water behaves like a stretched membrane. So, the iron needle floats due to surface tension.
• A detergent dissolved in water has two ends - hydrophilic and hydrophobic.
• The hydrophobic end attaches to grease and repels water molecules.
• This weakens the hydrogen bonds holding the water molecules together at the surface and thus reduces surface tension.
• ​Thus, the surface tension of water is decreased by adding detergent to it.
• The stretched membrane of the water surface gets weaker and can no longer hold the needle, and it will sink. Hence option 2 is correct.