Physics questions

1. Mercury is a better choice. It is because mercury is a good conductor of heat and can respond to temperature change more rapidly.

2. Water has a peculiar behaviour that its density is highest at 4'C. Hence, in temperature range from 0'C to 4'C, water in fact expands on cooling. If water were used as a thermometric liquid, we would observe that a rising water thread, instead of a falling one, when the temperature drops from 4'C to 0'C.

(1) Whether mercury or alcohol is used in a thermometer depends on the over range which temperature is to be measured. Mercury freezes at -39ºC and boils at 357ºC while alcohol freezes at -115ºC and boils at 78ºC. It is therefore essential to use alcohol thermometers in places such as northern Canada, Alaska and Russia, where winter temperature of -40C are not common. Alcohol also possesses the advantage of having an expansion of about six times that of mercury.

Apart from these advantages, mercury is to be preferred to alcohol as a thermometric liquid for the following reasons:
(1) It does not wet the glass. Alcohol tends to cling to the wall of the tube and this leads to lower reading when the thread is falling.
(2) It does not vaporize and distil on to the upper part of the bore.
(3) It is opaque and easily seen, whereas alcohol has to be coloured.
(4) It is better conductor of heat than alcohol, and therefore responds more rapidly to changes of temperature.


(2) Water is not suitable for thermometer for the same four reasons mentioned above.
The range is very restricted as water freezes at 0ºC and boils at 100ºC. Water also shows peculiar behaviour from 0ºC to 4ºC. It contracts instead of expansion. Snowing is very common in many places of the world. The ice formed around 0ºC will definitely break the thermometer if water is used.


2010-10-05 06:48:59 補充：
Correction:

where winter temperature of - 40C are common. (Delete the word "not")