December 12, 2019, 05:36:10 AM
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Topic: Does dissolving into solution count as a physical change of state?  (Read 509 times)

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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.

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