Elements of group- 15, 16, and 17 form compounds with hydrogen with general formula \(MH_3\), \(H_2M\), and \(HX\) (called hydrogen halides). The stability of these hydrides, along with \(M-H\) bond strength and basic nature decreases down the group. Boiling point of these hydrides mainly depends on the strength of their intermolecular forces.

Which of the following is liquid at \(20^oC\)?

A. \(HCl\)

B. \(HI\)

C. \(HF\)

D. \(NH_3\)

E. \(HBr\)

Choose the correct answer from the options given below:

C only

The correct answer is option 3. C only.

The state of matter (solid, liquid, or gas) of a substance at a given temperature depends on its boiling point and the temperature at which it is being considered. At \(20^\circ C\), a substance is in the liquid state if its temperature is below its boiling point at atmospheric pressure.

Let us examine the boiling points of the given compounds:

A. \(HCl\) (Hydrogen Chloride):

The boiling point is around \(-85.05^\circ C\). At \(20^\circ C\), \(HCl\) is in the gaseous state.

B. \(HI\) (Hydrogen Iodide):

The boiling point is around \(-35.36^\circ C\). At \(20^\circ C\), \(HI\) is in the gaseous state.

C. \(HF\) (Hydrogen Fluoride):

The boiling point is around \(19.54^\circ C\). At \(20^\circ C\), \(HF\) is in the liquid state.

D. \(NH_3\) (Ammonia):

The boiling point is around \(-33.34^\circ C\). At \(20^\circ C\), \(NH_3\) is in the gaseous state.

E. \(HBr\) (Hydrogen Bromide):

The boiling point is around \(122.48^\circ C\). At \(20^\circ C\), \(HBr\) is in the gaseous state.

Therefore, at \(20^\circ C\), only \(HF\) (Hydrogen Fluoride) is in the liquid state. The correct answer is 3. \(C\) only