Answer the following:
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)?
The triple-point of water is a standard fixed point because it is unique and reproducible — at this point, all three phases of water (solid, liquid, and vapour) coexist in equilibrium at a specific temperature and pressure. The melting point of ice and boiling point of water are not suitable as standard fixed points because they depend on pressure. At different pressures, ice melts and water boils at different temperatures, making these points non-unique and less reliable for precise thermometry.
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?
The other fixed point on the Kelvin absolute scale is absolute zero, which is assigned the number 0 K. This is the temperature at which all molecular motion theoretically ceases.
The absolute temperature (Kelvin scale) T is related to the temperature on the Celsius scale by . Why do we have 273.15 in this relation, and not 273.16?
The value 273.15 appears in the relation because the Celsius scale was originally defined with 0°C assigned to the melting point of ice at standard atmospheric pressure (1 atm). This melting point corresponds to 273.15 K on the absolute scale. The triple-point of water (273.16 K) is slightly higher at 0.01°C. Thus, the conversion uses 273.15, not 273.16, to maintain consistency with the original Celsius scale definition.
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 triple-point of water is 273.16 K. On the absolute scale where the unit interval size equals that of the Fahrenheit scale, the relationship is: 1 K = 1.8 Fahrenheit units (since 100°C = 180°F interval). Therefore, the temperature of the triple-point on this scale = = 491.69 units (approximately).
Explanation
This question tests understanding of temperature scales and fixed points in thermometry. Part (a) explains why triple-point is preferred over pressure-dependent phase change points. Part (b) identifies absolute zero as the second fixed point. Part (c) clarifies the distinction between triple-point (273.16 K) and ice point (273.15 K). Part (d) requires converting the triple-point temperature to a scale with Fahrenheit-sized units, yielding approximately 491.69 units (similar to the Rankine scale).