Help with a Science Fair Project

[bradleyshanrock-solberg]

I'm also reading Louise's information, as it's been about 20 years since I studied the H2O crystal structure. Water is quite unusual in how it behaves compared to other materials.

I've got nothing to add at this time, beyond encouraging you to keep going.

[Louise]

I am reviewing the information suggested by Louise. Wow - this is really complex material.

Any other suggestions - please let me know.

Thank you again.....Michael

Yes, this is complicated! If you don't understand something, please ask us questions. This is a really nice project because it demonstrates several effects clearly, and there is a lot of data out there that you can reference. Some of the material is written at a pretty advanced level (like the page on the different types of ice), and you probably don't need to understand every detail, but it is very interesting to think about all the different types of ice that can be made. ("I'd like my pepsi served with ice-eleven, please." )

If you have trouble understanding how to read the 'phase diagrams' please let me know. These diagrams help you predict exactly what material you have in your mixture at certain conditions.

Water is really interesting, because even pure water forms many different kinds of ice depending on the temperature and the pressure. Even liquid water is very complicated, and many research groups still study plain water so they can understand hydrogen bonds and how they change with time.


Louise

[deleted-71447]

Hi Micheal,
In addition to Louise's excellent info and advice, make sure to follow up on Paul Descarli's suggestion in the other thread to read about the freezing of sea water. That info should be relatively simple to understand, and the processes are very relevant to your experimental results. For example, as saline water freezes, it fractionates so that the initial ice formed has a lower salt content than the surrounding water. You should be able to find many references to this online. Also, solid ice expands at different rates depending on the salt concentration. These coefficients of expansion are also available online.

There may be other factors aside from salinity that could affect your results. For example, did both types of ice (saline and fresh water) appear equally cloudy? If it is cloudy, do you see tiny bubbles in the ice? Formation of air bubbles in freezing water affects the final volume of ice, and you might see some differences in air content between the saline versus fresh water ice, or in the different layers of ice in the saline trials.

Great experiment! Keep up the good work.

[Michael]

Hello. Hope everyone is doing well especially with the holidays quickly approaching.

I've been reviewing the suggested material from Louise and feel I have a much better understanding. As a result, I need to correct
my prior trial results that were done at -70 degrees celsius. When I took the cans out of the freezer, they were actually "supercooled". They actually froze out of the freezer. I didn't completely understand what was happening. I believe they ended up freezing when they reached the appropriate temperature.

I decided to contact the local lab in my neighborhood and got permission to do another trial using their scientific freezer but this time at -20 degree celsius.

I took Chris' advice and conducted the test this time using balloons.
The fresh water balloon definitely expanded more than the salt water balloon. But, what I found interesting was that the salt water was a cloudy color whereas the fresh water was almost crystal clear. In addition, when I observed the salt water I found that the water ice crystals formed in one area, but in the total opposite area of the ice crystals were clusters of salt. You could also see small air bubbles in the fresh water and the ice was completely smooth.

I learned that my hypothesis was correct.

The more "salt" in water, the lower its freezing point. Basically, salt "disrupts" the bonding between the Hydrogen atoms and Oxygen atom. As a result of the disruption between the Hydrogen and Oxygen atoms, the salt water solution freezes at a lower temperature and expands less.

Please let me know your thoughts and any other suggestions.

Best regards....MICHAEL

[deleted-71588]

There are some other possibilities that you need to think about. The solubility of a given salt in water is often a function of temperature and typically decreases with temperature (hot water will typically allow more salt to disolve than cold water).

Since you observed clumps of salt crystals in some samples, what might have occurred is that salt precipitated out of solution as the temperature was lowered but before it froze. Similar techniques are often used to seperate out various protiens in biochemistry applications. I maybe wrong, but I seem to recall that "freeze drying" was utilized as a step in commercial separation of insulin from pancreatic cells before people figured out how to DNA splice bacteria to produce insulin.

Ice from pure distilled water is a crystal structure that has a very regular pattern so its optical properties are uniform which translates into transparency.

Frozen salt water has an irregular structure so its optical properties are not uniform which translates into optical scattering which is translates into translucent behavior (cloudy appearance).

[bradleyshanrock-solberg]

Yes...the salt solution will likely have examples of all of the following

1. essentially pure ice
2. essentially pure salt
3. frozen salt/water solution

The phase diagrams referenced in early links show that if you heat it up and then cool it down (at certain concentrations of salt) you would expect to get all 3 states.

