Filter Paper and Semipermeable Membrane
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,
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.
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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Update: June 2012