Well, first of all you all know that water freezes at 0 degree celsius. Right?

You also know that the water boils at 100 degree celsius.

Now, here the best part is that, if you know that water freezes at zero degree celsius (°C) and it boils at 100 degree celsius (°C), then you already know the definition of celsius scale.

Now the definition of celsius scale will look very easy to you.

**Simple definition:** The temperature scale that represents the boiling point of water as 100 °C and freezing point of water as 0 °C is defined as celsius scale.

Celsius is a unit of temperature scale, and it is represented by “°C” (pronounced as degree celsius, “degree cel-see-uhs”.)

In some countries, Celsius is also known as centigrade.

**Why are there 3 different temperature scales?**

Interesting question, isn’t it?

Well, you already know that there are three major temperature scales which we use in our daily life as well as these three temperature scales are also used in industrial applications.

These three scale are;

- Celsius scale
- Fahrenheit scale and
- Kelvin scale

Now, the big question arises. Why are these three scales used by us?

**Answer:**

There are three scales on temperature measurements based on their different applications.

For example; Celsius scale was developed on the basis of freezing and boiling point of water.

Water freezes at zero degree celsius and it boils at hundred degree celsius. Hence this scale was developed keeping the freezing and boiling points of the water as standard from 0 to 100.

Now, the Fahrenheit scale was developed based on the temperature measurement of the atmospheric air.

Generally the temperature of the atmospheric air ranges from 0 fahrenheit to 100 fahrenheit.

Hence, as the temperature of the atmosphere ranges from 0 to 100 fahrenheit, the fahrenheit scale was discovered.

Now what about the kelvin scale?

Just think where have you used the kelvin scale more?

In your school, right?

At 0 kelvin temperature, the molecular motion stops and the entropy of the substance becomes zero.

Hence, the kelvin scale was developed as it is good to study about the molecules, their properties, etc.

**Celsius and fahrenheit show the same temperature at…?**

The short answer:** -40 degrees**

Celsius and fahrenheit scale coincide at -40 degrees.

Want to know the proof behind this?, then read below.

**Reason why -40 celsius is same as -40 Fahrenheit (Explained with calculation)**

Here is the calculation to prove that -40 °C = -40 °F.

The formula of celsius to fahrenheit conversion is,

Fahrenheit = (Celsius * 1.8) + 32

Now, to know the temperature at which both the celsius and fahrenheit scale coincide, we have to take their values as same during calculations.

Hence, taking Fahrenheit = Celsius in above equation, we get;

Celsius = (Celsius * 1.8) +32

So, Celsius – (Celsius * 1.8) = 32

Celsius – (Celsius * 9/5) = 32

(5 Celsius – 9 Celsius)/5 = 32

(-⅘) celsius = 32

Celsius = -(32*5)/4

Celsius = -40 degrees

Hence, at **-40 degrees**, the values of Celsius scale and Fahrenheit scale become equal.

Cross check by replacing celsius instead of fahrenheit.

So we have the equation,

Fahrenheit = (Celsius * 1.8) + 32

Here, replacing the celsius as a fahrenheit, so we will get;

Fahrenheit = (Fahrenheit * 1.8) +32

So, Fahrenheit – (Fahrenheit * 1.8) = 32

Fahrenheit – (Fahrenheit * 9/5) = 32

(5 Fahrenheit – 9 Fahrenheit)/5 = 32

(-⅘) Fahrenheit = 32

Fahrenheit = -(32*5)/4

Fahrenheit = -40 degrees

Here also we get the same result. Thus, at -40 degrees celsius and -40 degrees fahrenheit are the same temperatures.

**Freezing and boiling points of water in celsius and fahrenheit scales**

In celsius scale; freezing and boiling point of water are mentioned below;

**Celsius scale: **

- Freezing point: 0 °C
- Boiling point: 100 °C

**Fahrenheit scale:**

- Freezing point: 32 °F
- Boiling point: 212 °F