23
$\begingroup$

This is something that just occurred to me. If heavier elements sink, then how can the entire ocean be salty? Shouldn't the 'salt', because of its density, all sink to the bottom of the ocean? In theory, only the deepest parts of the ocean should be salty, while the top of the ocean is not. Yet, the only water in the world that isn't salty comes from rain and rivers. How can this be?

$\endgroup$
  • 5
    $\begingroup$ You will have better luck with this on the physics stack. This question is entirely about what a solution is. $\endgroup$ – John Jul 22 at 17:40
  • 15
    $\begingroup$ Sometimes it does, in brine pools at the bottom of the ocean. Watch this eel risk its life exploring one: youtube.com/watch?v=ZwuVpNYrKPY $\endgroup$ – Chloe Jul 22 at 21:49
  • 7
    $\begingroup$ This question clearly has Earth Science-specific answers and should stay open here. Voting to remain open! $\endgroup$ – uhoh Jul 23 at 6:36
  • 2
    $\begingroup$ I wander if the ocean brine pools have any cool fossils in them. I suppose lot of fish may not survive venturing in one and end up being conserved by the salt forever. $\endgroup$ – Tomáš Zato Jul 23 at 12:07
  • 4
    $\begingroup$ @TomášZato Interestingly, there are brine pool loving bacteria living there - not sure what they get their energy from, but they may not reject a fish from time to time. $\endgroup$ – Volker Siegel Jul 24 at 8:05
34
$\begingroup$

When dissolved in water, salt breaks up into sodium and chlorine ions, which combine with water molecules so they cannot easily sink. However, there is a tendency for streams of fresh water to float on salt water and rise to the top. This caused problems for British submarines in the Dardanelles Straits during WW1. Moving from almost fresh water to the denser salt water, they suddenly became more buoyant and rose involuntarily to the surface, making them visible to Turkish gunners on the shore. There are also parts of the ocean where there are pools of very salty water lying on the bottom in such a way as to clearly show the pool to any diver who happens to see it, as though it were a pool on land, so in some circumstances very salty water can sink.

$\endgroup$
  • 8
    $\begingroup$ Note that those brine pools that you refer to are not caused by salt solidifying out of seawater, but of salt coming up from the seabed. $\endgroup$ – Jan Doggen Jul 23 at 12:17
  • 1
    $\begingroup$ But isn't this independent of whether it is salt? More like differences in concentration (different density) and temperature (different density)? $\endgroup$ – Peter Mortensen Jul 23 at 21:05
  • 1
    $\begingroup$ @PeterMortensen Yes, it works for any solute. Indeed, it's not like salty water is 100% NaCl + water. But what Michael said still applies just as well - all solutes behave the same way. The main thing is how well the different solutions mix - if you let the ocean water settle, you'd get a pretty uniform distribution over time, since solute from the "saltier" part would migrate to the "sweeter" part (the difference in density isn't enough to overcome the energy favourability of the lower solute concentration) - though I'm sure that's not true for a kilometre deep column of water. $\endgroup$ – Luaan Jul 24 at 8:22
  • $\begingroup$ The submarine reference is fascinating but I cannot find any immediate source. Does anyone have a reference? $\endgroup$ – Edgar H Jul 24 at 10:05
  • $\begingroup$ One of my main interests is military history, and I have read endless books on it and watched many TV documentaries, but I can't remember which ones I saw this anecdote in. However, I did see it, I have definitely not made it up. Your best chance of finding an original source would be to read books or internet articles on the Gallipoli campaign, especially the naval aspects. Submarines and sea mines. played an important role. $\endgroup$ – Michael Walsby Jul 24 at 10:16
22
+100
$\begingroup$

Why does the salt in the oceans not sink to the bottom?

Because there isn't any "salt", per se, in the ocean. Salt, as the compound sodium chloride (NaCl) does not exist as a solid in the ocean. It is dissolved into sodium and chloride ions (charged atoms) that exist within the ocean as a homogenous phase (that is, a "thing").

That said, water with sodium chloride dissolved in it is indeed denser than pure water, because after all, sodium and chlorine atoms are denser than atoms of hydrogen and oxygen. This leads to an interesting phenomenon: you can have layers of more-salty water and less-salty water that do indeed rise and sink. There are several YouTube videos that demonstrate this very well. For example this video shows dyed salty and fresh water, separated by a barrier:

enter image description here

and then when the barrier is released, the salty water sinks down:

enter image description here

Source.

Some other videos: one and two.

This phenomenon is extremely important for planet-scale ocean circulation, and has strong influence on our climate.

