Video: Stability of the Reading of a Thermometer

Sofia wants to measure the temperature of some water that has been heated to near its boiling point. She uses a thermometer that has been on a table in the room for an hour. She places the bulb of the thermometer in the water and immediately reads the temperature on it, as shown in the diagram. She determines that the temperature of the water is 19°C.

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Video Transcript

Sofia wants to measure the temperature of some water that has been heated to near its boiling point. She uses a thermometer that has been on a table in the room for an hour. She places the bulb of the thermometer in the water and immediately reads the temperature on it, as shown in the diagram. She determines that the temperature of the water is 19 degrees Celsius.

Okay, before going on to our question, let’s consider this diagram. In it, we see this thermometer with its bulb, the temperature-sensitive part, submerged in some hot water. We’re told that this particular thermometer has been sitting on a table in the room for an hour before this measurement was made. And we’re also told the measurement took place immediately after the thermometer was put into the water. Now, we can see that at this moment, where the temperature was measured, the position of the red liquid inside the thermometer is at this level. It’s tricky to see, but we’re told that this level is 19 degrees Celsius. One other piece of information we’ll want to keep track of is the fact that this water, whose temperature is being measured, has been heated to near its boiling point. Now, keeping all this in mind, let’s go on to our question.

Which of the following statements explains why this answer is incorrect? A) She should hold the bulb of the thermometer just above the surface of the water. This is where the temperature is highest. B) She should submerge the whole thermometer in the water. And C, she did not wait for the reading to stabilize. The temperature of the water is actually far higher than 19 degrees Celsius.

All right, so let’s recall that Sophia recorded the temperature of this water to be 19 degrees Celsius, that that was the reading on the thermometer at this instant. We’re told that this answer is incorrect, and we want to explain why that is. Answer choices A, B, and C give us options for doing that. Let’s consider these options one by one.

Option A says that Sophia should hold the bulb of the thermometer just above the surface of the water. In other words, option A is saying that the thermometer should have been positioned something like this, where the bulb, the part that’s sensitive to temperature, is not actually in the water being measured. Rather, it’s just above that surface. The trouble with this explanation is that the bulb of the thermometer, in a correct measurement, is placed where it’s surrounded by the substance or material whose temperature we want to measure.

If we wanted to measure the temperature of the air right above the surface of the water, then indeed this would be a good position for our thermometer. But what we really want to measure is the temperature of the water. Set up this way, our thermometer would not measure that temperature. So we’ll cross option A off our list of possible explanations. This doesn’t mean, by the way, that we’re saying that 19 degrees Celsius is a correct temperature reading for the water. But we’re only saying that option A is not a correct explanation for why this number might be incorrect. Let’s move on to option B.

This option says that Sophia should submerge the whole thermometer in the water. Well, if she could find a thermometer small enough to do that, then that would be possible. And in that case, the temperature-sensitive part of the thermometer, it’s bulb, would indeed be submerged in the material whose temperature we want to measure. But in order to make an accurate temperature reading, it’s not necessary for the entire thermometer to be submerged in this material, in this case, the water. It’s only the bulb, the temperature-sensitive part of the thermometer, that must be. Option B is implicitly saying that the reason 19 degrees Celsius is not a correct temperature reading for the water is because the thermometer was not entirely submerged in the water. As we’ve seen though, that’s not necessary for an accurate reading. Therefore, we’ll cross option B off our list too.

Our last choice, option C, says Sofia did not wait for the reading to stabilize. The temperature of the water is actually far higher than 19 degrees Celsius. If we remember, this thermometer that Sophia used was at room temperature and had been for about an hour. We were told that immediately after the thermometer bulb was submerged in the water, Sophia took the reading on the thermometer. And that reading she recorded as 19 degrees Celsius. We have good reason to believe, though, that 19 degrees Celsius is not the temperature of the water. That’s because we’re told in the problem statement that the water is heated to near its boiling point. And we can recall that the boiling point of water on the Celsius scale is 100 degrees Celsius. If this water is near boiling, and we’re told it is, then we would expect a temperature reading near 100.

So the fact that the water temperature is actually much higher than 19 degrees Celsius. And the fact that we’re told Sofia made the temperature reading immediately after the thermometer was put into the water points us to the conclusion that she did not wait for the temperature reading to stabilize. And that indeed the real temperature of the water is actually much higher than 19 degrees Celsius. This, then, is our answer as to why it is that 19 degrees Celsius is an incorrect reading of the water temperature. It’s incorrect because Sofia did not wait for the reading to stabilize. And the temperature of the water is actually much higher than 19 degrees Celsius.

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