rock candy

[02josh10]

my son, a 6th grader, is trying to grow rock candy x3 to see how temperature effects crystal growth. He put one outside (on a porch close to the house), one at room temp. and one in the refrigerator. The problem we are having is that we keep ending up with a glassful of solid " rock". So far we have made 2 attempts. Thought we may have boiled the solution too long on the first attempt, ( I am helping with the stove), but the second was even worse. Any ideas on what we are doing wrong? His project is due Jan. 23, 2014. We would really appreciate any help you can offer before we make another attempt. Thank you and Merry Christmas.

[tdaly]

Hello,

I'm sorry to hear that you are having trouble with this project. Did you preheat the jars before adding the sugar solution? The amount of sugar that can be dissolved in water depends on temperature. At lower temperatures, less sugar will dissolve in water. If the hot sugar solution is saturated when you add it to the jars (i.e., you can't dissolve any more sugar in it) and the jars are colder than the solution, then I would expect sugar to quickly crystallize on the walls of the jar, instead of the string. (This assumes you have a string dangling in the middle of the sugar solution.) Once a sugar crystal has nucleated, it is usually energetically favorable to grow an existing than nucleate a new one. So, if you get crystals on the walls of the jar, then the crystals will grow inward, filling the jar with crystals, instead of growing outward from the string.

If you did heat the jars, let me know and I'll think about this some more. It's also possible that the abrupt cooling when you put the jars in the refrigerator is triggering crystallization. Did you notice whether the refrigerator jar had more crystals on its walls than the room temperature jar?

[02josh10]

Thank you for replying to our post. Don't remember if we heated the jars for our first 2 attempts. On our next 2 attempts we did, but not sure that was the problem. We did a little more research and decided to increase the amount of water we used -- 3 cups of water to 6 cups of sugar. There are crystals forming on the jar that is at room temp. The one outside and the one in the refrigerator have no crystals. Just a syrupy liquid covering the skewer. Thanks again for your help.

[tdaly]

Hi 02Josh10,

I'm sorry to hear you're still having problems! Given that it's Jan. 15th and the project is due Jan. 23rd, you might not have time to grow another batch of crystals. It can take ~10 days to grow rock candy, even when all the conditions are "just right". That being said, your project is not a failure if you do not have crystals growing on the strings. You still did multiple trials of your experiment, and you made observations/collected data on each one. The result was not what you expected, but that is OK. Scientists (myself included) often get results that don't match our expectations. The next step is to try to figure out why the experiment gave different results than expected. That usually involves looking more closely at the fundamental principles underlying the processes at work in the experiment. So, if you can explain the science behind growing rock candy crystals (e.g., why a supersaturated solution is required, why crystals grow, etc.), you should be in good shape. It sounds like you followed all the steps of the scientific method.

If you want to try growing another round of crystals, take a look at the advice/suggestions that Donna Hardy, a fellow Expert, gave to other students/parents. Instead of repeating her suggestions/questions, I'll point you to the topic (link below). Have you tried the things Donna suggests? In particular, did you seed the string?

https://www.sciencebuddies.org/science- ... rock+candy