Question

The data in the options lists the melting points and the boiling points of four different substances. Which substance is a liquid at ${0^ \circ }C$ ?
A) Melting point: $ - {219^ \circ }C$ ; Boiling point: $ - {183^ \circ }C$
B) Melting point: $ - {7^ \circ }C$ ; Boiling point: ${58^ \circ }C$
C) Melting point: ${98^ \circ }C$ ; Boiling point: ${890^ \circ }C$
D) Melting point: ${1083^ \circ }C$ ; Boiling point: ${2582^ \circ }C$

Answer 1

Hint: To solve this question, you need to consider that at melting point, there is coexistence of solid and liquid phase and above it, there is mainly liquid phase, similarly, at the boiling point, there is a coexistence of the liquid and the gaseous phase, below it, mainly liquid phase exists. Hence, the correct option would have a melting point lower than the liquid phase at the asked temperature and a boiling point, higher than the asked temperature.

Complete step by step answer:
As explained in the hint section of the solution to the asked question, the correct option is the option in which in the melting point is below ${0^ \circ }C$ while the boiling point is higher than ${0^ \circ }C$ . The reason is explained below:
Melting point is the temperature at which the solid and liquid phases of a substance coexist and are at equilibrium with each other. If the temperature goes below the melting point, i.e. the surroundings become more cooler, the substance starts being in the solid phase more and more and in the liquid phase less and less. Thus, if the current temperature is lower than the melting point, the substance will majorly be in the solid phase or solid state. Similarly, if the current temperature is higher than the melting point, the substance will majorly be in the liquid phase or liquid state. Hence, for the correct option, the melting point of the substance should be less than ${0^ \circ }C$. If we have a look at the given options, only options (A) and (B) have melting points lower than ${0^ \circ }C$. Hence, options (C) and (D) cannot be the answer.
Similarly, boiling point is the temperature at which the liquid and the gaseous phases coexist and are at equilibrium. If the temperature is lower than the boiling point, i.e. the surroundings become colder, the substance starts being more and more in the liquid phase or the liquid state and less and less in the gaseous phase or gaseous state. So, the correct answer should have a boiling point which should be higher than ${0^ \circ }C$. If we have a look at the options (A) and (B), we can see that option (A) has a boiling point lower than ${0^ \circ }C$, while, the option (B) has a boiling point higher than ${0^ \circ }C$.
Hence, the correct answer is option (B)

Note: The major mistake that many students do is that they only check the melting point and since the option (A) also has melting point lower than ${0^ \circ }C$, they tick it as the correct one, neglecting the fact that the boiling point is also lower than ${0^ \circ }C$ and thus, it’ll be in the gaseous phase at the given temperature of ${0^ \circ }C$.