[zimrock]

when can there be elevation in freezing point? when the number of particles are decreased?

I dont quite get it.

For example my book says, when mercuric iodide reacts with potassium iodide to form nesslers reagent, the number of particles decreases and hence freezing point is raised, osmotic pressure lowered etc.

how can this be explained?

[Arkcon]

This is part of the Colligative properties of a solution.  When you add something to something, like salt and sugar to water, you lower the freezing point, right?  Each ionic species, counts as a "particle" for colligative properties, right?  Sugar counts as 1, NaCl counts as 2, ammonium phosphate as 4, etc.  The question's setup explains the rest, draw a balanced equation.

[zimrock]

ok are trying to say... consider mercuric iodide, potassium iodide etc to be particles in water and hence since the number of such particles are decresing, the freezing point of solution increses but is still less than freezing point of water?

[Arkcon]

Ah, i suspecting there was a communication problem... Yes, the solutes still lower the freezing point below pure water, just less so, because there are fewer "particles" after the reaction than before.

[zimrock]

oh ok thanks. I understood