Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/5/84; site alberta.UUCP Path: utzoo!watmath!clyde!cbosgd!ihnp4!alberta!ken From: ken@alberta.UUCP (Ken Hruday) Newsgroups: net.cooks Subject: Re: freezing hot water Message-ID: <504@alberta.UUCP> Date: Tue, 21-May-85 17:58:09 EDT Article-I.D.: alberta.504 Posted: Tue May 21 17:58:09 1985 Date-Received: Thu, 23-May-85 01:20:05 EDT References: <188@sdcarl.UUCP> <334@we53.UUCP> Reply-To: ken@alberta.UUCP (Ken Hruday) Organization: U. of Alberta, Edmonton, AB Lines: 64 In article <334@we53.UUCP> bmt@we53.UUCP ( B. M. Thomas ) writes: >I know I shouldn't, but... > >If you start with, say, 140 degree water, and want to go to 32 degrees, >then how are you going to get there without going through every lower >temperature in between? And if it takes a certain time to get from >140 degrees to that lower temperature(say, 45 degrees), how is it going to >take less time to go to 45 degrees and then to 32 degrees than to start at >45 degrees? A similar situation exists with the boiling issue. It is very >true that the initial heat gain or loss is much faster with the greater >difference in temperature, but once you've gotten to the other starting >point, you've got exactly the same distance to go that you had from there. >My mathematical skills are not what they should be, but I don't believe that >you can prove that X plus Y is ever LESS than X, given that X and Y are >both positive. Nor can you show that the distance from one point to another >changes as a result of whether you start from there or are just passing through. > >I used to run a car wash. Now the folk around there were not the great erudite >sort that we are communicating with here, but when they came up with that line, >and I explained it to them, they eventually listened. Is it possible that there >are some things so obvious that the more intelligent just can't see them? > >brian OK. After reading the umpteenth million article on this topic I guess I should get my two cents worth in. I don't claim to know whether hot water freezes faster than cold but I have heard (many years ago) that hot water does freeze faster. The reason that was given (to the best of my recollection) is that when cold water is set to freezing, it first forms a crust of ice st the top of the tray. This is persumably because the ice at the top can release its heat into the atmosphere more readily. The consequence of this is that this crust becomes an impediment to the radiation of further heat from the tray. Hot water, on the other hand, tends to form a steeper thermal gradient in the tray with the water at the top being warmer than that at the bottom (much warmer than in the case of cold water freezing) - thus the formation of ice is different ie. it may be possible that ice forms from the bottom up (don't quote me on this). Thus heat from the tray can radiate more readily hence more quickly. This probably makes no difference in the case of metal trays but may make a difference in a tray with insulative value - ie. plastic. In any case, I don't think that the answer to the question is as obvious as some claim since you need to consider the effects of radiation and convection, and conduction - a simple comparison of temperatures is inadequate in any reasonable analysis of the problem. Sorry for being the umpteenth million + 1 person to clutter this newsgroup with unrelated trivia. P.S. While I'm at it, someone mentioned that they belived hot water contains less minerals than cold because they've seen mineral buildup inside a hot water tank. This buildup is most likely due to evaporation, and, hence, concentration of the minerals to the point of their precipitation on the walls of the tank. Ken Hruday University of Alberta