Thursday, 26 May 2011

Surface tension for first graders

I have three nieces and a younger brother and sometimes it is nice to explain to them what I do when ask about my job.  So one way to explain my job as a scientist (mad scientist I like to say to them) is by doing experimentation.  Here is one experiment that can explain surface tension without any devices.  Unfortunately in Europe we do not have pennies so I had to get one from an American friend.

  •  Penny
  •  Water
  •  Paper towel (or small dish)
  • Medicine dropper (or plastic pipette)
  • salt,
  • dishwashing detergent,
  • clean glass jars (or beakers),
  • measuring spoons. 

1) How many drops of water will fit on a penny before the water runs off? You can calculate the prediction by measuring the area of one drop with a ruler and calculating the surface area of the penny.
2) Place the penny on a piece of paper towel (or on a small dish)
3) Drop water onto the penny using the medicine dropper (or pipette), counting the number of
drops as you go.  Be consistent with the drops because there can be some variation.  Also use the same penny for all the experiments head side up.
4) Keep dropping water and counting the drops until the water finally runs off the penny.
5) Record the number of drops in a table (the last drop that caused the water to run off
should not be included in your total).
6) Perform a second trial (I get them to do about 30 for good statistical averages and to account for variations in the drop size), record the number of drops in the table, and calculate the average
number of drops that the penny held in your trials.
7) Compare your results to your calculated prediction.  Repeat but add a surface tension lowering substance like a drop of dish soap (1 drop of liquid dishwashing detergent in 1 liter of water; do not shake–cap the container and gently tip it back and forth to mix. Or add different concentrations of salt 1 teaspoon in 100 mL of water vary it with 1/8, 1/4, 1/2 a teaspoon or whatever. 

Aux 8) coat the penny with some vegetable oil

Why does this work? 

The cohesive forces of the water will allow it to stay on the surface of the penny.   The roughness of the penny and if there are any surface irregularities or dirt will have an effect.  The one thing that is important here is that the water does not roll off the penny until the penny is completely covered.  The salt (if it is sodium chloride it might depend on the type of salt e.g. Hofmeister series ) will raise the surface tension slightly whereas the detergent will lower surface tension (that's how we clean dishes).  Coating the penny in vegetable oil will repel the water from the surface of the penny as the vegetable oil is hydrophobic.

Postscript: Although amazed the kids still do not have a clue what I do.
Postscript 2:  If you are a first grade teacher teach the kids how to draw a proper raindrop (see other post about this)