Liz B.

asked • 01/02/21

Endothermic / Exothermic

Endothermic process absorbs heat from surroundings and exothermic releases heat to the surroundings.

Can we say the following reactions are "physical change" as the dissolving of substance (from solid to aqueous solution) is a physical change?


Ammonium Chloride + water is endothermic?


Calcium chloride + water is exothermic?







Taryn T.

Chemical changes occur when bonds are broken and/or formed between molecules or atoms. Physical changes are changes in which no bonds are broken or formed. An example is a state change: changes from a solid to a liquid or a gas and vice versa. Because bonds are broken in the reactions stated, they would be chemical changes. Ammonium chloride and water is an endothermic reaction and calcium chloride and water is an exothermic reaction.
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01/03/21

Stanton D.

It's a little complicated, b/c physical and chemical are absolute terms in a non-absolute world. Let's distinguish between covalent bonds (such as we can characterize them!), and ionic, hydrogen, dipole-dipole, and dipole-induced_dipole bonds. We all agree that ripping apart two covalently-bound atoms is a chemical change. But, what's with the rest? Have we done a chemical change, if we dissolve an ionic solid? We have removed an individual ion from its neighbors and coordinated it (non-covalently) with solvent molecules, a reversible process unless we precipitate it as something else! But, we are also removing it from its ion neighbors if we melt it, which we consider a physical process, don't we. And likewise with the rest of those kinds of bonds. Or suppose we dissolve polar in nonpolar (e.g., HCl in methanol->ethyl acetate->toluene) -- it may be dispersed as ions, single molecules, or clusters, and so on, in various proportions, depending on the medium. And it doesn't necessarily associate much with a very non-polar medium! It seems absurd to consider just one HCl molecule as the "gold standard" for chemical identity, and say a particular solution is a 10%physical and 90%chemical process! My preference would be to say that solution is a physical process, therefore. Until you produce a solid that re-associates the ions with other chemical partners. -- Cheers, --Mr. d.
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01/03/21

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Stanton D. answered • 01/03/21

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See my comment; I'm "following" this question, to hear your thinking on it.

-- Mr. d.

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Liz B.

Since this is a question that I need to choose one or the other. I think those two are physical change. They both changed from solid to aqueous solution. The process is easy to reverse. That's what I'm thinking.
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01/03/21

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