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Physical properties and states of matter

Chapter: Properties of Matter

Question 1 of 2 NDA MCQ

Which one of the following statements about surface tension is correct?

Detailed Explanation

Correct Answer

✓ Option (b) is correct. Surface tension decreases with an increase in temperature.

📖 Governing Law / Core Concept

Surface tension (\( \gamma \)) is defined as the force per unit length acting along the surface of a liquid. It can be expressed mathematically as:

\[ \gamma = \frac{F}{L} \]

Where \( F \) is the force acting along the surface, and \( L \) is the length over which the force acts. SI unit of surface tension is \( N/m \).

🧪 Step-by-Step Breakdown

As temperature increases, the kinetic energy of the molecules in a liquid also increases. This leads to an increase in intermolecular spacing, thereby reducing cohesive forces. The relationship between surface tension and temperature can typically be represented as:

\[ \gamma = \gamma_0 \left(1 - \frac{T}{T_c}\right) \]

Where \( \gamma_0 \) is the surface tension at temperature \( T = 0 \degree C \), and \( T_c \) is the critical temperature for the liquid.

🔍 Option Analysis

  • Option A: Incorrect. Surface tension typically decreases with increased temperature.
  • Option C: Incorrect. Surface tension changes with temperature variations.
  • Option D: Incorrect. While it decreases, it's not stated that it must decrease with de/increase in temperature only.

⚡ Mnemonic / Speed-Run

Remember: "Heat loosens; tension decreases."

6. Visual Suggestion

6. Visual Suggestion

Illustrate a liquid droplet on a surface, showing the forces acting on the surface and how it changes shape with temperature increase. Use arrows to demonstrate molecular motion and intermolecular forces.

📖 Factual Verification & Reference

📖 Factual Verification & Reference:

Verified against NCERT Class XI Physics, Chapter 2 (Fluid Mechanics).

Question 2 of 2 NDA MCQ

Tyndall effect is a phenomenon of

Detailed Explanation

Correct Answer

✓ Option (A) is correct. The Tyndall effect is caused by the scattering of light by colloidal particles.

📖 Governing Law / Core Concept

The Tyndall effect states that when a beam of light passes through a colloidal solution, it is scattered by the particles, making the path of light visible. This phenomenon can be explained by Rayleigh scattering principles.

🧪 Step-by-Step Breakdown

Light scattering occurs when light interacts with particles whose dimensions are comparable to its wavelength. The intensity of scattered light can be described by the following relation:
\[ I \propto \frac{1}{\lambda^4} \]
Where: - \( I \) = Intensity of scattered light - \( \lambda \) = Wavelength of light (in meters) The short wavelength light (blue) is scattered more than longer wavelengths (red).

🔍 Option Analysis

- Option (B): Refraction of light refers to the bending of light waves as they pass from one medium to another, unrelated to Tyndall effect. - Option (C): Dispersion of light involves the separation of light into its constituent colors, also not linked to Tyndall effect. - Option (D): Refraction by dust particles is not the mechanism of Tyndall effect, which specifically deals with colloids.

⚡ Mnemonic / Speed-Run

To remember Tyndall effect, think of "Colloidal Clarity" – light becomes visible when scattered by colloids.

6. Visual Suggestion

A schematic diagram showing a beam of light entering a colloidal solution, with scattered light waves represented, illustrating the visibility of the light path due to Tyndall effect.

📖 Factual Verification & Reference

📖 Factual Verification & Reference:

Verified against NCERT Class XII Physics, Chapter on Optics.

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