# Why is kerosene preferred over other non-polar solvents to store sodium in?

I think we all heard this again and again in our early schooling days:

Sodium is always stored under kerosene so that it doesn't react with air, and corrode itself.

But lately I've been wondering, why kerosene? Why not benzene? Why not cyclohexane? Why not $\ce{CS2}$? What is that makes kerosene preferred?

Is kerosene the cheapest non-polar liquid available? Or is there any particular reason that I'm not aware of?

• I always store lithium under mineral oil. I've never tried kerosene though because in case the metal "accidentally" catches fire, being surrounded by kerosene doesn't help O:) – paracetamol Jul 10 '17 at 16:58
• I'm not sure sodium is always stored under kerosene is even a true statement, at least not in the UK/USA. – NotEvans. Jul 10 '17 at 17:08
• I expect $\ce{CS2}$ to react with sodium, and quite eagerly. The rest are more or less interchangeable. – Ivan Neretin Jul 10 '17 at 17:08
• @paracetamol I suspect lithium is lighter than kerosene and stays afloat, that's why mineral oil is used for $\ce{Li}$ ($\pu{0.80 g/cm^3}$ (kerosene) vs $\pu{0.53 g/cm^3}$ (lithium)). – andselisk Jul 10 '17 at 17:10
• Regarding the question, I guess any cheap inert water-free/hydrophobic liquid with $\rho < \pu{950 g/cm^3}$ which is also miscible with lubricants or is a lubricant itself (if alkali metal needs to be cut or mechanically processed) will do just fine. – andselisk Jul 10 '17 at 17:31

Kerosene is a fraction from the distillation of petroleum, the one with boiling point 180° to 230°, containing hydrocarbons from $\ce{C_11}$ to $\ce{C_12}$. In reaction to what has been said in the commentaries (below the question), I note that mineral oil is a cloudy expression whereas kerosene seems to be the precise name of the more or less broader base material of this cloud.
The best means to store alkali metals, which are all strong reductants, is an alkane because they are inert (as indicated by the name “paraffin”) and cannot be reduced.$^1$ The reason why one does not take the lower boiling fraction (gasoline 40° to 180°, $\ce{C_6}$ to $\ce{C_10}$), cyclohexane (b.p. 81°) or hexane (b.p. 69°) is obvious: it would be too evaporable, which is dangerous since its evaporation when someone forgets to close the bottle sufficiently, would lead to spontaneous ignition of the alkali metal (especially in the case of potassium) by contact with moist air. (Additionally, cyclohexane would anyway be unnecessary expensive.) The reason why the higher boiling fraction (light gas oil, 230 to 305°, $\ce{C_13}$ to $\ce{C_17}$) is not used is that this is much more greasy than kerosene.
$^1$ In the case of lithium, which is much less reactive than the other alkali metals, the container should ideally be filled level with the top since it is swimming on the kerosene. But anyway the kerosene stays over as a protecting film on the lithium, which obstructs its reaction with moist air, even if it is not submerged.