cracking parafin issue

Discussion in 'Supporting Chemistry' started by Helen Jones, Mar 13, 2018.

  1. We got this practical back yesterday, which we are doing on a semi-microscale basis. The parafin is cracked over porcelain fragments in a test tube. On cleaning up, we noticed that a number of the rubber bungs holding the delivery tubes were damaged. The damage appears to be dissolving, not heat damage, as the bungs were still tacky / runny at the end that fits into the test tube some hours later, especially the ones where the glass tube does not project much beyond the end of the bung. I suspect the reaction products are condensing as they hit the cooler glass of the delivery tube and dissolving the bungs?

    I'm not keen to waste money replacing bungs every few times this practical runs, so is there an alternative? Would cork bungs work, or be too porous to safely contain the gaseous cracking products, for example? (possibly a daft question, but our chemistry-specialist technician is off at present).
  2. OK, our supplier has got back to us and concurs that it may well be a problem with organic condensates eating away at the bungs. The moral seems to be to make sure that the delivery tube projects far enough into the test tube that the gaseous organic products go into the tube before much of it reaches the bung. (Miffed, as the bungs were supplied with the tubes pre-inserted, so that shouldn't have been an issue, but a useful lesson learned). Second lesson seems to be 'don't use corks, may be too porous and will gradually get impregnated with the organics, making them a flammables risk'.
    Emma Pringle likes this.
  3. Have you had a go at the CLEAPSS microscale version?
    No bungs to melt! :)
  4. We always do this as a teacher demo so haven't had any issue with it yet....
  5. Although I do see the benefit of microscale quite often this does fail me. Fiddly set up for the kids. Ten spirit burners balanced on bricks. Hmm. I think we will stick to boiling tubes (pre prepared) clamped simply to a stand with a delivery tube attached. Sometimes its just not worth saving a few quid.
    007 likes this.
  6. I agree, that particular microscale prac looks fiddly for kids and uninspiring. I think they would forget the theory as soon as do it!
  7. I don't know why they have in the video it balanced on bricks! I wouldn't fancy that either!
    It can be a bit fiddly but if you've got a good group they quite enjoy it. Some groups we just go for the usual boiling tube and delivery tube set up.
  8. My thoughts on this were to use Silicone or Neoprene bungs, not rubber..Cracking and fract-dist kill them, very rapidly.Expensive, but it might pay back due to reduced need to replace.
  9. You can do this at "ground level". The film is several years old and you can improve. (Which is why I do not write a book.) Have the delivery tube at an angle. It goes into small test tube inside a beaker on the slope. My only issue is with air conditioning affecting the flame. I am very old, over 3 score and an 10 and can fill it. You need a little patience and that is no bad thing to teach. Once one pair get it to work the others will follow. You may fail the first time as with a lot of things in life, so you practice. This does not take long to do. The method was devised because certain LEAs wanted to ban it many years ago because of suckback explosions, even with teacher demos.

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