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Science: Experiments

Little Drips -- A Sip of Science

 

Today's Snack: We're going to be talking about drips, so let's make a dip! Mix one cup of peanut butter with one cup of plain vanilla yogurt. Add cup of unsweetened coconut. Serve with fresh carrots, celery, cucumber slices, or cauliflower tips. This is enough for 12 people. So cut the recipe down (make a smaller quantity of dip by mixing cup peanut butter with C. plain vanilla yogurt and just a tablespoon or two of coconut. Or make the 12-serving dip, and call every dip you know to share it!

 

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Supplies:

 

Scratch paper and pencil

Eyedropper

Plastic straw

Dime, nickel and quarter

Plastic checker or other small disk

Other items that are small, flat and water-resistant

Small see-through glass or cup

Straight pins

Water

 

 

Did you know there is an invisible "girdle" of air that keeps water together?

 

Does it make you "tense" to find out that there is surface tension in water? "Tension" does really have to do with headaches, although often, where there is emotional tension, there will be a headache.

 

No, "tension" means "stretching" or "straining." And that's what's happening on the surface of water - to literally hold it together. You can't see the tension, but you can see what it does to water drops:

 

 

 

You've seen how drops of water will stick together on a car windshield after a rain, for example. That's because there are many, many molecules in a tiny drop of water. And they stick together. Surface tension is a strong force that keeps water from spilling over.

 

Let's test it:

 

How many drops of water do you think you can place on top of a dime with an eyedropper before the water spills over?

 

Write down your guess on a piece of scratch paper.

 

Now, carefully drop some drops. Count them as you go. (If you don't have an eyedropper but are using a straw, dip the straw into the water and close off the top end of the straw with your finger to "suck" the water up into the straw, then release the finger briefly to let individual drops fall, one by one.)

 

Keep counting. When the water finally spills over, compare your guess with how many drops it actually took. Are you surprised? That's surface tension at work.

 

Now try the drip test with the larger coins - penny, nickel and quarter. Be sure to write down your guesses of how many drops each coin will hold BEFORE you try it. Afterwards, review: were your guesses a lot closer this time?

 

Now try the drip test on other round, flat objects that you found, including a checker. How is your guessing going now that you're getting some experience?

 

Last activity: fill a small glass or cup to the brim, JUST before you think it will spill over. You might want to use the eyedropper to fill it to the very, very fullest.

 

Now guess how many straight pins you can drop into the cup before it spills over. Write down your guess.

 

Now, gently drop pins in, one at a time. As you go, look at the cup from the side. What do you see?

 

Why doesn't it overflow?

 

How did your estimate compare to the result?

 

That's the principle of surface tension at work - just one more little "drip" of science knowledge filling your cup of academic excellence!

 

By Susan Darst Williams www.AfterSchoolTreats.com Experiments 2010

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