Cold Tea

Written by: Tony Harris, February 5, 2016



Latent Heat of Fusion must be one of the most boring topics in physics. They throw in words like ‘enthalpy’ and discuss ‘total energy’ and ‘pressure-volume systems’ and wonder why the class has fallen asleep.

I was fortunate. I had a magnificent physics teacher. He would make everything exciting and simple. We walked into class and he had an ice cube that was being held in the air by two clamps on his desk. There was a piece of steel wire with weights on each end. He placed the wire over the top of the ice cube and let the weights dangle so the wire began to cut into the ice.

He then asked us all what we thought would happen. Some people said that the ice would melt and fall off the clamps, I, like others, said that the wire would cut through the ice splitting it into two pieces.

We watched – spell bound. Not one of us had predicted what actually happened. The wire did indeed cut through the ice and fall onto his desk, but the ice cube had fused back together and was one piece of ice! How the hell did that happen? We certainly weren’t going to let our teacher out of the class room until he explained the magic behind his trick.

The simple question is: can you have water at zero degrees centigrade or is it ice? If you have some ice at zero degrees centigrade and heat it up, does it turn into water at zero degrees? If it is still zero degrees, where did the heat you put in go?

Well you can have ice at zero degrees and you can have water at zero degrees and you have to put energy into ice to turn it to water and vice versa. So the wire melts the ice as it cuts through. But it needs energy to do this, so it takes the energy from above it as it moves down through the ice cube. Because it takes energy away from the water above it, the cube re-freezes. So it’s not magic after all

You can have water at 100 degrees and steam at 100 degrees. You have to put energy into water at 100 C to turn it into steam. Are you still awake? This is called latent heat of vaporization. If you put your hand to your mouth and blow on it, your hand doesn’t really feel cold. Now lick your hand with your tongue, still not really cold.  Now blow on your hand where you licked it – cold huh? That is due to the latent heat of vaporization.

When you blow on your hand you feel the temperature of your breath, when you lick your hand you feel the temperature of your tongue. But when you blow on your wet hand, you cause the water to evaporate and it takes from your hand the heat required to vaporize – or the latent heat of vaporization.

So if you have a nice hot cup of tea and you leave it out in the open, the air will move across the cup and vaporize the liquid, taking from the tea that is left in the cup, the heat required – the latent heat of vaporization.

And so you have a cold cup of tea.

“Ah” you may say, but maybe the waiter just gave me cold water in the first place. The problem with that argument is that you can’t make tea with cold water. The making of tea actually involves a chemical process called infusion. In order for the herb, in this case tea, to release its active ingredients it must be allowed to stand in boiling or, at very least, hot water.

Oh I’m sorry – I think I may have poured cold water on your argument!

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