Explanation

The phenomenon where a chemical element exists in multiple crystalline forms is called allotropy

Allotropy refers to the ability of an element to exist in two or more different forms in the same physical state.

This is different from polymorphism, which is a broader term referring to any substance (not just elements) that can exist in multiple crystalline forms. 

Explanation

In electronic transitions, a direct transition from 2s to 1s is forbidden.

Due to quantum mechanical selection rules.

Explanation

The correct answer is (a) trichotomy property of real numbers.

The trichotomy property states that for any two real numbers a and b, exactly one of the following must be true: a > b, a = b, or a < b. 

Explanation

He had his breakfast at 9 o’clock.

The sentence refers to a specific time in the past ("at 9 o’clock"), so the correct tense is simple past.

"Had" is the simple past tense of "have," and it fits well with a completed past action at a specific time.

Explanation

Explanation

Thermionic emission is the process by which electrons are emitted from a hot metal surface.

This occurs when a metal is heated to a high temperature, causing the electrons to gain enough energy to escape from the metal's surface.

This process is also known as thermal electron emission or Edison effect.

Explanation

The frequency of AC (Alternating Current) supply in Pakistan is typically 50 Hertz (Hz).

AC electricity alternates back and forth in a direction 50 times per second

Explanation

The comparative rates at which solutes move in paper chromatography depend on Rf values. 

Rf values (retention factors) represent the ratio of the distance traveled by a solute to the distance traveled by the solvent front on the paper.

Since different solutes have different polarities and affinities for the mobile and stationary phases, they will travel at different speeds, resulting in unique Rf values.

Therefore, the comparative movement of solutes is directly related to their Rf values. 

Explanation

CaCO₃ (Calcium carbonate)

No, it is not hygroscopic.

It does not absorb moisture significantly → NOT a desiccating agent

Explanation

To find the resultant of two forces acting at right angles, use the Pythagorean theorem:

Resultant = √(3² + 4²) = √(9 + 16) = √25 = 5N