Because that oil is at a significantly higher temperature than the boiling point of water (depending on the type and grade of oil, potentially well over 200 C. Even my tiny chip frier goes to 190). The explosion is just water violently boiling.
Oil will not vapourise in boiling water, so thr reverse is safe.
Oil boils at a far higher temperature than water.
When water boils, it is expanding into steam. When water hits boiling oil, it gets to boiling quickly and expands quickly. This expansion pushes the oil out of the way and, if a lot of water was added, can do so in a way that looks like an explosion.
When you add oil to boiling water, the water is not hot enough to boil the oil. Thus, the oil just “mixes” with the water. It’s not a true mixture because as soon as the water stops boiling, the oil will rise to the surface and form a separate layer since oil and water don’t truly mix without an emulsifier.
Oil doesn’t boil in any practical sense, it burns. The boiling point for oil is MUCH higher than the smoking point and the smoking point for most oils is significantly higher than the boiling point of water which is 212°F/100°C.
The lowest smoking point for oil in this list is 325°F/165°C [https://www.seriouseats.com/cooking-fats-101-whats-a-smoke-point-and-why-does-it-matter](https://www.seriouseats.com/cooking-fats-101-whats-a-smoke-point-and-why-does-it-matter)
So, putting water in an environment that can quickly transfer energy to the water at above 212°F = water almost instantly turns to steam which is a pretty large expansion which is pretty much the definition of an explosion.
On the other hand, water cannot be above 212°F/100°C as a liquid under normal atmospheric pressures at sea level. So, adding oil to boiling water doesnt even bring the oil to the smoking point.
Water boils at 100 degrees C. Oil is commonly heated to double that in the case of deep frying… and can be heated much hotter depending on the situation.
Adding a small amount of water to this is going to cause the water to turn to a gas almost immediately. In the reverse, considering that water cannot get hot enough to turn oil into a gas, there’s really no danger of adding cold oil to boiling water.
Oil can get much hotter than the boiling point of water. That’s why you can fry things in oil but only boil or steam them in water.
When you pour water into superheated oil, there is enough heat energy present to boil the water instantly. The water breaks into tiny droplets as it enters the oil creating a large surface area which allows a lot of heat to transfer quickly meaning essentially instantaneously boiling water.
Lots of water boiling at once means a rapid expansion of gases which is an explosion.
Then the oil is blown out of the fryer into the air where *it* turns into little droplets. If there is any flame nearby (life if a grease fire was
the reason you dumped water in), those droplets ignite in a chain reaction which adds a bunch of fiery air expansion to the explosion resulting in a Michael Bay fireball that consumes your face.
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