Floating Tea Bags
Sometimes more is less.

Concepts
density, filtration, nucleation

Materials
2 standard tea bags (Bigelow Mint Medley works well), 2 600 mL beakers or two clear tall cups, water, stapler, stirring rod or spoon
Procedure
1. Carefully remove the string from both tea bags. Set one tea bag aside.
2. Gently remove the staple from the other tea bag. Above a trash can unfold the tea bag and remove the tea from the bag.
3. Fold the empty tea bag back up and staple it closed.
4. Fill both beakers (or tall cups) almost to the top with water; leave a little room so the water does not spill out.
5. Put the each tea bag (the one with tea and the one without) in a separate beaker of water. Observe.
6. With the stirring rod, push the tea bags so they are totally submerged under the water. Observe what happens.

Results and Discussion
Answer the following questions:
  • Compare the tea bags when they were initially placed in the water.
  • Compare the tea bags after they were submerged in the water.
  • How does the density of each tea bag compare to the water.
  • Make an inference to explain each tea bags behavior in the water.
Explanation
A normal tea bag consists of a filtration bag with ground up leaves inside of it. The bag is porous and typically made from paper, silk or plastic. When making a cup of tea, water flows into the bag, dissolves soluble matter from the tea leaves, andthen flows out of the bag with the dissolved components.

Density can be defined as the amount of matter packed into a given amount of space. Mathematically, it can be calculated by dividing the mass by the volume. Water is often represented as having a density of 1.0 g/mL. Anything put in water that is denser than it will sink (greater than 1). Anything put in water that is less dense than it will float (less than 1).

An interesting phenomenon is discovered when a full tea bag and an empty tea bag are placed in water. The full tea bag will float while the empty tea bag sinks. Since mass was removed from the tea bag one would expect it’s density to lower and for it to continue to float (or float even better). The difference between the tea bags has to do with the normal tea bag’s ability to inflate itself, significantly increasing its volume and hence lowering its density enough to float. The normal tea bag does this through a process called nucleation – air in the water naturally attaches itself to the ground up leaves in the tea. Once a few bubbles form on a leaf more and more air in the water attaches itself to the bubble and the air bubble becomes large enough that it actually stuck inside the bag. The tea bag that is empty does not have any particles for the air in the water to nucleate on and hence no air bubbles form inside the bag. Additionally, the density of the bag’s material is slightly denser than water and hence the empty bag will eventually sink to the bottom.

figure43.jpg
Figure 43: The empty tea bag (left) sinks in water. The normal tea bag (right) floats in water.

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