Question

Which of the following materials is opaque to thermal radiation?
(A) Carbon tetrachloride
(B) Sodium chloride
(C) Benzene
(D) Potassium bromide

Answer 1

Hint : An object which is opaque to thermal radiation is an object which absorbs or reflects radiation but does not allow thermal radiation to pass through. Investigate the properties of the materials given in the option

Complete step by step answer
The material which is opaque to thermal radiation is to be chosen among the options.
But first, thermal opacity, that is, in-transmissibility of thermal radiation has to be checked. Hence, an object which is said to be thermally opaque will either absorb thermal radiation, or reflect it (which can be diffused)
Now, investigating the options,
Carbon tetrachloride –this is a clear colourless liquid which is used as a solvents for fat and oils. It has a chemical formula of CCl4 or $CC{l_4}$ as it is written. It is a volatile liquid thus, evaporates pretty easily. It is quite absorptive of thermal radiation.
Sodium chloride – commonly called table salt, has a chemical formula of $NaCl$ . It is not known to be very opaque to thermal radiation
Benzene – this is a hydrocarbon which is known to be very stable. It’s chemical formula is ${C_6}{H_6}$ . These compounds are both chemically and thermodynamically stable, hence very opaque to radiant heat or thermal radiation.
Thus, the correct answer is option C.

Note
For clarity, note that a material which is opaque to thermal radiation is not necessarily opaque to visible light, and opacity to visible light also does not signify opacity to thermal radiation. For example, the atmosphere of the earth which allows visible light to pass through however, is very absorptive of infrared, which can be very thermal.