What Happens to Atoms in a Solution?

I was studying the concept of solutions, and I read where in salt the sodium and the chlorine come apart or actually dissociate, whereas it says in sugar the entire molecule stays intact, so there are two kinds of solutions. I almost understand the second kind: little molecules floating around in a liquid, but in the former 1)why when you put salt in water doesn’t the chlorine gas come flying out, poisoning everyone in sight,leaving pieces of the flammable sodium which, when the water evaporates will suddenly flare up? Now I read that it is the sodium ION and the chlorine ION that are separate, but how does this solve anything? In other words 2)is a chlorine ion, for instance, chlorine or isn’t it? 3) What is chlorine like when it is only an ion, and the same for other ions? I suppose an ion is an atom with one or two missing electrons, but 4)does it still have the chemical properties of the element that it is an ion of? Would an ion of sodium blow up in fire just like an atom of sodium?

No, a chlorine ion is not chlorine. Chlorine, the element, has a diatomic molecule with a single covalent bond, as well as 6 unshared electrons on each atom. Most importantly, chlorine molecules are electrically neutral. Electrical charge and electrons are what make chemistry happen, and it’s the electrons, the neutrality, and the bonds that make chlorine act like chlorine.

Similarly, sodium is a metal, and exists as neutrally charged atoms arranged in a crystal matrix, with lots of free electrons floating around. The free electrons are stripped when sodium is ionized, so that even if a chunk of sodium ions could exist, it would have entirely different chemical properties.

As you learned, sodium chloride is composed of ions, not atoms. You will not find chloride ions sitting in one corner and sodium ions in the other; the positively charged sodium ions and the negatively charged chlorine ions are very, very powerfully attracted to each other, and will repond to evaporation by re-crystallizing as salt.

The reason they don’t come flying out is that they are still mutually attracted to one another. So, diffusion keeps them from staying paired, but attraction keeps them from getting too far apart, or more importantly keeps them from forming regions where the concentration of one is higher than the other.

Er, if I’m not mistaken, sodium chloride is made up of formula units, and once it’s dissolved, it is then made up of ions?

If they’re so powerfully attracted to each other then how does water manage to disassociate them? Water is a polar molecule but I thought that was a pretty weak attraction.

First of all, the proper term for chlorine ion [sic] is chloride ion.

Chemically speaking, elemental chlorine gas (Cl[sub]2[/sub]) and chloride ion (Cl[sup]-[/sup]) are completely different. Similarly, elemental sodium metal and sodium ion are also completely different. Chemistry involves electron configurations of ions, atoms and molecules–the electron configurations of neutral elemental forms and ion forms are different.

Specifically, a chlorine molecule consists of two chlorine atoms with seven valence electrons each. The two atoms share these valence electrons, resulting in a relatively stable molecule. Elemental chlorine, however, is a good oxidizing agent, and has a tendency to react strongly so as to gain a valence electron (per chlorine atom). A chloride ion has gained a valence electron so as to result in a stable ion with 8 valence electrons.

Sodium metal has one valence electron. It is unstable and very reactive. It reacts so as to lose a valence electron, resulting in stable sodium ion.

In short, elemental sodium and chlorine are very reactive. Sodium ion and chloride ion are not.

Even solid sodium chloride is made up of ions. A formula unit is primarily a book-keeping convention to keep track of the fact that the number of sodium ions and chloride ions are equal, whether in solid form or aqueous form.

Yes, but there are a lot more water molecules present than sodium and chloride ions. (The concentration of water molecules in liquid water is ~55.5 moles/liter.) This allows many water molecules to “gang up” on a single sodium or chloride ion and keep them separate from oppositely charged ions in solution. However, if sufficient sodium and chloride ions are present, the solution will be saturated and solid NaCl will indeed form.

That is really well explained, robby
Saturated solutions flummoxed me in college.
I could cope with all manner of complex organic substitutions, fluorescence, sequestering agents, highfalutin’ physisorption isotherms, infinite axes of rotation and whatnot, but saturated solutions were a closed book to me.
The scales have fallen from my eyes.

