rock candy

[dtjank]

My son (4th) wants to do a twist on the project in which the presence of seed crystals on a string changes the growth rate of rock candy with my assistance. He came up with two projects he needs to choose between. We have discussed the many variables involved and have questions. (1) His first possible project is to see what effect changing the angle of the string would have on the amount of sugar crystals collected. We think it may be better to use skewers and clothespins instead of string since the horizontal string would require two strings tied at each end, but a clothespin could suspend a skewer horizontally or vertically. Problem: would changing the string to a skewer be a variable that would itself effect the growth of crystals? His plan is to use 6 containers that are identical and have enough depth and width to hold identical skewers either horizontally or vertically. Problem: If the containers are wider than they are vertical or vice-versa, would this also create a new variable that could effect the outcome? We can certainly try to find containers that are equal in width and depth... problem: does it matter if the container is plastic or glass? another variable! Finally, he wants to use food coloring to make the crystals fun! Problem: Will food coloring in the water be a variable that effects the outcome? He plans on using two containers (one for horizontal/one for vertical) and since he wants to repeat the experiment three times he thinks he needs 6 identical containers. Is this the right approach? (2) The second experiment idea seems easier. He wants to see if rock candy grows best on a skewer vs. yarn vs. nylon string. Problem: this introduces many variables that may make it harder to compare the items equally. The yarn and nylon would require a weight but the skewer would not. He plans to use 9 containers of the same size so he can repeat the experiment three times. Again, he would like to use food coloring to make it fun so same question about the variable change. So many things to think about and we are running low on time!!! Please let us know how he can plan ahead for these variables and then decide which project to do! Thank you!

[jskanderson]

Hi dtjank,

All of the variables that you listed could possibly affect the result of the experiment if they aren't kept consistent throughout whichever project your son decides to do. For example, if your son would like to include food coloring (without that food coloring being the independent variable in the project), then he should include food coloring in all the jars he's testing.

If the first possible project that your son could do tests "what effect changing the angle of the string would have on the amount of sugar crystals collected," then he should use skewers throughout his entire experiment, not string and skewers. If your son's plan is "to use 6 containers that are identical and have enough depth and width to hold identical skewers either horizontally or vertically" (which is logical in this experiment), but you happen to not have containers that are equal in depth and width, that's fine as long as identical containers are used (i.e. all of them should be wider than they are taller or taller than they are wider). These containers should be identical in material as well (keep them all glass or all plastic)!

Long explanation short, everything should be kept as consistent as possible so that only the independent variable (aka the angle of the skewer) is tested. So an independent and dependent variable (which is probably the rock crystal growth rate) should be decided on, and any other variable in the experiment should be kept consistent.

Hope this helped,

JSK