The short answer is because the boiling point of water (or any liquid for that matter) varies across different atmospheric pressures.
We are all taught by our elementary science teacher that water boils at 100 °C. This, however, is only true in the standard atmosphere which is approximately equal to Earth’s average atmospheric pressure at sea level. In the standard atmosphere, the atmospheric pressure is 1 atm or 101.325 kPa (Don’t mind the numbers, it is not important). As has already been noted, atmospheric pressure is not constant at all altitudes. Atmospheric pressure is greater than the standard atmosphere (1 atm) below sea level and less than 1 atm above sea level. In simple terms, atmospheric pressure is higher at lower altitudes and lower at higher altitudes. This is because the air molecules nearer to the center of the earth experience greater gravitational force, thus, those air molecules get closer together. When air molecules are closer together, the air becomes denser resulting in higher pressure. This is the same principle as to why we experience greater pressure further down a swimming pool than at the surface of the water.
What does the atmospheric pressure have to do with water’s boiling point?
To answer this question, we need to understand first what boiling point is. We know that the boiling point is the temperature at which the liquid boils and reaches its maximum temperature. In more sciency terms, it is the temperature at which the liquid’s molecules have enough energy to escape into the gas phase.
What heat does is that it adds energy to liquid molecules. This energy causes the liquid molecules to vibrate and spread out, increasing the distance between them. As the liquid molecules become more distant from each other, the liquid becomes less dense and takes more space. As this heating process continues, the liquid becomes less and less dense until it ultimately vaporizes into the gas phase with its vapor pressure similar to the surrounding atmospheric pressure. This is why boiling water turns into a gaseous phase called steam, and this transformation continues until no more liquid is left unless the energy source (heat) is removed.
The atmospheric pressure directly influences the boiling point of water because pressure determines the liquid’s resistance to vaporization. Increased pressure forces the water molecules closer together. When water molecules are closer together, greater energy is required to spread out the molecules until the water becomes so light that it turns into steam. A greater energy requirement for vaporization means greater applied heat.
Putting it all together, greater atmospheric pressure packing the liquid molecules tighter means greater energy is needed for those liquid molecules to escape the liquid phase and turn into the gas phase. The greater the atmospheric pressure acting on liquid, the higher its boiling point.
What is the relevance of atmospheric pressure and boiling point to a temperature sensor-equipped electric kettle?
Now that we know the relation of atmospheric pressure to a liquid’s boiling point, we can understand why an electric kettle that would rely on a temperature sensor is impractical. In the standard atmosphere where we are normally (approximately at sea level), this version of an electric kettle would work properly. The problem comes in when we use this electric kettle to boil water above or below the standard atmosphere.
If we take this electric kettle somewhere below sea level where the boiling point of water becomes 101 °C, the temperature sensing system would still work as intended to trigger at 100 °C. However, the water would never boil because as soon as it reaches 100 °C (still below the 101 °C boiling point), the temperature sensing system would turn off the kettle.
On the other hand, if we take this electric kettle on top of a mountain 1,000 meters above sea level where the boiling point of water becomes 99 °C, the temperature sensor would never detect 100 °C because 99 °C would be the maximum temperature of water. In this case, the temperature sensing system would never trigger to switch off the kettle because it would forever be waiting for the water to reach 100 °C, which would never happen. As a result, the water would not stop boiling until everything vaporized into steam.
The simple yet brilliant engineering of electric kettles
Luckily, the actual electric kettles are in no way similar to my imagined temperature sensor-equipped electric kettles. The electric kettles that we have in our own homes do not use temperature sensing systems to detect that the water has boiled. The simple engineering behind this electric kettle assures that it will always turn off the kettle whenever the water boils anywhere in the world at any boiling point. To check out how it works, click here.
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