Video Transcript
A thermometer is placed in a pan containing a mixture of fruit juice and sugar that is being heated to be turned into jam. The mixture is heated to over 100 degrees Celsius. The thermometer is in the figure shown. What temperature reading does the thermometer give? (A) 100 degrees Celsius, (B) 104 degrees Celsius, (C) 105 degrees Celsius, (D) 110 degrees Celsius, (E) it is not possible to get an accurate reading from the thermometer.
Okay, so we’ve been given this picture of a thermometer which we’re told is placed in a pan that contains a mixture that’s being heated to over 100 degrees Celsius. We can recall that a thermometer is a device that can be used to measure temperature. At the bottom of this thermometer, we see a reservoir or bulb that’s filled with something red. And then this red substance also extends up the scale of the thermometer. This substance is mercury, which is a liquid that expands as its temperature increases.
Now, mercury expands with temperature in a known and predictable way, which means that depending on the temperature of this bulb, the mercury in it will expand a certain amount. And so it will rise a certain height up the thermometer scale. The result of this is that the height of the mercury in the thermometer indicates the temperature. The question is asking us what temperature reading this thermometer gives. To have a reading of the quantity temperature, we need a value and we also need a unit. If we look at the top of this thermometer, we can see that it has written on it degrees C. This tells us that the units of the thermometer scale are degrees Celsius.
The scale itself is shown here on the right-hand side of the thermometer. We have these bigger numbered marks at intervals of five degrees Celsius. And then between each of these bigger marks, we can see that there are four smaller marks. We can see then that each of these smaller marks must correspond to one degree. In order to read a temperature in degrees Celsius of this thermometer, we just need to see how far the red line comes up the temperature scale.
However, at this point we might notice that we’ve got a bit of a problem here. Notice that the highest numbered mark on this thermometer’s temperature scale is this one right here, which is labeled as 100. We then have four smaller marks above this, and we know that each one of these smaller marks corresponds to one degree. Since we know the thermometer scale is in units of degrees Celsius, this means that the highest temperature that can be measured with the thermometer is equal to 104 degrees Celsius. Another way to phrase this would be to say that the measurement range of the thermometer only extends up to a temperature of 104 degrees Celsius. This means that we can’t accurately read any temperature value over 104 degrees Celsius using this thermometer.
If we look at the height reached by the red line in the thermometer, we can see that this height is clearly above the highest 104-degree mark on the thermometer’s temperature scale. What this means is that the temperature we’re trying to measure using this thermometer is outside of the instrument’s measurement range. Since the height of the mercury in the thermometer extends past the 100 degrees Celsius mark and the 104 degrees Celsius mark, it’s clear that we can eliminate answer options (A) and (B).
However, the important thing to realize is that it’s not actually possible to get an accurate reading outside of an instrument’s measurement range. In this case, the measurement range of the thermometer is from zero degrees Celsius up to 104 degrees Celsius. Since the temperature scale stops at 104 degrees Celsius, we can’t accurately say whether the temperature is 105 degrees Celsius, 110 degrees Celsius, or any other temperature. We can therefore eliminate answer options (C) and (D). This just leaves us with option (E).
And at this point, we can notice that our statement that we cannot get an accurate reading outside of the thermometer’s measurement range matches the statement given here in option (E). Our answer to this question then is the statement given in option (E). It is not possible to get an accurate reading from the thermometer.
