# Why is sand that insoluble?

Why is sand that insoluble?

Sand is a polar molecule but does not dissolve in water. Is it because of the big structure that sand makes?

# What is Sand?

In order to give a good answer, we first have to decide on the type of sand we want to talk about, because its composition is highly dependent on that. Is it beach sand? If so, where from? All these questions should have been addressed in the question. I will limit the scope of this answer to pure silicium dioxide $\ce{SiO2}$ sand.

## Is Sand a Molecule?

Sand is, strictly speaking, not a molecule. It's a crystal. As such it has properties such as the lattice energy. Maybe have a look at that link, it might clarify it all for you already.

So now we have talked about what we want to talk about and clarified the basics, so let's dive right in, shall we?

# Why does $\ce{SiO2}$ not Dissolve in Water?

Basically this is an energy calculation. On one hand you have the crystal, with its lattice energy, and on the other hand you have the solvated $\ce{SiO2}$ units. A tentative reaction equation would look like this: $$\ce{Si_{n} O_{2n} <<=>[E_\text{lat}] SiO2 <=>[E_\text{solv}] SiO2(aq)}$$

The energies I've indicated are the lattice energy $E_\text{lat} = 12535~\mathrm{kJ\,mol^{-1}}$ (Source) and the solvation energy $E_\text{solv}$ (see figure below, where I've also included sodium chloride as a substance to compare with).

Data from here (PDF) ($\ce{SiO2}$) and here ($\ce{NaCl}$). It is obvious that the solvation of $\ce{SiO2}$ monomers is highly unfavored (if I got all the sign conventions right, that is...)

Basically, for ease of understanding, imagine that the $\ce{SiO2}$ unit in the middle has to decide on which side it wants to go. Either it forms a crystal with other units (to the left) or it gets solvated by (surrounded by molecules of) water. Which way will it go? We already know that it will take the route to the left. This means that the whole system can release more energy this way. That is why $\ce{SiO2}$ crystals are stable in water.

Or somewhat. Some theoretical investigations point towards the possibility that the surface of quartz crystals undergo chemisorption of water, thereby generating $\ce{SiOH}$ terminal groups, which would then generate more favorable interactions between the crystal surface and the surrounding water.

Coming back to (a part of) your question:

Is it because of the big structure that sand makes?

You are kind of on the right track here. It is because of the lower energetical state as a crystal that these big structures (crystals) are formed, vs. the units just being dissolved (solvated by water).

There are many reasons, due to which it is insoluble in water.

High lattice energy:

Due to polar interaction with water, it get suspended but remain undissolved due to high lattice energy

size of sand particle:

The size of sand particle is bigger than $0.3 \mathrm{nm}$(Again as commented it depend on type of sand) which is difficult to dissolve.

Some things which are bigger than 0.3 nm can squeeze inside and cause unfavourable entropic effects but significantly more favourable than being at the surface or slightly more than being in normal solution. However, sand particles are significantly larger and entropically it prefers being suspended.

(Yahoo)

Because of bond:

Sand is not able to dissolve in water, because water can not break bonds between silica (sand / $\ce{SiO2}$ / Silicon dioxide)

Sand is insoluble in water and organic solvents. There are no possible attractions which could occur between solvent molecules and the silicon or oxygen atoms which could overcome the covalent bonds in the giant structure.

(Chemguide)

P.S. sand is a non-polar molecule.

• I think your point #2 is invalid. It is possible to dissolve a large crystal of NaCl, although the dissolution will be slower (since it depends on the surface area, which is larger for granular substrates). – tschoppi Oct 12 '14 at 17:08
• @tschoppi I have written "difficult to dissolve" which also play role what i have heard. But i can remove if it is wrong. :) – Freddy Oct 12 '14 at 17:11
• @tschoppi I agree with #2 and i also got link which explain that in detail – user7484 Oct 12 '14 at 17:51
• @RutvikSutaria I don't believe in Yahoo much but as they have also mention source possible they are right. BTW I have heard about it many times. – Freddy Oct 12 '14 at 17:54
• @RutvikSutaria Never a day without learning something. Thank you for the reference. – tschoppi Oct 13 '14 at 12:22

By sand I'd guess you mean $\ce {SiO2}$.

You see, sand or $\ce {SiO2}$ for what it matters, don't appear in nature as single molecules, they're crystals and the crystal arrangement defines if it will be quartz, sand, silica-gel and other stuff, and this crystalline structure also defines it's solubility in water. So even if a single molecule of $\ce {SiO2}$ could be polar due to the electronegativity difference between Si and O, the sand won't be exactly polar because they only hang together as crystals, and so the polarity will vary as a function of it's structure.

PS: I never heard of single $\ce {SiO2}$ molecules in labs neither.

• Apparently, molecular $\ce{SiO2}$ does exist in the gas phase, and can be produced by oxidizing silicon monoxide gas in an inert matrix. It's still not a particularly common or stable species, at least on Earth (although I suspect it may have some astrochemical relevance). – Ilmari Karonen Oct 13 '14 at 9:05

Surely the correct answer is to apply a variation of the anthropic principle: sand can be made of a variety of different crystals, some of which are water soluble and some of which are not. The reason you only see the insoluble crystals when you go to the beach is that the soluble ones were dissolved into the ocean millions of years ago.

If you look at different sand compositions, you'll see that desert sands have a higher proportion of water soluble components. For example, some parts of the Sahara have significant amounts of gypsum. White Sands is almost entirely gypsum. But gypsum never makes up more than a trace of beach sand because it dissolves in water. (Indeed, it dissolves better in cold water like the ocean than in warm water.)

• In extension to the anthropic principle, one could state that the energy is lower for sand in suspension as opposed to being dissolved. But this argument is so easy and kind of superficial, I do not like it. – tschoppi Oct 13 '14 at 14:26
• injecting gypsum "sand" into this really digresses from the question. – MaxW Oct 24 '15 at 20:16