When frying, bubbles are formed that project oil into the air. Source: pxfuel.com, public domain image.
This is something that is a little difficult to prevent from happening, although we can control that it is not so excessive, my mother's advice was not to throw the food into the pan but to place it carefully, another advice is to try to dry the food with absorbent paper to remove excess water or put a lid on the pan, and perhaps each cook has his own trick.
But why does this happen?
The short answer is that the water in the food vaporizes almost instantly when it comes into contact with the hot oil, forming steam bubbles that explode and project the oil in all directions. But let's analyze this hated effect a little more, since it has to do with the physical and chemical properties of the two substances involved, water and oil.
When frying food, the intensity of the oil splashes depends largely on the humidity of the food, and can range from the formation of slight bubbles to a violent splash.
Although colloquially we refer to the oil jumping, the correct thing would be to say that the water that falls on the oil jumps, and as a consequence, it drags part of the oil. It's a process we all know, but let's do this experiment, proceeding very cautiously, we're going to heat some oil in a pan, and once it's hot we add a drop of water, which we observe is described in the following animated image.
The result is as expected, when the drop of water falls there is immediately a violent reaction that expels drops of oil in all directions and we hear the characteristic sound produced when frying, known as crackling. But let us see what happens if we act now in the opposite way, and first heat some water, and when we observe that it reaches its boiling point we let a drop of oil fall.
As we see, the result is that nothing happens, the drops of oil on hot water remain supernatant on the surface, there is no projection of material and no noise.
To explain this difference when reversing the process, which explains why the oil jumps, we must resort to the analysis of three properties of these two substances: miscibility, density and boiling point.
Let's describe these three properties separately.
Miscibility
Miscibility refers to the ability of some liquids to mix in any proportion, forming a homogeneous solution[1].
And it is a well known fact that these two substances do not mix, this is due to the molecular structure of both molecules, so they are said to be immiscible.
Oils, as well as fats, are made up of molecules called triglycerides, which are triesters of fatty acids, that is, esters of glycerine and long chain organic acids, and because of the shape of this molecule it is a substance of a non-polar nature. Oils are formed by different quantities and types of fatty acids, saturated and unsaturated, so we cannot speak of it as a pure substance but rather as a mixture of several.
On the contrary, water is a pure substance constituted by two atoms of hydrogen and one of oxygen, in which the electrons of the hydrogens have been displaced towards the atom of oxygen, concentrating the electronic density on this atom and conferring polarity to the molecule, that is, a negative pole appears.
Influence of the polarity of the molecules on the miscibility of water and oil Source: 
The polarity of the molecules influences their miscibility, therefore a chemical saying says "like dissolves like", which means that substances with polar molecules dissolve in polar substances, such as ethanol in water, but then non-polar substances such as oil do not dissolve well in polar solvents. Therefore each substance seems to repel the other.
Density
Another known fact is that the two substances also have different densities. As many of us know, the density of water at room temperature is about 1 g/cm3 and oils used in cooking can have a density ranging from 0.840 to 0.960 g/cm3[2].
Therefore we see, as in the previous image, that if we pour oil on water it will be on top, since it is less dense it will float on the water, but if we proceed in the opposite way, and we pour water in oil, the water will sink. A simple way to experience the difference in density of these two liquids is by doing this simple experiment: we can take a container with oil and place an ice cube; since water in solid form is slightly less dense, the ice floats over the oil, but as the oil melts, the liquid water is deposited at the bottom of the container, as we can see in the following figure.
Boiling temperature
The other aspect to be discussed is the boiling point of the substances, or well, mainly the boiling point of water; Since oil does not have a boiling temperature properly, it can reach temperatures between 150 and 200 °C[3] without boiling, and above that they begin to decompose, that's why when we leave a pan alone with oil for a long time over the fire, we see how a large amount of smoke begins to form, corresponding to the vaporization of fats with the lowest boiling points, remember that oil is a mixture of several similar substances, so it is more appropriate to talk about another type of temperature, the smoke temperature.
Water, on the other hand, being a pure substance, boils at a constant temperature of approximately 100 °C if we are in areas at sea level, a much lower boiling temperature than that which can be reached when heating oil for frying. So it will vaporize almost instantly if we add it to a pan of hot oil.
Let's gather the facts
Now, let's see how the three properties of the substances we have mentioned influence the explanation of the phenomenon that interests us, why the oil jumps.
When we have a pan with hot oil, let's say at 150 °C, due to the difference in density of both liquids, the tendency of the water is to sink into the oil, immersing itself into a medium with a temperature higher than its boiling point, so the water boils suddenly, and since the water is immiscible in the oil, instead of dispersing, a drop forms where the water vaporizes in a localized manner, and when it explodes it carries with it some oil.
The combination of these three properties is responsible for "skipping the oil", note that if we remove only one of them, this would not happen. For example, if the water were not denser than the oil (but immiscible and with a lower boiling point), so that it would float over the oil instead of sinking, the sudden vaporization would occur on the surface and would not produce the projection of oil everywhere, something similar to what happens when a drop of water falls on a very hot sheet of metal.
On the other side, if the water had a higher boiling point (maintaining that it is immiscible and with a higher density), it would only submerge without boiling in the oil, without showing a noticeable effect; very similar to what happens when you add the drop of oil to hot water, we only see that the drop remains without boiling. And if in the last case, the water and oil were miscible with each other, maintaining the difference in density and the boiling point, the oil drop would be diminished by the dispersion of the water in the oil.
Conclusion and contribution
This post allows us to socialize how the properties of the substances influence the daily phenomena, allowing to discuss how from the structure of the molecules to the physicochemical properties of the substances influence the observed fact.
But this analysis also has other applications, since the oil that is projected and shot in all directions is a great danger because it can even reach our face, and we know that it is very hot, so it is the cause that we must be careful when frying food.
It is also the reason why it is not recommended to use water to extinguish a fire caused by the inflammation of the oil; by a carelessness a pan with oil can start burning, and if by nerves we run to sprinkle water to suffocate it, it only makes things worse, causing fine drops of oil to jump with much violence, and this steam ignites causing a great flame. It is best to cover the pan with its lid or with a damp cloth (well drained, not dripping water) and remove it from the kitchen. This way, when the oxygen is consumed, the fire will be extinguished.
Flame for the imflamation of oil vapors. Source: Pickpik.com, image for free use.
As we can see, thanks to the fact that oil can be heated to a higher temperature than water and because of the immiscibility of both liquids, frying food gives us a very fast cooking method, as well as being tasty, but the science behind this cooking method also allows us to establish relationships between the structure and properties of the substances, which are the basis for safety measures in the kitchen.
Well friends, I hope you liked the post. And remember, just as in the laboratory, in the kitchen there are substances and methods that require attention, not for everyday we must be careless. Until next time!
References
- Wikipedia.com. Miscibility.
- Las Valdesas Olive Oil. Which is the density of oil?
- Wikipedia.com. Fritura