$\endgroup$
  • 1
    $\begingroup$ "because after all, sodium and chlorine atoms are denser than atoms of hydrogen and oxygen." My understanding is that volume(water+salt) < volume(water)+ volume(salt), so that would also make it more dense. $\endgroup$ – Acccumulation Jul 24 at 15:22
  • 1
    $\begingroup$ @Acccumulation's point is very important for a liquid. For a gas, it is the molecular weight that is more important than the individual hydrogen and oxygen atoms: as water they are a combined molecule H<sub>2</sub>O. In either case, "sodium and chlorine atoms are denser than atoms of hydrogen and oxygen" is not a good description of density. $\endgroup$ – Bryan Krause Jul 24 at 22:14
  • $\begingroup$ @Acccumulation that’s molar volume, disregarding massed. $\endgroup$ – Gimelist Jul 24 at 22:29
  • $\begingroup$ @BryanKrause it is an excellent description for the layperson because no matter how you look at it, NaCl is whatever form is much denser than H2O in whatever (yet equivalent form). An accurate technical description is unnecessary in this case. $\endgroup$ – Gimelist Jul 24 at 22:31
  • 1
    $\begingroup$ @Gimelist Is the density of dissolved NaCl well-defined? To measure the density, you have to divide the mass by the volume, but how do you measure that volume of a solute? Since, as I said, volume is subadditive, how do you define how much of the volume of the solution is coming from the solute? Volume is a macroscopic property. Individual atoms don't have a volume, in a sense that corresponds to the ordinary meaning. "Volume" refers to the amount of space for which an object excludes other objects, but an atom does not necessarily exclude other atoms from occupying the same space. $\endgroup$ – Acccumulation Jul 24 at 22:40
7
$\begingroup$

I'm a regular from the Physics Stack Exchange reporting for duty.

Why this is a serious question

This is a bigger question than you might be giving it credit for. The question is ultimately similar to asking why all the air molecules in the atmosphere do not fall to the floor. Your question comes from a very solid principle in physics which could be called the minimum energy principle.

The basic derivation is that if you define the power exerted by a force $\mathbf F_i$ on a particle with velocity $\mathbf v$ to be $$P_i=\mathbf F_i\cdot\mathbf v = |\mathbf F_i|~|\mathbf v|~\cos\theta,$$then Newton’s law that the sum of forces on a particle $\sum_i \mathbf F_i = m~\dot{\mathbf v}$ is the mass times the change in velocity per unit time, directly implies that the sum of powers exerted on a particle $\sum_i P_i = \dot K$ is the change in kinetic energy per unit time. Drag forces exist and they oppose relative motion, so their $\cos \theta$ is negative and they will decrease kinetic energy, $\dot K < 0.$ Since energy is a conserved quantity (a “stuff,” if you’d like: if you find more or less of it in a box, then it must have come from somewhere else where there is less or more of it), drag forces eventually rob energy from a system until it ends up at the minimum energy.

And it is a very useful principle, for example you can use it to very easily derive the principle of buoyancy and the effective force that must be created by the displaced water to produce that effect; you can't do Newton’s laws easily when there are that many tiny little forces of little water molecules but you can absolutely compare total potential energy when an object is at the bottom of the ocean, the middle, and the top. It fails to describe certain things like static friction (why is my laptop on my desk and not on my floor?!) because it does not tell you how long such things take and requires an assumption of noise to eventually perturb you out of “local minimums” and such.

But surely the air has had enough time to fall to the ground if that were what it wanted to do. The air does not want to fall to the ground. And we can’t steal our normal solutions for other things like “why don’t clouds fall,” “well what you think of as a cloud is actually more like a waterfall, there is constant movement of water droplets, the water gets a boost upward from heating the air around it as it condenses but it does tend to eventually fall but when it falls beneath a certain flat surface it evaporates again and becomes invisible and so the visible puff is constantly being fed by new water droplet formation and constantly sapped by falling water that becomes invisible…”—no. These are concrete particles that somehow avoid falling to the ground and we have to actually solve the problem.

Fluctuation-Dissipation theorem to the rescue

The minimum-energy principle describes something that we would call dissipation, energy leaving one system to end up in another system. These sorts of gates are always bidirectional: energy goes through in both ways. But mostly you don't notice it, and that’s key to how the principle helps us describe things: energy always flows out, it never flows back in.

Until, well, it does. Energy of a bouncing ball spreads out among all of the different degrees of freedom of the floor, the air, but if it really goes all the way to 0 and sits perfectly and completely still, very soon the air will bump it and start it jostling and vibrating and moving again—just not moving very much. The same things that allow energy to dissipate must also be contributing constant energy fluctuations that prevent energy from going all the way to 0.

These fluctuations are collectively understood as temperature. Temperature is technically only defined for a system where all of its degrees of freedom in the ways it can move have come to the same average energy, and it is measured as that average energy. Temperature defines this average energy and the size of these fluctuations. So at room temperature for example we would say that every degree of freedom has 26 meV, 26 "milli-electron-Volts" of energy, or 0.026 of the energy that an electron would have if accelerated by a one volt battery.

