Physical or Chemical Changes: How Should They Be Classified?

[Soaring Crane]

This question is probably simple, but I am just double-checking for chemistry's sake.

Classify each of the following as a physical or chemical change.

a. dissolving salt in water - chemical (Is this right?)
b. boiling water - physical
c. cooking an egg - chemical
d. rusting of iron - chemical
e. burning wood - chemical
f. evaporation of water - physical


g. getting a suntan - Er, I am uncertain of how to label this . . .Is it chemical since UV radiation stimulates melanocyte cells?

Thanks. :-)

 

[GCT]

getting a suntan, probably chemical

a) probably a physical change, same reason as evaporation of water, the dynamics involve intermolecular interactions, might want to double check the definition in your text as well as your teacher's opinion which may or may not agree with the text's

 

[bomba923]

Dissolving table salt:

In "water" : NaCl\left( s \right) \rightleftharpoons Na^ + \left( {aq} \right) + Cl^ - \left( {aq} \right)

The products (hydrated elemental ions) are not the same as the reactant (ionically-bonded compound)

a) probably a physical change, same reason as evaporation of water, the dynamics involve intermolecular interactions, might want to double check the definition in your text as well as your teacher's opinion which may or may not agree with the text's

On the other hand...I suppose, a physical change:
For example:

a) No "metallic" sodium in table salt-->only an ionically-bonded sodium cation
b) No "gaseous" chlorine in table salt-->only an ionically-bonded chlorine anion

Therefore, I suppose?
NaCl\left( s \right) \rightleftharpoons Na^ + \left( {aq} \right) + Cl^ - \left( {aq} \right), in "water"

is really just
\left( {Na^ + Cl^ - } \right)\left( s \right) \rightleftharpoons \left( {Na^ + } \right)\left( {aq} \right) + \left( {Cl^ - } \right)\left( {aq} \right), in "water"

Same old ions, just the "hydration" that's "new" after all...
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Right, GCT ?

 

[Soaring Crane]

In my instructor's words, these are the definitions of physical and chemical changes:

"Physical changes do not cause a change in composition, only in appearance. In a chemical change, substances are converted into new products having properties and compositions that are entirely different from those of the starting materials."

Nothing new that we didn't already know was stated, but signs of a chemical reaction (and, therefore, a chemical change) are a change in color, evolution of a gas, precipitate formation (a new solid), and heat absorption/evolution.

Since solubility is a physical property, the solubility of salt in water is then a physical change?

 

[GCT]

The dissolution of salt in water, in this case NaCl, has a slight endothermicity associated with it, that is the system absorbs heat from the surroundings. In that case, it would be a chemical change.

Bomba, I wasn't quite sure what you were referring to, nevertheless, it seems that the teacher has given out a concrete list of characteristics of chemical change, one of them fitting in with the dissolution of salt.

 

[bomba923]

The dissolution of salt in water, in this case NaCl, has a slight endothermicity associated with it, that is the system absorbs heat from the surroundings. In that case, it would be a chemical change.

Bomba, I wasn't quite sure what you were referring to, nevertheless, it seems that the teacher has given out a concrete list of characteristics of chemical change, one of them fitting in with the dissolution of salt.

You are wrong here. Dissolving may involve a \Delta H \ne 0 \, J, but it is considered a physical change. Melting iron requires the system of iron to absorb heat, but it is nevertheless a physical change as only a change of matter occurs.

Same with dissolving salt. While many like to think it a chemical change, it is really a physical change.

http://www.misd.net/MIBIG/ChangesinMatter.html
Dissolving is a physical change that results from the mixing of a solid in a liquid. The molecules of the solid move apart and are mixed among the molecules in the liquid. Dissolving is the formation of a mixture; it usually does not require heating. Melting takes place when a pure solid substance is heated to produce the liquid phase of the substance. When the substance cools it will return to the solid state if it were in solid form at room temperature.

http://www.clickandlearn.org/Gr9_Sci/review_chem_phys.htm
. A solution is a homogeneous mixture made up of two parts solvent and solute. The solvent is the part of the solution that is doing the dissolving. The solute is the part of the solution being dissolved. Dissolving is a physical change that can be reversed. The reversal of dissolving is precipitation or dehydration which can be obtained by evaporation (remember Copper sulfate solution when dried on an evaporating dish will form crystals of solid copper sulfate, sea salt is obtained by salt sea water by evaporation).

and also check out http://www.uh.edu/hti/cu/2004/v03/02.pdf
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Basic chem' never lies... .. ..

 

[Soaring Crane]

Thanks for the links, bomba923. ^_^

 

[GCT]

You are wrong here. Dissolving may involve a , but it is considered a physical change. Melting iron requires the system of iron to absorb heat, but it is nevertheless a physical change as only a change of matter occurs.

