Help with a Science Fair Project

[Michael]

Thanks Louise - You're the Best!!!!!!!!

I will include some great pictures that I took of the different type of crystals - you can really tell best by the shape.

I will incorporate your other suggestions. But I just want to make sure, you're okay with the information / conclusion layout above.

Thank you again.
MICHAEL

[Louise]

Thanks Louise - You're the Best!!!!!!!!

I will include some great pictures that I took of the different type of crystals - you can really tell best by the shape.

I will incorporate your other suggestions. But I just want to make sure, you're okay with the information / conclusion layout above.

Thank you again.
MICHAEL

I would remove the sentence "the salt slows down the molecules from expanding and freezing". I think you don't have any evidence of this and I think you cannot find a reference/citation that would support this statement. Also, make sure you are clear when you are talking about hydrogen bonds and covalent bonds. (hydrogen is always _covalently_ bound to oxygen; this makes up a water molecule. Salt only disrupts the h-bonds BETWEEN molecules, not the bond WITHIN a molecule)

If you can include a little more about the phase diagrams and you have pictures of the different shapes of crystals, I think you will have a very complete report!

Also, do you have error bars for your measurements? I think you have done many trials?

I'm sure other experts will have suggestions too.

I'm not sure exactly what this is for? Your board, your paper? Your paper should probably have citations, but maybe you did not include them to post here?

I'd love to see your photos of the ices. If you have them on a website, you can put a URL here. Otherwise, if you email the pictures to sciencebuddies (scibuddyATsciencebuddiesDOTorg) and they can forward them to me.


Louise

[Michael]

Okay - I have alot of information and thought I should be brief and to the point. Let me try again with:

Salt Water:

-As you add salt to water, there are fewer water molecules in the liquid because some of the water has been replaced by the salt.

-Salt is a soluble material that separaes into ions with positive or negative charges when they dissolve.

-Since these positive and negative salt ions take up some of the positive and negative sides of the water molecules making it more difficult for them to bond and form into the hexagonal (6 sides) crystalline structure of ice. It forms an irregular structure of cubic crystals with 4 sides.

- For this reason, salt water requires a lower temperature to freeze. Here's what happens.
1. Tiny platelets and needles of ice form over the surface of the liquid.
2. The ice crystals incorporate water, but leave salt behind.
3. Finally, the flat ice crystals grow together. In the gaps between the crystals, are small pockets of brine. A cloudy, brittle frozen slush forms.
4. Crystals of ice and salt both start to form at the eutectic temperature (the temperature which the mixture of salt and water freezes -21.1 Degrees Celsius).
5. At -21.1 Degrees Celsius, the water freezes pushing the salot out of the solution. The salt begins to crystallize out of the solution.
6. You're left with ice crystals and salt crystals. They're no longer in the solution, they're separate.


Let me know if I should run with this..

Thank you. ... MICHAEL

[bradleyshanrock-solberg]

Strep always seems to happen in the busiest times. Take care of yourself.

I agree that the phase diagram needs to be part of your display, especially if you can show photos or something similar that demonstrates the different types of solids you got in your experiment, corresponding them to areas of the phase diagram.

[Louise]

Sorry for the late response. With the forum upgrades, and my own 'slacker-ness', I hadn't had a chance to read your latest comments carefully

-As you add salt to water, there are fewer water molecules in the liquid because some of the water has been replaced by the salt.

Don't say liquid, say 'solution'.

-Salt is a soluble material that separaes into ions with positive or negative charges when they dissolve.

I know we don't have spell checking on the forums, but make sure that you proof-read your paper careful. Have someone else read it for spelling too!

-Since these positive and negative salt ions take up some of the positive and negative sides of the water molecules making it more difficult for them to bond and form into the hexagonal (6 sides) crystalline structure of ice. It forms an irregular structure of cubic crystals with 4 sides.

A cubic structure isn't irregular. It is just of a different symmetry. You could say 'it forms a different type of structure, with cubic crystals'.

- For this reason, salt water requires a lower temperature to freeze. Here's what happens.

1. Tiny platelets and needles of ice form over the surface of the liquid.

2. The ice crystals incorporate water, but leave salt behind.

What temperature does this occur at? What mass % salt does it occur at?

