 # The triple point of water is a standard fixed point in modem thermometry

(i) The triple point of water is a standard fixed point in modem 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)?
(ii) 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?
(iii) The absolute temperature (kelvin scale) T is related to the temperature tc on the Celsius scale by\$t_{ c }\$ = T - 273.15. Why db we have 273.15 in this relation and not 273.16?
(iv) 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?

The melting point of ice as well as the boiling point of water change with change in pressure. The presence of impurities also changes the melting and boiling points. However, the triple point of water has a unique temperature and is independent of external factors.
(ii) The other fixed point on Kelvin scale is absolute zero,
which is the temperature at which the volume and pressure of any gas become zero.

(iii) As the triple point of water on Celsius is 0.01°C (and not 0°C) and on kelvin scale 273.16 and the size of degree on the two scales is same, so
\$t_{ c }\$ - 0.01 = T-273.16
\$t_{ c }\$=T- 273.15
(iv) One degree on fahrenheit scale
= 180/100=9/5 divisions on Celsius scale.

But one Celsius scale division -is equal to one division on kelvin scale.
Triple point on kelvin scale (whose size of a degree is equal to that of the fahrenheit scale) = 273.16 x 9/5 = 491.69