# What's the difference between frozen at -20C and -70C? **URL:** \ **Category:** Factual Questions\ **Created:** [March 18, 2005, 4:44am UTC](https://boards.straightdope.com/t/whats-the-difference-between-frozen-at-20c-and-70c/295116 "2005-03-18T04:44:10Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![Nunavut\_Boy](https://avatars.discourse-cdn.com/v4/letter/n/e95f7d/32.png) [@Nunavut\_Boy](https://boards.straightdope.com/u/Nunavut_Boy)\ **Post date:** [March 18, 2005, 4:44am UTC](https://boards.straightdope.com/t/whats-the-difference-between-frozen-at-20c-and-70c/295116/1 "2005-03-18T04:44:10Z")
At work we have two different freezers: one for regular stuff and one for stuff that needs to be at -70C (human plasma, lyophilized bacteria). The question I have is what changes between -20 and -70? I mean, frozen solid is frozen solid, right? ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [March 18, 2005, 4:50am UTC](https://boards.straightdope.com/t/whats-the-difference-between-frozen-at-20c-and-70c/295116/2 "2005-03-18T04:50:13Z")
> [@Nunavut Boy](#): > > At work we have two different freezers: one for regular stuff and one for stuff that needs to be at -70C (human plasma, lyophilized bacteria). The question I have is what changes between -20 and -70? I mean, frozen solid is frozen solid, right? Freezing doesn’t _stop_ chemical processes - like proteins degrading - it only slows them down. The colder it is, the slower the chemical reactions take place. My WAG is that certain things at your work must be kept at -70 in order to be kept from degrading within a reasonable time frame. ---
**Author:** ![Polycarp](https://avatars.discourse-cdn.com/v4/letter/p/82dd89/32.png) [@Polycarp](https://boards.straightdope.com/u/Polycarp)\ **Post date:** [March 18, 2005, 4:53am UTC](https://boards.straightdope.com/t/whats-the-difference-between-frozen-at-20c-and-70c/295116/3 "2005-03-18T04:53:16Z")
I do **not** know the answer here; a medical professional or biologist is going to have to give the Straight Dope. But I’ll hazard the guess that breakdown processes will occur, slowly, at -20 and be all but stopped at -70. There are certain foods that will spoil at a 0 F (~-18 C) freezer if left there, frozen, too long. So biological and histological specimens are probably in much the same state. ---
**Author:** ![Sage\_Rat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sage_rat/32/399_2.png) [@Sage\_Rat](https://boards.straightdope.com/u/Sage_Rat)\ **Post date:** [March 18, 2005, 6:24am UTC](https://boards.straightdope.com/t/whats-the-difference-between-frozen-at-20c-and-70c/295116/4 "2005-03-18T06:24:46Z")
Just to add a bit of argument to blow away the idea that “sub-zero = frozen” I would note that 0 degress celcius is the point at which _water_ freezes. If you want to freeze mercury, I believe you need to get things a bit colder. So even beyond the biological realm, the difference between two temperatures can make a lot of difference dependent on the affected materials. Water-centrism is a human invention and should not be viewed as having any particular magic which makes the laws of water have to apply to everything else. ---
**Author:** ![Mathochist](https://avatars.discourse-cdn.com/v4/letter/m/c89c15/32.png) [@Mathochist](https://boards.straightdope.com/u/Mathochist)\ **Post date:** [March 18, 2005, 7:27am UTC](https://boards.straightdope.com/t/whats-the-difference-between-frozen-at-20c-and-70c/295116/5 "2005-03-18T07:27:34Z")
> [@Sage Rat](#): > > Just to add a bit of argument to blow away the idea that “sub-zero = frozen” I would note that 0 degress celcius is the point at which _water_ freezes. If you want to freeze mercury, I believe you need to get things a bit colder. So even beyond the biological realm, the difference between two temperatures can make a lot of difference dependent on the affected materials. Water-centrism is a human invention and should not be viewed as having any particular magic which makes the laws of water have to apply to everything else. Even for water, there are differences in the solid state depending on temperature and pressure, as indicated in [this phase diagram](http://www.lsbu.ac.uk/water/phase.html). Admittedly, this really matters most at significantly higher pressures than I’d expect occur in the OP’s freezers, but still… ---
**Author:** ![Smeghead](https://avatars.discourse-cdn.com/v4/letter/s/f1d935/32.png) [@Smeghead](https://boards.straightdope.com/u/Smeghead)\ **Post date:** [March 18, 2005, 8:43am UTC](https://boards.straightdope.com/t/whats-the-difference-between-frozen-at-20c-and-70c/295116/6 "2005-03-18T08:43:32Z")
Nucleic acids are much more stable at -70 than they are at -20. We have the same setup in our lab. ---
**Author:** ![Harmonix](https://avatars.discourse-cdn.com/v4/letter/h/3ec8ea/32.png) [@Harmonix](https://boards.straightdope.com/u/Harmonix)\ **Post date:** [March 18, 2005, 8:47am UTC](https://boards.straightdope.com/t/whats-the-difference-between-frozen-at-20c-and-70c/295116/7 "2005-03-18T08:47:56Z")
Well, as a rule of thumb reactions occur 2x faster for every 10 degrees up in temperature. I’m not sure if this rule holds even reasonably true for sub-zero zero conditions but 1/2^5 = 32x slower then a -20 freezer.