[Moi.Lenours]

Hi,

I used to be a student in prep school in France (PC* for those who know), it was more than 10 years ago... I'm not working in the domain of chemistry at all but today, while I was cleaning up some old stuff, I came across the subject of a practical exam I had taken (for the Ecole Polytechnique, probably the most renown school in France - I didn't pass, unfortunately ). I remember that I did not do very well, maybe someone will provide me with an answer today...

The objective was to find 4 components: A, B, C, D, that would respect the following rules:
A+B -->  précipité 1 (solid)
C+B --> stay liquid, no color change, no gaz emission
A+D --> précipité 2 (solid)
C+D --> an irritating gaz

It was possible to use other standard components as well, heat the mixture or stir it. Of course, the 4th reaction was to remain theoretical  . There are several answers possible, but the objective was to find one that could be illustrated with components that can be commonly found in a lab.

Will someone be able to provide an answer to this ten years old puzzle?

LeNours

[m3lm4k]

My guess - rather inorganic one

A=Sodium
B=Oxygen
C=Hydrogen
D=Sulfur

[Moi.Lenours]

Well, really, I don't remember if the reactions were supposed to take place in the "organic" world... What I know is that we only had to manipulate liquid solutions. So I guess that your solution would be something like: A= NaOH, B=H2O, C=? D=H2SO4 ? Geese, that's really a long time ago...

[Dan]

Uuum, perhaps:

A. Silver Nitrate
B. Sodium Chloride
C. Hydrochloric acid
D. Sodium Cyanide

To call HCN an irritating gas is a bit of an understatement though....

Edit: Scratch that, HCN is water soluble. I´ll put D. Sodium Sulfide

[Moi.Lenours]

Ok, so I guess that the reactions would be:

AgNO3 + NaCl --> AgCl (s) + NaNO3 where AgCl is a white solid
HCl + NaCl --> HCl + NaCl (nothing happens)
AgNO3 + Na2S --> Ag2S (s) + NaNO3 where Ag2S is a black solid
HCl + Na2S --> ?
Cannot find the last one... Is Cl2 the "irritating gaz"?

[Dan]

You irritating gas in this example is H₂S, the stuff stinks. Really, really stinks. And it´s toxic. http://en.wikipedia.org/wiki/Hydrogen_sulfide

ie. HCl_(aq) + Na₂S_(aq) ----> NaCl_(aq) + H₂S_(g)

There are of course many possible answers to this puzzle!

[Custos]

The last one might be hypochlorite solution. In reacting with HCl you would get chlorine gas, which is an irritant. H₂S is irritating in that it stinks, and HCN is irritating in that it kills you, but Cl₂ is an irritant in the sense that it inflames mucous membranes.