These will be visually different. How much you get of each may well depend on how fast you cool it. The -70 degree freezer will give a different mix than a -20 degree freezer, assuming the samples go in at room temperature.

This technique is used in a lot of metallurgy - steel for example is a mix of iron and carbon, carefully heated and cooled to cause a precise mix of different combinations of iron and steel (crystal structures) in the final product which gives it a desired mix of hardness and toughness. You get a very different result if you heat up your carbon/iron mixture and suddenly cool it in water versus letting it cool down slowly in the air.

You're doing kind of the same thing by using different refrigerator temperatures. The water will go from liquid to solid state faster in the cooler freezer, and that will change the percentage of the three materials you see (probably less pure ice and salt, more mixed salt/ice if you cool quickly vs cool slowly - depends on the phase diagram and the amount of salt in the solution)

[Louise]

Hello. Hope everyone is doing well especially with the holidays quickly approaching.

I've been reviewing the suggested material from Louise and feel I have a much better understanding. As a result, I need to correct
my prior trial results that were done at -70 degrees celsius. When I took the cans out of the freezer, they were actually "supercooled". They actually froze out of the freezer. I didn't completely understand what was happening. I believe they ended up freezing when they reached the appropriate temperature.

I decided to contact the local lab in my neighborhood and got permission to do another trial using their scientific freezer but this time at -20 degree celsius.

I took Chris' advice and conducted the test this time using balloons.
The fresh water balloon definitely expanded more than the salt water balloon. But, what I found interesting was that the salt water was a cloudy color whereas the fresh water was almost crystal clear. In addition, when I observed the salt water I found that the water ice crystals formed in one area, but in the total opposite area of the ice crystals were clusters of salt. You could also see small air bubbles in the fresh water and the ice was completely smooth.

I learned that my hypothesis was correct.

The more "salt" in water, the lower its freezing point. Basically, salt "disrupts" the bonding between the Hydrogen atoms and Oxygen atom. As a result of the disruption between the Hydrogen and Oxygen atoms, the salt water solution freezes at a lower temperature and expands less.

Please let me know your thoughts and any other suggestions.

Best regards....MICHAEL

Michael,
I'm glad to see that things are going well with your experiment. The results with supercooling is really neat! Did you see the control (pure water) do the same thing? (And why didn't you mention this observation before?? This is an important observation!). I hope your experiments with the -20 freezer work well! Looking at the phase diagram though:
http://www.ucalgary.ca/~kmuldrew/cryo_c ... ap6_1.html

I think you not be able to freeze all samples. If you look at the 0-40% salt part of the phase diagram, it looks like -20 is very near a phase transition between getting a solid of [H2O ice and NaCl*H2O2] and having a mixture of solid (either pure H2O ice or the NaCl hydrate solid) and brine. You should determine the temperature of the freezer very carefully because a few degrees cooler than -20 is a very different result from a few degrees warmer.

I think you need to be careful about how you word your last paragraph. Not all ice that forms is 'disrupted' ice. As the phase diagrams show, some of the ice forms the regular 'pure' water ice, with the normal density and structure. In terms of conclusions, I would probably rely more on the phase diagrams. Hydrogen bonds are important- and they do dictate the type of ice structure you get as well as the properties of the liquids and solutions, but you don't directly observe them. The colligative properties can be understood with out knowing the details of the molecular interaction- you can understand freezing point depression without knowing that a particular solvent does or does not form hydrogen bonds. I would recommend you look at this site again:
http://en.wikipedia.org/wiki/Freezing-point_depression

I would also recommend taking photographs of the different types of ice if possible. This would look really cool on your poster board, and would help illustrate the complexity of the ice/water/salt behavoir.

Good luck with the rest of your experiments.

Louise

[Michael]

Louise had confirmed the following:

"The 6 sided ice crystals build one on another to form sheets of ice. When salt (sodium chloride) is mixed into the water, chlorine ions grab the hydrogen atoms in H2O that interferes with the ice crystal building. It's difficult then for the ice crystals to connect and so they move slower to freeze."


Please clarify if "chlorine ions" in the above quote should read "chloride ions".

Thank you.

