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Leonie
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| Posted: May 27 2008 at 6:59pm | IP Logged
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This experiment is an oldie but a goodie. 
Son Thomas did this last week - made one chocolate bag and one vanilla. Yum!
And he learned about ionic compounds....Thought someone else might like the link...
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Rachel May
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| Posted: May 27 2008 at 8:07pm | IP Logged
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Bless you! Ice cream in a bag came up randomly this past week, and I ended up with a variety of soggy bags of sweetened milk on my poarch and wasn't sure why. Now we can do it properly and learn something.
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Rachel
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Mary G
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Leonie -- what a great end of the year experiment!
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MaryG
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Leonie
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| Posted: May 27 2008 at 8:51pm | IP Logged
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Let me know if it works this time, Rachel....
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Leonie in Sydney
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Kristie 4
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Excellent Leonie...just what the doctor ordered to spice up our days right now 
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Kristie in Canada
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Rachel May
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Can you help me understand this? It's been a while since college chem, but I think I understand this part:
Quote:
Explanation
Ice has to absorb energy in order to melt, changing the phase of water from a solid to a liquid. When you use ice to cool the ingredients for ice cream, the energy is absorbed from the ingredients and from the outside environment (like your hands, if you are holding the baggie of ice!). When you add salt to the ice, it lowers the freezing point of the ice, so even more energy has to be absorbed from the environment in order for the ice to melt. |
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This is saying that the point at which ice usually freezes is 32 deg F/0 C. Adding salt to the ice makes the freezing point lower, so the ice needs to absorb more heat to melt. This keeps the ice from melting so quickly, right?
But this:
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| This makes the ice colder than it was before, which is how your ice cream freezes. |
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Does that mean that the ice in the outer bag is colder than the ingredients in the inner bag so that the outer bag is sort of like a "freezer" or "icebox" that the inner bag is in? And moving the bags helps it all freeze evenly instead of having it freeze the outer layer only? Or am I missing something?
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| Ideally, you would make your ice cream using 'ice cream salt', which is just salt sold as large crystals instead of the small crystals you see in table salt. The larger crystals take more time to dissolve in the water around the ice, which allows for even cooling of the ice cream. |
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Why? This is just my personal interest here, but what does the size of the salt have to do with the evenness of the cooling? Does this have to do with the salt dissolving into the 2 seperate elements because that's where I really got lost.
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| You could use other types of salt instead of sodium chloride, but you couldn't substitute sugar for the salt because (a) sugar doesn't dissolve well in cold water and (b) sugar doesn't dissolve into multiple particles, like an ionic material such as salt. Compounds that break into two pieces upon dissolving, like NaCl breaks into Na+ and Cl-, are better at lowering the freezing point than substances that don't separate into particles because the added particles disrupt the ability of the water to form crystalline ice. |
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I was good until "crystalline ice." What is it, and why do I want that ability of water disrupted? Also, could you remind me of another kind of salt?
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| The more particles there are, the greater the disruption and the greater the impact on particle-dependent properties (colligative properties) like freezing point depresssion, boiling point elevation, and osmotic pressure. The salt causes the ice to absorb more energy from the environment (becoming colder), so although it lowers the point at which water will re-freeze into ice, you can't add salt to very cold ice and expect it to freeze your ice cream or de-ice a snowy sidewalk (water has to be present!). |
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Pretty much totally lost here.
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| This is why NaCl isn't used to de-ice sidewalks in areas that are very cold. |
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This is an interesting fact. If I could undertand the colligative properties thing, I could have a very interesting conversation with Bill about this. If I can come up with another kind of salt, I can have a very interesting conversation with my kids about this (and save the rest for later ).
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Rachel
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Maryan
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Here are my answers:
1st quote question - That's what I understand!
2nd quote question - I think so -- but they lost me a bit too.
3rd quote question - ??
