Physics

Inverted Celsius

The Swedish astronomer Anders Celsius (1701-1744) invented the Celsius temperature scale. However, he initially appointed the freezing point of water to 100 degrees Celsius and the boiling point at 0 degrees.
After 2 years this was reversed and since then 0 degrees Celsius has been defined as the freezing point of water and 100 degrees was defined as the boiling point of water, both at a pressure of one standard atmosphere with mercury being the working material.
This is still mostly teached as being the definition of the Celsius scale, however international agreement defines the unit “degree Celsius” and the Celsius scale by two different points: absolute zero and the triple point of VSMOW. This definition also precisely relates the Celsius scale to the Kelvin scale, which defines the SI base unit of thermodynamic temperature.

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Hot water freezes faster than cold water

Mpbemba effect

The phenomenon that hot water freezes faster than cold water is called the Mpemba effect . The Mpemba effect is named after the student who discovered this phenomanon of hot water freezing quicker than cold water. The high school student Mpemba from Tanzania, Africa discovered in 1969 that hot water freezes faster than cold water while making icecream.
His teachers did not believe it was possible, and it took several years until university professors finally accepted his discovery.

Why does hot water freeze faster than cold water?

It is unclear what the exact cause of this phenomenon is, but there are several factors that have influence on the mpemba effect:

A part of the hot water vaporizes which already cools down the remaining water and of course has the effect that there is less water left to be freezed.
Besides that, it has to be taken into account that it’s possible that the hot water holds less dissolved gasses, there is a difference in convection currents when cooling down and of course the surroundings can have effect as well.
All this leads to the fact that hot water freezes faster than cold water.

Source: http://www.physics.adelaide.edu.au/~dkoks/Faq/General/hot_water.html

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