Dissolving/Dissociating

[theun4given]

Sugar dissolves which is a physical change. So here are my questions:

1. How does sugar dissolve if it does not dissociate into ions?

2. When something like NaCl dissociates in water, is that a physical or chemical change?

thanks for the help.

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[exmissionary]

2. When something like NaCl dissociates in water, is that a physical or chemical change?

I would think that NaCl would be undergoing a physical change because its chemical identity as NaCl is still the same.

Chemical changes are the changes in a substance through chemical reactions. The chemical reactants form a new product with equal mass. -http://en.wikipedia.org

A physical change involves the change in a substance that does not involve a chemical reaction, as opposed to a chemical change. Since no reaction occurs, there are no chemical substances present after a physical change that were not there before the change. Because of this, a physical change is often said to be reversible. However, this definition is sometimes misleading, as many physical changes are difficult to reverse (such as cutting paper) and some chemical reactions reverse very easily (see reversible reaction).
-http://en.wikipedia.org

 

[doc toothache]

Sugar dissolves which is a physical change. So here are my questions:

2. When something like NaCl dissociates in water, is that a physical or chemical change?

Wasn't this covered in elementary school science?

 

[theun4given]

Wasn't this covered in elementary school science?

you are not helping.

any thoughts on the sugar?

thanks.

 

[RyanF]

If you have the Kaplan Blue Book check out page 277 under the solvation section. Nonionic solutes are solvated by favorable van der waals between solute-solvent.

 

[doc toothache]

you are not helping.

any thoughts on the sugar?

thanks.

In the case of sugar, it may have something to do with all of those OH groups in sucrose.

 

[Twiigg]

I would think that NaCl would be undergoing a physical change because its chemical identity as NaCl is still the same.

Chemical changes are the changes in a substance through chemical reactions. The chemical reactants form a new product with equal mass. -http://en.wikipedia.org

A physical change involves the change in a substance that does not involve a chemical reaction, as opposed to a chemical change. Since no reaction occurs, there are no chemical substances present after a physical change that were not there before the change. Because of this, a physical change is often said to be reversible. However, this definition is sometimes misleading, as many physical changes are difficult to reverse (such as cutting paper) and some chemical reactions reverse very easily (see reversible reaction).
-http://en.wikipedia.org

No, I am pretty sure the dissolution of NaCl in water is a chemical reaction, not physical. Physical reactions involve state changes, breaking something into bits and pieces, etc. When NaCl is placed in water it reacts with the water. It disassociates into two ions: Na+ and Cl-. In fact, H20 forms intermolecular bonds (ion-dipole) with the ions, which is why they dissolve in the first place. Just my thoughts. 🙂

 

[Twiigg]

Oh and I think sugar dissolves due to the London Dispersion forces b/w the sugar molecules and the H2O.

 

[doc toothache]

No, I am pretty sure the dissolution of NaCl in water is a chemical reaction, not physical. Physical reactions involve state changes, breaking something into bits and pieces, etc. When NaCl is placed in water it reacts with the water. It disassociates into two ions: Na+ and Cl-. In fact, H20 forms intermolecular bonds (ion-dipole) with the ions, which is why they dissolve in the first place. Just my thoughts. 🙂

Oh and I think sugar dissolves due to the London Dispersion forces b/w the sugar molecules and the H2O.

This must be "new concepts in chemistry".

 

[bigstix808]

solubility is a physical property of different substances that is governed by the molecular properties of the solvent and solute - period.

sugar dissolves because it is polar, and we all know "like dissolves like". it does not dissociate because the bonds that hold the sugar together are covalent. a chunk of sugar consists of many many sugar molecules all just "stuck" together (different than ionic salts) Some of the H's from the OH groups do come off causing a "dissociation" of sorts, but the pKa's of those H's are too high to really matter.

when salt dissolves in water, becomming dissociated, it does not undergo any type of chemical RXN. the Na and Cl ions are just simply solvated (surrounded by h2o molecules). if you don't believe that, then just make some salt water and boil it and then tell me what you see left behind on that pot. 20 bux says it's that same salt you put in there the first time (in a little bit more pure state for those that want to get technical 🙂) if you look at a grain of salt, it is made of a "lattce" structure with alternating Na and Cl atoms (Na-Cl-Na-Cl) and does not contain a whole bunch of single NaCl molecules just hanging out around each other, like you would see with a sugar.

this concept holds for all solvents, but each solvent also has a set of salts that will NOT dissolve in it. eg: all sulfates dissolve in water except for any sulfate that is "lattaced" with a Pb, Ag, Ba, Sr, or Ca ion.

Na-SO4-Na-SO4-Na-SO4-Na-SO4 <----- good solubility in water
Pb-SO4-Pb-SO4-Pb-SO4-Pb-SO4 <----- bad solubility in water