Alka-Seltzer in a Latex Glove

Set-Up

A latex glove is fit tightly over the open end of a 300-ml glass beaker half full of water. The glove contains four alka-seltzer tablets. The tablets are in the part of the glove that is hanging over the edge of the beaker so that they cannot fall into the water. The beaker and glove are set on a digital scale which measures the weight in newtons. In a large class, the reading on the scale can be projected onto a screen with a video camera. Our scale has a computer interface so that we can project the reading from a computer. In the photo below, the glove is tightly held to the beaker by a black elastic. This is important, otherwise the pressure of the expanding gas in the first part of the demonstration may cause the glove to pop off of the beaker.

Part 1

Lift up the glove so that the alka-seltzer tablets fall into the water. The tablets react with the water and produce carbon dioxide gas. The gas inflates the glove. Before doing this, ask the class to predict what will happen to the reading on the scale.

As can be seen in the figure above, the reading on the scale has gone down by 0.014 N. This is a good time to point out to the class that the scale is not actually measuring the "weight" of the beaker and its contents. The scale measures the force with which the beaker is pressing down on it. The weight of the beaker is the downward force of gravity and has not changed since its mass has not changed. All of the atoms in the carbon dioxide gas were originally in the alka seltzer tablets and the water. The upward buoyant force of the air on the beaker has increased since its volume has increased. The increase in buoyant force causes the downward force on the scale to decrease, as observed.

Part 2

Cut a slit in one of the fingers of the glove, releasing the carbon dioxide gas. Before doing this, ask the class to predict what will happen to the reading on the scale.

As can be seen in the picture above, the reading on the scale has gone down further by an additional 0.007 N. Since the volume of the beaker has decreased, so the upward buoyant force of the air has decreased. However, the loss of the carbon dioxide gas causes the weight to also decrease. The decreased buoyancy would cause the reading on the scale to increase, and the decreased weight would cause the reading on the scale to decrease. The buoyant force decreases by the weight of the air which was displaced by the lost carbon dioxide gas. The weight of the beaker decreases by the weight of the lost carbon dioxide gas. Since carbon dioxide is heavier than air, it wins and the reading on the scale decreases, as observed.