Naked Science Forum
Non Life Sciences => Chemistry => Topic started by: goofkid on 22/12/2005 21:20:02
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Hey all! [:D]I have a question related to solubility.
Well, most ionic compounds are strong elecrolytes right? Meaning they won't form any kind of precipitate because each unit of the ionic compound is separated, and therefore, dissolved. Now NaCl is an ionic compound. How is it that this ionic compound is commonly found in lakes and seas? Doesn't it dissolve? The quantity would have to be HUUUGE in order for it be easily found like that wouldn't it?
Srry if this sounds silly, im not really good with chemistry. [^]
Goofkid
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Sorry I don't understand your question so can't help to answer it please repeat it in a different way expressing your ideas more clearly
Learn, create, test and tell
evolution rules in all things
God says so!
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I assume you're asking why supersaturated solutions will precipitate salts.
You are wrong to say that they will just infinitely dissolve. No solvent has the capacity to dissolve an infinite quantity of anything, there will always come a time when it stops dissolving the particular solute (depending on the nature of the solvent and the solute). Water can dissolve a great deal of NaCl, and it does, but if some of that water later evaporates, then there is less water left in which to dissolve the same quantity of salt, so the water starts to precipitate some of the salt.
As you say, the sodium and chlorine are ionised, and so as they precipitate they will be attracted to each other to form regularly arranged crystalline structures.
I'm sure there are guys here who can give you a far more detailed explanation than that, but I think that is what you are asking, is it not?
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quote:
Originally posted by another_someone
I assume you're asking why supersaturated solutions will precipitate salts.
Thx! [:D] That is exactly what i meant to ask. And the explanation made alot of sense, if only i had thought about it a bit more. [:(]
Goofkid
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quote:
Originally posted by another_someone
I assume you're asking why supersaturated solutions will precipitate salts.
You are wrong to say that they will just infinitely dissolve. No solvent has the capacity to dissolve an infinite quantity of anything, there will always come a time when it stops dissolving the particular solute (depending on the nature of the solvent and the solute). Water can dissolve a great deal of NaCl, and it does, but if some of that water later evaporates, then there is less water left in which to dissolve the same quantity of salt, so the water starts to precipitate some of the salt.
As you say, the sodium and chlorine are ionised, and so as they precipitate they will be attracted to each other to form regularly arranged crystalline structures.
I'm sure there are guys here who can give you a far more detailed explanation than that, but I think that is what you are asking, is it not?
Good explanation. I just have a couple of minor points to add.
You are correct if you're talking about a liquid dissolving a solid. However, other solutions exist as well. Air is a gas/gas solution. Metal alloys are solid/solid solutions. Vodka is primarily a liquid/liquid solution. Many of these types of solutions are infinitely soluble. You can mix any amount of gases and they will be soluble. Likewise with ethyl alcohol and water.
Finally, I have some data for you. The solubility limit for sodium chloride (NaCl, normal table salt) in water is about 26%, so you can mix 26 grams of NaCl with 74 grams of water. Add any more salt and it will just sink to the bottom.
DrDick
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quote:
Originally posted by DrDick
Good explanation. I just have a couple of minor points to add.
You are correct if you're talking about a liquid dissolving a solid. However, other solutions exist as well. Air is a gas/gas solution. Metal alloys are solid/solid solutions. Vodka is primarily a liquid/liquid solution. Many of these types of solutions are infinitely soluble. You can mix any amount of gases and they will be soluble. Likewise with ethyl alcohol and water.
Finally, I have some data for you. The solubility limit for sodium chloride (NaCl, normal table salt) in water is about 26%, so you can mix 26 grams of NaCl with 74 grams of water. Add any more salt and it will just sink to the bottom.
DrDick
I always understood that liquid/liquid mixtures were miscible, not soluble.
The whole point about a solution is that you have a solvent and a solute – when both are of a similar type, then which is the solute and which is the solvent?
I did not understand metal alloys are not solutions either, but simply alloys. Since alloys are inherently solid, the idea of dissolving or precipitation can have no meaning.
Gas can be dissolved in liquid, just as solids can, and they conform to roughly the same rules.
Gasses will freely mix with other gasses, but I did not think they interact in the way that a solute interacts with a solvent.
In any case, the original question was with regard to ionic compounds in water, and thus gas/gas, or solid/solid mixtures would not fall into that category (unless one is talking of steam or ice).
George
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quote:
Originally posted by another_someone
quote:
Originally posted by DrDick
Good explanation. I just have a couple of minor points to add.
You are correct if you're talking about a liquid dissolving a solid. However, other solutions exist as well. Air is a gas/gas solution. Metal alloys are solid/solid solutions. Vodka is primarily a liquid/liquid solution. Many of these types of solutions are infinitely soluble. You can mix any amount of gases and they will be soluble. Likewise with ethyl alcohol and water.
Finally, I have some data for you. The solubility limit for sodium chloride (NaCl, normal table salt) in water is about 26%, so you can mix 26 grams of NaCl with 74 grams of water. Add any more salt and it will just sink to the bottom.
DrDick
I always understood that liquid/liquid mixtures were miscible, not soluble.
The whole point about a solution is that you have a solvent and a solute – when both are of a similar type, then which is the solute and which is the solvent?
I did not understand metal alloys are not solutions either, but simply alloys. Since alloys are inherently solid, the idea of dissolving or precipitation can have no meaning.
Gas can be dissolved in liquid, just as solids can, and they conform to roughly the same rules.
Gasses will freely mix with other gasses, but I did not think they interact in the way that a solute interacts with a solvent.
In any case, the original question was with regard to ionic compounds in water, and thus gas/gas, or solid/solid mixtures would not fall into that category (unless one is talking of steam or ice).