4. Crystals of ice and salt both start to form at the eutectic temperature (the temperature which the mixture of salt and water freezes -21.1 Degrees Celsius).

You may want to define 'eutectic' in your report.

5. At -21.1 Degrees Celsius, the water freezes pushing the salot out of the solution. The salt begins to crystallize out of the solution.

6. You're left with ice crystals and salt crystals. They're no longer in the solution, they're separate.

Do not use contractions in your report. Maybe instead... 'The solid salt and solid water are separated'. 'The salt/water solution is transformed in to separate domains of salt (solid) and water (solid).'

Let me know if I should run with this..

Run with it! You've done an incredible amount of work on your project, and I think your understanding of the science behind it has improved a ton! I'm sure your project will be a great success.

I put typos in bold, so if something is quoted without comment, look for a bold word.


Louise

[Michael]

Hi, Louise.

I'm feeling alot better today and will take your valuable input and finalize my display board. I've definitely learned alot from this project but your guidance was most valuable and am eternally grateful. I've included an acknowledgement of science buddies and a special thanks to you on my display board. I also shared this wonderful experience with my teacher and classmates.

Thank you for being there for me.

I want to wish you and everyone at Science Buddies a Happy New Year 2008!

MICHAEL

[Louise]

Hi, Louise.

I'm feeling alot better today and will take your valuable input and finalize my display board. I've definitely learned alot from this project but your guidance was most valuable and am eternally grateful. I've included an acknowledgement of science buddies and a special thanks to you on my display board. I also shared this wonderful experience with my teacher and classmates.

Thank you for being there for me.

I want to wish you and everyone at Science Buddies a Happy New Year 2008!

MICHAEL

I'm sure I speak for all the experts when I say that I am really happy you feel good about your project and that you've learned a lot through the process. It is very clear to us that you learned a ton, it is great that you see this too! Thanks for including sciencebuddies in your acknowledgements; I don't need any thanks other than the sciencebuddies credit.

I'm glad you told your friends and teacher about the site. I hope we see some of your classmates next year!

Please let us know how your science fair goes and best of luck. If you have any last minutes questions, we'll be here!


Louise

[Michael]

Hello and Happy New Year to Everyone.

Would you recommend including the phase diagram of salt water solution (chart) on the display board?

Thank you.

Michael

[Louise]

Hello and Happy New Year to Everyone.

Would you recommend including the phase diagram of salt water solution (chart) on the display board?

Thank you.

Michael

Yes! Absolutely put up the phase diagram. And mark the areas you explored. You could put dots at the room temp- % salt for all the starting points. You could even color code- you have pictures of different phases- use a colored border around the photo, then mark that area on the phase diagram with that color.

Louise

[Michael]

Hello.

The question came up while practicing my presentation that I did a good job on explaining why salt water and fresh water freeze at a lower temperature but I did not fully explain why salt water expands less. I thought I was clear with my layout, but the feedback I'm getting is saying I'm not. It was my understanding that salt water expands less when frozen due to hydrogen bonding as illustrated below. Appreciate if you could clarify again. THANK YOU!!! MICHAEL

Fresh Water:

When water freezes, it expands because it takes up more space due to “hydrogen bonding”.

 Water is a “polar” molecule. Polar means that there is a charge difference throughout the molecule – that is the oxygen atom has a negative electrical charge while the two hydrogen atoms are positive. This is important because it means that the charged ends of a water molecule are attracted to oppositely charged portions of other molecules it encounters. This attractive force is called hydrogen bonding.

 As the water freezes, the molecules start holding on to each other tightly. They rearrange themselves to form a crystal that takes up more space than of the liquid water molecular arrangement. The crystal consists of molecules in a very precise, repeating arrangement, hexagonal (6-sided) structure like a snowflake.