4th quote question - Well... all I know is when my ice cream melts and refreezes it loses that "fluffy" aspect and gets yucky and sorta flat. So I'm guessing that "crystalline" qualities must refer to that aspect of ice cream iykwim.
5th & 6th quote questions - Hmm... brain is not functioning at full tilt. Having lived in very cold areas, I knew there was a limit to salt's melting potential -- which is why sand is used more often than salt up north. But...the rest of all that chemistry gave me an ice cream headache. 
Disclaimer: I have no chemistry background... you were just speaking about my favorite pasttime, food, passion, etc.
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Maryan
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Mary G
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| Posted: May 29 2008 at 3:36pm | IP Logged
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We just did this today and it was so fun -- we made both vanilla and chocolate! We thought it'd be a great end to our "school year" and my dh is a PhD in biology and done lots of biochem so that helped ... altho the kids could care less ... they just wanted the ice cream!
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MaryG
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Mary G
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| Posted: May 29 2008 at 8:19pm | IP Logged
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Oh, we tasted the ice cream tonight -- vanilla and chocolate -- and boy, it was the best ice cream! It doesn't make much but that's probably a good thing !
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MaryG
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Rachel May
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Another question Maryan and Mary G's husband:
Could you use Icemelt to make ice cream? Or sand? Would it be more effective? Inedible?
I thought of another salt--magnesium sulfate, also known as Epsom Salts. Should we try making the ice cream with both kinds and see which works better? I have 11 kids in mind to make this and about 11 questions about salt to test....
Maryan, I think I get it now about the crystalline ice. I was thinking of the ice in the outer bag, but we are actually trying to prevent the crystalline stuff from forming in the ice cream, right?
Off to look for ready made ice cream to get a headache too. 
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Rachel
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Maryan
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Rachel May wrote:
| Another question Maryan and Mary G's husband: |
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This made me laugh. Not only because all my ice cream chemistry observations (based on my eating experiences) were off.. but because...
I've seen Mary G's husband's credentials in all the John Paul the Great school's literature... and I SO don't have them!
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Maryan
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Mary G
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Rachel May wrote:
Could you use Icemelt to make ice cream? Or sand? Would it be more effective? Inedible?
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Per dh: icemelt would work but sand would be inefficient and sand would not go into solution (or dissolve).
Rachel May wrote:
I thought of another salt--magnesium sulfate, also known as Epsom Salts. Should we try making the ice cream with both kinds and see which works better? I have 11 kids in mind to make this and about 11 questions about salt to test....
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Epsom Salt would work but it's not something you want around food. It's actually an emetic.
Basically, any kind of salt in solution with water cause a freezing point depression; what you want to be careful of though is that you want to eat the final product and some salts are not compatible with the human system!
Isn't my dh smart?
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lapazfarm
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re the crystalline ice.They are talking about the ice in the outer bag.
Reworded in English: The dissolved salt lowers the freezing point, so the water re-freezes into ice crystals at a colder temperature.
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Theresa
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Mary G wrote:
| Per dh: icemelt would work but sand would be inefficient and sand would not go into solution (or dissolve). |
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I've always wondered why sand is used on ice. Does it actually make the ice melt or does it just provide some traction? Or is it just that at some temps salts don't work at all so inefficient sand is the best option? (I'm from So Cal so I grew up "going to the snow"
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Epsom Salt would work but it's not something you want around food. It's actually an emetic. |
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! OK, won't try that.
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Isn't my dh smart?
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YES!
(And so is Maryan about other aspects of ice cream )
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Rachel
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Rachel May
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lapazfarm wrote:
re the crystalline ice.They are talking about the ice in the outer bag.
Reworded in English: The dissolved salt lowers the freezing point, so the water re-freezes into ice crystals at a colder temperature. |
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Thank you!
Quote:
| Compounds that break into two pieces upon dissolving... are better at lowering the freezing point than substances that don't separate into particles because the added particles disrupt the ability of the water to form crystalline ice. |
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This sounds to me like it's saying the salt is keeping the ice from reforming. So we do or do not want the ice to reform in the colder crystalline form? Why can't you use just plain ice?
