# Is gravity an obstacle for homogenous solvation due to diffusion?

If you have a glas with a spoon of salt ($\ce{NaCl}$) in it and you carefully add the water($15^\circ\mathrm{C}$). A lot of the salt will not be dissolved immediatelly, it will take a while for full dissolution.

When the salt dissolves, it is said that the salt is mixed equally in the water. But $\ce{NaCl}$ is split in two ions $\ce{Na+}$ and $\ce{Cl-}$. $\ce{Cl}$ has a mass of $35$ and Na $23$. But due to gravity (or is really curved space in the glass a better word) you should expect that the $\ce{Cl}$ atom will be more at the bottom then the $\ce{Na}$ ion. Is that right or is there someting I'm missing?

So can you say that after an hour the salt is evenly diffused through the water and that there are probably more more $\ce{Na+}$ ions at the top of the glass, so that the top of the salt water will taste more salty due to presence of sodium, which has more power of giving salty tast?

• The difference in concentrations caused by gravity is totally negligible for any reasonable purpose, and most certainly can't be detected by taste. – Ivan Neretin Mar 3 '16 at 16:47
• A 50kg ball and a 5gm ball fall at the same rate (acceleration due to gravity is independant of mass). So shouldn't Na+ ion also try to move down with an equal rate? – Surya Teja Mar 3 '16 at 17:48
• I presume that it is a rethorical question Surya? But indeed they has the same speed because of gravity and enertia of the equivelence principe but there is still a difference due to upward forces of the second law of Newton or/and Archimedes force which are higher for heavier objects what them make slower falling – Marijn Mar 3 '16 at 19:10
• Since the salt is dissolving without stirring, a different factor in microgravity (earth orbit) is that the salt wouldn't sink to the bottom. It would float in the solution while dissolving. Since the salt isn't all at the bottom, it should dissolve faster in microgravity. – MaxW Mar 17 '16 at 15:14
• The normal gravity differential just isn't enough. But if you had NaI and CsCl in a 4 inch diameter pipe which was 100 miles high, then some ionic differentiation would happen due to gravity. So your idea isn't wrong, you just misunderstand the scale needed. – MaxW Mar 17 '16 at 15:33

At normal Earth gravity, for all practical purposes homogeneous solvation is not hindered by gravitational force.

However: For some solutes such as sucrose and $\ce{CsCl}$, though, the "artificial large gravity" of centrifugation is capable of establishing concentration gradients in their solutions. One example in common laboratory use is buoyant density centrifugation:

One of the most common methods of fractionating DNA is to exploit differences in density between different types of DNA in concentrated salt solutions. Caesium chloride is the most commonly used. The method involves prolonged high speed centrifugation of purified DNA in concentrated caesium chloride. The centrifugation produces a gradient of salt concentration in the centrifuge tube and the DNA migrates to occupy a position in the gradient which corresponds to its own buoyant density.

This is actually a very interesting question! The reason why NaCl dissolves (and dissociates) at all is that there are intermolecular interactions between Na and Cl ions and water molecules. Compared to these forces (ion-dipole forces), the force due to gravity on the ions is negligible.

On a related note: Brownian motion (the seemingly random, erratic, and gravity-defying motion of small pollen particles in air) was one of the key pieces of evidence confirming the existence of atoms and molecules in the first place!

https://en.wikipedia.org/wiki/Brownian_motion

It is worth noting, however that when liquids aren't miscible, the force due to gravity does have a noticeable effect on our chemical system.