Re: Why did the ice melt faster in salt water than in plain water?

Apologies!

Remind your son that water, when it is liquid, can't be any colder than
32 degrees (F).  If it does get colder, it turns into ice.  Salt, however,
allows the water to be colder than 32 degrees without turning into ice.

So if you put ice into normal water it will cool down, but won't get below
32 degrees.  If you put ice into salt water, it will cool down even
colder than 32 degrees.

Ice melts faster at higher temperatures.  For example, a snowball will last
a while on a 40-degree day, but it won't last long at all on an 80 degree
day, and it will melt almost instantly on a 100 degree day.

Since the ice in the normal water is going to be at a higher temperature
than the ice in the salt water, it will melt faster.

A very important piece of data that will help explain this experiment is
the temperature of the water *throughout* the melting.  Have him keep a 
thermometer in both vessels and record the temperature every 3-4 minutes.
He will see that the salt water will get colder than the non-salt water.