MCQ Detailed View

The waterproof properties of materials used in rain-proof coats and tents are primarily due to surface tension. Surface tension is a physical phenomenon that occurs at the boundary between a liquid and another surface, caused by cohesive forces between the molecules of the liquid.

In the case of water, molecules are strongly attracted to each other, forming a kind of invisible “skin” on the water’s surface. When water droplets come into contact with a raincoat or tent made of a waterproof fabric, the material is designed to increase the contact angle, preventing water from spreading or soaking through. Instead, water beads up and rolls off the surface due to surface tension.

Manufacturers often treat fabrics with hydrophobic coatings, such as wax, silicone, or special polymers. These coatings reduce the adhesive force between water molecules and the fabric, making water unable to penetrate. The interaction between the coating and water relies heavily on the properties of surface tension.

The other options are incorrect:

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  Viscosity: Refers to the internal friction of a fluid, determining how easily it flows, but it does not make fabrics waterproof.

  
  


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  Specific gravity: Measures the density of a substance relative to water and has no direct effect on waterproofing.

  
  


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  Elasticity: Refers to a material's ability to stretch and return to its original shape, unrelated to repelling water.

  
  

Waterproof materials are designed to repel water molecules by exploiting their cohesive forces. The droplets maintain their spherical shape on the surface instead of spreading out, thanks to high surface tension. This principle ensures that rainwater slides off raincoats and tents, keeping the person or objects beneath them dry.

In everyday physics, understanding surface tension explains many phenomena such as floating insects on water, formation of water droplets, and waterproof fabric technology. Thus, the correct answer is surface tension, as it is the key property that gives rain-proof materials their effectiveness.