Falling Test Tubes?
Objective: To demonstrate adhesion and cohesion.
Standards: Properties of Earth Materials (physical and chemical properties of
water) and Structure and Properties of Matter (atoms interact with each other by
sharing or transferring electrons; the physical properties of compounds reflect
molecule interaction).
Materials: two glass test tubes (one small, one large), water, and something to
catch water.
Procedure:
(1) Make sure that both test tubes are clean and that the smaller tube fits
inside the larger one. It may be a good idea to experiment with
different pairings of sizes to determine which ones work the best.
(2) Fill each test tube full with water.
(3) While holding both of the test tubes over the catch pan/container lower
the smaller one inside of the larger one and release your grip on the
small test tube.
(4) Invert the larger test tube.
(5) Observe.
Science Behind It: The smaller test tube will not fall out of the larger one. Water will
drop out of both of the tubes and the smaller one will “rise” up into the larger one. This
demo is made possible by the principles of adhesion and cohesion. Because water
molecules are polar and attracted to each other, water forms beads and drops and has
surface tension. This type of intermolecular force is known as cohesion. Cohesion
occurs when the same type of molecules or materials are attracted to each other. An
attraction that occurs between different types of molecules is known as adhesion. This is
demonstrated by the attraction between the water and the glass of the test tube. Both
forces of cohesion and adhesion are seen when the water is bulged and hanging out over
the glass lip of the test tube (when they are inverted). In this demo, both attractive forces
of cohesion and adhesion are stronger than the force of gravity and this is the reason why
the small test tube does not fall when inverted.