Video Transcript
Chloe wants to measure the
temperature of some water in a beaker that is near to its boiling point. She holds the thermometer above the
surface of the water, as shown in the diagram, and holds it there until the reading
has stabilized. She determines that the temperature
of the water is 77 degrees. Which of the following statements
explains why this answer is incorrect? (A) The thermometer must be fully
submerged in the water in order to measure the temperature of the water. The temperature of the water is
higher than 77 degrees. (B) The reading must be taken as
soon as the thermometer is above the water rather than several minutes later. (C) The bulb of the thermometer
must be submerged in the water in order to measure the temperature of the water. The temperature of the water is
higher than 77 degrees.
Looking at the diagram, we can see
that Chloe has placed the thermometer above the surface of the water rather than in
the water. We know that the boiling point of
water is 100 degrees Celsius. This question tells us that the
temperature of the water is near boiling. So we should expect the temperature
to be very near to 100 degrees C. Chloe must be making a mistake
because the reading she takes is 77 degrees C, which is much lower. To measure correctly, Chloe should
put the bulb of the thermometer inside the water, without making contact with the
container and only making contact with the water.
Let’s look now at the answers and
see which best reflects this practice. Answer (A) is not entirely correct
because the thermometer must not be fully submerged. Only the bulb which contains the
reservoir of liquid has to be submerged. Also, if the thermometer was
totally submerged and the bulb was in contact with the container, this would also
measure the temperature of the container, which would be different from the
temperature of the water.
Option (B) is also not correct
because the liquid in the thermometer that indicates temperature needs time to heat
up. If you remove the thermometer too
quickly, the measured temperature will be too low and immediately begins
dropping. Also, the thermometer being above
rather than in the water means that Chloe is actually measuring the air temperature
above the water.
This means answer (C) must be
correct. When the bulb of the thermometer is
immersed in the water, the reservoir of liquid inside the thermometer that expands
or contracts to give a temperature reading along the scale is submerged. This would indicate the correct
temperature of the water and only the water. If this is done correctly, it is
likely that the measured temperature would be higher than 77 degrees Celsius.
