Competency #3
Answer and Explanation
The correct answer is A.
Before discussing the answers, first consider the process of boiling. According to Hewitt et al, Boiling is when “bubbles of vapor form in the liquid and are buoyed to the surface, where they escape.1” Now, let us consider each statement, in reverse order:
D. The water is not really boiling.
The water was “bubbling vigorously.” This indicates that bubbles were being formed and were rising to the surface. This sounds just like the description of boiling given above. Even though it is counter to our common experiences, the water must be actually boiling.
C. The water has increased to 100°C and is boiling.
We know that water typically boils at 100°C, so it is common to think that somehow the water must have gotten hotter for it to boil. However, no energy has been transferred into the water. In fact, boiling actually removes energy from the liquid, so a noticeable drop in temperature of the liquid will be observed if this demonstration is carried out for sufficient time!
B. The pressure has increased enough that the water can boil at room temperature.
Removing air from the sealed chamber will decrease the chamber pressure. Nothing was done that would have caused an increase in the chamber pressure. Further, an increase in the chamber pressure would have served to raise the boiling point of the water, meaning it would have to be heated beyond 100°C to get it to boil.
A. The pressure has decreased enough that the water can boil at room temperature.
This is the correct answer. As stated above, removing some of the air from the sealed chamber will decrease the chamber pressure. Boiling is the formation of vapor bubbles in the liquid that then rise to the surface. Lowering the pressure in the chamber allows the bubbles that are formed at low temperature to persist and rise to the surface. At atmospheric pressure, these same bubbles would not have been able to resist the pressure exerted on them, and thus the water would not have boiled.
1Hewitt, Paul G., John Suchocki, Leslie A. Hewitt, Conceptual Physical Science, 3rd Edition, San Francisco: Addison Wesley, 2004.