Paper and pencil item: A container with a little hole at the top is placed over a hot plate. There is water in the container. A deflated balloon is attached to the hole. The hot plate is turned on. The water starts boiling and the balloon inflates (see picture):
What is the balloon filled with?
Oxygen and hydrogen gas
Air and water vapor
Consider the combined mass (amount of stuff) of the container, water, and balloon (deflated or inflated) and what the balloon contains. When the water boils:
Mass (amount of stuff) stays the same because__________________________
Mass (amount of stuff) decreases because_____________________________
Mass (amount of stuff) increases because______________________________
There is no way to predict.
Interpretation: These questions test whether students believe that what escapes from the boiling water is material, whether they apply conservation of mass to this situation, and what they think escapes from the water. The proportion of students who believe that what escapes from boiling water is still water increases through middle school. Some students believe that what escapes from the boiling water is air, or they might say that the water breaks down into oxygen and hydrogen (evincing a confusion between chemical and physical transformations). Students may correctly apply conservation of mass and predict that when the liquid water changes state, there is no change in mass. Alternatively, they may believe that the mass of the gas will be less than the mass of liquid because gases are thought to be light or weightless.