A month or so ago, a friend of mine was told that if she poured table salt on an ice cube that she’d placed on the back of her hand, it would result in a fascinating sensation. When she tried it, she burned or froze (or something) a pretty large area of skin–about one square inch of affected, it looked burned clean off, although the area was not raised, and it took a long time to heal. The skin was reddish pink, and eventually became infected before finally fading into a faint scar that wasn’t too noticible.
I know that salt changes the freezing point of liquids, but I don’t know what caused this burning/freezing reaction on the back of my friend’s hand. Could anybody tell me what caused this? Thanks.
I’m not exactly sure why (I think it might be something to do with latent heat), but adding salt to ice (and thus melting it) causes it to absorb more heat than otherwise from its surroundings - old fashioned ice cream making machines employed exactly this method.
Perhaps because the salt created liquid water from the ice that was colder that normal liquid water (less than 32 degrees), this water was more likely to cause frostbite.
My WAG is that the skin became stuck to the ice, I once had a weekend job at a supermarket whilst at uni. Someone had left a metal trolley in one of thoise huge walk-in freezers and I dared one of my co-workers to lick it. He did. The result was his tongue became stuck to the trolley and when he managed to get it off you could see that he left some of the tongues surface behind too.
I learned the same thing the hard fall, falling victim to the bet of “who can squeeze an ice cube and some salt in your fist the longest?” with a bunch of guys. Yeah, good times!!
The above answers are all on target, adding salt to ice allows it to refreeze quickly at superlow temperatures - in effect a faster sucking of more and more heat (a more effecient heat sink, I think) out of your hand and creating a lower temperature than is possible with just ice, so frostbite happens.
Well the salt itself won’t change the temperature of the mixture, it will only change the melting/freezing point. As you can see in the winter (or even in your kitchen, although it’s harder to see), sprinkling salt on ice causes it to melt. The reason for this is that it depresses the freezing point. Unless you somehow cool down the area around your hand and the ice cube, nothing is getting supercooled…
The effect of salt on the freezing/thawing temperature of water is not relevant to this discussion. Your friend placed a chunk of ice on the back of her hand, at which time the temperature of the ice was X deg. After sprinkling some table salt on that ice, its temperature where it touched her skin was… um, precisely X deg. As for the cooling of her skin by the ice, the effect over the same time would have been exactly the same if no one had even mentioned the word “salt.”
There she sat, with a chunk of salted ice on the back of her hand – for how long? Was this ice cube soaked perhaps in a concentration of one of our more pleasant legal beverages? (Alcohol also lowers the freezing temperature of water, BTW… Of course, it also lengthens the span of time someone might spend sitting with an ice cube on the back of her hand.)
My guess is that either (1) your friend suffered some mild skin damage due to localized hypothermia, or (2) she reacted badly to the concentrated salt/water mixture that no doubt soaked the back of her hand while she was sacrificing herself in the name of science.
This is what I was referring to, sorry I changed the wording slightly. The water/ice isn’t changing temperature, if anything it is getting warmer from the body heat. Also, if the melting point is lower, it takes less heat to melt it, so it would actually be “sucking” less heat from your hand.
It does, and anyone who is saying it doesn’t is flat wrong. This is precisely the mechanism in an ice cream maker, wehre salt and ice are used to lower the temperature of the mixture enough to freeze it. Typically the temperature gets down to around -7[sup]o[/sup] C or thereabouts.
oops. Guess I should have paid more attention in pchem…
But is that going to be enough to cause frostbite in a short period of time? I’m not quite masochistic enough to try it myself
AH… I just realized what my problem in thinking was, I wasn’t considering the ice/water gradient… so let me see if I got this right… the heat from the water goes to melt the ice, which dissolves the salt. The salt lowers the melting/freezing point of the salt water, so it shifts the equilibrium towards making more water (because the water can’t refreeze). The water is getting colder… am I right?
Thanks for the input, guys. And to answer TBone2’s questions…stupidity, absolutely. We ranked on her for weeks. Secondly, no, she says the concoction only contained salt and ice…(and later on frozen bits of skin).
I’m not sure of the exact length of time she actually sat there freezing off the skin on the back of her hand, but I’d imagine not more than a two or three minutes. The short increment of time was the reason why I had doubts that frostbite would set in so quickly…but I guess I stand corrected, unless it was more of a “stick your tongue to the telephone pole and peel it off” sort of thing, which it really didn’t look like. Thanks again.
When I did the bet I wussed out after less than a minute when I sensed something screwy was going on, the “winner” squeezed for only about 4 minutes and had a pretty good burn.