Salt Water:

When salt water freezes, the expansion is less because:

 As you add salt to water, the salt (NaCl) “disrupts” the bonding between the hydrogen molecules found in water (H2O) making it more difficult for the water molecules to bond and form ice. As you add salt to water, there are fewer water molecules in the solution because some of the water has been replaced by the salt.
 Salt is a soluble material that separates into ions with positive or negative charges when they dissolve. Since these positive and negative salt ions take up some of the positive and negative sides of the water molecules making it more difficult for them to bond and form into the hexagonal (6 sides) crystalline structure of ice. It forms a different type of structure with cubic crystals (4 sides). For this reason, salt water requires a lower temperature to freeze.
The phase diagram below illustrates what happens to the salt water solution:
 As the solution cools, the composition of the solution changes. When the temperature drops enough, the ice crystals incorporate water but leave salt behind. The lowest temperature possible for liquid salt solution is -21.1° Celsius. At that temperature (called the “eutectic point”), the water freezes pushing the salt out of the solution and the salt begins to crystallize.
 Below the eutectic point, the solution will freeze into ice crystals and salt crystals. The solid salt and solid water are separated. The salt water solution is transformed into separate domains of salt (solid) and water (solid).

[Louise]

Hello.

The question came up while practicing my presentation that I did a good job on explaining why salt water and fresh water freeze at a lower temperature but I did not fully explain why salt water expands less. I thought I was clear with my layout, but the feedback I'm getting is saying I'm not. It was my understanding that salt water expands less when frozen due to hydrogen bonding as illustrated below. Appreciate if you could clarify again. THANK YOU!!! MICHAEL

Fresh Water:

When water freezes, it expands because it takes up more space due to “hydrogen bonding”.

well, water is h-bonded too!

 Water is a “polar” molecule. Polar means that there is a charge difference throughout the molecule – that is the oxygen atom has a negative electrical charge while the two hydrogen atoms are positive. This is important because it means that the charged ends of a water molecule are attracted to oppositely charged portions of other molecules it encounters. This attractive force is called hydrogen bonding.

Correct

 As the water freezes, the molecules start holding on to each other tightly. They rearrange themselves to form a crystal that takes up more space than of the liquid water molecular arrangement. The crystal consists of molecules in a very precise, repeating arrangement, hexagonal (6-sided) structure like a snowflake.

this arrangement is very stable (due to hydrogen bonds) but takes up more space than other arrangements. Most molecules take up _less_ space when they go from a liquid to a solid. They become 'more dense' and the volume shrinks. Water is one of the few molecules where the opposite is true. They take up more space in the solid, and the volume expands.

Salt Water:

When salt water freezes, the expansion is less because:

 As you add salt to water, the salt (NaCl) “disrupts” the bonding between the hydrogen molecules found in water (H2O) making it more difficult for the water molecules to bond and form ice. As you add salt to water, there are fewer water molecules in the solution because some of the water has been replaced by the salt.

This is probably a very important point at high fractions of salt, but not at low fractions.
I think I gave you this link before, but look at the section on density of water.
http://www.lsbu.ac.uk/water/phase.html

Ice is 0.92 g/cm3. Water (liquid) is 1 g/cm3. From the wikipedia, the density of NaCl(s) is: 2.16 g/cm³

This is just a crude estimate... the density of a mixture may not be the same as the sum of its parts. You have to stack the salt crystals against the water crystals. If they fit well, then the density will be high. If they not fit well, then the density will be low.

Look at the pictures on this page:
http://en.wikipedia.org/wiki/Crystal_structure

Imagine having blocks of these shapes. Can you stack them tightly? Obviously, it depending which ones you have... any of the cubics will stack together, but a triclinic and a cubic won't. So, there will be less good packing, and it will take more space for the same number of molecules.

 Salt is a soluble material that separates into ions with positive or negative charges when they dissolve. Since these positive and negative salt ions take up some of the positive and negative sides of the water molecules making it more difficult for them to bond and form into the hexagonal (6 sides) crystalline structure of ice. It forms a different type of structure with cubic crystals (4 sides). For this reason, salt water requires a lower temperature to freeze.

Salt crystals also have a cubic structure, so I wonder if the cubic water and the cubic salt can fit together well. Think of stacking blocks again. How well can you pack hexagonal with cubic? Having the same structure makes it easier. However, at some conditions you have hydrated salt- not sure what the structure (or density) of that is. I'll look tomorrow when I am at school. I can't find out with normal search engines, so I'll try the university library.


Anyway, do these thoughts help?

Louise

[Michael]

Louise,

What's the difference in volume between a bunch four-sided crystals and a bunch of six-sided crystals, if both bunches have the same mass?

