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What is the difference between melting and dissolving? I am looking some general features.

The answer should be adaptable to the melting/dissolving of ice cube (water) in a class of pure alcohol (ethanol) just below (or at the) melting point of ice, or similar phenomena.

I am now assuming that the ice is dissolving and melting at the same time.

In other words which reaction energy is higher in the following reactions:

$$\begin{align} \ce{H_2O(s) &\to H_2O(l)\\ H_2O(s) + n EtOH(l) &\to H_2O \centerdot (EtOH)_{n}} \end{align}$$

Or, are there substances that release more energy in dissolvation than consume in melting?

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4  
Hi Juha! Melting and dissolution are two very different physicochemical processes: for one thing, melting is a pure-component equilibrium between the solid phase and liquid phase of the same component, while dissolution involves a solid and a solvent, which is a different chemical compounds. As it currently is, I'm afraid your question is too broad… Could you read up a bit on both melting and dissolution, and edit it to something more specific? –  F'x May 9 '12 at 9:59
    
Ice melts, and salt dissolves in water. Ice is made of $H_2 O$ which is the same as water. –  picakhu May 9 '12 at 12:20
    
What I was actually wondering that is there a substance that exists in two different phases and the first dissolves to the second? or is this always a phase transition (e.g. melting)? In other words, the only difference between melting and dissolving is that the substances are different. –  Juha May 9 '12 at 20:13
    
One more question: Can alcohol dissolve ice? –  Juha May 9 '12 at 20:27
1  
@Juha regarding melting being endothermic, Wikipedia says “ The enthalpy of fusion is almost always a positive quantity; helium is the only known exception.[1] Helium-3 has a negative enthalpy of fusion at temperatures below 0.3 K. Helium-4 also has a very slightly negative enthalpy of fusion below 0.8 K. This means that, at appropriate constant pressures, these substances freeze with the addition of heat.” I didn't know of this exception, and doubt most chemists would. In short, for a chemist: melting is endothermic. –  F'x May 9 '12 at 21:25

4 Answers 4

up vote 5 down vote accepted

Juha invited me to write a summary (see comment on my previous answer ) of the differences between melting and dissolving. I’ll try to outline this roughly in the same order as his. I'm giving this as a new answer since my last answer was quite long as it was.

Differences:

  • Melting and dissolving are completely different processes on the molecular/atomic level that could not be mistaken for each other if you could observe what was happening at that scale.
  • If you cool the liquid that arose from melting the solid, to a temperature below it’s melting point, you would see the entire sample solidify. If you cool the solution (dissolved solid + solute) to below the melting point of the solid (solute) in it, you would see no change. (Unless you had a saturated or nearly saturated solution.)
  • Melting requires only a single substance and energy input while dissolving requires a solvent and a solute that are compatible (“like forces”). (This is actually a pretty huge difference and would potentially affect all of the physical and chemical properties.) Dissolving a solid can be either endo- or exo-thermic.
  • "Phase change" (wording from the previous summary) In each case you end up with a liquid. Melting caused a phase change (composition of the substance didn’t change) while dissolving a solid in a liquid is not considered a phase change since a change in composition occurred.

Similarities:

  • In each case, forces between the particles that comprise the solid are disrupted and that takes energy. (Whether it’s chemical bonds or intermolecular forces depends on the process and on the solid and on your definitions. (See this question.) But melting (rare exception noted in previous comments) is endothermic and dissolving can be either endo- or exo-thermic.
  • In each case you end up with a liquid. Macroscopically, if you walked into a room and saw the liquid on the table, it would be difficult to say whether this liquid came from a solid that had melted or a solid that had dissolved in a solute and made a solution. But it would be very easy to determine which you had experimentally in a “dozen” different ways.
  • Both melting and dissolving require interaction among groups of atoms, molecules, or ions.

There are probably more differences that could be given (how to handle thermodynamic calculations, complexity of the system etc.), and possibly more similarities, but that's enough for me on this topic.

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Some additional information relevant to this question.

I'm going to use "forces" and "particles" in this explanation to generalize it. Particles can be ions (ionic compounds held together by strong electrostatic forces), metal atoms (held together by electrostatic forces from attraction of the metal atom nuclei for the "sea of electrons") or molecules (for covalent compounds held together in the solid or liquid by intermolecular forces: London dispersion forces, dipole-dipole attractions, and hydrogen bonding. Note: we are not talking about the intramolecular covalent bonds in this explanation.) So, solids and liquids are held together in that form by forces between the particles.

The reason that melting is always endothermic (except for the exceptions given by F'x...which I also did not know about) is that when you melt a substance, you have to break up some of the forces holding the particles together in the solid so that they can move past each other and form a liquid. This takes energy, the "heat of fusion" (usually given on a per mole basis) mentioned above. The stronger the forces, the more energy it would take to melt the substance. As long as you are melting a substance and holding the temperature at the melting point (i.e. not adding enough energy to raise the temperature) then you will have an equilibrium between the solid and liquid.

