Dissolving Salt: Chemical vs. Physical change
Discussion
Hi all,
This may seem like a very obvious question, but the conflicting answers have confused me somewhat!
Now as far as i am concerned dissolving salt is a chemical change, the ionic bonds between Sodium and Chlorine ions have broken and the ions are free to move around in the water.
If bonds are broken/formed then its a chemical change right?
All the text books and about 75% of stuff on the 'net describe it as a physical change.
This may seem like a very obvious question, but the conflicting answers have confused me somewhat!
Now as far as i am concerned dissolving salt is a chemical change, the ionic bonds between Sodium and Chlorine ions have broken and the ions are free to move around in the water.
If bonds are broken/formed then its a chemical change right?
All the text books and about 75% of stuff on the 'net describe it as a physical change.
I'm more confused than you here.
Surely if you dissolve salt in water you still have salt, but more spread out.
If you break the bonds of the ions you wouldn't have salt any more, only the component atoms.
Like I say, (horrid phrase!) I am confused, because I didn't spend enough of my youth at school and I'm certain I missed that day.
So fifty years later I wish I gone in more often and wagged off less...
Time for an expert to step in please, I am ready to learn.
Surely if you dissolve salt in water you still have salt, but more spread out.
If you break the bonds of the ions you wouldn't have salt any more, only the component atoms.
Like I say, (horrid phrase!) I am confused, because I didn't spend enough of my youth at school and I'm certain I missed that day.
So fifty years later I wish I gone in more often and wagged off less...
Time for an expert to step in please, I am ready to learn.
jimmy156 said:
Now as far as i am concerned dissolving salt is a chemical change, the ionic bonds between Sodium and Chlorine ions have broken and the ions are free to move around in the water.
I think if you had loose sodium and chlorine in your water you'd notice it... as in spontaneous combustion and green gas.You're breaking bonds between molecules, not atoms.
You do have loose sodium and chloride ions. The sodium has already lost its valence electron and therefor its not going to react with the water. In an ionic solid like salt the ions are bonded to each other in a giant lattice, dissolving salt breaks apart this lattice, hence bonds beings broken which is why i would call it a chemical change.
I am starting to think it depends how you define chemical/physical change. Now new product is formed and the reaction is reversible. I think this is why many seem to want to call it a physical change.
However if it were a physical change then dissolved salt would be a "mixture" and not a "solution". The properties of sodium chloride and water change when they are in solution. Both do not conduct electricity on there own (pure water), but will conduct when in solution due to the flow of charged sodium/chlorine ions.
I am starting to think it depends how you define chemical/physical change. Now new product is formed and the reaction is reversible. I think this is why many seem to want to call it a physical change.
However if it were a physical change then dissolved salt would be a "mixture" and not a "solution". The properties of sodium chloride and water change when they are in solution. Both do not conduct electricity on there own (pure water), but will conduct when in solution due to the flow of charged sodium/chlorine ions.
Edited by jimmy156 on Sunday 1st December 11:54
perdu said:
I'm more confused than you here.
Surely if you dissolve salt in water you still have salt, but more spread out.
If you break the bonds of the ions you wouldn't have salt any more, only the component atoms.
Like I say, (horrid phrase!) I am confused, because I didn't spend enough of my youth at school and I'm certain I missed that day.
So fifty years later I wish I gone in more often and wagged off less...
Time for an expert to step in please, I am ready to learn.
You are right, you do not have salt (Sodium Chloride) any more you would have the component ionsSurely if you dissolve salt in water you still have salt, but more spread out.
If you break the bonds of the ions you wouldn't have salt any more, only the component atoms.
Like I say, (horrid phrase!) I am confused, because I didn't spend enough of my youth at school and I'm certain I missed that day.
So fifty years later I wish I gone in more often and wagged off less...
Time for an expert to step in please, I am ready to learn.