So why does the air stay up? It is, basically, because the molecules of the floor are kicking the air molecules with enough energy to hit the upper reaches of the atmosphere. They do not actually go straight there; one air molecule bumps into other air molecules over a very short distance scale: but it transfers that energy and momentum to other particles which transfer that energy and momentum to other particles and in the end the air "prefers" to "hang out" near the ground but the fluctuations cause it to get bumped to an average height given by our temperature. So if you take the mass of nitrogen N2 of 28 amu, and the acceleration due to gravity of 9.8 N/kg, you can find out that this 26 meV temperature means that the atmosphere is ~9 km high on average, which does get you a good chunk into the troposphere where the air starts to thin out dramatically. Actually the theory says that if nothing else were to happen and the random kicks were to just launch a particle up into the atmosphere, it would have a random height sampled according to an exponential probability distribution, $P(h) \sim e^{-h/(9\text{ km})}$.

Similarly why don't the salt molecules fall to the ocean floor? Well, they do, and then they get kicked back up. The water at the ocean floor is saltier. The key difference is whether the salt in question dissolves in water (if it sticks to water better than it sticks to itself) or precipitates in water (it sticks to itself better): larger chunks of a piece of stuff that get bound together will tend to act as big massive chunks and then that thermal energy cannot kick it as high.

This is the general idea of the fluctuation-dissipation theorem, which states that fluctuation and dissipation (under some extremely broad assumptions called “detailed balance”) always go hand-in-hand. Anything which can absorb light (dissipation) must radiate light into space (blackbody radiation, a sort of fluctuation). Every electrical resistor is also a noise source (Johnson noise). If energy can flow out of a system into some environment, then it will only flow out until they have the same average energy levels, and if you try to go lower, energy fluctuations from the environment flow back into the system.

$\endgroup$
  • $\begingroup$ Interesting. I’ll have to read it several times until it will sink in (pun very much intended). $\endgroup$ – Gimelist Jul 25 at 11:53
  • $\begingroup$ I don't see how this helps at all. The theory of hydrodynamics (HD) is an average over newtonian kicks and short mean free-paths. More importantly, HD is perfectly able to explain static stratification and density gradients via flux differences of up minus down. The FD-theorem which you quote is usually only of importance when the mean-free path of particles becomes comparable or greater than the system scale, otherwise HD is fine. Both for the ocean and atmospheric structure up to 100km height. $\endgroup$ – AtmosphericPrisonEscape Oct 10 at 20:25
  • $\begingroup$ @AtmosphericPrisonEscape I think you are overthinking it? HD is the mean field approximation but FD is the fundamental cause; OP was asking for the fundamental cause. $\endgroup$ – CR Drost Oct 11 at 11:57
  • $\begingroup$ I guess that's up to OP to decide that. I just don't think that a question to which the understandable answer would be, "salt is dissolved, and fluids of differing densities behave in the following way", can be expanded much by handwaving it away with FD. $\endgroup$ – AtmosphericPrisonEscape Oct 11 at 12:15
5
$\begingroup$

Turbulence, because seawater is, almost, always on the move saltier water is mixed with fresher by wave action and, to a lesser extent in surface waters, by Brownian motion. In Fjordland the annual rainfall is so high (up to 8000mm) that there is a permanent freshwater layer several metres thick that you can drink from sitting over the salt water from the Tasman in the sheltered inlets. Even there this layer doesn't have a clear cut boundary but rather a mixing layer where the salt and fresh water exchange particles and homogenise over time. In bodies of water that don't experience regular circulation stagnation and anoxia set in over time but chemical solution of a number of dissolved salts still occurs.

$\endgroup$
5
$\begingroup$

Saltier water has higher mass density, so the gravitational energy can be lowered that way. The concentration differences go up until the free-energy of creating that big a concentration difference balances the gravitational energy change.
Department of Physics, University of Illinois at Urbana-Champaign

Making some simplifying assumptions, they find:

the equilibrium concentration goes up exponentially with depth, by a factor of e for each 10 km or so.
The actual oceans are stirred by currents, so this equilibrium concentration difference isn't present in them.

Basically they saying that it takes energy to separate a homogeneous solution into parts which are more or less concentrated (and hence more or less dense). Taking into account the gravitational energy, it follows that the least energy state of a column of water is saltier at the bottom.

