Why does adding water to boiling oil cause an explosion but nothing happens when adding oil to boiling water?
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When you pour water on to boiling oil, a couple of different things happen at near enough the same time.
The first is that the water sinks. The water is a heavier/more dense liquid than the oil, so just like dropping something heavy like a marble into water will cause the item to sink right to the bottom, the same will happen to the water you drop into oil.
Alongside this though, oil typically boils at a higher temperature than water does. This means that when the water sinks down into the oil, it is suddenly surrounded on every side by a liquid that is hotter than the 100°c water will reach before it starts to boil itself. This heat causes the water to boil pretty much instantly, and when water boils it turns into steam.
Combine these two things and you have an amount of water that has sunk down below your oil flash boiling and expanding into steam in an instant. That steam needs to go somewhere, but because it is trapped under the latter of oil it had nowhere to expand to – until it pushes the oil out of the way. Because this happens very suddenly and violently, the steam won’t slowly bubble up through the oil, but very violently throw the oil up and outwards as it expands from below.
Do this when the oil is on fire (such as if a pan of oil being used to fry food) and you have a flammable liquid getting thrown up into the air and igniting, which can make quite the fireball.
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