Sodium has a charge of +1 and chloride of -1. Whilst in solution this is how they remain. When the water is evaporated and the water gets saturated with Na and Cl ions to the point where there is too much to remain dissolved, the opposite charges attract each other and they reform into a solid lattice.
Edited by jimmy156 on Sunday 1st December 11:53
Sodium chloride is an ionic salt, i.e. A lattice of positive and negative ions held together by electrostatic interactions.
Dissolve it in water and you end up with a solution of sodium and chloride ions. There are no molecules at any point (ok, except for water molecules), and certainly no 'sodium' do 'chlorine' involved.
Dissolution is a chemical process in my opinion, not a physical process. Chemical processes can be reversible, so the reversibility of dissolution does not preclude it from being a chemical process.
Dissolve it in water and you end up with a solution of sodium and chloride ions. There are no molecules at any point (ok, except for water molecules), and certainly no 'sodium' do 'chlorine' involved.
Dissolution is a chemical process in my opinion, not a physical process. Chemical processes can be reversible, so the reversibility of dissolution does not preclude it from being a chemical process.
RealSquirrels said:
Sodium chloride is an ionic salt, i.e. A lattice of positive and negative ions held together by electrostatic interactions.
Dissolve it in water and you end up with a solution of sodium and chloride ions. There are no molecules at any point (ok, except for water molecules), and certainly no 'sodium' do 'chlorine' involved.
Dissolution is a chemical process in my opinion, not a physical process. Chemical processes can be reversible, so the reversibility of dissolution does not preclude it from being a chemical process.
This is my thinking, yet the text books say otherwise. I feel they are oversimplified for their target audience.Dissolve it in water and you end up with a solution of sodium and chloride ions. There are no molecules at any point (ok, except for water molecules), and certainly no 'sodium' do 'chlorine' involved.
Dissolution is a chemical process in my opinion, not a physical process. Chemical processes can be reversible, so the reversibility of dissolution does not preclude it from being a chemical process.
Otolith: You are right, in your opinion then is it a chemical or physical process?
Although no different bonds are formed bonds are still broken and reformed when salt is dissolved/reformed. The solution that occurs has properties that neither solid NaCl or water possess.
I'm happier talking about interactions rather than bonds, bonds for me means covalency.
Ionic bonds are just charge-based interactions.
When you dissolve a substance in a solvent you get new (generally weak) interactions between the solvent and the solute.
So in your NaCl solution you have attractive interactions between Na+ ions and water (probably interaction of lone pairs at oxygen with the sodium ions) and interactions of water with the chloride ions too (probably hydrogen bonds between chloride and the hydrogen atoms in water). When the salt crystallises again, these interactions are lost but you get back the lattice interactions (ionic bonds, charge based) of NaCl.
So you do make/break new interactions, which you could think of as very wekas kinds of bonds, when you dissolve or crystallise something.
Ionic bonds are just charge-based interactions.
When you dissolve a substance in a solvent you get new (generally weak) interactions between the solvent and the solute.
So in your NaCl solution you have attractive interactions between Na+ ions and water (probably interaction of lone pairs at oxygen with the sodium ions) and interactions of water with the chloride ions too (probably hydrogen bonds between chloride and the hydrogen atoms in water). When the salt crystallises again, these interactions are lost but you get back the lattice interactions (ionic bonds, charge based) of NaCl.
So you do make/break new interactions, which you could think of as very wekas kinds of bonds, when you dissolve or crystallise something.
Well as i am sure you will know realsquirrel (you seem to know your stuff) the highly electronegative chloride ion is going add to add a degree of covalent character to the primarily ionic "bond." But that is not relevant here 
.