$\endgroup$
  • 2
    $\begingroup$ So to paraphrase, salt does sink to the bottom, but only somewhat, so that the bottom of Challenger Deep is nearly three times as salty as the surface? $\endgroup$ – Tanner Swett Jul 23 at 10:47
  • $\begingroup$ @TannerSwett: This sounds correct. "Exponential growth" isn't really very impressive between $0$ and $1$. $\endgroup$ – Eric Duminil Jul 23 at 15:23
  • 3
    $\begingroup$ The answer is no! Salinity almost does not change with depth. Here is an example from the Challenger Deep: link.springer.com/article/10.1007/s10872-005-0053-z $\endgroup$ – arkaia Jul 24 at 18:38
  • $\begingroup$ @arkaia As the University of Illinois page states, the actual oceans do not reach the equilibrium state because of currents. Gravitational differentiation (over thousands of km) is significant inside the Earth, resulting in the heavier elements sinking to the core. $\endgroup$ – Keith McClary Jul 24 at 18:58
  • 3
    $\begingroup$ Then the answer to the formulated question is that it does not sink because is dissolved and the currents (and temperature gradients, by the way) prevent equilibrium from happening. I just don't want people to get a misconception. Salt in the ocean is pretty evenly distributed (except near rivers). The differences in salinity with depth are minimal compared with the gradients in temperature. $\endgroup$ – arkaia Jul 24 at 19:19
1
$\begingroup$

But it does, but according to each salt's solubility and density. Soluble salts tend to mix into the water and keep suspended. Insoluble salts separate from the solution and creates deposits in the oceanic floor.

One famous example was the "de-ironing" of the seas, when iron salts were deposited in the bottom due to the oxigenation of the oceanic water, by the time of the emergence of aerobic, photosynthetic organisms.

Great Oxidation Event: https://en.wikipedia.org/wiki/Great_Oxidation_Event

"The oxygen then combined with dissolved iron in Earth's oceans to form insoluble iron oxides, which precipitated out, forming a thin layer on the ocean floor". https://en.wikipedia.org/wiki/Banded_iron_formation

$\endgroup$
  • $\begingroup$ Geochemists use “salts” most commonly to talk about soluble compounds. I have never heard anyone refer to insoluble compounds (and iron oxide) as a “salt”. So while this answer is technically correct, it may be a bit misleading. $\endgroup$ – Gimelist Jul 24 at 12:09
  • $\begingroup$ Salt, in the parlance of this response, is a result of a reaction between an acid and a base (in the broadest sense, i.e. Lewis acid, etc). So Iron is an electron "giver", i.e., a base, and Oxigen an electron "taker", i.e., an acid. The reaction "Fe+2 + O2 -> Fe+3 + O-2" (unbalanced, the signs are electric charge of the ion) means that Fe2O3 is a salt, in this sense. $\endgroup$ – Luiz P. O. Pereira da Silveira Jul 29 at 20:33
  • $\begingroup$ no, in the parlance of geochemistry and ocean geochemistry in particular, “salts” are always soluble compounds like halides, nitrates and some sulfates. Your definition of a Lewis acid includes too many inorganic compounds to make it useful in the field of geochemistry. $\endgroup$ – Gimelist Jul 29 at 20:57
  • $\begingroup$ Sure, geochemistry has a specific nomenclature. Chemistry in general has many definitions of salt, and the most basic is the result of a reaction between an acid and an alkaline compound. The answer does explain how some of the salt gets deposited in the ocean's bottom, though. $\endgroup$ – Luiz P. O. Pereira da Silveira Aug 12 at 14:39
0
$\begingroup$

And then there is the saturation issue. Salt can be dissolved in water to a certain degree only. Once that degree is exceeded the salt begins to fall out and sink to the ground. If I remember well the limit for water is something like 35g per litre (depending on the temperature)

$\endgroup$
  • $\begingroup$ This seems to suggest that (contrary to the question) salt does sink to the bottom. Can you back this up, i.e. shows that this effct actually takes place in the oceans? $\endgroup$ – Jan Doggen Jul 25 at 13:33
  • $\begingroup$ not directly related to this answer but when seawater freezes(in the arctics)the salt will be consentrated and sink down to the bottom of the ocean,this does lower the salt consentration in the ice. $\endgroup$ – trond hansen Jul 26 at 10:08
0
$\begingroup$

Salt does sink to the bottom in the oceans.

Why? Your question referring to salt. Salt is a solid chemical compound. Take a lump of rock salt of sodium chloride, throw it into the water: it will sink to the bottom. The reason is that the density of sodium chloride with more than 2 g/cm3 is higher than the density of seawater less than 1.1 g/cm3.

Of course, the salt lump will be dissolved sometime and no longer exist. But then it's no salt anymore. Then there are only fast and somehow moving loose cations and anions in the water.

$\endgroup$
0
$\begingroup$

Salt does not sink to the bottom in the seas and oceans, because it dissolves in water! If you want to get salt from the seas and oceans, try to vaporize them :-)...

New contributor
Cyril is a new contributor to this site. Take care in asking for clarification, commenting, and answering. Check out our Code of Conduct.
$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy