The atmospheric pressure is 101 KPa, which is the same as the saturated vapor pressure of water at 100°C. Is there any connection between them? !
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At 100°C, the saturated vapor pressure of water is 101324.72 Pa, which is roughly equal to the atmospheric pressure exerted by the atmosphere. Is there any connection between the two? ! Thank you! !
The saturated vapor temperature of water varies with atmospheric pressure. For example, it is impossible to cook rice on plateaus at several thousand meters above sea level, as the temperature of boiling water is below 100 degrees.
This post was last edited by ylb913 on 2016-12-15 at 12:53. This is the definition of 100°C – the temperature at which water boils under 1 standard atmosphere of pressure is 100°C. If the actual atmospheric pressure is higher than the standard atmospheric pressure, water will not boil at 100°C; if the atmospheric pressure is lower than the standard value, water will boil before reaching 100°C.
I know that. What I want to ask is, is there a strong connection between the atmospheric pressure in Earth’s atmosphere and water? !
The pressure of a vapor that is in phase equilibrium with a solid or liquid at a certain temperature under sealed conditions is called the saturated vapor pressure. In an open environment, the saturated vapor pressure is the partial pressure of the vapor phase of a substance at the liquid surface during gas-liquid equilibrium at a certain temperature. For example, at 30°C the saturated vapor pressure of water is 4245.5 Pa. Thus, if a closed container contains only water, the pressure of the gas inside the container at 30°C will be 4245.5 Pa. In an open environment, the pressure above the water surface is the atmospheric pressure, namely 101 KPa. However, the vapor pressure of water above that surface remains 4245.5 Pa, with the remaining partial pressure coming from air. At the temperature you mentioned of 100°C, the saturated vapor pressure of water is 101324.72 Pa, which is equal to the atmospheric pressure. This means that the vapor pressure of water at the surface of the water is 101324.72 Pa; there is no other air present. However, since it is an open system, air will diffuse to the surface of the liquid. Therefore, the liquid water must continuously vaporize in order to maintain a vapor pressure of 101324.72 Pa at the liquid surface. But in an open system, the pressure at the water surface cannot exceed the atmospheric pressure, so the temperature of the water cannot rise any further – this temperature is thus the boiling point of water. Therefore, at the boiling point temperature of a standard atmosphere, the saturated vapor pressure of any liquid is equal to the atmospheric pressure. The saturated vapor pressure of ethanol at 78.3°C is 101.33 kPa, which is also the atmospheric pressure. So the 100°C you mentioned is just the boiling point of water at standard atmospheric pressure. If the atmospheric pressure is 4245.5 Pa, the saturated vapor pressure of water at 100°C remains 101324.72 Pa; however, the boiling point of water in this case is 30°C. But if water is heated in an open container, it will never reach 100°C.
““If water is heated with the lid off, it will never reach 100°C.” But when I use a kettle with an open lid to heat water, it can boil! !
It’s a waste of time talking to you like this. What I’m saying is, what if the atmospheric pressure is 4245.5 Pa?
Boiling doesn’t necessarily mean 100 either℃
In other words, at the temperature at which water boils, the saturated vapor pressure of water equals the atmospheric pressure. The reason you think there is a connection between atmospheric pressure and saturated vapor pressure is that when you heat water in an open container, the pressure at the surface of the water cannot be higher than the atmospheric pressure, as the difference in pressure between the atmosphere and the space above the heated water is quite large. If a pressure cooker is used, when heated to 100°C, the pressure inside the cooker equals the saturated vapor pressure of water, which is equal to atmospheric pressure. As the temperature continues to rise, the pressure inside the cooker becomes greater than the saturated vapor pressure of water (ignoring the small amount of air present in the cooker, whose partial pressure is negligible). Returning to your question, the atmospheric pressure is 101 KPa, which is the same as the saturated vapor pressure of water at 100°C. It’s simply because the boiling point of water under standard pressure is 100°C; therefore, the saturated vapor pressure of any substance under standard pressure at its boiling point is equal to the atmospheric pressure. It’s just because water is the most common and its boiling point is 100°C. In a mixture of ethanol and water inside a sealed container, at a certain temperature, the partial pressures of these two substances in the gas phase are their respective saturated vapor pressures at that temperature.