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The Mystery of Boiling Point: Why do liquids boil at different temperatures under different pressures?
When we boil water in the kitchen, we may not think that the boiling point of the water is affected by the surrounding pressure. The boiling point of water at standard atmospheric pressure is 100°C, but this data varies at different ambient pressures. For example, the boiling temperature of the water decreases when we are at high altitudes, which is one reason why cooking is especially important when climbing.
Boiling point is the temperature at which a liquid transforms into a gas when its vapor pressure reaches equal to the surrounding ambient pressure.
In a low-pressure environment, the boiling point of a liquid decreases. For example, at an altitude of about 1905 m, the boiling point of water drops to 93.4°C. This is due to the fact that at that altitude, the atmospheric pressure decreases, the vapor pressure of the water reaches that pressure more easily, and therefore the water starts to boil more easily.
Relationship between boiling point and environmental pressure
The boiling point of a liquid increases with increasing ambient pressure. As the pressure increases, the required vapor pressure of the liquid rises and therefore the boiling point. This phenomenon can be clearly seen in pressure cookers where the pressure inside is much higher than the external atmospheric pressure, which allows the liquid to boil at higher temperatures.
The difference between thermal evaporation and boiling is that evaporation is a phenomenon that occurs on the surface of a liquid, while boiling proceeds throughout the liquid.
In the process of boiling, the liquid produces air bubbles that form throughout the liquid and rise to the surface to form boiling. Thermal evaporation is the process in which molecules on the surface of a liquid escape into gas as the temperature rises. This process can also occur at lower temperatures.
Saturation temperature and pressure
The saturation temperature of a liquid is the boiling temperature it can achieve at the corresponding saturation pressure. Simply put, a liquid, under specific conditions, can be considered “saturated,” meaning that if there is additional heat energy input, the liquid will begin to boil. At high altitudes, the saturation pressure decreases and therefore the saturation temperature decreases accordingly.
Under different standards, the common boiling point of water is 100 °C (212 °F), and the actual boiling point at 1 atm is 99.97 °C (211.9 °F).
This means that in practical applications, the measurement of the boiling point needs to be calibrated according to the ambient pressure at the time. For example, at the height of Mount Everest, the boiling point of water is only 71°C (160°F). This not only affects the safety of drinking water for climbers, but also affects the time and effectiveness of food cooking.
Boiling point of a chemical
Each chemical has its unique boiling point, a characteristic that can be used to indicate the volatility of the substance. In general, a lower boiling point often means that the substance is more volatile. For example, helium has the lowest boiling point, while tungsten and steel have very high boiling points, over 5000 K. The boiling point characteristics of these substances play an important role in many industrial processes.
Understanding the characteristics of the boiling point and its variations under different pressures and environments is essential for the fields of chemistry, physics, and engineering.
Impurities and Boiling Points
In addition, in solutions containing nonvolatile impurities, the boiling point is increased, a phenomenon known as boiling point elevation . For example, the boiling point of salt water is higher than that of pure water. This characteristic is very useful in culinary and industrial applications and can influence process design.
Conclusion
Understanding how boiling point is affected by environmental stress is important not only for everyday cooking, but also has profound implications for scientific research and industrial applications. How many interesting scientific principles are hidden behind this seemingly ordinary phenomenon?
