Why do we use mercury for a conventional thermometer and not water?

Question

IS it related to thermal expansion?

Answer 1

Most thermometers use some kind of red tinged alcohol. It's not as harmful to the environment as mercury (face it, thermometers break) and has a reasonable operating range for most mundane applications.

My guess is it's probably ethanol which has a higher boiling point than methanol and is a stronger solvent for dyes than heavier alcohols.

Answer 2

Lots of other answerers - go to the bottom of the class!

Water has a much higher coefficient of thermal expansivity than mercuty - for instance, at 20C water is around 200E-6, mercury is 60E-6, 75% lower. Its density is irrelevant - alcohols is lower and it is used in thermometers. A thermometer expands a bulb of liquid into a capillary so can aimplify expansion.

There are two reasons to use mercury over water. The first is obvious - you want something that can measure freezing point and boiling point, so water is a non starter. Mercury freezes at around -38C and boils at a handy 356C. Water freezes at, well, freezing point and blows the end of the thermometer at 100C.

The second is that water has a hopelessly non-linear coefficient of expansion. As you cool it it actually stop contracting at about 4C and then starts expanding again. Pretty useless for a thermometer.

Answer 3

The mercury is preferred as a thermometric material more than many other materials (like water) becouse:
1) It expand regularly
2) It is affected by slight change in temprature
3) It is opaque so it can be easily seen
4) It doesn't wet glass during its expansion & contraction
5) It has a wide thermal range (freezes at -39 C & boils at 357 C)
6) It has small specific heat so it doesn't absorsb a great amount of heat from the meduim

BUT

Water conflict with most of these properties:
1) Its expansion is not regular
2) It is not affected by slight change in temprature
3) It is transparent so it is not easily seen
4) It wets glass
5) it don't have wide range for expansion (freeze at 0 C & boils at 100 C)

i hope u got ur point from my explanation

Answer 4

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The standard thermometers in use in the late 19th and early 20th centuries weren’t that much different to the ordinary thermometers we have today. In fact, the design has barely changed in 350 years. The earliest continual temperature record we have is one called HadCET (HADley Central England Temperature) which dates from 1659. From the outset readings were taken using the new fangled sealed bulb thermometers. The predecessors to the sealed bulb instruments had relied largely upon the expansion of either air or a liquid such as water or alcohol. The configuration of the instrument meant that air or liquid in a container would expand and contract according to the changing temperature. This expansion and contraction would either push or draw a column of liquid through a glass tube. A couple of drawbacks to these thermoscopes were that they weren’t calibrated or have a scale, and they were sensitive to changes in atmospheric pressure. Placing the liquid in a sealed vessel eliminated the problem of changing air pressure and the introduction of a calibrated scale enabled instrumental readings to be taken. In the early days, each thermometer was individually calibrated; no two thermometers would give the same reading. It wasn’t until a universally accepted scale based on the freezing and boiling points of water was introduced that readings from different thermometers could be compared. With two precise fixed points to work from it was easy to calibrate a thermometer. But they did still have a drawback, the irregular coefficient of expansion of the liquids used. It wasn’t until the early 18th century that Daniel Fahrenheit produced a precise scale by using thermometers filled with mercury, am element with a constant coefficient of expansion. By now precision engineering was possible and thermometers made to exacting standards could be produced in sizeable numbers. With standardised temperature scale in place, thermometers could be calibrated either by comparison with other instruments or by comparison with the two fixed points. The longer the capillary, the greater the degree of accuracy that could be achieved. A thermometer one metre long could quite easily be calibrated to an accuracy of 0.1°C, any more would mean changes indiscernible to the human eye (0.1°C would equate to 1mm on a 1 metre long thermometer with a 212°F range). Throughout the 18th and 19th centuries the British were obsessed with scientific discoveries and advances. Their navy traversed the oceans and many explorers took off to the far-flung corners of the world. Many of these voyages would incorporate scientists and it wasn’t at all unusual for astronomers and meteorologists to be a part of the crew. As a result observations were taken from around the world, both on land and at sea. Weather stations were established throughout the Empire and most of these records were accurately maintained until the host country gained independence. There weren’t any great advances in temperature observations until the advent of satellite telemetry in the late 1970’s. Today this is widely used to map temperatures right across the surface of the planet. There are still something like 10,000 Stephenson’s Screens that make up part of the Global Historical Climatology Network and many thousands more reporting stations at airports, onboard ships and at universities. The accuracy of the conventional thermometer record hasn’t changed and the weakness is still the person taking the readings. It’s standard practice to take readings to one decimal place and in this respect the accuracy today is the same as it was some 200 years ago; the thermometers themselves are generally accurate to 0.01°C. The digital, automated and satellite readings are more accurate and these make up an ever greater proportion of the modern global temperature record. Although one single measurement may only be accurate to 0.1°C, by the time there are several hundred or thousand readings from a site, any slight inaccuracies will be averaged out. Over time, a correctly sited and maintained station should be accurate to within 0.001°C. The Recording Thermometer referenced in your question isn’t an instrument I’m familiar with other than being aware of it’s existence. I do have a little weather station in the garden and it would be great to have one of those devices but alas, it’s just a standard max/min and wet bulb thermometer.

Answer 5

yes - mercury has a high amount of expansion during minor temperature variations. As compared to it, water would freeze.. you know the first thermometers were much greater than a glass with coiled glass tubes around it. In these coiled tubes, were Alcohol. now there are somem thermometers that use tingled alcohol in them and it is not much harmful as mercury is.. :)

Answer 6

there are 3 basic reasons:

HIGH TEMPERATURES
the freezing point of mercury is much lower than the freezing point of water. a water thermometer could not measure a temperature lower than zero degrees Celsius (it would crack).

LOW TEMPERATURES
the boiling point of mercury is much higher than the boiling point of water. a water thermometer could not measure a temperature above 100 degrees Celsius (it would explode).

SIZE
due to the different rates of expansion for water and mercury, a water thermometer would need to be much bigger than an equivalent mercury thermometer. the size of the water thermometer would make it more difficult to use.

Answer 7

beside thermal expansion the water absord and give thermal energy lower than Hg and in water there is amount evaborated not like Hg

Answer 8

Water has less specific gravity as compared to mercury.
If we use water thermometer then we have to use atleast 100mts length of thermometer which is very difficult to handle.
So, we use Mercury.

Answer 9

yes - mercury has a high amount of expansion during minor temperature variations ( water would freeze )

Answer 10

yes since mercury expands more than water for little inc in temp.
this is one reason. also it doesnt sticks to the walls of thermo meter and it is easier to take the readings.