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Why 273 Kelvin is used for temperature conversion
As you already know, Celsius, Fahrenheit and Kelvin are the units for measuring temperature. A temperature value can be expressed in any one or all of the above units independently. The values can be converted from one unit to other unit still keeping the meaning same.

We use the number 273 (273.15 exactly) to convert from Celsius to Kelvin and vice versa. .

Many people wonder what this 273 means and why is this 273 and not any other value.

The following explanation would make this clear.
Let me start by briefing about the Celsius temperature scale. In Celsius temperature scale, the freezing point of water is taken as 0 and the boiling point of the water is taken as 100.
Celsius temperature scale:

Freezing point of water   =      0 C
Boiling point of water      =      100 C

Kelvin temperature scale:

In Kelvin temperature scale, the zero point is shifted from freezing point of water to the absolute zero point.
Absolute zero is the theoretical lowest temperature achievable for any substance. It is a point at which all molecular movements of any substance stops.
Since 1 unit C = 1 unit K, the 100th point also shifts.

Why 273?

To get the idea, Let’s do a virtual experiment. For this, you need
1. Celsius thermometer.
2. Boiling Water.
3. A microscope to see the molecular movement. (Imagine !)

Now, Use the thermometer to find the temperature of boiling water. It will read 100C. At the same time, see the movements of water molecules. You can see a very rapid movement. Next remove the heat from the water. The temperature drops and when the thermometer reads 0, the water freezes. If you look at the molecular movement at this stage, it will move slowly. Now keep on removing the heat from the water/ice till you see no molecular movement in your microscope. When you go to that stage, the thermometer
would read -273.15 C.

This point is taken as 0 point in Kelvin scale. It is called the “absolute zero”.
Now you know how we got the value 273.15 !!!
Remember, 1 unit C = 1 unit K.

Lets reverse the process and add the heat to the water/Ice. When the temperature reaches 0C the Kelvin scale would read 273.15 K. Keep adding heat and at 100 C the Kelvin scale would read 373.15 K.
Here are some example conversions.

Remember, 0 K = -273.15 C or 0 C = 273.15  K.

So 10 K = 273.15 - 10 = -263.15 C.
       100 K = 273.15 - 100 = -173.15 C.
       600 K = 273.15 - 600 = 326.85 C.

Similarly, 10 C = 273.15 + 10 = 283.15 C.
                      100 C = 273.15 + 100 = 373.15 C.
                      600 C = 273.15 + 600 = 873.15 C.

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