What does it mean when something dissolves?

[axp107]

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Seems like a simple question, but I have the hardest time understanding solubility... I can do the math.. but blindly. It'd be great to understand it conceptually...

So when something dissolves completely, all of the solute forms bonds with the solvent right?

What about in water then...

For ex: HCL --> H+ +Cl-

I don't see any bonds forming with water..

Also, when mixing multiple solutes into a beaker of water..
what's the answer the a question that asks

1) Which solute dissolves first?

and

2) Which precipitates first?

On the first PS passage of AAMC CBT3, theres a question asking which "anion" precipitates first... how can an anion precipitate.. I thought only solids did

 

[Chuck's Right Foot]

Seems like a simple question, but I have the hardest time understanding solubility... I can do the math.. but blindly. It'd be great to understand it conceptually...

So when something dissolves completely, all of the solute forms bonds with the solvent right?

What about in water then...

For ex: HCL --> H+ +Cl-

I don't see any bonds forming with water..

Also, when mixing multiple solutes into a beaker of water..
what's the answer the a question that asks

1) Which solute dissolves first?

and

2) Which precipitates first?

On the first PS passage of AAMC CBT3, theres a question asking which "anion" precipitates first... how can an anion precipitate.. I thought only solids did

Bonds aren't forming with water, but the solid is no longer a solid, the crystalline structure has broken down and now exist as ions in solution. H20 has a molecular dipole that helps this process by solvating the ions. No bonds with water, but also no longer attached at the hip to it's partner.

Ksp is a measure of how much a solute will dissolve. The bigger the number, the more that will dissolve into the water. So a LOW Ksp that approaches 0 means the solid won't dissolve easily (The Ksp is temperature dependent). So, something with a low Ksp will precipitate (reform the solid) before something with a higher Ksp. Ksps are exactly the same as a Keq FYI.

 

[vicinihil]

When something dissolves...it disassociates...you got that

It disassociates by being SOLVATED by water molecules which forms a SOLVATION shell around the anion or cation stabilizing it in solution to prevent it from re-turning to its previous "partner." Take NaCl....Na is surrounded by H2O molecules with the O's facing the positive Na

The positive H's of H2O face the negatively charged Cl.

For something to dissolve first...hmm...I don't know if there's something that dissolves first but there's certainly something that dissolves more and that depends upon its Ksp value. This value deals with now much energy is required to break the bonds in the solute molecule because bond breaking is endothermic. The more energy needed, the smaller the Ksp and the harder it is for it to dissolve. This is also why raising the temperature increase the dissolution of a substance.

You can visualize this by talking about acids. Strong acids are strong because the H+ ions dissolve easily in solution because the Conj Base is stabilized. Weak acids are weak because H+ ions don't dissolve as easily because its conj base is NOT as stable. That's why HF is a weak acid but HI is damn strong.

I hope this answers your first questions. I'm not sure about the second one. I would think the anion that precipitates first is the anion of a solute with a low Ksp. It makes sense because the conjugate base (the anion) is not stable in water so it wants its buddy back.

 

[themule]

OP I'm assuming that you are talking about AAMC 3 PS Q4 and what the question is asking is when adding the cation to the solution which precipitate will form fastest. The way the question is worded might confuse you but the answer is in the passage on how the different precipitates were created.

TM

Also don't forget about the common ion effect.

 

[medicalCPA]

I seem to vaguely remember a similar question. I got it wrong. But I also remember that the smaller a solute's Ksp, the faster it will precipitate out of solution. I don't know if that will help you very much, though.

 

[ArchieMoses]

Also don't forget about the common ion effect.

this is key

 

[themule]

I seem to vaguely remember a similar question. I got it wrong. But I also remember that the smaller a solute's Ksp, the faster it will precipitate out of solution. I don't know if that will help you very much, though.

Not necessarily faster but at a lower concentration.

TM

 

[BrokenGlass]

Bonds aren't forming with water, but the solid is no longer a solid, the crystalline structure has broken down and now exist as ions in solution. H20 has a molecular dipole that helps this process by solvating the ions. No bonds with water, but also no longer attached at the hip to it's partner.

Ksp is a measure of how much a solute will dissolve. The bigger the number, the more that will dissolve into the water. So a LOW Ksp that approaches 0 means the solid won't dissolve easily (The Ksp is temperature dependent). So, something with a low Ksp will precipitate (reform the solid) before something with a higher Ksp. Ksps are exactly the same as a Keq FYI.

When something dissolves in water, bonds with water are formed, but they are INTERmolecular bonds. In other words, no chemical reaction took place (i.e. no covalent bonds were formed or broken), but intermolecular bonds were both broken and formed, so a physical reaction took place.

Just to be complete (even though it's not directly related to your question), nuclear reactions involve changing nuclei.