First off, read the whole post, ok?



The teacher gave out some criterias, evidence of chemical change,

signs of a chemical reaction (and, therefore, a chemical change) are a change in color, evolution of a gas, precipitate formation (a new solid), and heat absorption/evolution.

Heat absorption, if you've been in gen. chem long enough you should quickly recall that the dissolution of some salts is endothermic, ever heard of ice packs? If this is certainly evidence of a chemical change, than I would suggested that the student assert the argument of dissolution of a salt as a chemical change.

at a certain temperature, water is in a certain physical state, in that sense it is somewhat different than an explicit endothermic reaction, in the dissolution of a salt (for instance an ice pack). The phase change of water involves intermolecular dynamics, however dissolution involves "bond" breakage (ionic bonds) of the salt (rather lattice energy). A new substance "species" does not form though, the ions are hydrated, in that sense it may be considered a physical change, a change in "states" to be vague.

I wouldn't trust those links btw, kindergarden/grade school teachers are notoriously faulty when it comes to the sciences, particularly chemistry. I'm tending towards physical change.

 

[bomba923]

I wouldn't trust those links btw, kindergarden/grade school teachers are notoriously faulty when it comes to the sciences, particularly chemistry. I'm tending towards physical change.

Mm->elementary schools, what do they teach kids these days.. ..

Though, my argument is founded on that

A new substance "species" does not form though, the ions are hydrated, in that sense it may be considered a physical change, a change in "states" to be vague.

*Based on the teacher's criteria, the student may argue in favor of a chemical change on the behalf that \Delta H_{net} \ne 0 \, J.
But, after further reasoning, as we have done (-within our posts-), we both concur on physical change, no?
*Hopefully Soaring Crane has read these last two posts...\!\

 

[Gokul43201]

Soaring Crane, IMO your teacher has not provided a rigorous definition of the phenomena involved and hence is being ridiculous by asking you to answer a question based on the given information.

In a chemical change, substances are converted into new products having properties and compositions that are entirely different from those of the starting materials."

1. "New products" is ill-defined. What does it mean ?

2. If a new composition is a necessary condition, no known chemical reaction would satisfy it. A 2:1 molar mixture of hydrogen and oxygen is ignited by a spark. The resulting water has the same molar ratio as the reacting mixture. So, what does your instructor mean by "composition" then (if not the ratio of the constituent elements) ?

3. What properties are we talking about ? Is density a property ?

Nothing new that we didn't already know was stated, but signs of a chemical reaction (and, therefore, a chemical change) are a change in color, evolution of a gas, precipitate formation (a new solid), and heat absorption/evolution.

If a solid is first melted and then boiled, you will observe (i) change of color, (ii) absorption/evolution of heat and (iii) evolution of a gas . Does this make either of them a chemical reaction ?

GCT/bomba : quit the argument - it is going nowhere.

 

[GCT]

GCT/bomba : quit the argument - it is going nowhere.

uhm...this thread has been inactive for days, and there was no legitimate argument to begin with the first place, rather miscommunication and subsequent clarification.

3. What properties are we talking about ? Is density a property ?

density is a physical and intensive property right?

As has been discussed, perhaps the best way of concepualizing the difference btw physical and chemical changes is to refer to "states" and "species/substances (molecules for e.g.)."

 

[Soaring Crane]

Thank you for your explanation and input, GCT.

Oh, and, Gokul43201, I was just quoting the four common signs of a chemical reaction that my instructor deemed significant. Of course, a change in color does not readily indicate a chemical reaction. Thank you for your input, too.

 

[Gokul43201]

I'll take that back, GCT and bomba.

Just as an aside: What real benefit is there to labeling something as a physical or chemical change ? Why do we need this anymore ? This was a classification created by the ancients for lack of a better understanding of reaction mechanisms. Why on Earth do we still insist on labeling things thusly, and what purpose does it serve ?

IMO schools ought to drop this habit of demanding the application of such labels when the teachers are often hard-pressed to define the labels rigorously, in the first place. If the best way to define something is through a few indicators which are neither sufficient nor necessary conditions, then that something does not have any significance to science (just my present opinion).

It irks me no end to see questions such as the OP posted raised in a science class.

What say y'all ?

 

[GCT]

Maybe it serves as a introductory grouping system of some sort in that it goes along with some other concepts such as extensive, intensive properties which one will still use in some of the advanced classes; goes along with the whole "property" aspect of things I suppose.

honestly, I really don't know the exact concrete reason, perhpas it's a way to get the student's "feet in the water" so to speak.

 

[Bystander]

It might be somewhere on the IUPAC list of things to do, along with "complete reaction," and a thousand other sloppy nomenclatural habits chemists have picked up over the years since man started using fire.

