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It is easy to drink cold drinks, soft drinks with straw but when I tried to drink tea with straw then that was very unpleasant and even for time I lost the sense from my tongue.

My question is why it is difficult to drink hot liquids with straw?(Drinking cold liquids is very pleasant!!!)

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    $\begingroup$ because it is hot? $\endgroup$ – Mohammad Feb 5 '14 at 6:53
  • $\begingroup$ @user689 but it is pleasant when drinking tea in a cup $\endgroup$ – Singh Feb 5 '14 at 6:57
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    $\begingroup$ I'll leave you with the idea that it is pleasant to drink iced tea through a straw. $\endgroup$ – gregsan Feb 5 '14 at 7:47
  • $\begingroup$ What do you mean by "unpleasant" and "difficult"? With "difficult" are you referring to the straw collapsing when drinking a hot beverage? Since this is caused by the affect heat has on the structural strength of the plastic of the straw. $\endgroup$ – fibonatic Feb 5 '14 at 8:16
  • $\begingroup$ @fibonatic Sir, with "difficult" I mean it will hurt you when using straw to drink tea and with "unpleasant" I just mean unpleasant $\endgroup$ – Singh Feb 5 '14 at 9:51
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I think user689's answer is basically correct, so you should regard this as just an extension/clarification of their answer.

If you place a small volume of hot tea in contact with your tongue then the large thermal capacity of your tongue will cool the tea a lot and your tongue will heat up a little. This is essentially what happens when you sip tea. You pull in a certain volume of tea, this tea is spread out over the tongue and stays in contact with it for a second or so. In this time the tea cools and your tongue heats. However the heating of the tongue is relatively small because the heat from the tea is spread over a large area.

If you suck the same volume of tea through a straw, then because the cross sectional area of the straw is small the velocity of the tea in the straw is high (as user689 points out) so the whole volume of the tea rapidly hits, and heats, a small area on the tongue. This causes much greater heating of that small area and scalds the tongue. Exactly what the sensory effects of the scald are I will leave to our medical/biological colleagues.

This also suggests a reason why drinking cool fluids through a straw is pleasant. The same effect means that when drunk through a straw a cold drink causes more intense local cooling than if it was drunk without a straw.

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  • $\begingroup$ Agreed, that was my reasoning too. $\endgroup$ – DumpsterDoofus Feb 5 '14 at 14:37
  • $\begingroup$ Also, evaporative cooling cools the top layer of the liquid quite effectively. Sipping is a process that takes the liquid from the top, whereas with a straw you tend to get the hotter liquid from the center. $\endgroup$ – Martin J.H. May 17 '14 at 12:56
  • $\begingroup$ +1 for the nice reasoning. Additionally, by the same reasoning, if you are standing in 0 deg C weather and drinking water maintained of same temperature, sipping it through a straw would be more unpleasant than ''gulping'' the same volume (because the leaky thermal regulation processes of the human body would try to maintain the temperature inside mouth at a value much higher than 0 deg C, so the hot and cold part of the above answer are reversed). So apparently, there is a subjective biological side to pleasantness as well. :) $\endgroup$ – 299792458 Jul 4 '14 at 11:33
  • $\begingroup$ But I didn't get one thing. Fluid moving at a faster velocity will also spend lesser time with the tongue, which means lesser time available to lose that heat. How do we incorporate that within this answer? $\endgroup$ – 299792458 Jul 4 '14 at 11:34
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This is called Venturi effect:

venturi tube

The Venturi effect is the reduction in fluid pressure that results when a fluid flows through a constricted section of pipe.

Applying Bernoulli equation we get:

$\frac{P}{\rho \space g} + \frac{V^2}{2g} = cst$

and we know that:

$Q = V\space S =cst$

So as S(surface) decreases (using a straw) velocity increases and pressure decreases.

It is unpleasant because the hot tea will flow with a high speed to your mouth, such a rapid change causes this unpleasant feeling. While when you drink with a cup you control the flow of tea to your mouth.

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  • $\begingroup$ This has nothing to do with the question, since this does not explain the difference between drinking a hot and cold beverages. $\endgroup$ – fibonatic Feb 5 '14 at 8:09
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    $\begingroup$ @fibonatic: I think the answer is along the right lines though. Using a straw means you get a fast jet of hot fluid concentrated on one spot in the mouth. That causes much faster heating of that small area than sipping the hot fluid. $\endgroup$ – John Rennie Feb 5 '14 at 8:22
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I'm going to venture a few hypotheses, since we can't 'know' off the bat.

  1. The tea was hotter than usual on your tongue: the evidence for this is that sensation on your tongue was reduced even after drinking the tea. Possible physical reasons:
    1. You removed an important safety device: the temperature sensors in your lips. When you sip, your lips will tell you very quickly that the cup and/or tea is too hot. Without that, you could easily drink it too soon and not realise it is too hot.
    2. You did not slurp: slurping is an effective method of reducing the average temperature of the liquid by bubbling air through it. It also draws from the top layer of the tea.
    3. You drank from the bottom of the cup instead of the top: Thinking about it, this is my top candidate; the top surface of the tea will be much cooler than the bottom, and the straw almost certainly was near the bottom of the cup.
  2. You got more tea by drinking it through a straw: A larger volume of tea would stay hot for longer in your mouth, so instead of immediately cooling when it met your saliva, it could instead heat your saliva and remain hot on contact with your tongue. Possible mechanism:
    1. If you draw liquid up a straw, there will be some overrun when you stop sucking. So it is difficult to drink just a little, as you have to wait for it to arrive before you know to stop sucking. This way, you could easily draw in much more tea than you would by sipping.

In sum, I would hypothesise that some or all of the above occurred, particularly that you drank hotter tea from the bottom of the cup. Putting them together, though, is a recipe for disaster; you accidentally draw in more tea, the tea is hotter, it remains un-cooled by any slurping and you have much less warning from your lips that it is so hot. Result is burnt tongue.

By contrast, you want the coldest, most carbonated part of a cold drink to quench your thirst, so a larger quantity of the drink from the bottom of the cup with no chance to warm up is better, rather than worse.

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  • $\begingroup$ +1 for the observation that when we drink from a cup, it is from the cooler top layer, and the straw is dipped in the hot interior. :) $\endgroup$ – 299792458 Jul 4 '14 at 11:37

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