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Topic: Instant sugar crystals experiment?  (Read 1497 times)

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Offline Cynthia Moore

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Instant sugar crystals experiment?
« on: September 15, 2022, 04:03:08 PM »
This is an odd request. I hope it's appropriate. If there is a better place to post this, please let me know.

My grandson, age 7, is fascinated with crystals. He has a couple of crystal growing kits.

Unless my memory is foggier than I thought, I recall an experiment way back in high school chemistry. The instructor created a super-saturated beaker of sugar water by heating the water, dissolving a lot more sugar than the water would take at room temp. He then suspended a weighted string into the beaker and set it aside to cool. At the end of class, he tapped the side of the beaker and a fairly large sugar crystal formed almost instantly.

I wanted to try this for my grandson. I searched for instructions, but in all the ones I found, the crystals take anywhere from several hours to several days to form.

Is it possible to do the experiment I think I remember? If so, any hints as to how?

Thanks

Offline Borek

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Re: Instant sugar crystals experiment?
« Reply #1 on: September 15, 2022, 04:59:01 PM »
I suppose when you say "sugar" you mean sucrose (one of many sugars). Could be that's not what the instructor used, there are plenty of other substances that produce similarly looking crystals.

If you can't find the recipe you can always try to experiment a bit - try several amounts of sugar dissolved in the same amount of water and see if any combination produces expected result., Actually doing it with your grandson can be a great lesson for him. Trying it alone first so that you know what to expect would be even better ;)

According to wikipedia at 25°C 100 mL of water dissolves around 200 g of sucrose, at 100°C it can dissolve twice that amount. I would try adding 250g and 300g to 100 mL samples of water (or smaller ones, like 125 and 150 g per 50 mL, just keep the ratio) and see if it works as you remember. Beware: such a syrup has a much higher boiling point that just a pure water and is a bit sticky, can easily burn skin.
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Offline Cynthia Moore

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Re: Instant sugar crystals experiment?
« Reply #2 on: September 15, 2022, 05:09:06 PM »
I remember that he used regular table sugar.

That was more or less my plan. I thought I'd start with cold water and dissolve as much sugar as I can. Then slowly increase the temperature so he can see that hot water can "hold" more sugar. One website said to slowly add the sugar at or near the boiling point, stirring and adding sugar until just a few grains remain. Then add a bit of water to get it all dissolved.

As remember it, the teacher said that the sides of the glass need to be super clean. If the suspoended sugar finds anything to grab, it will come out of solution. Once cool, the tapping on the glass jars it out of solution.

He'll be home from school in an hour. Then we'll give it a try.

Thanks

Offline jeffmoonchop

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Re: Instant sugar crystals experiment?
« Reply #3 on: September 15, 2022, 05:09:55 PM »
I did my PhD in crystal growth of sugars and organic acids. The problem you have is that yes you can make a highly concentrated sugar solution, but if its too concentrated the solution will become very viscous, so even if you add something to initiate crystallization like a string, the molecules can't move quickly enough to form crystals at the size you are envisioning. So for the most rapid crystallization you need to have the solution be concentrated enough without it being too viscous. Heating reduces the viscosity, so its possible in your class the crystals formed as it was cooling.

What may have happened in that experiment you remember is less that the crystal formed instantly, but that it formed as the class was progressing. The tap on the glass would have done little to form it. And even then its lucky it did at all. Sugars are quite difficult to crystallize predictably (wide metastable band widths, probability driven nucleation within the metastable zone, which is why you add a string to get it going. Look those terms up if you're interested).

Tapping on the glass of a supersaturated solution sometimes initiates a crash, but rapid precipitation results in small crystals, not large. The instructions online tell you it takes days, because you have to be very careful when growing a large crystal, that you don't crash out the concentrated solution and form small crystals around or on the surface of the large crystal.

You can test the tapping experiment by putting a bottle of water in the freezer for a while, then carefully taking it out and tapping on the side. If the water is supercooled sufficiently, it will rapidly freeze. That might be an interesting one to try with your grandson.

Offline Cynthia Moore

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Re: Instant sugar crystals experiment?
« Reply #4 on: September 15, 2022, 05:16:11 PM »
Wow, that's very helpful. Thank you. I'll read your reply to him. He loves stuff like that so we can have a little fun looking up the terms.

Is there a different type of sugar that would work better?

Are there any other crystal experiments we can do?

Thanks for this very helpful reply.

Offline Corribus

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Re: Instant sugar crystals experiment?
« Reply #5 on: September 15, 2022, 07:45:11 PM »
Google "homemade rock candy". You should find a lot of DIY information.
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Offline Borek

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Re: Instant sugar crystals experiment?
« Reply #6 on: September 16, 2022, 03:13:08 AM »
You can test the tapping experiment by putting a bottle of water in the freezer for a while, then carefully taking it out and tapping on the side. If the water is supercooled sufficiently, it will rapidly freeze. That might be an interesting one to try with your grandson.

Word of warning: freezing water increases its volume, so basically that's a grenade waiting to explode. Don't put too much water in the bottle (in general about 3/4 is perfectly safe, unless the bottle is shaped fancy), don't use glass bottle, don't close it tightly.
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