A laboratory mixture of sand and naphthalene can be separated effectively by using which separation technique?

Introduction / Context:
This practical chemistry question tests your knowledge of basic separation techniques for mixtures of solids. Choosing the correct method depends on specific physical properties of the substances involved, such as volatility, solubility and sublimation behaviour.

Given Data / Assumptions:

• The mixture consists of sand and naphthalene.
• Sand is an inorganic, non volatile solid that does not sublime.
• Naphthalene is an organic solid known to sublime on gentle heating.
• We are asked to identify the best method to separate these two substances.

Concept / Approach:
Sublimation is a process in which a solid changes directly to vapour without passing through a liquid state, and then condenses back to solid on cooling. Certain organic compounds like naphthalene, camphor and iodine readily sublime on heating, whereas sand does not. If one component of a mixture sublimes and the other does not, sublimation is the ideal separation technique. Distillation and chromatography are meant for different types of mixtures and are not efficient here.

Step-by-Step Solution:
1) Recognise that naphthalene is a volatile solid that undergoes sublimation on gentle heating.2) Sand, mainly composed of silica, is non volatile at such temperatures and will not sublime.3) If the mixture is placed in a sublimation apparatus and heated, only naphthalene will vaporise and move as vapour.4) The vapours of naphthalene are cooled on a cold surface, where they condense and deposit as pure solid naphthalene crystals.5) Sand remains in the original container, so the two substances are effectively separated by sublimation.

Verification / Alternative check:
We can check the other methods. Fractional distillation is used for separating liquid mixtures based on boiling point differences, so it is not appropriate for a solid solid mixture like sand and naphthalene. Chromatography is generally used for separating components dissolved in a mobile phase, not for bulk solid mixtures of this type. Simple filtration alone would not work because both components are solids and are not in different phases. Therefore sublimation is clearly the most suitable technique in this scenario.

Why Other Options Are Wrong:
Fractional distillation: This technique requires both components to be liquids or vapourisable liquids; it is not suitable for solid solid mixtures like sand and naphthalene.
Chromatography: Used mainly for separation of small amounts of components in solution; it is not a standard method for bulk separation of sand and naphthalene.
Filtration only: Since neither sand nor naphthalene is dissolved in a liquid here, filtration does not separate them effectively.
Crystallisation from hot water: Naphthalene is not very soluble in water and might not give clean separation using this method, whereas sublimation is straightforward and standard for this mixture.

Common Pitfalls:
Students sometimes choose distillation or filtration out of habit, without first identifying the special property of naphthalene, namely sublimation. Another misconception is to think that any solid mixture must be separated by sieving or filtration, but in reality the best method depends on properties like ability to sublime or differences in solubility. Always start such questions by listing key properties of each component and then match them with the most suitable separation technique.

Final Answer:
The mixture of sand and naphthalene can be separated most effectively by sublimation.