Thursday, April 21, 2011

How Many Drops of Water Can Fit on a Penny?

Materials
• penny
• dropper
• water in a container (small plastic/paper cup)
• plate/paper towel (optional)

Directions
• Have students observe a penny with a hand lens.
• Have students predict the number of drops of water that can fit on a penny.
• Use an eye dropper to place drops of water one at a time on the penny until the water runs over the edge of the penny.
• Record the data.
• Older children can repeat the experiment a few more times and calculate the average.

If it is their first time doing this experiment, children will probably be amazed at how many drops can fit on a penny. Most students will underestimate the number.  Water molecules are attracted to other water molecules (cohesion). Molecules near the surface of a liquid pull toward each other. The cohesion of water molecules forms a kind of "skin" on the surface.  A drop keeps its shape because of this surface tension.

Older students can brainstorm ways to change this experiment. Perhaps use a nickel or quarter. Try the back of the penny. What other ideas can students come up with?

Another variation of this experiment is to use a cup of water filled to the brim. Have the students predict how many drops of water can fit in the cup without the water spilling over.  Have them use a dropper to add drops of water to the cup. They should count the drops and record their data.

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Let's try another experiment with  cohesion and surface tension.

Materials
• water
• pennies
• cup
• plate/pan/paper towel to catch any overflow

Directions
• Place the cup on a desk or table with a pan/plate underneath to catch any overflowing water.
• Fill the cup with water to the rim.
• Predict how many pennies you can add to the full cup.
• Slowly and carefully drop a penny into the cup.
• Continue adding pennies in the same manner until the water spills/overflows.
• Record the data.

The water level actually rises above the top of the cup due to surface tension. The surface tension forms a kind of "skin" on the top of the water to hold the water together. Water drops are more elastic than we think!

1. Thanks for the great ideas. They are always easy to exucute and I always have the supplies right on hand! And I do love the explanation about how they work out the way they do!

2. Wow, your blog is the first new, genuinely interesting one I have seen in a long while!

Refreshing!! Well done.

I am following you now and will make a point of coming back!

Please feel free to drop over to our kid friendly blog.
Maybe we could arrange a guest post in the future?

Thanks again!

http://beourbest.blogspot.com/

3. Such a cute experiment!! I will be trying it with my son :)

Love your corner of the internet!!

Came back to return the love and am now following your blog :)

xo

4. I'm a new follower from the hop! I'd love a follow-back! Have a super weekend!!

~Holly
http://www.twinsplusone.com

P.S. I teach 2nd grade. :)

5. Cool looking experiment. Following from Wed hop!

6. i am going to try this with the kids! thank you.
http://www.tanyetta.com/2011/04/follow-me-friday.html

7. I LOVE these experiments! You always have such great ideas and directions! Thanks for all you share! Deb @ LivingMontessoriNow.com

8. We haven't tried that one - it looks like it could be fun!

9. I haven't seen it with adding pennies to cups. What a great idea.

10. I did these experiments during President's week in my preschool class. http://www.brennaphillips.com/how-many-pennies-fit-in-a-cup-of-water-before-spilling-water-out
http://www.brennaphillips.com/how-many-drops-of-water-fit-on-a-penny