Glad your brother is helping you! Water with salt freezes at a lower temperature than pure water for a reason called 'freezing point depression'. (depression= lower, so it is when you make the freezing point lower temperature)Jonathan wrote:Hello.
My brother is helping me do the charts. The question i have is why does salt water freeze at a low temperature thats negative.
Thank you
Jonathan
http://en.wikipedia.org/wiki/Freezing-point_depression
That is pretty complicated, but basically, when you mix things together, they no longer act like the original parts. They have different properties. There are many different ways you can think about this process. You probably won't learn about most of the terms until you are in higher grades. On that page, it says that this is an 'entropic' effect. Entropy is a fancy science word that basically means 'messy' or 'random'. One of the physical laws of the universe is that entropy increases (things get messier). So, in a mixture of salt and water things are pretty messy. You've got some salt here, some water there, everything is jumbled together. As you observed, the water/salt mixture formed ice needles on the top and then had wet stuff and maybe salt chunks. The interesting thing is, when you freeze salt water, you are actually making it un-messy. The ice is PURE water, and the salt is pure salt, and the remaining liquid is actually saltier than the starting water. To do this requires that you remove more energy (temperature= energy) from the system, so it freezes at a lower point. Maybe this page will help:
http://antoine.frostburg.edu/chem/senes ... -ice.shtml
Again, this is a pretty complicated topic, so if what I said doesn't make sense, or you cannot understand those pages please ask some more questions.
As you found, if you add a lot of salt, the freezing point can go to very low temperatures. In fact, your limit was probably your freezer only going to -19... if your freezer could go to -50 *C, I'm sure your final values would be colder. Those pages I linked to before say that the lowest you can make it is -23 *C with a lot of salt.
If you live in an area where it snows, you may see people putting salt in front of there house when the weather gets bad. This is for exactly the same reason- salt plus water/snow means you won't get ice unless it gets very, very cold. So, putting salt down prevents ice from forming (unless it gets to -19 *C!) or helps to melt the ice. (As an aside, people use sand for a different reason. Sand doesn't prevent ice, but it can make the ground very rough, instead of smooth (with ice), so you won't slip. Some places don't use salt any more because the high amount of salt can kill the plants nearby, so sand can be used instead. But, the reason sand works is different from the reason salt works.)
Hope this helps.
Louise