Re: How do you separate salt & sugar if accidentally mixed together?

It depends a lot on what are the rules of the task.

Do you have to get both substances back again? Or only one of them? Do you 
have to use a procedure that will leave the substances uncontaminated and 
safe to eat?

When you have to separate two substances like salt and sand, it is fairly 
easy. Salt dissolves in water, and sand does not. So you shake up the 
mixture of salt and sand in water, and pour it through a filter. The sand 
is trapped in the filter, and salty water passes through. You can get the 
salt back by evaporating the water away from the solution that has passed 
through the filter.

But this will not work for salt and sugar, because both salt and sugar are 
very soluble in water.

One way around this problem is to use something other than water to 
dissolve one thing and not the other. Sugar will dissolve in petroleum 
spirit (I think you might call it paraffin in America? It is sometimes 
called white spirit too). Salt will not. So you could use the filtering and 
evaporation trick to separate them using petroleum spirit instead of water.

But if you do that, the products might not be safe to use as food. There 
might be a lasting taint from the petroleum spirit even after it had all 
evaporated (because of impurities that would not evaporate). I certainly 
would not recommend it.

You can recover salt that is safe to eat by another method, if you are 
prepared to sacrifice the sugar. Put the solid mixture into an old 
saucepan, and cook it up on the hottest gas burner you can manage for half 
an hour or so. You want the base of the saucepan glowing red! The sugar 
will burn. If you do the job thoroughly, it will burn away altogether, but 
more likely you will be left with a lot of soot as well as salt in the 
bottom of the saucepan. When you add water, the salt will dissolve. Any 
soot can be filtered off, and the salty water evaporated to recover quite 
edible salt.

I can not think of any method of recovering sugar that I would be prepared 
to eat from a mixture of salt and sugar, especially not using things that 
would be available in most homes. 
But then -- this is not my exact field of scientific research :-)
         -- I am overweight, and shouldn't be eating sugar anyway ;-)