Chemistry MCQs

Sublimation is the specific physical process where a solid transitions directly into a gas. When one component of a solid mixture possesses this property while the others do not, heating the mixture allows the volatile component to sublime and be collected as a solid upon cooling (desublimation), effectively separating it from the non-volatile impurities remaining in the vessel.

Chromatography is a highly versatile analytical and preparative technique. It is widely used to separate complex mixtures into their individual components based on differential partitioning between stationary and mobile phases. It is effective for small-scale analytical samples, the separation of complex natural products like plant pigments, and the purification of industrial dyes, making all listed options valid applications.

Sublimation is a phase transition process where a solid changes directly into a gas without passing through the liquid phase. It is specifically used for the purification of solids that possess high vapor pressure. Conversely, chromatography, steam distillation, and standard distillation are standard laboratory techniques used to separate and purify components within liquid mixtures or solutions based on their boiling points or affinities for stationary phases.

Activated charcoal possesses a highly porous structure with a very large surface area. It functions by adsorbing molecules onto its surface through van der Waals forces or other intermolecular interactions. This physical process traps colored impurities and other contaminants, effectively removing them from the solution or gas phase without chemically altering the substance through oxidation or reduction.

Aniline is a weak base that reacts with dilute hydrochloric acid to form anilinium chloride, which is a water-soluble salt. Benzene, being a non-polar organic solvent, does not react with HCl and remains insoluble in the aqueous phase. This difference in solubility allows for the effective extraction and separation of the two components from the mixture.

Sublimation is the phase transition of a substance directly from the solid to the gas state without passing through the liquid phase. Ammonium chloride (NH4Cl) is a classic example of a substance that sublimes upon heating. In contrast, the other metal halides listed (ZnCl2, CuCl2, AgCl) typically melt or decompose before they vaporize under standard atmospheric pressure conditions.

Camphor is a substance that readily undergoes sublimation, meaning it transitions directly from a solid to a vapor upon heating. Benzoic acid does not sublime under the same conditions. By heating the mixture, camphor can be collected as a sublimate on a cool surface, leaving the benzoic acid behind. This physical separation technique is highly efficient for mixtures where one component has a significantly higher vapor pressure than the other.

Fractional crystallization is a purification technique that relies on the differences in solubility of substances in a given solvent at varying temperatures. By carefully controlling the temperature, one component can be made to crystallize out of the solution while the other remains dissolved. This process is repeated to increase the purity of the desired substance, making it highly effective for separating mixtures of solids with distinct solubility profiles.

Distillation under reduced pressure, or vacuum distillation, is utilized for liquids that are thermally unstable. By lowering the external pressure, the boiling point of the liquid is significantly reduced. This allows the substance to vaporize at a temperature below its decomposition point, thereby preventing chemical degradation during the purification process. This is essential for high-boiling-point compounds that would otherwise break down before reaching their normal boiling point.

In steam distillation, the total pressure of the system is the sum of the partial pressures of the organic substance and water. Boiling occurs when this total pressure equals the atmospheric (barometric) pressure. Therefore, the partial pressure of the organic compound (toluene) must be less than the total atmospheric pressure. This allows the substance to boil at a temperature lower than its normal boiling point, preventing thermal decomposition.