In thermodynamics, what is the temperature value of the triple point of water, where solid, liquid and gas coexist in equilibrium?

Introduction / Context:
The triple point of water is a famous reference point in thermodynamics and physical chemistry. It is the unique combination of temperature and pressure at which ice, liquid water, and water vapour can coexist in equilibrium. This concept is used to define temperature scales precisely and appears frequently in competitive exam questions. The question asks for the numerical temperature value of the triple point of water expressed in kelvin.

Given Data / Assumptions:

• The triple point of water refers to the condition where solid, liquid, and gas phases of water are in equilibrium.
• The answer choices include values in degrees Celsius and in kelvin.
• We assume standard thermodynamic definitions and the internationally accepted value of the triple point temperature.
• Pressure at the triple point is specific, but the question focuses only on temperature.

Concept / Approach:
The triple point of pure water is defined as 273.16 K, which corresponds to 0.01 °C at a specific pressure of about 611 pascals. The kelvin scale historically used the triple point of water as a fundamental reference. Among the options provided, only 273.16 K matches the precise triple point temperature. The value 0 °C corresponds to the normal freezing or melting point at 1 atmosphere, not the triple point. Negative 273.16 K is physically meaningless, and 273.16 °C is far too high and incorrect for this context.

Step-by-Step Solution:
Step 1: Recall that the triple point of water is approximately 0.01 °C, not exactly 0 °C. Step 2: Convert 0.01 °C to kelvin using the relationship K = °C + 273.15, giving a value of 273.16 K (approximately). Step 3: Compare this calculated value with the options given in kelvin and degrees Celsius. Step 4: Identify that 273.16 K is listed explicitly among the options, while 0 °C and other values do not match the defined triple point. Step 5: Select 273.16 K as the correct temperature value for the triple point of water.

Verification / Alternative check:
Standard thermodynamics texts and international standards such as those from metrology organisations define the triple point of water as exactly 273.16 K in older temperature scale definitions. This value was used to fix the size of the kelvin unit before more recent redefinitions. No reliable source uses -273.16 K for any physical temperature, because absolute zero is 0 K and temperatures cannot be negative on the kelvin scale. Similarly, 273.16 °C would correspond to over 500 K and is unrelated to any phase equilibrium of water at normal laboratory conditions. These checks confirm that 273.16 K is the correct value.

Why Other Options Are Wrong:
The value 273.16 °C is incorrect because it is extremely high and does not correspond to the triple point or any common phase change of water. The value -273.16 K is impossible, since the kelvin scale starts at absolute zero (0 K) and does not allow negative values. The value 0 °C represents the normal freezing point of water at 1 atmosphere, not the triple point, which occurs at a slightly different temperature and at a lower pressure. Thus, none of these alternatives correctly represent the triple point temperature.

Common Pitfalls:
Students often confuse the triple point of water with its freezing point, assuming both are at 0 °C. Another common error is to ignore the distinction between degrees Celsius and kelvin, leading to careless selection of a numerically similar but incorrect value. To avoid these mistakes, remember that the triple point is slightly above 0 °C at 0.01 °C, which corresponds to 273.16 K. Also keep in mind that the kelvin scale is an absolute scale starting from absolute zero, so negative kelvin values are not physically meaningful.

Final Answer:
The temperature at the triple point of water is 273.16 K, at which ice, liquid water, and water vapour coexist in equilibrium under specific pressure conditions.