# Lab report questions, gas thermometer

Gold Member
Hi PF!
I have a lab report to do about a gas thermometer experience that I couldn't do. I'm very saddened about this because I should have manipulate mercury but the earlier group broke a container of mercury and so the class had to be disinfected and closed for some days.
Basically it consists of calibrating a gas (air) thermometer in order to evaluate the room temperature. (This picture explains how is the device : http://www.shef.ac.uk/physics/teaching/phy001/fig1.gif [Broken] . For further explanations of what is a gas thermometer, read this : http://en.wikipedia.org/wiki/Gas_thermometer)
So the professor gave us the results the first class obtained and told us to work on it to get the final result. Furthermore he asked a few questions to be answered in the lab report.
One of these questions is : What if the liquid used is water instead of mercury?
My thought is that water would go up more than mercury because it is quite less dense. So as the column of the thermometer is only 47 cm and that the mercury went up to 16.5 cm for a gas temperature of 98,488 °C, the thermometer of water wouldn't be of any use for high temperatures (it would go up more than 47 cm and spread on the floor). Am I right?

For low temperatures (0°C to 8°C), the water has a density's minimum at 4°C. Does it affect the lecture of measures? Also, if the air is say at -20°C, will the water freeze? Or not if I take the measure fast? I know it has a somewhat a low heat capacity so I wonder if the air temperature has a big effect.
Another doubt: the mercury seems to have to big heat capacity, so when I heat up the air to 100°C, the mercury expand quite a lot and the column of mercury should go higher than if it were at room temperature. Does it makes “false” results?
Is there anything else I can add to answer the professor's question?

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Gold Member
Ok maybe I've asked too many questions.
I don't understand why the gas thermometer that uses mercury as liquid is accurate. Indeed, when we heat the gas from 25°C to 100°C, the mercury that is in contact with the gas also heat up and expands. Meaning that the column of mercury is higher than it should. As we read the temperature looking at the height of the mercury's column, it is erroneous.
We should take this in count when analyzing the data but we do not. I feel I'm missing something here.

Staff Emeritus
Homework Helper
Hi,

Usually the change in density, with temperature, of liquids and solids is neglected because it is small. For mercury, it's a 2% change from 0C to 100C (13.60 vs. 13.35 g/cc).

Also, we should probably assume the temperature changes slowly enough so that the liquid will be the same temperature as the air. So you're right about water freezing below 0C.

Other issues with water: as you said, it could spill out of the tube (if the tube end is open) or fill up the tube (if the tube end is closed). Either way, a longer tube is required to get a reading.

Gold Member
Hi,

Usually the change in density, with temperature, of liquids and solids is neglected because it is small. For mercury, it's a 2% change from 0C to 100C (13.60 vs. 13.35 g/cc).

Also, we should probably assume the temperature changes slowly enough so that the liquid will be the same temperature as the air. So you're right about water freezing below 0C.

Other issues with water: as you said, it could spill out of the tube (if the tube end is open) or fill up the tube (if the tube end is closed). Either way, a longer tube is required to get a reading.
Thank you very much! I wasn't aware that liquid's density change with temperature was so "unnoticeable".
Thanks for the informations, now I can do the conclusion of my report.