Science Buddies: "Ask an Expert"

My son is growing Salt crystals for his science fair project. He wanted to see if the temperature will effect the rate of growth of the crystals. He has 2 test samples, one at room temperature and the other in a lower temperature using the refridgerator. The test sample in the refridgerator started growing crystals within 3 hours. The room temperature sample took 8 days before it started growing crystals. Can you explain why they grew faster in the lower temperature? Are the crystals forming at a lower temperature from evaperation, or are they solidifying? If this is the result why does salt melt ice?

Hi beckys160,
It sounds like your son has done a very interesting experiment. For a K-5 experiment, maybe it would be sufficient to explain that crystals tend to accumulate when the concentration of salt in solution exceeds the solubility. In this state of growing crystals, the solution is called "supersaturated". Supersaturation can be achieved by quickly changing the solubility, or by changing the concentration.

One way to change solubility is by changing the temperature. As temperature increases, some salts become more soluble, some become less soluble, and some maintain a relatively constant solubility. Here is a chart illustrating this principal
http://galileo.phys.virginia.edu/educat ... bility.htm
Here is a table of solubility versus temperature for alum, which is commonly used in these sorts of experiments
http://en.wikipedia.org/wiki/Alum#Alum_solubility
Did your son use table salt (sodium chloride) or some other salt, like alum (potassium aluminum sulfate)? Did he dissolve the salt at room temperature or some higher temperature?

One way to change concentration is by evaporating the water (and leaving behind the salt ions in solution). Did you observe any change in the water levels in his containers, especially the room temperature container? Was there any crust around the rim just above the water level?

If you or your son would like learn more about these sorts of processes, there are many fascinating aspects. For example, here is a page that discusses the competing processes of dissolution and precipitation
http://dwb.unl.edu/teacher/nsf/c09/c09l ... p01181.htm
Many people tend to think that dissolution and precipitation are exclusive processes (only one happens at a given time), but, in reality, they both occur simultaneously. The different rates of the processes often give the appearance to the human eye that only one or the other is taking place.

I hope that helps.
Chris

Hi beckys160,

Welcome to the forum!

I am going to assume that the experiment consisted of two solutions of table salt (NaCl) and water, with equal concentrations of salt mixed up in the same way (esp if any heating was involved), in identical containers, etc -- that is, all variables controlled except for whether the solution was kept in the refrigerator or at room temperature. If this is incorrect, let me know.

Can you explain why they grew faster in the lower temperature?

It makes sense that the solution would crystallize more quickly at a lower temperature because the crystalline state is more ordered. When molecules and atoms are at a higher temperature, they have a higher average kinetic energy, i.e. they are moving around more quickly. This makes it hard for them to enter and remain in an ordered, solid state. This is the same reason that a pure liquid such as water will crystallize below its freezing temperature and melt above. The same thing happens with most amorphous (non-crystalline) solids such as butter -- above some temperature, they become liquid. However, with the salt in your experiment, the two states are "crystalline solid" and "in solution" (rather than liquid). When the experiment is done, it might also be interesting to compare the sizes of the crystals obtained at the two different temperatures.

Are the crystals forming at a lower temperature from evaperation, or are they solidifying?

It's difficult to say without being certain of your experimental setup and seeing photos. If the crystals that have formed are under water, then no, they are not being formed by evaporation. They have precipitated out of the solution. If the only crystals you can see are on the inside of the container above the water line, then that sounds like evaporation. (Evaporation should occur more quicker at the higher temperature, other things being equal, but the humidity levels inside and outside of your fridge are almost certainly not the same.)

If this is the result why does salt melt ice?

Think of it this way: in your experiment, the crystals that are forming are made of salt -- there is no water in them. In the case of salt helping ice to melt, the crystals are made of water. So, even though the same two substances are involved (although road salt is often not NaCl), two different processes are occurring. The general idea is that atoms from the salt wiggle their way in between water molecules, making it more difficult for them to stay in their ordered, crystalline structure. Thus, less kinetic energy (a lower temperature) is required to induce melting.

Here are some resources for further reading:

https://www.sciencebuddies.org/science- ... p051.shtml
http://chemistry.about.com/od/growingcr ... ystals.htm
https://www.sciencebuddies.org/science- ... 2&start=15
http://antoine.frostburg.edu/chem/senes ... -ice.shtml
http://www.pa.msu.edu/sci_theatre/ask_st/030492.html
http://www.highlightskids.com/Science/S ... ltMelt.asp
http://en.wikipedia.org/wiki/Road_salt#Road_salt

Amanda