How does brewing tea work physically?
September 15, 2021 5:37 PM   Subscribe

So I pour hot (not boiling, in my case) water on my tea bag. During this process the water is really agitated so lots of water is soaking the tea bag and then mixing with other water. But then I leave the tea bag floating in a peaceful cup of water for another couple minutes. Nothing moving that I'm aware of. Shouldn't there be a strong layer of tea on the top of my cup, with the rest of the cup being less strong?
posted by lbergstr to Food & Drink (12 answers total) 5 users marked this as a favorite
Best answer: Water, especially hot water, circulates rapidly in a cup. It disperses and blends constantly even without agitation.

But also, tea flavor dissolves into water. So it will spread around quite a bit.
posted by bbqturtle at 5:40 PM on September 15, 2021 [3 favorites]

Best answer: Behold the magic of diffusion: "the net movement of anything (for example, atoms, ions, molecules, energy) from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in concentration."
posted by mandolin conspiracy at 5:45 PM on September 15, 2021 [14 favorites]

Response by poster: No such thing as a dumb question, right guys?

... guys?

(thanks for the answers)
posted by lbergstr at 5:59 PM on September 15, 2021 [13 favorites]

Several factors making the tea uniform:
* The water's unlikely to be entirely still. There's Brownian motion in even the most motionless of warm liquids, but currents can survive a long time. The agitation from pouring the water and putting the teabag in means there are vortices and flows which even if you don't see them are still moving all the water around.

* Density of liquid. The teabag, as you mentioned, is floating. As the water around it becomes tea, that water will get heavier (from the dissolved solids), and then fall, to be replaced by water-which-is-not-yet-tea, which is now in direct contact with the teabag. Also, this effect, in the unlikely event your liquid lacked internal currents, will set some up, as denser liquid moves to the bottom and displaces lighter liquid.

* Diffusion. As the water around the teabag becomes extremely tea-ful, the random motion of those dissolved molecular components of tea tends to move them from areas of higher concentration to lower concentration (that's basically a thermodynamic principle; entropy in action, essentially), so there's a high probability of random molecular movement turning strong tea in the midst of water into weak tea throughout.

Which is not to say concentration can't happen. I frequently cold-brew hibiscus in a large clear jug, using a tea ball heavy enough to sink. If my water's unusually still, then the combination of being in the fridge (slowing diffusion) and the tea ball being at the bottom (meaning I don't have density-driven circulation) sometimes means that going to look in on it the next day there is a very conspicuous color gradient from a paler pink at the top to a ruby red at the bottom. That gradient vanishes the moment I give the tea ball's chain a good yank to produce an upswell in the liquid.
posted by jackbishop at 6:00 PM on September 15, 2021 [4 favorites]

Incidentally, the technical term for bbqturtle's explanation is "convection". Convection currents are driven by pressure differences, caused by changes in density as the tea dissolves or thermal gradients as the tea starts to cool. Both convection and diffusion are likely to be important factors in helping the tea to mix without stirring.
posted by biogeo at 7:13 PM on September 15, 2021 [2 favorites]

Hot water looks like nothing is moving but it is not peaceful. All the molecules in a cup of hot water are banging around quickly and bashing into each other and moving quite a bit. This banging around is called Brownian motion, and there's a LOT more of it in hot water than cold water. Here is a video that shows the effect of all the banging around when you put something (in their case, dye but in your case, tea) in it.
The increased brownian motion of hot water also explains why sugar and salt dissolve faster in hot water than in cold - the water molecules are way more active in hot water and ram into the sugar/salt crystals to break them up.
Also helping: convection, currents, diffusion, changes in density, etc, all mentioned above.
posted by Vatnesine at 7:51 PM on September 15, 2021 [6 favorites]

You can do an experiment to see this! If you pour the hot water into your cup, wait for it to settle and then carefully add in your tea bag, you can see quite clearly how the tea disperses throughout the cup, at least at first. This will include the density effect, as jackbishop notes. A cup made of clear glass will make this easier to see.
posted by ssg at 9:11 PM on September 15, 2021 [2 favorites]

Do what ssg says with two glasses, one containing just-boiled water and one containing cold tap water. Compare and contrast. Ask yourself why the hot steep seems so much faster at getting a visible trail to emerge from the teabag than the cold steep does.

What is heat?
posted by flabdablet at 12:25 AM on September 16, 2021 [1 favorite]

Here's a YouTube video of Brownian Motion in hot water vs cold water. Not super scientific but the guy drops some ink into a glass of cold water vs a glass of hot water and the difference is pretty obvious to the eye.
posted by tiamat at 4:56 AM on September 16, 2021 [1 favorite]

You can observe this action by putting a drop of ink into a cup of water and watching it disperse -- very rapidly -- even in cold water. You could run the experiment by putting drops of ink into glasses having different temperatures of water and timing how long it takes for the ink to be completely distributed through the water of each glass.

Edit -- just what tiamat says. That's what I get for getting distracted and then clicking "comment" without refreshing the thread.
posted by gauche at 5:20 AM on September 16, 2021 [1 favorite]

Certain flavored teas with the teabag on the bottom of the cup you can see the now hot flavored water sit on the bottom of the cup and then needs stirring. So not everything diffuses throughout a hot cup of tea. A
posted by TheAdamist at 5:49 AM on September 16, 2021

It can be even simpler. There's hot water in the cup, the water on the top is evaporating and getting cooler. That makes it denser than the hot water left below it so the cooler top sinks and the warmer from down below rises. Brewing tea in orbit on the space station would be quite a bit different in many ways but one is that there's not the gravity driven hot/cold density convection happening.
posted by zengargoyle at 11:27 AM on September 16, 2021

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