Question

We did an experiment in class separating (and determining the % composition of) a mixture of NaCl, NH4Cl, and SiO2 by subliming the NH4Cl, then adding water to dissolve the NaCl, separating that from the SiO2 by decanting, and then heating both of those to evaporate the water.

They are asking whether these steps could be done in another order, such as adding water first to separate out the SiO2.

Answer 1

When you add water to mixture of compounds both NH₄Cl and NaCl will completely soluble in water and ​​​SiO₂ does not dissolve in water.So SiO₂ can be seperated by filtration. Ammonium chloride dissolves in water and forms Ammonium ion and chloride ions which is slightly acidic P^H. At this PH ammonia gas doesn't evolve during evoporation of water. How ever if PH of solution is becomes basic then ammonia gas will evolve. So we have to check the PH of solution. So evoporation of water gives solid NH₄Cl and NaCl. Then heating this mixture around 34​​​​0^oC NH₄Cl under goes sublimation and NaCl remains in the vessel.

Actually ammonium chloride does not under goes sublimation. They decomposes into Ammonia and HCl both are gases then they react at coller condition to from solid NH₄Cl. So it appears as sublimation. Hower sublimation is a physical change unlike this.

The NaCl will remain. So we can separate the mixture by this order also. How ever this is ittle difficult process than the previous order. So generally 1st seperate the sublimation product.