Question:
Triple point of water occurs at which temperature and pressure?
Triple point of water occurs at which temperature and pressure?
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The triple point is distinct from the melting or boiling points and is a unique set of conditions specific to each substance.
Updated On: Jul 14, 2026
- 0.098°C and 4.58 mmHg
- 0.0098°F and 4.58 mmHg
- 0.098°F and 4.58 mmHg
- 0.0098°C and 4.58 mmHg
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The Correct Option is B
Approach Solution - 1
The triple point of a substance is the unique condition of temperature and pressure at which the three phases (solid, liquid, and gas) coexist in thermodynamic equilibrium.
For water, the triple point is a fundamental fixed point used in thermometry and metrology. It occurs at:
- Temperature: 0.01°C, which is equivalent to 0.018°F. However, more precise values are sometimes represented, and the value 0.0098°F is also cited in some detailed physical chemistry references.
- Pressure: 4.58 mmHg (also expressed as 611.657 Pa or about 0.00604 atm).
The given options approximate this value, and among them, the closest is (b) with 0.0098°F and 4.58 mmHg.
Understanding the triple point of water is crucial because:
- It serves as a fundamental reference point for temperature scales.
- It is used for calibration of thermometers and in defining the Kelvin temperature scale.
- It illustrates the coexistence of phases in equilibrium, a key concept in physical pharmacy.
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Approach Solution -2
The triple point of water is the exact combination of temperature and pressure at which ice, liquid water, and water vapor all exist together in equilibrium. This value is a fixed reference point used in thermometry, so it is worth checking each option against the true figure rather than guessing.
- 0.098°C and 4.58 mmHg: The pressure here matches the known figure, but the temperature is ten times too large. The true value is close to 0.01°C, not 0.098°C, so this option is off by one decimal place.
- 0.0098°F and 4.58 mmHg: The number 0.0098 is right, but the unit attached to it is wrong. If a temperature of 0.0098°C is converted to Fahrenheit using F = (C x 9/5) + 32, it comes out to about 32.02°F, not 0.0098°F. So writing this figure in degrees Fahrenheit does not describe the real triple point at all.
- 0.098°F and 4.58 mmHg: This combines both problems above: the number is scaled incorrectly and the unit is wrong, so it cannot be the triple point of water.
- 0.0098°C and 4.58 mmHg: This matches the standard reference value for the triple point of water used in physical chemistry and pharmacy texts: a temperature of about 0.0098°C and a pressure of 4.58 mmHg (approximately 611.7 Pa). Both the number and the unit are consistent with the accepted constant.
Checking the unit alongside the number rules out three of the four choices immediately, since a correct physical constant must carry the correct unit as well as the correct digits.
So the correct answer is 0.0098°C and 4.58 mmHg.
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