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 (a liquid) 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?
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!
You may also be interested in my post on matter:
Have fun with science!
