Tag: filtration and crystallisation

Ammonium chloride changes directly from solid to gaseous state on heating. Hence, it can be separated from sand using sublimation.

Sublimation is the transition of a substance directly from the solid to the gas phase without passing through the intermediate liquid phase. Ammonium chloride, camphor, anthracene can sublime but common salt cannot sublime.

There are many substances which are converted into gas from solid when heated, and converted from gas to solid when cooled without converting into liquid. Such substances are known as sublime. For example, ammonium chloride, naphthalene balls, camphor, etc.

Sublimation is the transition of a substance directly from the solid to the gas phase without passing through the intermediate liquid phase. Ammonium chloride, camphor, anthracene, napthalene can sublime but common salt cannot sublime. Ammonium chloride evaportaes and forms vapours which rise up the funnel and get deposited in the cooler parts of the funnel. Sodium chloride remains in the china dish as no reaction takes place.

If the dish in not plugged properly during sublimation, the vapors will escape and cannot be converted into solid on cooling. Thus, separation becomes difficult.

Evaporation technique is used to separate salt from the filtrate. Common salt does not decompose during evaporation. As the water evaporates, fewer and fewer water molecules are present to keep the salt particles apart. The salt, therefore, recrystallizes and can be collected. 

Ammonium chloride doesn't actually undergo sublimation upon heating. It decomposes and forms ammonia gas and hydrogen chloride gas.Thus the given statement is true.

Sublimation is the process of transition of a substance from the solid to the gaseous state without passing through the liquid state. Non-volatile compounds cannot vaporise and hence, cannot undergo sublimation.

In crystallisation, the impure substance is dissolved in a suitable solvent to reach its nearly saturated solution at a temperature higher than the room temperature. At this high temperature, the solute has very high solubility in that solvent, so a much smaller quantity of hot solvent is needed for dissolving the solute than the solvent at room temperature.  When the solution is cooled, the pure substance is crystallised. The solution left behind is called mother liquor. All the impurities are left behind in the mother liquor. The purification method depends on the differences in solubility between the compound and the impurity.     

Fractional crystallization is the method of refining substances based on the differences in their solubility. If a mixture of two or more substances in a solution is allowed to crystallise, the least soluble substance will crystallise first. The proportion of the components in the precipitate will depend on their solubility product.