Cartesian Diver

INTRODUCTION

This is a great toy that illustrates the difference in density between water and air. Density is a measure of mass per volume. Water has a density of one gram per milliliter. Substances with densities greater than water will sink in water, and substances with densities less than that of water will float in water. Using the Cartesian diver students will be able to determine whether air’s density is greater or less than the density of water.

MATERIALS

• 2 Liter pop bottle
• Water
• Eyedropper

WHAT TO DO

1. Start with a clean 2 Liter pop bottle with labels removed. Fill the bottle completely with water.
2. Fill an eyedropper half-full with water. Drop it in the bottle of water. If the dropper sinks, remove the dropper from the bottle and squeeze some of the water out of the dropper then try again.
3. When the dropper is floating on top of the water, screw on the cap of the bottle. Squeeze the pop bottle with both hands.

QUESTIONS

1. Carefully watch the eyedropper (or diver). What changes do you observe?
2. Can you use your understanding of density to explain what is happening?

SUMMARY

Density is a measure of mass per volume. Objects that are less dense than water can float in it, but objects that are more dense than water will sink. The diver originally floats because the dropper contains part air and part water. Air is less dense than water, meaning that one liter of air has less mass than one liter of water, thus the diver is initially lighter or less dense than water. When the bottle is squeezed the water level in the dropper increases, which causes the density of the eyedropper to increase to the point that it is more dense than water. This causes it to sink to the bottom of the bottle.

SOURCE

Department of Chemistry, The Ohio State University

© S. Olesik, WOW Project, Ohio State University, 2000.