Diffusion is the process by which molecules spread from areas of high concentration to areas of low concentration. When you place a tea bag in hot water, the process of diffusion is clearly illustrated.
Initially, the tea leaves inside the bag are concentrated with flavor compounds, tannins, and color. The hot water surrounding the tea bag has a much lower concentration of these substances. As time passes, the heat of the water helps break down the walls of the tea leaves, allowing the molecules to escape into the water.
As the tea bag sits in the water, the flavorful molecules diffuse out of the bag and into the water, gradually increasing the concentration of flavor in the surrounding liquid. The stirring or movement of the water can further help in this process by mixing the tea with the water, which allows the molecules to spread more evenly.
Eventually, if left long enough, the diffusion will reach an equilibrium state where the concentration of the flavor molecules is equal throughout the water. This is why you notice that the longer you leave the tea bag in, the stronger and more flavorful the tea becomes – you are simply allowing more and more of those molecules to diffuse into the water.
In summary, diffusion is key to making tea, as it allows the flavors and colors from the tea leaves to mix with the hot water, creating the delicious beverage we enjoy.