The Physics of Tea

01Aug,2024

The Physics of Tea

BY : Rhiannon Nevinczenko

Different teas typically have different instructions for preparation. But what difference does a couple minutes of steeping make? And why do they always tell you to use freshly boiled water?

(Well, not always.)

There are a range of methods in tea preparation. The reasons behind them usually boil down to the physics of how tea is made. Steeping tea relies on the physical principle of diffusion. Diffusion is the net movement of particles from an area of high concentration to an area of low concentration. This relies on Brownian motion. Brownian motion is the random movement of particles in fluid or gas. So, the soluble compounds in tea leaves dissolve in water, ultimately diffusing throughout the fluid.

But why does the water have to be hot? Technically, it doesn't. You can cold brew tea, but it takes significantly longer (i.e., 8 hours) than if you were to use hot water (a few minutes). Heat, or thermal energy, is also kinetic energy (atoms vibrating and colliding in a substance). Therefore, increasing heat increases the rate of diffusion.

So, you've got your tea steeping in some hot water, with the bag floating at the top. Why is the tea not darker at the top, closer to the bag, the source of diffusion?

This is because of convection currents. Convection currents are driven by pressure differences. Tea is a solution, full of polyphenols, and therefore denser than pure water. This pressure difference causes the water and tea in your cup to swirl around. The tea circulates rapidly through the cup, dispersing and blending without the need for agitation (stirring or dunking). (Though dunking a tea bag does provoke the tea to steep faster!) Basically, as the tea bag floats and steeps, the denser tea falls, replaced at the top by water-that-isn't-tea-yet. This happens continuously through the steeping process. Basically, physics stirs your tea for you!

Photograph credit: Mareefe via Pexels.

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