George
"miscible" is the conventional term when discussing fluids, but it means the same thing as "soluble". Most people are more familiar with the latter term than the former, so I stuck with that.
The line between the terms "solute" and "solvent" do blur and even vanish when you get approximately equal amounts. For instance, in a 3% ethanol in water solution, it's clear that the ethanol is the solute and the water is the solvent. When it's 50/50, there's no clear solvent or solute.
The term "solution" simply refers to a homogeneous mixture, where you can't see the separate components. So an alloy or a gas mixture are both considered solutions.
You're right about the original question being about solutions of ionic compounds. I was responding to a statement that said, "No solvent has the capacity to dissolve an infinite quantity of anything".
DrDick
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No, Another Someone is right, liquids are miscible in other liquids but most likely do not dissolve in them. Ethanol is not soluble in water at all because they are not of like type (non-polar ethanol and polar water). To dissolve and thus be soluble in something a substance must dissociate (break apart into it's components). Ionic compounds do this extremely well. When you mix two liquids they mix just as gases mix in air but the one is not made to dissociate by the other and thus there is no solvent or solute and no solution. There is a fine line between a mixture and a solution and you seem not to have learned that. That may not be your fault, our education system is pretty terrible at the moment[:(!]--don't even get me started-- but you should know that little fact about solutions so you don't a) confuse more people with a fallisy, and b) don't make an mistake in that respect in the future.
"His mind is the ultimate weapon!"-MacGyver television series
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Actually DrDick is correct. Any combination of gas/liquid/soild can form a solution. There is a fine line between mixtures and solutions which is called homogeneity.
Ethanol is most definitely soluble in water. The term miscible essentially means that they will form a solution at all relative concentrations. The most obvious proof of this is that a volume reduction is observed upon adding say 50 mL water to 50 mL ethanol which will leave a solution of around 95 mL.
Secondly, dissociation is not required for solubility, hydrogen bonding and many other interactions lead to solubility.
Finally, in response to the original poster, the salinity of water bodies is often referred to as NaCl but is actually the dissolved ions Na+ and Cl- since in natural water it does not approach the 26% w/w solution required for precipitation.
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If I remember my chemistry right a solution is only that in which an ionic compound is disassociated into the to component ions, either compound or elemental. There are three types of ways that chemicals are combined, solution, suspension and mixture. Solution would be something like table salt (NaCl) and water, where the water dis-associated the salt into component radicals (Na+ & Cl-). The result is H2O, NaOH and HCl. Because the basic and acidic components are inherently equal, the pH remains a neutral 7. A suspension, like sugar, is where the solvent does not dis-associate the solvent, but instead uses weak secondary bonds to hold the chemical in the medium. Sugar's composition lends itself to hydrogen bonding, and therefore can "hang on" to a water molecule. Air is a pure mixture based on the completely random distribution of the molecules involved. There is no dis-association, and not bonding, even on a secondary level. This is evident by the change in air composition based on temperature or pressure. An example is a change in the percentage O2 concentration at high altitudes, because the O2 molecule is heavier than the N2 molecules and the other noble gases that make up Earth's atmosphere.
Again, its been a while since my last chem class.
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quote:
Originally posted by Mr Andrew
... liquids are miscible in other liquids but most likely do not dissolve in them. Ethanol is not soluble in water at all because they are not of like type (non-polar ethanol and polar water).
1.Mixing ethanol and water generates heat.
2.The volume decreases.
3.You cannot completely separate them with distillation (they form an azeotrop).
4.They form one single phase after mixing.
5.The ethanol is polar too (even if less than water); if it weren't, it wouldn't mix with water so well. Example: try to mix CCl4 with water!
And you say this is not a solution? What else can it be more than this?
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bityman - it isn't only ionic substances that can dissolve! Sugar is not an ionic substance! Many of these concepts are 'grey' areas but that is not one of them.
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I might like to add that sugar and water does form a solution, not a suspension. Hydrogen bonding is the strongest type of intermolecular force. Hydrogen bonding can only occur when a hydrogen atom joins with a oxygen, nitrogen or a fluorine atom. In sugar, such as glucose, the formula is C6H12O6. If you draw out the structural formula for that molecule then you will see some "OH" groups surrounding the carbon. This combined with water, H2O will form Hydrogen bonding. This intermolecular force is so strong that to separate the mixture, the water has to be evaporated to retrieve the sugar. It is not a suspension because the sugar WILL NOT settle down the bottom of a beaker if left sitting somewhere overtime. Just like you can't decant sugar from water like you do with sand and water.
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Regarding suspensions, you're correct. A suspension is when solid particles are held within a fluid and don't settle out. The solid particles have to be very small to do this, but not as small as isolated ions.
The bit about hydrogen bonding has some problems with it, however. It's a common misconception that hydrogen bonding is stronger than other forms of intermolecular forces. Each type (hydrogen bonding, dipole-dipole, London, etc.) has its own continuum. Sometimes, the hydrogen bonding forces are stronger, sometimes the London forces are stronger. Two examples:
water
- very strong hydrogen bonding
- very strong dipole-dipole attractions
- very weak London forces
olive oil
- extremely weak (essentially nonexistent) hydrogen bonding
- very weak dipole-dipole attractions
- very strong London forces
Boiling point is mostly dependent on the intermolecular forces present in the molecule. Which has a higher boiling point? It's olive oil (~300 °C). In this case, the London forces present in olive oil are much stronger than all of the combined forces in water.
One last example:
sucrose (table sugar)
- very strong hydrogen bonding
- strong dipole-dipole attractions
- very strong London forces
net result = very strong intermolecular forces, which helps make sucrose a solid at room temperature
Dick