Michael

[Louise]

Louise had confirmed the following:

"The 6 sided ice crystals build one on another to form sheets of ice. When salt (sodium chloride) is mixed into the water, chlorine ions grab the hydrogen atoms in H2O that interferes with the ice crystal building. It's difficult then for the ice crystals to connect and so they move slower to freeze."


Please clarify if "chlorine ions" in the above quote should read "chloride ions".

Thank you.

Michael

I don't think I 'confirmed this'; I actually think this is not the best description of what happens. I think you should write in terms of 'colligative properties', which is why I included a link to colligative properties this quote. Please see my early post to you today.

Chlorine is the element. Chloride is the negatively charged chlorine (-1). Chloride is technically correct, and you should use it. 'chlorine ion' would probably be understood by a chemist to mean chloride (which I am), but is not technically correct. So, if you choose to keep this paragaph in your paper, you should use chloride.

Again, I would recommend re-reading my last post and reviewing colligative properties.

Good luck with your paper!
Louise

[Michael]

Hello!

I'm trying to think of a good title for my display board that will catch people's attention.

Any ideas will be greatly appreciated.

Thank you... Michael

[Michael]

Hello again.

Can anyone think of a good title for this experiment?

Does the increase in salinity affect the water’s ability to expand when frozen inside an aluminum can?

I'm starting to put my information together..... Everyone has been so helpful. I couldn't have done this experiment without your direction.

THANK YOU!!!!! MICHAEL

[bradleyshanrock-solberg]

You might just try something a bit more general.

"Phase changes and effects on volume in saline solutions at different cooling rates"

Really what you're doing is exploring the phase diagram for salt and water at the liquid/solid boundary. You're doing it in a fun way, showing pressure exerted by volume change in a dramatic fashion, but the meat of your results is in the combination of pure ice, saline ice and salt that emerge from the different treatments.

[Louise]

You might just try something a bit more general.

"Phase changes and effects on volume in saline solutions at different cooling rates"

Really what you're doing is exploring the phase diagram for salt and water at the liquid/solid boundary. You're doing it in a fun way, showing pressure exerted by volume change in a dramatic fashion, but the meat of your results is in the combination of pure ice, saline ice and salt that emerge from the different treatments.

I really like this suggestion. I haven't posted back about a title, becuase I haven't had any good ideas!

I think this is a really good title, and the different cooling rates parts allows you to mention both the -20 and -70 data.

Louise

[Michael]

Hi, Everyone.

I've been sick with the flu / strep throat. What a time to get sick? With the holidays coming and the project being due on January 10th, I'm trying to finalize my project so that I can focus on the actual presentation. Of course, I value your input and look forward to your comments.

I spent alot of time reading the research material that was recommended especially the Solid-Liquid Phase Diagrams suggested by Louise. I feel it finally clicked in and am able to explain it. In brief, when freezing a salt water solution nothing happens until you get down to the eutectic temperature (the temperature which the mixture of salt and water freezes -21.1 degrees C) and it is at that point, that both ice crystals and salt crystals start forming. The end product is that you're left with ice crystals and salt crystals. They're no longer in the solution, they're separate.

So that I'm clearly answering the purpose of my experiment Does the increase in salinity affect the water's ability to expand when it freezes inside an aluminum can, I feel I need to include the following points in very simple terms so that my classmates (8th graders) understand and can follow what I'm saying:

Fresh Water:
When water freezes, it expands because of hydrogen bonding. The water molecules rearrange themselves to form a crystal that takes up more space than of the liquid molecular arrangement. The crystal consitsts of molecules in a very precise, repeating array, hexagonal (6 sided) structure like a snowflake. (Will include a great picture of this)

Salt Water:
When salt water freezes, the expansion is less because: As you add salt to water, the salt slows down the molecules from expanding and freezing. The salt interferes with the bonding between the hydrogen and oxygen atoms found in water making it difficult for them to bond. Salt water form cubic crystals (with 4 sides) whereas ice (fresh water) is hexagonal (with 6 sides).

After conducting the experiment, I concluded that my hypothesis was correct! As the salinity of water increases, the expansion decreases.

This is specifically shown in my experiment when the can with 0g of salt expanded 2 cm (in diameter), the can with 5g of salt expanded 1 ½ cm (in diameter) and the can with 10g of salt expanded 1 cm (in diameter) .

I learned that the reason for this is because the salt (NaCl) in the water interferes with the bonding between the Hydrogen and Oxygen atoms when freezing, causing the ice to expand less and freeze at a slower pace. As the water freezes, the dissolved salt lowers the freezing point of the water to less than 0°C.

