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.
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.