You’re quite welcome! I appreciate your thanks, and I thank you for your kind words.

Care to explain optical rotation to me? :wink:

<rolls eyes>
Your place or mine?
</rolls eyes>

Believe it or not, I was actually being semi-serious, and no pun was intended. You mentioned “axes of rotation,” a topic I never got. I believe it has something to do with “chirality” and optical activity/rotation.

That being said, <rolleyes> could surely be considered to be optical rotation as well. :smiley:

You were being serious?!?!
And there’s me being all flip and girly and making with the puns. Most unlike me.

A chiral molecule has optical activity an achiral compound has no optical activity. Achirality is the norm.

A chiral molecule has two physical forms, differentiated in English as left and right, so called because a solution of either bends plane polarised light the exact same characteristic amount, either to the right or the left. These are a specific type of isomer, which cannot be superimposed upon each other. The ususal example given in the texts is a pair of gloves’ you can’t put one directly on top of the other, the same way up without the thumbs being on the wrong side
I always thought that the gloves line was a bit of an odd one to choose. A Mercedes in Germany and a Mercedes in Dublin are facing each other and are identical, but if you put one on top of the other the steering wheels would be in the wrong place - They are optical isomers! Enantiomers!

The reason optical activity happens is that if you have an aqueous solution of L sucrose, all the molecules stack up nice and tidily and sit on each other’s knees, due to van der Walls forces and weak hydrogen bonds and the like
This forms soluble “layers” which act as a prism, so when the plane polarised light shines in, it is refracted due to the presence of this sucrose prism.
If your solution was D sucrose, the same “layers” effect would be seen but in this case, the prism is facing the other way, so the angle through which the light was refracted would be exactly the same number of degrees, but in the opposite direction.

If you added these two solutions together, the neat stacks would all be disrupted and the solution is now random, with no nice neat layers. This is a racaemic solution, made up of equal parts of both L and D. When this happens the layers are dispelled and no prism action can take place. Plane polarised light is not refracted and just shines through, in a straight line.

Chiral compounds are of enormous interest to the pharmaceutical industry, because usually, only the L (or is it the D?) of any pair of chiral compounds is pharmacologically active, the other one is generally inactive.
It is all to do with the body’s receptors for whatever the active is.
Square pegs and round holes, don’t you know.

When I mentioned infinite axes of rotation in my post I was talking about linear molecules in FTIR theory, which you will be most delighted to hear I ain’t going into now.
What a long and tedious post. That’s the last time I use a chemical pun as a come on!

So is sucrose (or medium-largish organic compounds in general) polar or weakly polar? And what are these weak hydrogen bonds of which you speak?

Sucrose has 8OH functional groups, so in aqueous solution, there will be a fair few electons bopping to and fro between the hydroxyl and hydronium ions in the water.
Weak hydrogen bonds between molecules are called van der Walls forces, so I was using a bit of repetitive embellishment there, sorry about that.

Any molecule is a sum of it atoms, so you neded to add up the electronegativities of all the atoms to see if the resulting molecule is polar or not. I have not got a periodic table to hand, so I can’t really help you on that one. Sugar is organic, soluble in water and moderately solvent in alcohol so I would hazard a guess at weakly polar.
But I would refer you to a periodic table and a diagram to draw the thing, that is what I always used to do and it generally worked for me.

A real organic chemist may be along any minute now to help you

That first line means 8 hydroxyl groups; 8 OH, not eighty hydrogen atoms

Good explanation.

Having taught general chemistry, I’ve got the basics (such as solubility) down cold, as well as how to explain them. However, my understanding of more advanced topics is considerably murkier.

Ah. Shows what I know… :smack:

However, I do know that sucrose is polar. :slight_smile:

Well, sort of serious, in that I know little of the topics you first mentioned.

Actually, in your subsequent response, I read:
<rolls eyes> = :rolleyes: (rolleyes/sarcasm)
instead of
<rolls eyes> = flip and girly

Hence the confusion. Hazards of internet communication and all. :smack: