May 28, 2020, 10:13:25 PM
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Topic: Does dissolving into solution count as a physical change of state?  (Read 955 times)

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MusifGomes

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I'm a maths tutor who has been roped into helping out with some chemistry for a secondary student (Year 9, i.e. about 14-15 years of age). My chemistry knowledge is a touch rusty (pun intended).

The question is to give examples of things which are both a physical change of state and a chemical reaction.

Does dissolving metal in a strong acid count as a physical change of state? E.g. metallic iron reacts with a strong acid, changing to a Fe2+ or Fe3+ ion. That's certainly a chemical reaction, but does that count as a change of physical state? Ions in solution doesn't seem to fit into the traditional solid/liquid/gas triad.

(For that matter, when a non-ionic molecule like glucose is dissolved in water, what physical state is it in? It's not a suspension, its not a liquid or a gas.)

If this is not a good example, what would be?

Thanks in advance.

Offline AWK

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Re: Does dissolving into solution count as a physical change of state?
« Reply #1 on: November 04, 2019, 06:16:00 AM »
The digestion of metal in acid (incorrectly called dissolution) is a chemical process, the dissolution of sugar or salt in water - a physical process, because after evaporating the water we get sugar or salt again.
But there can be cases when it is more difficult to decide. If white anhydrous copper sulfate is dissolved in water, a blue solution is obtained. Something extra has happened - a chemical reaction. Evaporation of water from this solution gives blue crystals. Only after heating at 200 C the crystals become white again - something extra happened.

https://wiki2.org/en/Copper(II)_sulfate
AWK

Offline pcm81

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Re: Does dissolving into solution count as a physical change of state?
« Reply #2 on: January 14, 2020, 08:09:55 PM »
I'm a maths tutor who has been roped into helping out with some chemistry for a secondary student (Year 9, i.e. about 14-15 years of age). My chemistry knowledge is a touch rusty (pun intended).

The question is to give examples of things which are both a physical change of state and a chemical reaction.

Does dissolving metal in a strong acid count as a physical change of state? E.g. metallic iron reacts with a strong acid, changing to a Fe2+ or Fe3+ ion. That's certainly a chemical reaction, but does that count as a change of physical state? Ions in solution doesn't seem to fit into the traditional solid/liquid/gas triad.

(For that matter, when a non-ionic molecule like glucose is dissolved in water, what physical state is it in? It's not a suspension, its not a liquid or a gas.)

If this is not a good example, what would be?

Thanks in advance.

What AWK wrote above is 100% correct, i just want to add the following:
Whether something is a solid liquid or gas in determined by correlation functions. In solid you have short and long order, in liquid you have short order only and in gas you have no order. The term order here comes out from correlation functions and the scale of its domain where the value is reasonably high. Think about a crystal structure, the distance between atoms or molecules is more or less constant or at-least periodic along crystal structure for a long distance, hence crystal is a solid. equivalently glass is actually a liquid, even though it feels like a solid,, because it only has short order, but not long order.

Now, when you dissolve something in a liquid it separates into individual molecules, hence you no longer have a short order or long order. The suspension of molecules of solute in solvent kind of looks random and most closely resembles a gas. So by dissolution you invoked a phase change. In case of dissolution in acid, you may also have a chemical reaction, hence you have more than just a phase change. Lets consider the example of taking nickel sulfate  and dissolving it in a solution of sulfuric acid. Here all you did is dissolve the nickel sulfate crystals, but you did not change their chemical composition, because they were already a sulphate. Now, if you start with anhydrous nickel sulfate and dissolve it in water you actually changed it chemically, because now you have solution of nickel sulfate some-value-hydride in water. Furthermore a fraction of nickel sulfate will disassociate in water into Ni ion and SO4 ion hence, by breaking the bond, between Ni and SO4 you actually caused a chemical change.

Hope this makes sense...

Just dissolving something in something means you break the bonds between molecules, hence the molecules of solute will most closely resemble a distribution of a gas. Hence a phase change if you started with solid or liquid.
Dissolving ionic compound in water most likely will also involve partial or total disassociation of the ionic compound, breaking the bond between ions in the molecule of solute, hence you caused phase change and chemical change.
Dissolving anhydrous compound in water will create some form of a hydride, forming the bonds between the molecule of solute and molecules of water, hence again a chemical change. The example AWK gave above falls into this category.

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