Question on purifying solid

Question

callisto · Answer

I know C and D are not correct, but why A is not correct while B is correct??

anonymous · Answer

KI doesn't melt till close to 700C. It's not going to form vapors and re condense on the ceiling. Besides the ammonium chloride decomposes to ammonia and hydochloric gas then re combines and solidifies at the top. Your not going to get KI to decompose into potassium gas. That would blow my mind.

callisto · Answer

The KI option is clear now. But why would ammonium chloride decompose to ammonia and hydochloric gas upon heating?

apoorvk · Answer

this seems more like a sublimation based separation ( i guess you already know how they do it). And ammonium chloride is the the only solid here that sublimates here normal pressures wit the help of a wee bit of heating. So, NH4Cl gets heated up, sublimates and solidifies on the surface of the funnel, and as it achieves thermal equilibrium with the contact surface, it solidifies. Separated!

callisto · Answer

Yup, I've also marked down the word 'sublimation' for this question. Though, I don't know why it can happen in this case.

apoorvk · Answer

told you why. and no it doesnt decompose to HCl and NH3 gas, as sharmala mentioned above (just a wee bit of dissociation since this is not an ideal case).

callisto · Answer

It is like whenever NH4Cl gets heated, it sublimes?

apoorvk · Answer

yeah, it is like camphor, that sublimates when left in the open, and when the temperature is increased to a certain extent, the sublimation increases. (ofcourse at a some very high temperature a sublimable compound may reach its kindling point. there's a lot more about the co-existence of the states of matter determined by the critical point curve) http://en.wikipedia.org/wiki/Critical_point_(thermodynamics)#Pure_substances:_vapor.E2.80.93liquid_critical_point

callisto · Answer

Okay! Thanks a ton!!!