Science type question for my first grade son
 

My son had his first science fair this week. His project was on bread mold ( the classic wet vs dry.. which molds first).

At the actual fair, his teacher asked him to check out the class experiment, which was does adding food coloring to water affect the time it takes to freeze the water. Their conclusion was that it made no difference.

Here's the fun part: Mrs Wynkoop asked Ethan if he could explain why the food coloring seemed to congregate in the center of the cup. It sort of seemed to have a darker color towards the middle of the cup, as if there was more food coloring there.

I pride myself on being sort of a figure it out kinda guy, so I thought for a few minutes and came up with an idea that perhaps the food coloring had a lower freezing point tha the water, and some of the FC would take longer to freeze. Since the cup would freeze from the sides (I guessed they would anyway, as the sides would stay cold and help freeze from out to in), I reckon that the denser food coloring froze last.

I was wondering if anyone agrees/disagrees/laughs and points accusatory finger.

I did try to Google a few phrases, but they were all parsed out and were of no help.

Help me Tilted Knowledge, you are my only hope.

Your reasoning makes sense to me. My father used to own an ice company. Part of it made huge 300 lb blocks of ice. Essentially the floor was a huge ice tray with removable copper molds held in a grid. The molds were surrounded by salt water that was chilled to well below freezing. The water in each mold would freeze from the outside in. Before each block was completely frozen, the unfrozen water in the center would be suctioned out as it contained all the impurities, now concentrated. That water would be replaced with fresh water and allowed to completely freeze.

Not a scientific answer but a practical experience that backs up your theory.

I suppose it depends on the composition of the food coloring. As an example The color additive FD&C Red No. 40 is principally the disodium salt of 6-hydroxy-5-[(2-methoxy-5-methyl-4-sulfophenyl)azo]-2-naphthalenesulfonic acid.

Electronic Code of Federal Regulations:

If I only knew the freezing point of this substance I might have an answer. I will hazard a guess and say that in a scientific experiment under strict laboratory conditions you would see a difference in the freezing point; however it is probably a very small difference and wouldnt have been detectable by your son.

Assuming I understand the situation, it may be similar to red sky at night. It would look lighter around the edges because the back light has less total food coloring to filter through. Where you are looking through a chord of the glass at 5 deg arc the light is only having to pass through very little food coloring. When passing through the chord closer to 180 deg it has to travel through the full diameter of coloring.

Hmmmmmm, on further thought if the question is "does food coloring freeze at a different rate than water" then I think I was on the right track before.

If the question is "Why does the food coloring congregate at the center of the cup" then other variables are in play. How the FC was actually introduced into the cup (if a dropper was used, it could be have been dropped into the center or along the sides) plus the composition, density and temperature of the FC.

I may not fully understand the situation either. Bleh

Perception is key here, light refraction and density of water. The food coloring "freezing" has nothing to do with it. Less light reaches the middle of the colored water because of the additional FC molecules dispersed throughout the H2O molecules so it appears darker and as though the color has "condensed" here, it's just refraction and density. imo.

You're probably right, surprisingly I seldom over-think a problem. :) Insomnia and illness may be mitigating factors. :paranoid:

The easy test of this is to to look at it from the top.

Another test would be to freeze the cup then cut the ice into the inside and outside pieces. Put all the inside pieces in one glass and the outside in another and let them melt. Then compare the colors.

However, cutting the ice might be difficult. You would need some sort of hot wire or something.

I asked my SO about this because he is a chemical engineering student who is almost finished with his degree. He said without knowing what the food coloring consists of, he can only speculate as to why, but given what he knows about, well, all sorts of stuff pertaining to this, he says that it's likely the food coloring doesn't freeze as quickly as the water, and so as the liquid freezes, it freezes from the outside in (as many of you noted), pushing the food coloring to the middle. So for those of you who thought this might be why, good work.

He was actually really excited about this question. :) I'm pretty sure he still wants to bust out his engineering pad and do some calculations.

A bread knife is good for cutting ice.

Do we have a definitive answer yet?

If the composition of the food coloring is a factor I wouldnt feel so dumb.<---(self-deprecating humor) :)

Well, for my SO to give a definitive answer, he'd have to know what kind of food coloring was used. Additionally, he doesn't really have the time right now to do this kind of recreational science, unfortunately. He's done with his engineering degree in three weeks and the senior project expo is on Friday of this week. Suffice it to say, I won't be seeing much of him until June 12th (and we live together).