Celsius (Or Centigrade) Scale:

Up to now, we have been talking instead loosely about temperature and generally have expressed it in terms of the Celsius or centigrade scale (°C). It depends on the behavior of water at the surface of the Earth under ordinary atmospheric pressure and at sea level.

When you have a sample of ice which is extremely cold and you start to warm it up, it will ultimately begin to melt as it accepts heat from the environment. The ice, and liquid water formed as it melts, is assigned a temperature value of 0°C by convention (The figure is as shown below). As you continue to pump energy into the chunk of ice, more and more of it will melt, and its temperature will remain at 0°C. It won't get any hotter since it is not yet all liquid and does not yet follow the rules for pure liquid water.

Once all the water has become liquid and as you keeps pumping energy into it, its temperature will begin to increase (figure is as shown below). For awhile, the water will stay liquid and will get warmer and warmer, following the 1 cal/g/°C rule. Ultimately, though, a point will be reached where the water begins to boil, and some of it changes to the gaseous state. The liquid water temperature, and water vapor which comes instantly off of it, is then assigned a value of 100°C by convention (figure is as shown below).

Now there are two perfect points-the freezing point of water and the boiling point-at which there exist two particular numbers for temperature. We can state a scheme to express temperature depend on these two points. This is Celsius temperature scale, named after the scientist who apparently first came up with the idea. At times it is known as the centigrade temperature scale as one degree of temperature in this scale is equivalent to 1/100 of the difference among the melting temperature of pure water at sea level and the boiling temperature of pure water at sea level. The prefix multiplier centi means "1/100," therefore centigrade literally means "graduations of 1/100."

Figure: (a) Ice melting into liquid water, (b) liquid water warming up without boiling, and (c) liquid water starting to boil.