Friday has arrived, and with it the weekly "A Taste of Physics" column, number 27.
This week — the triple point, water that boils and freezes simultaneously, and its fascinating connection to pressure cookers.
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The video shows water in a truly unique state.
Some of the water is liquid, some is evaporating, and some is turning to ice — all at the same time.
This phenomenon occurs only at a precise combination of temperature and pressure, and to understand it we first need to understand why water turns to steam or freezes in the first place.
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**What is temperature?**
Temperature defines the average kinetic energy of all the molecules in a substance.
The higher the temperature, the faster water molecules collide with one another — and the more likely they are to break free.
When we boil water in a pot, we keep adding energy to the molecules until their collisions are fast enough to allow them to escape into the air as steam.
Pressure is the opposing force.
The weight of the air presses down on the water's surface and prevents molecules from escaping as steam. The higher the pressure, the more kinetic energy the molecules need in order to break free.
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This means that to evaporate water, we can either keep adding energy — or we can reduce the pressure without changing the temperature at all.
When the pressure above the water is low, even a gentle molecular motion is enough to cause molecules to leave the liquid and become steam.
The triple point is the precise equilibrium at which only a minimal change in energy is needed to shift the substance from one phase to another.
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In the experiment shown in the video, the water is at a temperature of 0.01 degrees Celsius and a pressure of exactly 611.73 pascals.
Under these conditions, the vessel contains very low pressure. This causes the water to "boil" — shedding molecules into the surrounding space as steam in order to equalize the pressure with the near-vacuum.
The steam carries a large amount of energy away from the remaining water during evaporation, causing it to freeze.
When the steam condenses, it releases its energy, contracts, and causes the pressure to drop again — which causes the ice to turn back into water, then into steam, and so on in a continuous cycle.
The small amount of energy the vessel absorbs from the surrounding room is enough to keep the process going.
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In the real world, these facts have many practical implications — one of them being the pressure cooker.
The airtight lid creates especially high pressure above the water inside, preventing it from evaporating, so its temperature rises well above the normal boiling point and allows for much faster cooking.
Shabbat Shalom 😊
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