Why do tea bags float in cold water?

Factual Questions
1

I’ve noticed that when I make tea:

If I put the tea bag into cold water, (with the intention of popping the cup in the microwave), the tea bag does not easily absorb water, so floats on top. But,

If I put the tea bag into already heated water, it quickly absorbs the water and begins to steep.

Why?

2

Cold water is more dense than hot water.

3

I don’t think that the minute difference in density matters much at all. I found here a value for the thermal expansion coefficient for water: 1.53 * 10[sup]-4[/sup] K[sup]-1[/sup].

Assuming that you heat the water 80K, that still only means an expansion of 1.2%, which ought to be negligible.

I believe that there’s a mechanical side to it:

The tea-bag is made out of some kind of fibre, that probably gets less rigid as it heats up. I believe that the effect of heating up the material to near-boiling will make it a lot more flexible, so that it changes shape and lets out the air bubbles.
On a related note: Don’t put in the tea-bag in cold water! The most common misstake when making tea is when people use water that isn’t hot enough. (It’s tempting to say that the most common misstake is to use tea-bags, but it is possible to make half-decent tea with tea bags, if you’ve only got hot enough water, whereas no tea will taste good unless the water is hot.)
Try it one day!

4

Based entirely on unscientific observations, I would be more inclined to think that it’s the water getting in, rather than the air getting out, that’s the issue. Water absorbed by the tea bag quickly increases its density, so the bag sinks. If the water isn’t absorbed, it floats because the density of dry tea is very low.

Does capillary action work better for hotter liquids?

And it seems like water moves faster through a material that has already been saturated. Why is that? Am I wrong, and is displaced air the key?

As another tea snob, I agree with Popup . . . avoid bags! :slight_smile: But if you have to use them, the best way to make tea with a microwave is to heat the water to boiling in a separate container (like a measuring cup). You might want to consider sticking a coffee stirrer or a toothpick or something in the cup to avoid superheating. Then pour the boiling water into your cup pver the tea bag.

5

Oh, you tea snobs, you’re all alike. :wink: I know how uncouth my tea proclivities are, and I can make a great pot of tea when so inclined. But on occasion I’m just totally lazy.

So, more questions raised than answered. Here’s a synopsis of the hypotheses so far (paraphrased):

  1. Cold water is more dense than hot water.
  2. Flexibility of the bag is affected by heat. The more flexible the bag, the easier water can get in, thus letting air bubbles out.
  3. Similar theory, only that the water gets in and saturates the tea so it sinks, rather than sinking because air is released.

So, Podkayne’s question is in my head too - why is the tea bag more easily saturated when the water is hot?

By the way, I’ve tried a search of the Dope’s database for earlier versions of this question. Since we can’t search on three letter terms (tea, bag), I tried teabag. I don’t recommend it. :eek: