Soap Film Demonstrations

in chemistry one of the most important topics we cover is intermolecular forces what holds molecules together and what consequences that have well I think one of the most spectacular consequences is soap films and that is that hydrogen bonding the attraction between water molecules causes soap films that is what soapy Waters mostly make of it's got water in the middle and soap on the edges like this like a sandwich causes soap Helms to be as small as possible so if I take a given frame this is pretty simple right here and dip it into this is just some soapy water Dawn is what I like to use about about 5 to 10% solution um the film that forms on this is a nice flat one because that's wh That's What covers this frame with the least amount of material possible okay flat frame if I take something quite a bit more elaborate than that and say what would cover this frame with the least amount of surface area the least amount of material that becomes a much trickier problem you could actually solve that mathematically using high-speed computers it would take probably several weeks to solve entering each of these points of this weird shape into a threedimensional matrix whatever and solving that problem it would be able to give you the answer but soap films do it in an instant that is the solution to how to cover that particular frame with the least amount of material so soap films because of hydrogen bonding want to be as small as possible here's an interesting Arrangement it's a helix with a rod down the middle and a string connected Loosely to the Helix and to the rod going to dip this in there see if we can't get a good shot of that okay you get a nice little looks like one of those spiral uh driveways or whatever you have at a parking garage I'm going to pop part of it but this part's still down here so watch what happens as I zip the film up oh it B popped let me reestablish that zip it all the way up to the top and then zip it back down again the soap film always covering with the least amount of material there kind of remarkable that that tiny little film there that little sandwich of water molecules between layers of soapy molecules can be extended out to such a fine film as that okay here's a nice one this seems simple enough and this by the way was made from a soda bottle I cut out the middle section and then I trimmed this part down just a little bit so it fits inside the lower one I'll scoop up a little bit of soapy water here and and now my question is this what do you think the soap film will look like I'll set this down on the [Applause] table as I lift this straight up well the shortest distance between two points is a straight line so it make sense that as I lift it up every point to every Point down here would be a straight line it would just form a cylinder well that might make sense at first but soap films always give you the right answer let me do that again again it turns out that the best solution is one that pinches off in the middle that hourglass shape whoop went too far with it that hourglass shape is what uh is the best way to cover it okay now if I add this stipulation I'm going to add a little pressure to it to the center here I can and then cap it so that air is trapped inside there I can generate a nice cylindrical soap film it turns out that if I try to stretch it too much it tends to form like a bowling pin Arrangement there and eventually pinches off this might seem kind of new but it's actually was all written up every one of these so far was written up in a book written by CV boy over 120 years ago he did all these demonstrations he wasn't using a cut up soda bottle obviously but this one's been nicknamed the hoola bubble because it's got this nice little hoola dance going on there but uh if I let the air out it automatically pinches off recognize this that's right good old USA with 1 two 3 four five six seven eight major cities can you name them all hm well we got New York we got Atlanta and a couple others anyway um question if you had to connect these eight cities with pipelines or communication lines or roads using the least amount of linear distance how to do it well this will solve it because I made a sandwich between these and made those little rods between them so this is a way of solving that problem right away H to jiggle it a bit there there we go that gives do the best solution for how to connect those cities using the least amount of total roadway or communication lines and if you want to leave out one of the Cities say forget this one just pop that one and now it adjusts the answer to give you the best new answer okay so soap films can be used for answering some questions that pertain to again minimal surface area have a few more fun ones here this one to get a little straw here that's what I went okay brand new this by the way is the spiral that I took out of a spiral notebook the plastic spiral that came out just unthreaded so it's um kind of find a dip that in the soapy water lift it out slowly here it's got a soap film covering it like a little cylinder I got a little thing at the bottom I'm going to pop to get rid of it I didn't want to do that that is kind of neat though you saw the soap film pop I'll throw that again that wasn't what I'm going after here in fact I'll just do that right now so you can watch the soap film pop okay you don't get a chance to see a soap film as it pops but what I want to do is pop the little film that's across the bottom that forms for this demonstration let me see if this little there okay and now Now hook it up here I'm going to take and blow the tiniest little bubble here the bottom okay and now I'm going to pop the film below it and you're watching that little bubble race up to the top as the soap film again gets as small as possible did that show up well okay so kind of remarkable one there um this one is kind of temperamental but hey I'm feeling lucky and we saw this one before my little flat soap film and oh this one popped here let me do that again it's the bubble trampoline there we go in CV boy's book he talks about the fact that these soap films aren't even really coming in contact with each other there's electrostatic charges that or keeping them from ever even touching um if they do they usually tend to fuse together and that happens sometimes with that but uh this last one I've got oh there it is is going to involve a balloon and I usually have a volunteer come up that I rub their hair with this but uh I found this little wool patch on this glove Works probably just as well so wouldn't want to give anyone a bad hair day anyway so I'm going to charge up the balloon see if I can't get the bubble to levitate just to the right spot there wh so that's a nice little illustration by the way of Milan's oil drop experiment in which He adjusted electric field between two plates and caused oil droplets to just hover in place and from that was able to deduce the charge of a single electron the reason that was working by the way was because water is quite attracted by a static charge like that you can deflect a stream of water by charge balloon or a comb or something like that and it's much more impressive I think with a bubble though anyway those were all some wonderful demonstrations that show intermolecular forces through soap films

Channel: FlinnScientific

Views: 48,735

Keywords: Chemical Bonding, Flinn, Flinn Scientific, High School, chemistry, demonstration, demo, chem demo, science, experiment, science experiment, lab, laboratory, class, classroom, chemical, cool science experiment, awesome science experiment, flin, flin scientific, flin science, flinn science, chemistry experiement, fun science, fun experiement, how to, safety, lab safety, soap film, soap film demonstrations

Id: 52wVrtA5krY

Length: 10min 7sec (607 seconds)

Published: Wed Dec 19 2012