Science Project Planning

[Louise]

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

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)

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

[Jonathan]

Hello.

This is hard to understand. I have to wait for my brother or mom to help me. I just went back to look at the frozen salt water. I think I saw clumps of salt at the bottom. Do you know why else I did? I tasted the top and the bottom of the ice. The top was less salty and the bottom was very salty.

I'm also reading about what happens when water freezes. The molecules stick together and can't move. What does the hydrogen bonding do?

Thank you.
JONATHAN

[Louise]

Hello.

This is hard to understand. I have to wait for my brother or mom to help me.

It is complicated. Even now, people study this at universities. After your mom or brother have helped, please ask questions!

. The top was less salty and the bottom was very salty.

Cool! This is a really important observation!

I'm also reading about what happens when water freezes. The molecules stick together and can't move. What does the hydrogen bonding do?

Another set of complicated topics! Hydrogen bonds (h-bonds for short) make a molecule "sticky" so it will stick to other molecules. Water has a lot of hydrogen bonds, so it sticks to itself a lot. Hydrogen bonds can help organize the molecules in a certain pattern, so the ice of water has a very certain pattern of water molecules because of h-bonds.


Louise

[Jonathan]

Hello.

I thought I was doing an easy project because water is simple to understand. It's not!

Water freezes because of hydrogen bonding. My brother showed me a picture of the molecules when they stick together. The shape is like a snowflake. The water froze at 0 degrees celsius.

When the salt water froze, it did look very messy.

I mixed the salt in the water for a long time. It was all dissolved. You said "The ice is PURE water, and the salt is pure salt, and the remaining liquid is actually saltier than the starting water." Are you saying that the ice separates from the salt? I always saw some liquid on top of my frozen salt samples. Is this the remaining liquid that is saltier than the starting water?

JONATHAN

[Louise]

Hello.

I thought I was doing an easy project because water is simple to understand. It's not!

Yep, water is really complicated. It is kind of cool that something so normal as water is so complicated.

Water freezes because of hydrogen bonding. My brother showed me a picture of the molecules when they stick together. The shape is like a snowflake. The water froze at 0 degrees celsius.

Yes, it is very pretty. This structure is called 'hexagonal'. Hydrogen bonds aren't the only reason ice freezes, but it is a big part. (Other things freeze that don't have hydrogen bonds.) Water is special (and complicated) because it can hydrogen bond.

I mixed the salt in the water for a long time. It was all dissolved. You said "The ice is PURE water, and the salt is pure salt, and the remaining liquid is actually saltier than the starting water." Are you saying that the ice separates from the salt? I always saw some liquid on

Yes! Pure water freezes out of the salt water giving you pure water ice. You could actually use this method to get clean pure water to drink if all you had was salt water. You would pick out the ice, which would be less salty than the starting water. (Maybe you would have to do this several times, since the ice chunks have salt on the outside, but it could work.)

Is this the remaining liquid that is saltier than the starting water?

yes, the remaining liquid will be more salty and the ice won't be salty. This means your cups will have some very salty areas and normal salty areas (and ice which won't be salty) and maybe chunks of pure salt. Since you don't stir the cups after you put them in the freezer, when you taste different areas you taste more or less salty depending on whether ice is forming there. (The more salt in water, the heavier it is. This is called density. The saltiest water will eventually fall to the bottom. This could explain your observation that the liquid at the bottom was saltier than at the top.)

Louise

[Jonathan]

Hello.

Okay - My brother also helped me understand this. When the water freezes, the molecules line up in order. They're sticky so they stick together so tightly so they can't move. They freeze and form a hexagonal structure of ice with 6 sides. It almost looks like a snowflake.

For salt water, the salt makes it harder for the water to freeze. It gets in the way. Is this why salt water needs a lower temperature to freeze?

Thank you.
JONATHAN

[Louise]

Hello.

Okay - My brother also helped me understand this. When the water freezes, the molecules line up in order. They're sticky so they stick together so tightly so they can't move. They freeze and form a hexagonal structure of ice with 6 sides. It almost looks like a snowflake.

For salt water, the salt makes it harder for the water to freeze. It gets in the way. Is this why salt water needs a lower temperature to freeze?

Thank you.
JONATHAN

Yes, that is a good explanation. To freeze, water has to push salt out of the way so it can line up. This leads to 'freezing point depression'.


Louise

[Jonathan]

Hello.

I understand that salt water needs a lower temperature to freeze. I saw that in my experiment. I had to make my amounts of salt less because my original amounts of water and salt never froze in my home freezer. I read that salt water freezes at a -21.1 degree celsius. Is that because of the amount of salt in the water?

JONATHAN

[Louise]

Hello.

I understand that salt water needs a lower temperature to freeze. I saw that in my experiment. I had to make my amounts of salt less because my original amounts of water and salt never froze in my home freezer. I read that salt water freezes at a -21.1 degree celsius. Is that because of the amount of salt in the water?

JONATHAN

Yes. I was saying -23 degrees, but that is incorrect. It is -21.2 degrees. This is the coldest point that that salt/water/ice mixtures will get. If you have less salt, it can freeze at warmer temperatures.

Look at this picture again...
http://www.ucalgary.ca/~kmuldrew/cryo_c ... ap6_1.html

Maybe your brother can help you with this picture? It helps you see exactly what temperature a certain mixture of salt will freeze at.

By the way, I am very impressed with how carefully you have done this experiment and how hard you are working to understand this science. This is very hard stuff, and you are doing a great job!

Louise