The Ultimate Thermometer

The substance, water, without which life would be impossible, has been used to measure all temperatures, in effect.

When science, serious science began, it was realised that some sort of temperature scale would be necessary. Mr Faranheit came along with an arbitrary system. Then Mr Celsius saw common sense and invented another system, first calling it "Centigrade" before being modified to "Celsius".

He defined it very simply; with water.

"At normal temperature and pressure, let pure melting ice be zero Centigrade, and pure boiling water be one hundred Centigrade."

Just to make things easy. To convert from one to the other was fairly easy:

Celsius to Fahrenheit (°C × 9/5) + 32 = °F
Fahrenheit to Celsius (°F - 32) x 5/9 = °C

Or, in words,

°F to °C Deduct 32, then multiply by 5, then divide by 9
°C to °F Multiply by 9, then divide by 5, then add 32

Most Americans use Deg F (except for the scientists), whilst almost everyone else uses Deg C (because Celsius is the Universal unit of temperature as adopted by the Scientific Community - it is simpler).

One may have noticed that in the definition of zero and one hundred Celsius, zero is not simply "ice", nor is one hundred simply "steam".

There is a very good reason for this. Thermodynamics again. If you take a block of ice and heat it, it's temperature goes up like this:



There are five bits to it: three sloping bits and two flat bits. The sloping bits are easy. Over time, as more heat gets put into the substance, the temperature rises, the first slope is the temperature of ice rising, the second slope, that of water, the third, that of steam, as more and more heat is supplied.

But what about the flat bits? This is the interesting part. The flat parts of the graph are where the temperature stays constant because all the heat (therefore energy) is used in breaking chemical bonds, the first flat bit from ice to water (solid to liquid), the second flat bit from water to steam (liquid to gas).

Celsius used this simple fact to define his fixed points, the two flat bits, where the temperature is always the same.

So, if you want something to be cooled to EXACTLY zero degrees Celsius, stick it in a bag of MELTING ice, or if you want something to be EXACTLY 100, put it into a container of boiling water.

That is why water could be called "The Ultimate Thermometer"

( It is quite clear that the bigger the sample of ice, the more heat you have to put in to break the two types of chemical bonds. There is a measure for this was well, it is called "Specific Heat Capacity", but that is another story. )