A mixture of sodium chloride (salt) and ammonium chloride can be separated by _________.
sublimation
Separating different substances from a mixture is a fundamental concept in chemistry. The technique used depends heavily on the physical and chemical properties of the components in the mixture. In the case of a mixture of sodium chloride (\(\text{NaCl}\)) and ammonium chloride (\(\text{NH}_4\text{Cl}\)), we need to find a method that exploits a significant difference in their properties.
Separating Sodium Chloride and Ammonium Chloride Mixture
Sodium chloride, commonly known as table salt, is an ionic compound. Ammonium chloride is also a salt. Let's look at the properties relevant to common separation techniques:
- Solubility in water: Both sodium chloride and ammonium chloride are soluble in water. Therefore, simply adding water and filtering would not separate them, as both would dissolve.
- Melting and Boiling Points: Sodium chloride has a high melting point (\(\text{801}^\circ\text{C}\)) and boiling point (\(\text{1413}^\circ\text{C}\)). Ammonium chloride, however, behaves differently upon heating.
- Sublimation: This is a key property difference. Sublimation is the process where a solid changes directly into a gas without passing through the liquid phase. Ammonium chloride (\(\text{NH}_4\text{Cl}\)) is known to sublime readily upon heating to about \(\text{340}^\circ\text{C}\). Sodium chloride (\(\text{NaCl}\)) does not sublime under normal conditions; it melts at a very high temperature.
Using Sublimation for Separation
Since ammonium chloride sublimes on heating and sodium chloride does not, sublimation is an effective method to separate this mixture. The process involves gently heating the mixture. As the temperature rises, the ammonium chloride solid turns directly into ammonium chloride gas. The sodium chloride, which does not sublime, remains in solid form.
The gaseous ammonium chloride can then be cooled. When the ammonium chloride gas encounters a cool surface, it changes back into solid ammonium chloride directly from the gas phase (a process called deposition or desublimation). This allows the collection of purified solid ammonium chloride, leaving the sodium chloride behind.
Why Other Methods Are Less Suitable
- Filtration: As both substances are soluble in water, filtration cannot separate them from a solution.
- Chromatography: While chromatography can separate components in a mixture, it's typically used for smaller quantities or for analytical purposes, not usually for bulk separation of common salts like these based on this method.
- Distillation: Distillation separates liquids with different boiling points or a liquid from a dissolved non-volatile solid. Neither substance is a liquid at common temperatures, and the separation relies on the volatility difference upon heating, which is specifically addressed by sublimation in this case. Standard distillation procedures are not designed for separating two solids based on one's sublimation property.
Therefore, the most suitable method to separate a mixture of sodium chloride and ammonium chloride is sublimation, leveraging the unique property of ammonium chloride to change directly from solid to gas upon heating.
Revision Table: Common Separation Techniques
Understanding when to use different separation techniques is crucial.
Here is a brief overview:
- Filtration: Separating an insoluble solid from a liquid.
- Evaporation: Separating a soluble solid from a liquid solvent by boiling off the solvent.
- Distillation: Separating liquids with different boiling points or a liquid from a dissolved solid by vaporization and condensation.
- Sublimation: Separating a solid that sublimes from one that does not.
- Chromatography: Separating components based on differential partitioning between stationary and mobile phases.
Additional Information: Properties of Ammonium Chloride
Ammonium chloride (\(\text{NH}_4\text{Cl}\)) is a white crystalline salt. It is highly soluble in water. Its property of undergoing sublimation upon heating is due to its thermal decomposition into ammonia (\(\text{NH}_3\)) and hydrogen chloride (\(\text{HCl}\)) gases, which then recombine upon cooling to form solid ammonium chloride again. The chemical equation for its thermal decomposition (and recombination) is:
\(\text{NH}_4\text{Cl(s)} \rightleftharpoons \text{NH}_3\text{(g)} + \text{HCl(g)}\)
This reversible thermal dissociation is often what is observed and utilized in the separation process described as sublimation for ammonium chloride.
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