11.4 Answer the following :

(a) The triple-point of water is a standard fixed point in modern thermometry. Why? What is wrong in taking the melting point of ice and the boiling point of water as standard fixed points (as was originally done in the Celsius scale)?

(b) There were two fixed points in the original Celsius scale as mentioned above which were assigned the number 0 °C and 100 °C respectively. On the absolute scale, one of the fixed points is the triple-point of water, which on the Kelvin absolute scale is assigned the number 273.16 K. What is the other fixed point on this (Kelvin) scale?

(c) The absolute temperature (Kelvin scale) T is related to the temperature tc on the Celsius scale by tc = T – 273.15.

Why do we have 273.15 in this relation, and not 273.16?

(d) What is the temperature of the triple-point of water on an absolute scale whose unit interval size is equal to that of the Fahrenheit scale?

(a) The triple point of water has a unique value of 273.16 K. The triple point of water remains equal to 273.16 K for particular values of volume and pressure. The melting point of ice and boiling point of water depends on pressure and temperature and do not have constant values.

(b) The absolute zero or 0 K is the other fixed point on the Kelvin absolute scale.

The temperature of 273.16 K is the triple point of water. It is not the melting point of ice. The melting point of ice is equal to 0°C on the Celsius scale and 273.15 K on the Kelvin scale.

Hence, absolute temperature (Kelvin scale) T, is related to temperature tc, on the Celsius scale as: tc = T – 273.15.

(d) If TF is the temperature on the Fahrenheit scale and TK is the temperature on the absolute scale, then:

TF-32180=TK-273.15100    ....i

If TF1 is the temperature on the Fahrenheit scale and TK1 is the temperature on the absolute scale, then:

TF1-32180=TK1-273.15100    ....ii

Given that:

TK1-TK=1 K

Subtracting equation (i) from equation (ii),

TF1-TF180=TK1-TK100=1100TF1-TF=1×180100=95

 

The triple point of water = 273.16 K

The triple point of water on the absolute scale = 273.16×95=491.69