# automotive coolant **URL:** \ **Category:** Factual Questions\ **Created:** [November 10, 2001, 2:15pm UTC](https://boards.straightdope.com/t/automotive-coolant/92627 "2001-11-10T14:15:27Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![clifford\_leighton](https://avatars.discourse-cdn.com/v4/letter/c/dfb087/32.png) [@clifford\_leighton](https://boards.straightdope.com/u/clifford_leighton)\ **Post date:** [November 10, 2001, 2:15pm UTC](https://boards.straightdope.com/t/automotive-coolant/92627/1 "2001-11-10T14:15:27Z")
In a vented automotive coolant reserve tank with a 50/50 mix of anti-freeze and water, under normal operating conditions, does the water evaporate from the solution leaving a stronger concentration of anti-freeze? [cliffordleighton@hotmail.com](mailto:cliffordleighton@hotmail.com) ---
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [November 10, 2001, 8:49pm UTC](https://boards.straightdope.com/t/automotive-coolant/92627/2 "2001-11-10T20:49:37Z")
I bet practically not, because the container has lots of water and only a small vent hole. I bet if you filled an old plastic jug with water and put that size vent hole in the lid and left it sitting on a shelf for the several months you’re suposed to go without changing the fluid, you wouldn’t see the water level had changed enough to matter. Besides, maybe the antifreeze itself can evaporate. It has an odor, so some of it must be volatile. The concentration would only change according to the difference in the evaporaiton rates of the ingredients. ---
**Author:** ![clifford\_leighton](https://avatars.discourse-cdn.com/v4/letter/c/dfb087/32.png) [@clifford\_leighton](https://boards.straightdope.com/u/clifford_leighton)\ **Post date:** [November 11, 2001, 3:38pm UTC](https://boards.straightdope.com/t/automotive-coolant/92627/3 "2001-11-11T15:38:01Z")
So in other words you are not sure? I agree with tou but need some verifyable answers. Thanks for the input! Cliff ---
**Author:** ![robby](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/robby/32/11048_2.png) [@robby](https://boards.straightdope.com/u/robby)\ **Post date:** [November 11, 2001, 7:51pm UTC](https://boards.straightdope.com/t/automotive-coolant/92627/4 "2001-11-11T19:51:10Z")
Pure ethylene glycol (1,2-ethanediol), the major constituent of antifreeze, freezes at -12°C (10°F)\* and boils at 197°C (387°F), based on data from this reliable [source](http://webbook.nist.gov/cgi/cbook.cgi?Name=ethylene+glycol&Units=CAL&cTP=on). Because ethylene glycol has a boiling point that is so much higher than that of water, the water can be expected to evaporate more readily. Over time, the mixture should have a higher concentration of ethylene glycol. \*Interestingly, a mixture of 100 g of ethylene glycol and 100 g of water freezes at about -30°C (-22°F), which is of course lower than the freezing point of pure ethylene glycol or pure water. A layman’s explanation of this phenomenon can be found here: [http://www.gi.alaska.edu/ScienceForum/ASF6/680.html](http://www.gi.alaska.edu/ScienceForum/ASF6/680.html) (This is not the case for the boiling point, however. Ethylene glycol/water mixtures boil at a temperature that is above that of pure water, but lower than that of pure ethylene glycol.) ---
**Author:** ![clifford\_leighton](https://avatars.discourse-cdn.com/v4/letter/c/dfb087/32.png) [@clifford\_leighton](https://boards.straightdope.com/u/clifford_leighton)\ **Post date:** [November 12, 2001, 6:59pm UTC](https://boards.straightdope.com/t/automotive-coolant/92627/5 "2001-11-12T18:59:13Z")
Thank you for your reply. This is the best information I have found to date. But I am still puzzled! When a 50/50 solution of ethylene glycol and water freezes the water does not freeze out first, the mix freeze’s as a single enity. So why would the water evaporate out of the same solution? Shouldn’t it remain stable until the boiling point [plus the required latent heat] evaporates the mixture as a whole?? Thank you again; Cliff ---
**Author:** ![robby](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/robby/32/11048_2.png) [@robby](https://boards.straightdope.com/u/robby)\ **Post date:** [November 12, 2001, 8:41pm UTC](https://boards.straightdope.com/t/automotive-coolant/92627/6 "2001-11-12T20:41:10Z")
You are mixing up boiling with evaporation. Also, what is confusing the issue is that our mixture is not in a closed container, so components can leave the system. Any liquid evaporates more and more readily as the temperature is increased. (We are assuming the liquid is in an open container.) It need not be at the boiling point to undergo a phase change to a vapor. The whole situation is very close to that of distillation, but with evaporation instead of boiling.