Video: Properties of Liquids Used in Thermometers

Analog thermometers often rely on the thermal expansion of a liquid to infer the temperature. Which of the following properties would make a liquid a bad choice for use in a thermometer? [A] Low vapor pressure [B] Strongly colored [C] Low reactivity [D] Nonlinear thermal expansion [E] Low toxicity

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Video Transcript

Analog thermometers often rely on the thermal expansion of a liquid to infer the temperature. Which of the following properties would make a liquid a bad choice for use in a thermometer? A) Low vapor pressure. B) Strongly colored. C) Low reactivity. D) Nonlinear thermal expansion. Or E) Low toxicity.

Traditionally, all thermometers were analog given continuous readings and not having digital readouts. Analog thermometers you might come across will probably have a liquid bulb at the bottom, have a colorful liquid inside, and the whole thing will be made out of glass. Analog thermometers are generally used where high accuracy isn’t a priority where a difference of a degree or two is acceptable. But you can get really accurate analog thermometers. These thermometers generally rely on a liquid which changes its volume depending on the temperature. Each thermometer has graduations indicating the temperature of the liquid inside.

In this scenario, we’re being asked about the liquid inside of the monitor. And we have to pick which property would probably be a bad property for the liquid to have. Let’s go through them one by one. A vapor pressure is the pressure of gas above a liquid outside if it’s the only thing in the container. For instance, if we had a container where the only thing inside was water, no air, the vapor pressure would be about three percent of atmospheric pressure. In a thermometer, some of the liquid will evaporate. But it doesn’t really make a difference to the function of the thermometer. However, if we had a particularly high vapor pressure liquid, it’s conceivable that at high temperatures the internal pressure might break the glass. So actually having a low vapor pressure is probably a little bit safer. But all in all, it’s not likely to make a great deal of difference.

What about being strongly colored? The truth is being strongly colored is a great thing for a liquid inside of the monitor. It provides contrast, making it easier to read the thermometer. Since it’s actually a good rather than a bad choice, we can move on to the next one. Having a low reactivity liquid inside the thermometer actually makes it safer if the thermometer breaks. It’s also going to be easier to make the thermometer in the first place. So what about nonlinear thermal expansion? Firstly, what does nonlinear thermal expansion mean?

Well, its easier to understand if we’d look at the linear thermal expansion. In a linear thermal expansion, an increase in temperature produces a consistent increase in the volume. Using a liquid with linear thermal expansion in a thermometer means it can be made very simply with even graduations across a glass tube of even thickness. Meanwhile, if we had the same thermometer design but with a liquid with nonlinear thermal expansion, the graduations would have to be uneven. This is not automatically a problem. But it does make it more expensive to make the thermometer. This causes even greater problems when the temperature and volume relationship is much more complex.

So we’ve found an undesirable property for the liquid inside a thermometer, nonlinear thermal expansion. But let’s look at the last option just in case. It should be fairly easy to see that having a low-toxicity liquid in our thermometer makes it a good choice rather than a bad one. It’s safer and it’ll be easier to make the thermometer in the first place. So we have our answer. So the answer to which of the five properties we’ve been given would make a liquid a bad choice for use in a thermometer is nonlinear thermal expansion.

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