Academic journal article The Science Teacher

Try This Tops Idea!

Academic journal article The Science Teacher

Try This Tops Idea!

Article excerpt

water domes ... adapted from COHESION/ADHESION #13 BY TOPS Learning Systems

1. Set a dry penny on a dry bottle cap.

2. Count how many drops of water you can heap on the penny without spilling. Go for a record!


3. Build a 20-drop water dome.

a. If you poke it with a clean, dry pin, does this break the cohesive (attractive) bonds between water molecules?


b. Dip the pin point in liquid soap, then poke the water dome again. What happens?


4. How does soap affect surface tension (the cohesion of water)?

[c] 2008 by TOPS Learning Systems. Photocopies permitted if this notice appears. All rights reserved.


To observe that cohesive forces hold water together on a penny as a rounded dome; that adding soap significantly weakens this cohesion.


Steps 1-2. Start with a clean, dry penny. Traces of moisture may cohesively attract the dome to spill over the edge too soon. A bottle cap raises the penny above water that spills during repeated trials.

Students will naturally compete to see who can heap the most water on their pennies. Drops should free fall onto the dome for counting accuracy.


2. A penny typically holds 20 drops of water (1 mL) or more before spilling over the side. …

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