Taken together, my experiment proves that the increase in salinity does affect and decrease the expansion of the water when frozen.


This project can apply to real life in helping to determine the causes of erosion:

 The expansion of ice is responsible for much of the erosion that makes our soil. During the winter, water that has found its way into cracks in rocks freezes. As the water freezes, it expands, and the cracks are forced open and enlarged by the ice. This turns big rocks into little rocks and makes mountains into molehills.

 Erosion can occur when water freezes and glaciers are eroding the surface of the earth.


Appreciate your thoughts of any changes or if I can go forward with the above.

All the best....Michael

[Louise]

Hi, Everyone.

I've been sick with the flu / strep throat. What a time to get sick? With the holidays coming and the project being due on January 10th, I'm trying to finalize my project so that I can focus on the actual presentation. Of course, I value your input and look forward to your comments.

I spent alot of time reading the research material that was recommended especially the Solid-Liquid Phase Diagrams suggested by Louise. I feel it finally clicked in and am able to explain it. In brief, when freezing a salt water solution nothing happens until you get down to the eutectic temperature (the temperature which the mixture of salt and water freezes -21.1 degrees C) and it is at that point, that both ice crystals and salt crystals start forming. The end product is that you're left with ice crystals and salt crystals. They're no longer in the solution, they're separate.

So that I'm clearly answering the purpose of my experiment Does the increase in salinity affect the water's ability to expand when it freezes inside an aluminum can, I feel I need to include the following points in very simple terms so that my classmates (8th graders) understand and can follow what I'm saying:

Fresh Water:
When water freezes, it expands because of hydrogen bonding. The water molecules rearrange themselves to form a crystal that takes up more space than of the liquid molecular arrangement. The crystal consitsts of molecules in a very precise, repeating array, hexagonal (6 sided) structure like a snowflake. (Will include a great picture of this)

Salt Water:
When salt water freezes, the expansion is less because: As you add salt to water, the salt slows down the molecules from expanding and freezing. The salt interferes with the bonding between the hydrogen and oxygen atoms found in water making it difficult for them to bond. Salt water form cubic crystals (with 4 sides) whereas ice (fresh water) is hexagonal (with 6 sides).

After conducting the experiment, I concluded that my hypothesis was correct! As the salinity of water increases, the expansion decreases.

This is specifically shown in my experiment when the can with 0g of salt expanded 2 cm (in diameter), the can with 5g of salt expanded 1 ½ cm (in diameter) and the can with 10g of salt expanded 1 cm (in diameter) .

I learned that the reason for this is because the salt (NaCl) in the water interferes with the bonding between the Hydrogen and Oxygen atoms when freezing, causing the ice to expand less and freeze at a slower pace. As the water freezes, the dissolved salt lowers the freezing point of the water to less than 0°C.

Taken together, my experiment proves that the increase in salinity does affect and decrease the expansion of the water when frozen.


This project can apply to real life in helping to determine the causes of erosion:

 The expansion of ice is responsible for much of the erosion that makes our soil. During the winter, water that has found its way into cracks in rocks freezes. As the water freezes, it expands, and the cracks are forced open and enlarged by the ice. This turns big rocks into little rocks and makes mountains into molehills.

 Erosion can occur when water freezes and glaciers are eroding the surface of the earth.


Appreciate your thoughts of any changes or if I can go forward with the above.

All the best....Michael

Sorry to hear that you were sick. I'm glad the phase diagrams "clicked"! I think your explanation of the phase diagram can also be included in your report and perhaps a copy of it included in your display with either dots or lines representing your experimental trials (show what part of the phase diagrams you studied). Remember too, your report and presentation are not just for your classmates, but also teachers and judges. I don't think you should simplify too much just because your classmates are in the 8th grade; your classmates can learn a lot from what you present.

I like your paragraph about erosion. It is nice that you are thinking about the larger picture.

Lastly, did you see the different types of crystals? Could you tell that you were forming cubic rather than hexagonal? I'm not sure you could see specific structures by eye, but did you see differences in the surfaces or shapes? (I know some looked cloudy, but that is for a different reason.)

One last, tiny detail... It is usual to put a space between the number and the unit. Thus '5 g' and not '5g'.

Louise

[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/ ... 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