Why does adding water to boiling oil cause an explosion but nothing happens when adding oil to boiling water?
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Two big reasons: Oil boils at a higher temperature than water, and oil floats on water.
If you have a pot of boiling water, it’s 212 degrees (100 C), and adding oil just puts some hot, but not nearly boiling, oil, floating on top of the water. That might splash a bit, and make a mess, but it’s not really dangerous.
If you have a pot of boiling oil, it’s around 570 degrees (300 C), and if you put water in it, the water sinks *and* boils at the same time. It boils very fast, because the oil is much, much hotter than the boiling point of water. That creates steam at the *bottom* of the oil. A lot of steam. A given volume of water makes about 1700 times that volume of steam. That’s like a teaspoon of water becoming 10 liters of steam. That steam pushes the oil up and out, like an explosion.
Now, that oil is still really hot, and a lot of stuff, like cloth, paper, and even wood, can burn if it hits them. The oil can catch on fire, too, so now you have a mess of hot, burning oil flying around your kitchen, setting stuff on fire.
Multiple things:
1g of water takes up about 1mL of volume (depends on temperature). 1 g of steam (at 1 atmosphere of pressure, also depends on temperature, but it doesn’t change the overall result.) takes up about 2700 mL. So if we add water to oil that is above the boiling point of water, then the water will boil into steam. Even worse, vegetable oil can be 100⁰F above the boiling point of water before it catches fire. That means the water can boil near instantaneously, expanding over 2700x its volume in a fraction of a second.
Also, water sinks in oil. Oil is less dense and floats on top. Meaning not only does the steam expand at a tremendous rate, it can push 300⁰F oil up and out along with it. If the oil is that hot, and it’s a gas stove, there’s a pretty good chance that it will combust. The rate of combustion is determined by the surface area of the oil. Since fire needs both fuel and oxygen (and heat, but we’ve got that covered already) the more fuel is in contact with the air, the faster it burns. So spraying tiny droplets of oil all over your kitchen, if it catches fire, it’s going to engulf in flames. If you drop about 1.5 tbsp of water, that expands to roughly a human-sized fireball in under a second. That’s a bad day.
Now let’s go the other way and put oil into boiling water. The water cannot be above 212⁰F because it wouldn’t be water anymore and we already know that oil can make it up to 100⁰ above that without issue, so no explosion. Hot things do expand a bit, but we’re talking a few percent, not a few thousand times. And some oil may have enough inertia to dip below the surface, but it will bob right back to the top so even if it did explode, it’s not going to push anything out with it.
This reminds me of something they told us when I worked in an aluminum foundry. They warned us to never ever throw water or icicles in the crucibles, because that would cause explosions. The principle is the same, the water sinks, boils and rapidly expends as a result, which displaces the oil (or melted metal) and it splashes everywhere.
Vegtable oil boils at around 300 celcius, water boils at 100 celcius.
If you place water into boiling oil it will vapourize and expand to 1700 times its previous volume carrying with it boiling oil and creating an explosion.
If you place oil into boiling water it merely gets warmed. Whilst the heat capacity (the energy needed to heat up said item 1 degree) is higher for water than the oil meaning the oil will heat up quicker, if the energy is not there to boil it, it won’t.
Water sinks beneath the oil, turns to stream and expands by about 1700x. Somethings gotta give and its the oil. It gets displaced and breaks up into smaller droplets which in turn creates a greater surface area for burning if an ignition source comes into contact with it.
Boiling oil is much hotter than boiling water. Add water to boiling oil, it sinks to the bottom and rapidly turns into steam. The steam expands quickly and turns the very hot oil into a mist, and the tiny droplets of already hot oil are easy to ignite if they get near an open flame (and if one catches fire, its close neighbors will to, and suddenly the whole mist of oil catches).
If you add oil to boiling water, the water is much cooler than the oil and the oil floats on top of it like a raft. Rather than all the water turning to steam very quickly, you get pockets of steam that rise up and push the oil aside. The steam doesn’t explode out turning the oil to a mist and there’s much less chance of the oil catching fire, unless it sloshes over the side of the container and drips into the flame. Even then, instead of millions of tiny fireballs hurtling through the air, you have a puddle of oil that takes time to heat up and catch fire.
Imagine you heat oil to 150ºC. It doesn’t boil, but it’s really hot. Now you pour water on it. The water heats above 100ºC really, really fast. So fast that it’s an explosion, which is just gasses expanding really fast.
Now, you heat up water to 100ºC. Now you pour oil on it. The oil heats up to 100 degrees really fast, but it doesn’t turn into gas, so it doesn’t explode.
This is one of the best real word examples of using a thought experiment:
A: Oil can be heated well above the boiling temp of water – and oil is flammable..
B: Boiling water has a temp limit (per physics) and lets add substance that does not do much at that temp.
Yeah I learned all about this when a ice cube slipped out my hands right into some hot oil I was using for spring rolls… Thought hmm that ice cube sank directly to bottom instead of floating…. That’s when my brain came back online and immediately grabbed a pan to cover the impending flash of oil/water that came soon after I slapped that pot ontop
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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