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Q10.4: NCERT Physics - Part 2 THERMAL PROPERTIES OF MATTER 10.4 Solution: Answer the following: (a) The triple-point of water is a…
⚠️ Common Exam Traps & Conceptual Pitfalls
Students frequently use the triple‑point temperature (273.16 K) as the offset when converting Celsius to Kelvin, forgetting that the Celsius scale is defined by the ice‑point (273.15 K) at 1 atm; this leads to an incorrect relation $t_c=T-273.16$. Remember: the Kelvin scale is anchored at the triple point, while the Celsius scale is anchored at the melting point of ice, so the correct conversion is $t_c=T-273.15$. Additionally, avoid treating the boiling point as a fixed point because it varies with pressure, unlike the triple point.
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? (b) There were two fixed points in the original Celsius scale 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) is related to the temperature on the Celsius scale by . 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?
Step-by-Step Verified Solution
**Answer:** (a) Triple point is unique; melting/boiling points depend on pressure. (b) Absolute zero. (c) 273.15 is the ice point. (d)
**Step-by-Step Explanation:**
(a) The triple point of water occurs at a unique temperature and pressure (273.16 K, 0.006 atm). The melting point of ice and boiling point of water depend on pressure, so they are not unique fixed points.
(b) The other fixed point on the Kelvin scale is absolute zero (0 K).
(c) 273.15 K is the ice point (melting point of ice at 1 atm), while 273.16 K is the triple point. The Celsius scale was originally defined using the ice point and steam point, so the relation uses 273.15.
(d) On the Fahrenheit scale, the ice point is 32�F and the steam point is 212�F, so there are 180 divisions between them. The triple point is at 32.018�F. On the Rankine scale (absolute Fahrenheit scale):
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