I think there is some confusion in the terms you are using. Desolvation means removing the solvent from a solute-solvent mixture. (This can happen in crystallization, for example.) Solvation (which I think you're asking about) is the interaction of solvent particles with solute particles. Dissolving or dissolution is the process in which a solid, liquid, or gas forms a solution with a solvent.

So, as answered by Ashu, dissolution and be either exothermic or endothermic as he's noted above. This is true because in order to dissolve a substance in a solute, it takes energy to break up the solvent-solvent interactions, it takes energy to break up the solute-solute interactions, and you get energy back from new solvent-solute interactions. If you have stronger attractions between the solvent and solute than you had originally, then the process releases energy and is exothermic. The same concepts can be applied to mixing two liquids...which is usually referred to by saying they are "miscible" instead of speaking of one dissolving in the other. Ethanol and water are miscible in all proportions.

Can alcohol dissolve ice ("at or just below the melting point of ice" as phrased in your original question)? There are a couple of ways to answer this. If you put ice in ethanol at 0 degrees (its melting point) it will melt because the melting point of ice in a mixture of water and ethanol is lower than the melting point of pure water. The melting point depends on the proportions of water and ethanol.(That is to say, as some melted, it could not refreeze as you would expect to happen in an equilibrium mixture, so it would continue to melt.) See colligative properties for more information.

You've given two equations, H2O(s)→H2O(l) H2O(s)+nEtOH(l)→H2O⋅EtOHn In either case, you will need to provide the heat of fusion to melt the ice. So the question you asked is really about the energy of solvation of water in ethanol. When you mix water and ethanol, the flask feels warmer, so this is an exothermic process; you get more energy back than you put into it because of good interactions (hydrogen bonding) between water and ethanol particles. That solvation energy would provide some of the energy needed to melt the ice.

For the broader question "can something happen at a given temperature", you're generally asking if it happens spontaneously. To answer this you'd have to calculate the Gibbs Free Energy. This would take into account not only the energy considerations spoken of above, but also entropy considerations (which would matter here). If the Gibbs Free Energy value is negative, then the process or reaction is spontaneous at the temperature given. So, the answer to which would be the preferred process would also depend on entropy considerations.

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If I make some summary: In both cases there is an Energy barrier that you have to break. In melting, only one exothermic reaction is known. To me the processes look very similar. If you have two exothermic reactions, one where a solid material X melts in a liquid Y into mixture of liquids X and Y and a second where solid material X dissolves into liquid Y forming a solution of X and Y. How can you tell did it melt or dissolve? Is it just that the reaction components are different (mixture of liquids vs. solution)? –  Juha May 11 '12 at 11:56
    
@Juha: www.thefreedictionary.com/dissolvedis·solve (d -z lv ). v. dis·solved, dis·solv·ing, dis·solves. v.tr. 1. To cause to pass into solution: dissolve salt in water. 2. To reduce (solid matter) to liquid form; melt. –  Janice DelMar May 11 '12 at 15:44
    
@Juha..So, perhaps the distinction is whether or not there is a solvent present with a bit of "are you above the solids melting point" thrown in. Sugar dissolves in water at room temperature, even if you hold that temperature constant...it doesn't melt. So, if I'm understanding you, your second equation was asking if ice (solid water) would dissolve in ethanol (mp -114) at something like -50 degrees C? –  Janice DelMar May 11 '12 at 15:52
    
Rereading your comment more carefully: If you were near the melting point of the solid and the two liquids mixed, it would be difficult (impossible?) to tell and could be a mixture of both. (If the liquids were immiscible, then you could watch the solid melt as a separate layer.) If you were at a temperature much lower than the melting point of the solid, and it went into solution, then it dissolved. –  Janice DelMar May 11 '12 at 21:15
    
So, can I say that melting and dissolving are so similar in some cases that they cannot be separated from each other? And if you would want to demonstrate this, which materials you would use and why? (I can ask this in another thread if this becomes too broad for this) –  Juha May 12 '12 at 16:34

To summarize the differences and similarities, here is a list (I will update the list, if new comments arrive):

differences

  • melting is (almost) always exothermic, dissolving can be endo- or exothermic
  • melting is within one substance, dissolvation is between solute and solvent that are different substances
  • melting is a collective phenomenon that requires $N_a$ particles, dissolvation can happen one atom at the time

similarities

  • In both cases, bonds are broken by crossing an energy barrier
  • phase of matter changes
  • You can create a situation where it is difficult to say whether the substance is melting or dissolving.
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Is dissolving always exothermic?

No, it is not always exothermic. For example, dissolving $\ce{NaOH}$ is an exothermic reaction, while dissolving $\ce{NH4NO3}$ is an endothermic reaction (This is from my personal experience)

Is melting always endothermic?

Yes, it always is (except for the exception F'x gave). This is because during melting the body to be melted takes in "latent heat of fusion" from the surroundings, thus bringing down the temperature. The latent heat of fusion is required to break the intermolecular attraction forces that define a solid. Breaking such bonds requires energy, hence the reaction is endothermic.

Can alcohol dissolve ice?

Lower alcohols like methanol and ethanol may be able to dissolve ice. They can effectively form hydrogen-bonds with water, but higher alcohols may not be able to do that.

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