You are given a mixture of NaCl and NH4Cl. Name and define the technique which can be used to obtain NH4Cl from the above mixture.

We have a mixture of ammonium chloride (NH4Cl) and sodium chloride (NaCl).  These are both white solids that will be essentially visually indistinguishable from each other.  The name of the technique we can use to separate them is called sublimation.  Sublimation is the act of a solid passing directly from the solid phase to the gaseous phase while completely bypassing the liquid phase.  Ammonium chloride is capable of sublimating while sodium chloride is not.  So if we take the mixture and heat it, the ammonium chloride will turn into a gas and separate from the solid sodium chloride which will not undergo any change.  The gas vapors rising up in the air can be condensed on a cold surface to convert back into a solid.  This re-condensed ammonium chloride can then be scraped off the cold surface to collect the separated solid.