Question

Given below are two statements :
Statement I: Sublimation is used for the separation and purification of compounds with low melting point.
Statement II: The boiling point of a liquid increases as the external pressure is reduced.
In the light of the above statements, choose the correct answer from the options given below :

• A. Both Statement I and Statement II are false
• B. Statement I is false but Statement II is true
• C. Statement I is true but Statement II is false
• D. Both Statement I and Statement II are true

Hint:

Boiling occurs when Vapor Pressure = External Pressure. If you lower the external pressure, the liquid needs less heat to reach that pressure, hence a lower boiling point.

Answer 1

The Correct Option is A

Let's evaluate each statement in the given question:

1. Statement I: "Sublimation is used for the separation and purification of compounds with low melting point."
   • Sublimation is a process where a solid changes directly into a gas without passing through the liquid state. It is typically used for separating mixtures where one of the components sublimes upon heating.
   • It is especially useful for separating substances that have a relatively higher tendency to sublime, not necessarily those with low melting points.
   • For instance, substances like naphthalene, ammonium chloride, and iodine are separated by sublimation because they readily change from solid to vapour.
   • Thus, Statement I, which claims that sublimation is used for compounds with a low melting point, is false because sublimation is more about volatility than melting point.
2. Statement II: "The boiling point of a liquid increases as the external pressure is reduced."
   • The boiling point of a liquid is the temperature at which its vapour pressure equals the external pressure.
   • As the external pressure is reduced, the vapour pressure required for a liquid to boil is achieved at a lower temperature, thus decreasing the boiling point.
   • Therefore, as conventional wisdom and scientific understanding tell us, Statement II is false. The boiling point decreases with a decrease in external pressure.

Based on the analysis above, both statements are incorrect:

• Statement I is incorrect due to a misunderstanding of sublimation criteria.
• Statement II is incorrect as it contradicts fundamental principles of physics regarding boiling points and pressures.

Hence, the correct answer is: Both Statement I and Statement II are false.