Kitchen Chemistry Experiment 25
The Surface Tension of Water

Students explore the surface tension of water by using pennies, paperclips, a medicine dropper, and a small drinking glass

Materials	Substitutions
small wide mouth bottle	juice glass
medicine dropper	medicine dropper
20-30 pennies
small paperclips
water

Procedure

1. Place a penny on a flat tabletop. Add water to the juice glass. Use the medicine dropper to deposit drops of water on top of the penny one-at-a-time. Count the number of drops you are able to keep on the penny without spilling water over the edge. Note the peculiar shape that the water assumes. Why does this happen?
2. Fill the juice glass completely full of water. It should be so full that any more would spill over the edge. Now, carefully slide a penny into the water. Don't let the water spill!!! The best technique may be to place the penny on the lip of the glass the slowly release it. Continue to add pennies one-at-a-time counting the number that you are able to submerge without spilling any water. Note the shape the water forms on the surface.
3. Refill the glass with water. Fill it completely. Try to float several paperclips on the water surface.

 

Extensions

 

1. Why do we call water a dipole ? What is hydrogen bonding ?
2. How many pennies were you able to add? What is surface tension ?
3. What is a meniscus ?
4. Would it make a difference if the paperclips or your hands were dirty or oily? What effect does oil have on the surface tension? Test your hypothesis.

 

Teacher's Notes

1. The water molecule is often referred to as the water dipole because of the distinct positive and negative regions in its molecular shape. The prefix "di-" means two and the "pole" refers to the positive and negative ions. Hydrogen ususally attains a positive charge while oxygen is assigned a negative charge. (see diagram above)
   
   These dipoles are attracted to each other. This mutual attraction causes water molecules to associate, or join together as groups of molecules.
   
   A hydrogen bond is a weak chemical bond between a hydrogen atom in one polar molecule and a very electronegative atom of a second polar molecule. The hydrogen of one water molecule will be attracted to the oxygen of another water molecule. The are usually 4-8 molecules per group in liquid water. The surface tension of water is due to the hydrogen bonding in the associated groups of water molecules.
   
   
2. The meniscus is the curved surface of a liquid in a graduated cylinder or any other small diameter glassware. Water adheres to the sides of any container creating a "cup" of surface tension. When reading volumes in a graduated cylinder, one should take an accurate measure from the bottom of the meniscus.
   
   
3. Water covers about 75% of the earth's surface. Significant quantities of water are frozen in glaciers, which cover almost 10% of the land surface.
   
   Water is essential to life, comprising 70-90% of the weight of living things. Water in a liquid state is probably unique to Earth. There is ice on Mars and water vapor in Jupiter's atmosphere. Ice is found on comets and on two of the moons of Jupiter, Europa and Ganymede. Water is one of the only substances that will expand one-ninth in volume as it solidifies. Because ice has air in between its crystalline arrangement, ice is less dense than water. This fact enables aquatic life to survive winter in a frozen lake or stream. It is thought that all life began in water!