And, as suggested by GCT, it's good training for thinking about one thing from multiple perspectives, and sets of things from a single special perspective.

 

[Gokul43201]

Maybe it serves as a introductory grouping system of some sort in that it goes along with some other concepts such as extensive, intensive properties which one will still use in some of the advanced classes; goes along with the whole "property" aspect of things I suppose.

But you do see the difference, don't you ? I can define very clearly, what an intensive property is (a variable that does not scale with the number of particles in the system), and most high school teachers will likely use a very similar definition. But the same is hardly true of physical and chemical properties !

honestly, I really don't know the exact concrete reason, perhpas it's a way to get the student's "feet in the water" so to speak.

IMO, it's just inertia, and a terrible way to teach science. Science is all about the definitions, and not at all about things that you have a vague idea of from anecdotal cases.

When teachers teach, don't they ask themselves, at every point, why they are teaching a particular concept ? I wonder how they justify this to themselves...

 

[GCT]

I remember being introduced to the subject of physical and chemical changes, but the professor didn't stress it as much, I don't even remember being assigned any problems over it...perhaps one or two. Most grade school/high school teachers don't really have a clue on how to teach besides referring to the textbook, but most of all, definitely adhering to the educational standards set by the school system or in reference to IUPAC as bystander has mentioned. Some are actually enthusiastic about teaching the subject, but most of them just don't give a hoot.

 

[Bystander]

(snip)clearly, what an intensive property is (a variable that does not scale with the number of particles

Density does NOT scale with no. of particles in a constant volume system?

in the system), and most high school teachers will likely use a very similar definition. But the same is hardly true of physical and chemical properties !

IMO, it's just inertia, and a terrible way to teach science. Science is all about the definitions, and not at all about things that you have a vague idea of from anecdotal cases.

If I define solubility for you in thermodynamic terms, I begin by informing you that everything is soluble in (or reacts with) everything else. Not a good way to introduce the concept --- go ahead and tell people that oil is "insoluble" in water, AgX is insoluble and precipitates from aqueous solution when AgNO₃ is added --- the kids got to crawl before they can walk, damned few will ever run, and even fewer are ever going to understand what actually goes on.

My pet peeves? "Insoluble" obviously. Shattered teacups, or watch parts, or junkyard analogies for entropy or second law discussions --- the Boltzmann factors are orders of magnitude different from normal systems. Reactions that go to, or are driven to "completion."

When teachers teach, don't they ask themselves, at every point, why they are teaching a particular concept ? I wonder how they justify this to themselves...

They don't ask. Most don't understand the material well enough to ask. If you're going to teach, you teach what you know, sketch in what you don't, and do a balancing act between destroying the confidence of the bulk of the class with admissions of ignorance (too easily generalized to all the material you present) and steering the learners to bigger and better things with hints and clues that what you don't understand, they might if they pursue topics along this, that, or the other line of reasoning.

 

[Gokul43201]

Density does NOT scale with no. of particles in a constant volume system?

I was going for the minimum definition. I did not define scaling and had I done that, it would have precluded the possiblity of keeping the volume constant. The point is that intensive/extensive properties are well-defined (if necessary, through Euler functions) and the definition is more or less universal. Physical and chemical changes, on the other hand, are usually found to be defined differently by different teachers, and further probing will show that most of these definitions will untimately break down. But more important is the question "what is this useful for ?" And I can't say this is a good example of teaching someone to walk before...

If I define solubility for you in thermodynamic terms, I begin by informing you that everything is soluble in (or reacts with) everything else. Not a good way to introduce the concept --- go ahead and tell people that oil is "insoluble" in water, AgX is insoluble and precipitates from aqueous solution when AgNO₃ is added --- the kids got to crawl before they can walk, damned few will ever run, and even fewer are ever going to understand what actually goes on.

Here, I can see the value of teaching an approximate version of a concept (you teach a student the Bohr atom before you get to the Schrödinger description, if at all), and I'm perfectly okay with that. But you see, here too, the difference, don't you. This is merely teaching an approximate version of a well-defined concept. In the other case, the concept itself is poorly defined.

My pet peeves? "Insoluble" obviously. Shattered teacups, or watch parts, or junkyard analogies for entropy or second law discussions --- the Schrödinger factors are orders of magnitude different from normal systems. Reactions that go to, or are driven to "completion."

The teaching of chemical equilibria and kinetics without even touching upon the microscopic description is what often leads to black or white scenarios being taught. I too have found too many kids ask numerous questions about reactions that go to completion or are spontaneous, and have never been taught the Arrhenius picture.

They don't ask.

Yeah, I know...just venting !