Melting Points of Water and Drinks
Most recent answer: 01/23/2011
- Audrey (age 8)
Kingwood, Texas, U.S.A.
Audrey- That's a great question. It sounds like you've done the experiment carefully. You've given us a very clear description of the results, and you've put your finger on the most puzzling part of the results.
That's the good news. The bad news is that we're just about as puzzled as you.
(but see below for possible idea)
As you say, it makes sense that the cranberry juice melted first. There's a lot of sugar in it so that lowers the melting point. Even though the sugar is not really part of the ice, it's sitting in little pockets throughout the ice, helping melt it. The same applies to the milk, which I guess doesn't have quite as many molecules dissolved in it.
What would make filtered water melt before tap water? Maybe as the water is filtered it gets more air dissolved in it. If some of that air is trapped in the ice as it freezes, that could help promote melting. That's just a guess, because I can't think of any better reasons.
Distilled water also can have gas dissolved in it. CO2 is a particular suspect, because it partially ionizes in water, making a weak acid. That could help keep it trapped as the ice forms. If you have some way of measuring the pH of the waters, you might check to see if the tap water, filtered water, and distilled water are different. (Off-topic: As we increase CO2 in the atmosphere it acidifies the oceans, killing off corals and other animals with shells.)
Mike W.
Aha- I forgot one of our old answers! () Under some circumstances dissolved salt etc. can actually slow the melting. It still lowers the melting point, but it leaves the ice floating in cold melted water on top of warmer saltier water. I'm not sure why the melting point effect is more important in some cases and this other effect is (possibly) more important in others. You've got the interesting finding that lots of solute does speed melting but (apparently) small amounts slow it down.
(published on 01/23/2011)
Follow-Up #1: Why does filtered water melt faster?
- Audrey (age 8)
Kingwood, Texas, U.S.A.
Have you tried letting each of the types of water sit for a long time at room temperature before freezing them? Maybe some other dissolved gases are involved, and if they sit long enough those will reach equilibrium. Just scrambling here.
Mike W.
(published on 01/30/2011)
Follow-Up #2: melting times of different types of water: results
- Audrey (age 8)
Kingwood, TX USA
Mike W.
(published on 02/14/2011)