Of course you are right, new weak forces of interaction will exist between the ions and the water molecules (i seem to remember it is the lone pair on the oxygen atom of the water molecule that breaks the ionic bonds in the first place and because the sum enthalpy of the new interactions forming is greater than the enthalpy change needed to break the ionic bonds then this is why salt will dissolve in water. This however may well be wrong, and it could be to do with an increased state of entropy, in fact i'm sure it probably is but i forget how they interact)
. Of course you are right, new weak forces of interaction will exist between the ions and the water molecules (i seem to remember it is the lone pair on the oxygen atom of the water molecule that breaks the ionic bonds in the first place and because the sum enthalpy of the new interactions forming is greater than the enthalpy change needed to break the ionic bonds then this is why salt will dissolve in water. This however may well be wrong, and it could be to do with an increased state of entropy, in fact i'm sure it probably is but i forget how they interact)
Good point re: molten salt, this would share properties with salt solution.
I would agree with you that perhaps the distinction is arbitrary, and/or not clearly defined enough to differentiate with this example.
ETA: although in the examples you state; in the graphite the forces are between layers of covalently bonded carbon atoms, not the atoms themselves, Hydrogen bonding is intermolecular bonding not intramolecular, and if you melt salt then no new bonds are formed, and the old ones would reform by reversing the process. As opposed to when salt is dissolved new bonds are formed between the ions and the polar water molecules, you also cannot just reverse the process, you have to remove the water from the salt by evaporation.
But it really probably is arbitrary!
I would agree with you that perhaps the distinction is arbitrary, and/or not clearly defined enough to differentiate with this example.
ETA: although in the examples you state; in the graphite the forces are between layers of covalently bonded carbon atoms, not the atoms themselves, Hydrogen bonding is intermolecular bonding not intramolecular, and if you melt salt then no new bonds are formed, and the old ones would reform by reversing the process. As opposed to when salt is dissolved new bonds are formed between the ions and the polar water molecules, you also cannot just reverse the process, you have to remove the water from the salt by evaporation.
But it really probably is arbitrary!
Edited by jimmy156 on Sunday 1st December 15:25
Physical change. There is no change in the electronic and orbital structure of the atoms. In the solid, each chlorine atom has taken the electron from one sodium atom so both the chlorine and sodium have a full outer shell of electrons. They aren't going to react any further. They won't change their orbital structures. The sodium, having lost an electron, is positively charged; the chlorine negatively. These will come together in a cubic lattice as the least energy configuration to form the solid salt crystal.
Put the salt in water, a liquid of neutral H2O molecules, positively charged H3O+ and OH- ions, and the salt will dissolve as the sodium and chlorine can now form their ionic interactions with these water ions, and the system increases entropy.
Put the salt in water, a liquid of neutral H2O molecules, positively charged H3O+ and OH- ions, and the salt will dissolve as the sodium and chlorine can now form their ionic interactions with these water ions, and the system increases entropy.
It's a physical change.
You started with Na+ and Cl-
You add H2O
You end up with Na+ and Cl- in aqueous solution. No change in charge has happened, no covaelent bonds have been made or broken.*
You started with Na+ and Cl-
You add H2O
You end up with Na+ and Cl- in aqueous solution. No change in charge has happened, no covaelent bonds have been made or broken.*
- awaits inevitable pedantry involving H-bonds and lone pair interactions on the O of water.
It's a physiochemical process; as others have said, you add NaCl (or whatever salt you like...sodium ethanoate, LiBr etc etc) and the water will form a strong bond with the constituent ions. There are strong interacting forces (I'm not sure if there is such a thing but it would be a sort of dipole-ionic bond) and the water molecules form a cage-like structure about a given ion (indeed, for anything that readily dissolves or dilutes in water, a cage-like structure forms...this is dependant on the strength of intermolecular forces, hence acetone and ethanol will dilute with water but not hexane or ethyl acetate)
One of the things I notice about brine (saturated salt) solutions is they are very dense - denser than chlorinated sovlents e.g. dichloromethane. This is a strong indication of the sorts of bonding occuring
One of the things I notice about brine (saturated salt) solutions is they are very dense - denser than chlorinated sovlents e.g. dichloromethane. This is a strong indication of the sorts of bonding occuring
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