Of the choices listed, the best would be sublimation.
Sublimation is a physical process in which a substance changes from a solid directly to
a gas without passing through the liquid phase. A common example of this process is dry
ice. When you put a piece of dry ice into a room, you immediately see a cloud forming
around the dry ice. What is happening is that the dry ice is very cold and the CO2 gas
coming off the piece of solid dry ice is cold enough to cause the moisture in the air to
condense, forming the cloud you see.
Mothballs and iodine
are two other substances that will sublime. To separate the salt and iodine, place both
in a beaker and place a watch glass or evaporating dish on top of the beaker. Put some
ice in the top of the evaporating dish. Now gently warm the bottom of the beaker and
the iodine will sublime into a gas. As it rises and hits the bottom of the cold
evaporating dish it will deposit on the underside of the dish as iodine crystals. You
can then scrape the iodine crystals off the dish and put them into a container. The
salt, being a crystalline solid, has a very high melting point and will remain in the
beaker, unaffected by the gentle heating.
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