Expert: Dr. Jeffery Raymond - 1/2/2012

Question
I'm trying to help my son (4th grader) with his science project. He has to take two clear 16 ounce cups and put 9 ounces of water in each. Then we each had a teaspon in hand. One had a tspn of sugar and the other had a tspn of salt. We dumped them in at the same time to seperate cups of room temperature water and had to time which one dissolved faster. It appeared that the salt cup just went cloudy right away and the sugar sunk right to the bottom of it's cup. So now I''m confused on how to help my son articulate the amount of seconds it took. The sugar took a very long time between sinking to the bottom and finally dissolving and the salt seemed to have dissolved immediately. The question he has to answer is which dissolved first and in how long?

Answer
The clouding you saw was the salt being first suspended before dissolving.  A suspension is not the same as a dissolution - suspension is the physical placement of one thing into another thing so that it disperses in distinct parts.  For a solution to be formed, the distinct parts have to break down into its smallest form and become chemically dispersed.  So, for talking to him, point out that with the salt, he observed

first: salt as a solid, and water as a liquid
second: adding the salt you saw a cloudy solution which was the solid salt being suspended (instantaneous)
third: once the water turned clear again, that meant that a new substance had been formed: salt water - where there is no more liquid water or solid salt but rather a liquid solution called salt water. (have him note how long it took to turn clear --- that is the dissolution time)

Sugar does not naturally suspend in water, so it just settles and must go about dissolving without an intermediate suspension phase.

A neat thing to do is to have him put a low power LED light (like the one on a laser pointer or on some key chains) through the water glass and shine it on to a white surface behind (make sure he does not shoot the light into his eye).  Then, when the salt is added, the light will be scattered.  When the scattering stops and the light gives its original pattern again, then you have dissolution.  Best performed in the dark.