With regards to a variation of this post that was covered some time ago…
I got a little egg on my face this past weekend when my buddy claimed that it was common knowledge that boiled tap water somehow changed it’s molecules so that when it was brought back down to room temperature and placed side by side with tap water of room temperature into a freezing environment, the previously boiled water would freeze first.
We set a timer and checked our cups of equal volumes every 15 minutes. At around the 45 minute mark I was shocked that while both cups were in the process of freezing over, the previously boiled water had more substantial and thicker ice build up than the non boiled water. We let the test continue and every time the cups were examined, the previously boiled water was showing signs of freezing faster.
What’s the deal??? While I don’t believe his assumption that the molecules of the previously boiled water have changed, I’m more inclined to believe that the boiled water released dissolved gases thus allowing it to freeze quicker. But that is just speculation on my part and I have no science background that would back that statement up.
Any help?
I know this has come up before but I haven’t seen it discussed after an actual experiment like yours that isolated this effect from evaporation, etc. Here’s the Usenet FAQ on the subject (most straightforwardly written version that caught my eye). In particular:
“Dissolved Gasses: Water always contains dissolved gases such as oxygen and carbon dioxide. These impurities have the effect of lowering its freezing point. When water is heated, much of these gases are driven out because their solubility in water is less at higher temperatures. When the hot water cools it then has less dissolved gas than water which was not heated so it has a higher freezing point and freezes first. This is often cited as a an important factor but no references to experimental evidence or quantitative analysis have been given.”
So you at least get points for the mechanism, though “dissolved gasses = lower freezing point” isn’t that dissimilar from the phrase “changed its molecules” I suppose! If you had a thermometer in each water glass when the water started to freeze, you’d see if yours had a lower freezing point and could quantify this effect (e.g. the amount of gasses driven out) some more.
How do people not know about the Mpemba effect? I know this question has come up recently.
This is not the same thing. In the OP’s case, previously-boiled water was compared to not previously-boiled, but both started the experiment at the same temperature. The article you point to specifically states that the hot versus cold were both previously boiled to remove that effect. So here we have two independent effects isolated by two different experimental designs that both contribute to the “hot water freezes faster than cold”.
Thanks for this thread, my kid was asking about this just yesterday and now I can giver her (dramatic pause)… the Straight Dope.
Thanks for the reply Squidfood. While I’m a little disappointed that the science in my head doesn’t relate well to the real world, I’m glad there is a reasonable explanation for this. Guess I’ll officially admit defeat and do a little research before challenging my buddy again 