MICHAEL

[Michael]

After reading all of the research material, I believe salt water expands less than fresh water when frozen because the salt influences the crystal structure of solid ice in such a way that bonds take up less space then if there were no salt.

Any other thoughts?

MICHAEL

[Louise]

After reading all of the research material, I believe salt water expands less than fresh water when frozen because the salt influences the crystal structure of solid ice in such a way that bonds take up less space then if there were no salt.

Any other thoughts?

MICHAEL

Well, it isn't the crystal structure of just ice. Cubic ice has the same density as hexagonal ice. It probably has to do with the packing of salt and ice- you have to stack up both sets of blocks! Plus, the salt is more dense, so the salt block take up less volume.

Louise,

What's the difference in volume between a bunch four-sided crystals and a bunch of six-sided crystals, if both bunches have the same mass?

MICHAEL

That is actually not an answerable question! The atoms or molecules in a 'unit cell' (the repeating block) have a certain shape, but the forces that hold the block together are different from molecule to molecule. So a cubic cell of salt (NaCl) may be much smaller than a cubic cell of KCl. I have no idea if that is true... I'm just using two salts as an illustration. The forces that help form a crystal are very complicated- in addition to hydrogen bonds, you can have ionic attractions and something called 'van der waals' forces. Basically, it is very difficult to predict what type of crystal you will get, unless someone grows it. The density (mass/volume) of each substances crystal will be different, and has to be experimentally measured.

One of my friends got her PhD (doctoral degree in chemistry) doing exactly what you are doing for your science fair project. She didn't study salt water, but rather a new inorganic compound (like NaCl, but with more than two components) that she made. She then studied it in all conditions to generate the phase diagram, and measured what the crystal structure looked like in certain phases. This took her 5 years! And she didn't know _why_ she necessarily got the crystals types that she did.

So, to sum up... I would say that the hydrogen bonding of water (and water with salt) is probably a very important factor in dictating what structure you get (both in liquid and in solid phase). However, this isn't something you experimentally observed. What you observed is the density (expansion) vs. temperature and morphology (what the stuff looked like) vs. temperature. You can make arguements about the packing of the different crystals vs. temperature based on some simple arguments about the density of the component parts- i.e., salt is more dense than ice. Cubic crystals might pack better against each other than cubic and hexagonal. You can show photos to illustrate the different morphologies of the crystals. One last thing, can you calculate the density of your samples? How accurately do you know your volume change? If you aren't sure how to do the calculation, could you post one set of data here? One pure water, and one mixed? Okay, I guess that is two datasets.

Louise

[Louise]

Salt crystals also have a cubic structure, so I wonder if the cubic water and the cubic salt can fit together well. Think of stacking blocks again. How well can you pack hexagonal with cubic? Having the same structure makes it easier. However, at some conditions you have hydrated salt- not sure what the structure (or density) of that is. I'll look tomorrow when I am at school. I can't find out with normal search engines, so I'll try the university library.
Louise

The structure of hydrated salt (NaCl*2H20) is monoclinic, which is not what I would have guessed. The guys (Bernt Klewe and Bjorn Pedersen) who grew the crystal and solved the structure, made it using exactly your conditions- saturated salt water at -20 C. The paper was published in 1974 (In Acta Cryst.). However, this crystal was first grown and reported (though the structure wasn't totally known) in 1829 (by a guy named Mitscherlich). Some trivia for you.

Also, I found a table of specific densities of brine (salt and water solutions) that might be useful. Everything is in english units, so it is annoying.

http://seagrant.oregonstate.edu/sgpubs/onlinepubs/h99002.pdf

Specific gravity is the ratio of density of the material to the density of water at a specified temperature. So, if the specific gravity is 1, it is as dense as water. The chart of specific gravities in this table shows that even your solutions become more dense as you add salt. For example, at 25% salt (by weight) the specific gravity is ~1.2. (This relates to why it is very easy to float in very salty water like the dead sea) I don't remember how you made your solutions... did you start with a fixed volume of water and add a weight of salt, or did you put salt in a volume, and add water until you reached a certain total volume?

I realize I've just thrown out a bunch more stuff that you haven't really thought about- just when you thought you were done. I hadn't thought about your project this way before- sometimes you come up with new thoughts when you look at stuff again. Anyway, don't stress if this is too much. Your project, as is, sounds really good.

Louise