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Name: Thanish
Status: student
Grade: 9-12
Location: N/A

How does the filter paper of a tea bag act as a semi-permeable membrane when the bag is placed in a cup of hot water?

Paper is made up of tiny fibers. The fibers overlap, but do not make a completely solid barrier. Like a sweater, filter paper looks solid from a distance, but if you look up close, paper has little holes too. Objects larger than the holes cannot fit through (like bits of tea). Smaller objects, like water molecules or flavor molecules, move through easily. The fibers themselves are attracted to water too -- when put in contact with water, they readily become wetted (coated with water), which allows water and water-soluble flavors to move through the bag.

Hope this helps,
Burr Zimmerman

Hi Thanish,

Actually, the filter paper of a tea bag does not act as a semi-permeable membrane at all. It is just a simple filter that allows water and everything that is dissolved in it, to pass through easily, but blocks passage of the "big lumps" like tea leaves.

Bob Wilson


By definition, a semi-permeable membrane allows selected substances to pass through the membrane while preventing others from passing through. In this sense, the filter paper is preventing the tea leaves from passing through while allowing water and the tea extracts to pass back and forth through the paper.

Greg (Roberto Gregorius)

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