Have any of you ever sucked on lollipops of the type: Please Register or Log in to view the hidden image! You my have noticed that as you suck, the lollipop kid of melts in your mouth or more secifically, it crumbles into a wet powder. Have you also noticed that while you are sucking, it feels very cold on your tongue? What is the reason for this? At first thought, it seems that the breaking of the sherbet bonds in the lollipop requires energy and this energy is taken from the tongue where you can feel it as a kind of endothermic cooling. Any thoughts?
If the candy is a decent conductor of heat, even at room temperature it can feel cold, because your mouth is much hotter than room temperature, and the sensation of coldness only depends on how quickly heat is being removed from your mouth, not the actual temperature of whatever's inside- same reason why a metal surface feels colder than a wood surface even at the same temperature. Could also be a hallucinatory feeling kind of like how a breath mint gives that cool numbing sensation.
It's not this as it doesn't happen with other candies or lollipops which should be better conductors - for example hard glazed candy. Compared to these, the compressed sherbet powder seems to be quite the insulator Please Register or Log in to view the hidden image! Not this either as it is not cold at the start when you just put it in your mouth and it is not cold when it is just a wet mush of sherbet. This is why I have the theory that it extracts heat from your tongue only when it is dissolving leading to the cooling effect.
I am in year 10 and for science as part of revision we did a lesson on chemical reactions one of the sample reactions was the reaction between citric acid and bicarb of soda with added water (you may have done this with vinegar and bicarb). The lesson was actually about chemical compounds but the teacher remarked that this type of reaction with a base and an acid cools rather than heats. I am eating one at the moment and reading the post realized that simply holding it in my mouth (which would cool sufficiently if it was transferring the heat like metal) gave sufficiently less cool than crunching it and therefor adding saliva that mixed the two (base and acid) and produced the cooling sensation. To make homemade sherbert you add citric acid and bicarb of soda to icing sugar then when you suck it from the spoon you get a slight fizz. I know the pictured lollies as sherbert lollipops. So my theory is the cooling sensation is caused by a chemical reaction between a base and an acid that react when saliva devolves and combines them.
I just run down to my local grocery store and buy some frozen sherbet whenever I want to eat some.Please Register or Log in to view the hidden image!
Yes, I also suspect the cold sensation may be due to an endothermic process occurring. I have an idea dissolving glucose is markedly endothermic - more so than sucrose, though both are endothermic I think. I'm not sure about the enthalpy change when sherbet reacts (i.e. when tartaric or citric acid reacts with sodium bicarbonate). That may be endothermic as well.
Whether or not sugar dissolving is endothermic (I think it is), I'd be very surprised if it was enough so that you could feel it on your tongue. And I'm pretty sure an acid neutralizing a base is exothermic, so that wouldn't explain it. What everyone seems to be forgetting, though, is that melting is VERY endothermic. I'd guess the cold sensation is from the melting sherbert.
Citric acid reacting with sodium bicarbonate appears to be significantly endothermic, acc. attached: http://www.dsisd.txed.net/DocumentCenter/View/12986 As for "melting" sherbet, nothing melts in this case - if it did, your mouth would need to be above the melting point of sugar - about 180C! The "melting" everyone (loosely) talks about is dissolving. It's a lesson to us all, though, how complicated such an everyday phenomenon can be!
Thanks for the link! That surprises me - the canonical example of sodium bicarb neutralizing HCl is exothermic, so I always assumed bases neutralizing acids were generically exothermic. Guess I was wrong. Please Register or Log in to view the hidden image! But melting is also important. Sherbet is a frozen desert; in addition to the sugars and fruit flavors in it, it contains a lot of water and dairy, which are frozen to give it the creamy-ish texture that makes it so good. The human mouth is definitely above the melting point of water, so it's not just dissolution that's happening as you eat sherbet. It would be interesting to actually calculate how much ice melting and acid neutralizing contribute to the endothermicity (is that a word?) of sherbet consumption.
Actually, I don't see why there need be any general rule about acid/base reactions being exothermic. The wild card in all these things is the entropy change, isn't it (dG =dH-TdS)? In this case we have not only an acid/base neutralisation but the decomposition of the HCO₃⁻ ion to CO₂ and water, and the formation of aqueous citrate ions from a trivalent carboxylic acid. Not sure what this should lead us to expect, but quite a complicated change. On your reference to a frozen desert, we seem to be talking at slightly cross purposes. The OP refers to a type of lollipop, well-known in the UK at least where I live, which is a dry product, kept and eaten at room temperature. It is not frozen.
Oh, that does change things! In the states, sherbet is basically like ice cream with less dairy, so I was thinking of frozen treats as soon as I read the title of the thread. If these candies are room temperature, you're right that the acid/base reaction must explain nearly all of the cold feeling.
