What powder-like substance when poured on water will keep your hands dry?

[curiousguy19]

What powder-like substance when poured on water will keep your hands dry? The explanation behind thtis was that as you placed your hand into the water, your hand came out dry because the powder did not let you break the water's "skin". I remember seeing this experiment done on Mr. Wizard's World. If Im not mistaked it was a yellowish color.

[theprof]

It's called Lycopodium powder and you're right - it is yellowish in colour.

I'm occasionally teach chemistry and biology in a couple of local schools now and again and I've shown this phenomenon to teenagers. Invariably, they are gobsmacked!

[theprof]

Oops that should say "I occasionally" not "I'm occasionally". Now you know why I'm not teaching English!

In simple terms, lycopodium powder floats on water because water has a high surface tension. Lycopodium powder is a hydrophobic or water-hating substance because there is no attraction between the lycopodium molecules and the water molecules. Due to this hydrophobic property, the lycopodium molecules adhere to each other and form what is effectively, a water-resistant glove around the hand, preventing the hand getting wet.

Incidentally, lycopodium powder is very dangerous in it's dust form when dissipated in air due to the high surface area of the powder and the presence of oxygen. Combustion can occur very rapidly, which may lead to an explosion.

[Toleman]

Explosion? Send some my way...

you said it floats on water because the high surface tension. Would this change if you added to washing up liquid?

Chris

[theprof]

If you mean adding washing up liquid or other detergent to it, yes it does change Chris.

If you drop a single drop of washing up liquid on to the lycopodium layer, it falls through the powder until it reaches the interface between the water and the powder. The droplet immediately lowers the surface tension of the water causing the water to effectively expand which in turn results in the lycopodium powder being pushed away towards the sides of the container. If it's done carefully with the right size droplet, a very neat hole will remain in place for quite a little time.

We use a fair amount of lycopodium powder in biochemical applications at university but it has to be treated with respect. For example, if you chuck a pinch towards a candle flame it will explode with a extremely hot vivid flash that dissipates very quickly. However, when plenty of heat and oxygen are in the vicinity or it's mechanically fed into a flame, it readily forms a fireball.

So I'm afraid Chris, I can't send any your way!

[chakka35]

You can buy the powder in a good conjuring or stage-effects shop.
Conjurors produce those flashes of flame by waving their hands in the air, in the process scattering lycopodium powder over a (sometimes hidden) flame.
I have a box of it here which I bought at the Dolmar theatre shop in Covent Garden and have often amused children with it.
The powder actually consists of the spores of club moss.

[theprof]

Lycopodium powder can indeed be bought via magicians shops etc but you do need to be careful when using it. There's a knack to knowing where to stand and how much to scatter towards a flame if you want to walk away with your eyebrows intact.

Some years ago, I saw a commercially available box of the powder that had been bought in a trick shop. Although the box stated the need for caution when scattering the stuff in a flame, no mention was made of the danger of Lycopodium powder to those that had respiratory problems such as asthma. Because the powder is so fine and the fact that spores can cause allergic and/or anaphylactic reactions, people with respiratory problems should keep well away from the stuff.

[chakka35]

Gosh,the prof, I wonder how many of those devices and substances I used to buy as a lad in The Wizard's Den in Liverpool would now be banned on health & safety grounds in this nanny state.
As for the nitric and sulphuric acids in my chemistry set....

[theprof]

I know exactly what you mean chakka35.

I think back fondly to those days of playing around with ammonium carbonate, ammonium chloride, nickel sulphate, cobalt chloride and the many other chemicals in the umpteen "Merit" chemistry sets I went through.

I spent almost a morning last week looking for a rare and expensive chemical in the main chemistry stores at my university - I've walked around smaller villages. I knew the chemical was there somewhere even after the stores men couldn't find it on their computerised inventory lists. While I was hunting around, I looked at some containers of the very substances I played with as a boy. The hazard warning labels on some of these containers nowadays is downright frightening to say the least. All the same, I'd be the first to acknowledge that we need to be aware of these things.

I often think of the "stink bombs" I made all those years ago. Nowadays, experiments using hydrogen sulphide gas, the key ingredient, has to be confined to a fume cupboard or area of high automated ventilation due to the toxicity of the gas.

I don't think we were ever "safe" when using these chemistry sets despite the fact that the experiments were all said to have been vetted by a man with letters after his name - at the time we simply knew no different.

The other thing that comes to mind about all this is that years ago, the carcinogenic properties of chemicals was largely unrecognised. Yes, we knew about Benzene and its relations, but not many others. As for teratogenic effects, forget it.

Count yourself lucky that I didn't know you had nitric and sulphuric acids in your chemistry set - I'd have hunted you down to get my hands on those! I had to resort to tipping out the contents of old car batteries!