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Rachel
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lapazfarm
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| Posted: May 30 2008 at 1:10pm | IP Logged
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It just means that the water has to absorb more energy in order to melt, because it is colder (lower freezing point). So, it absorbs energy from your ice cream, making it freeze.
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lapazfarm
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I found an easier to understand explanation here:scitoys
For ice to melt, it has to get heat from something. In our ice cream project, it gets the heat from the ice cream mixture (and from your hands, which is why they get cold while holding the bag). When the ice is melting, it is at 0 degrees Celsius (32 degrees Fahrenheit).
When ice is melting, the surface of the ice is wet. At the surface, there is solid ice on one side, and liquid water on the other. The surface is exactly at the freezing point. This means that some water molecules are leaving the ice and moving into the water, but it also means that some liquid water is refreezing onto the ice. We say that the system is in equilibrium when the rate of melting is equal to the rate of freezing, and this happens at 0 degrees Celsius.
At equilibrium, the heat lost by the water as it freezes is equal to the heat gained by the ice as it melts.
Because plain ice can only barely cool something to the freezing point of water, we will need to do something to make it much colder than that, since our ice cream mixture freezes at a lower temperature than water.
The ice cream freezes because the salt and the ice mix to make a substance with a lower freezing point than ice alone. This means that the ice and salt mixture must get even more heat from somewhere in order to melt.
Salty water freezes at a lower temperature than plain water. But the ice is made of plain water, so it melts at 0 degrees Celsius. Since the ice keeps melting, but the water no longer freezes (because there is only salt water, which doesn't freeze at 0 degrees), the temperature goes down.
The heat gained by the ice as it melts is no longer offset by the heat given up by freezing water (since the water is no longer freezing back onto the ice). The heat gain has to come from somewhere else. It comes from the ice cream and your hands.
The sodium and chlorine in the salt split apart into charged ions, and these ions attract water molecules to form weak chemical bonds.
The resulting compound has a freezing point of -21.1 degrees Celsius (-5.98 degrees Fahrenheit). This is 21.1 degrees colder than ice (37.98 degrees Fahrenheit colder than ice).
When people put salt on the ice on a sidewalk or a road, the ice mixes with the salt, and the mixture of the two solids (ice and salt) produces a liquid, but the sidewalk actually gets colder than it was before.
If we add a different chemical to the ice, such as calcium chloride, we can get an even lower temperature (-29 degrees Celsius, or -20 degrees Fahrenheit).
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JodieLyn
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sand is traction.. besides.. salt isn't that good for the environment to be spreading it on roads constantly all winter long.. sending it into the soils and water supply come spring.
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We tried this yesterday and it turned out really well!
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Rachel May
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Theresa, I was saying to myself today that I wish you were the About.com chem expert, except then you wouldn't have time to explain chemistry and other things too to me here so I'm very grateful you're not. This explanation not only was very clear but also answered all my questions, I think.
This:
lapazfarm wrote:
| Since the ice keeps melting, but the water no longer freezes (because there is only salt water, which doesn't freeze at 0 degrees), the temperature goes down. |
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I'm going to have to meditate on a bit. The solution gets colder because the freezing point is lowered?
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| If we add a different chemical to the ice, such as calcium chloride, we can get an even lower temperature (-29 degrees Celsius, or -20 degrees Fahrenheit). |
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I looked up calcium chloride out of curiosity, and it is something like icemelt!
This whole ice cream thing has been really interesting, and I haven't had a bite yet. Thank you all for entertaining my questions and giving such great answers. I can't wait to teach the 4th grade and below version. I'm printing this thread to go with the original recipe/experiment for my science notebook.
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Rachel
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4Real Forums : Exploring God's Creation in Nature and Science
Topic: Ice cream in a baggie
