How salt melts ice
Salt lowers the freezing point of water so that ice forms at lower temperatures. If ice is already present, adding salt to it raises its melting point so that it melts more quickly. Salt affects how water freezes by altering the speed at which water molecules move.
To test this out we put three ice cubes in two bowls. In the bowl on the left we added salt and on the right we didn't. This is our ice cubes after 14 minutes. The bowl on the right had made good progress from the house being warm but the ones on the left were almost gone.
This is the reason highway departments put salt on the roads in the winters. The salt only melts the ice though if the temperature is over -21 degrees. Any colder than that and the salt doesn't create enough movement of the water molecules to cause any meltage. While that sounds really cold, and it is, there are times in our little corner of the world that it gets colder than that and we will hear on the news that the road crews are not salting the roads because it is too cold.
The effect salt has on water freezing
Using two glasses we marked one salt and the other not salt. Each glass has a cup of water and in the salt one Dancer put a tablespoon of salt. We set them ever so careful so not to have a mess in the freezer and left them for about 45 minutes.
When we pulled them out we had the results we had anticipated. The salted cup was slushy and easily poured right out because the salt had effected the freezing temperature of the water.
The glass without salt had frozen on the top and sides and was well on it way to becoming a solid mass.
The effect of salt on the boiling temperature of water
For this experiment we put a quart of water in a pot and brought it to boil. The temperature of the boiling was was 202 degrees. This could be because we have a faulty candy thermometer or our elevation level makes water boil at a lower temperature for us. That doesn't matter though for this because we only needed a base number of the boiling water to see any difference when salt was added.
Dancer added one tablespoon of salt to the already boiling water. At first it bubbled up more and then stopped boiling. In a few moments it started boiling again. The new temperature of the water was now 205 degrees. This was because the water had to get hotter to boil the salted water. When the next tablespoon of salt was added the same thing happened and the temperature of the water rose to 207 degrees.
When adding salt to cooking water to say boil pasta, the pasta will cook faster in salted water than plain because the water temperature is higher. I am not sure how much time it saves, probably not a noticeable amount, that would be another experiment.
How salt makes objects float in water
When salt is added to water it makes it denser. Objects float better in denser water so the salt makes objects that normally wouldn't float, float. The denser the water the heavier the object that will float.
In a bowl with two cups of water and 11 teaspoons of salt an egg would float. It was a fresh egg just laid last night and gathered before the experiment. An older egg will float to the top because it has air pocket in it. Sadly, this egg rolled off the table during the rest of the experiment and found it's way into the cat bowl. From the cats perspective that was a lucky turn of events.
Eleven teaspoons also made an apple float half above the water. Good trick to know for bobbing for apples.
We tried to make other objects float, but after adding 22 teaspoons we couldn't get any more salt to dissolve. We even thought heating it in the microwave would help dissolve the salt but it didn't. The dice floated a wee bit above the bottom of the bowl but the marble and the rock, nope. The marble seemed like it rolled around real easy but it never made it off the bottom.