goodcheer wrote:
......I would say that dissolving NaCl in water is a physical change, not a chemical change. I spoke to our current and past high school chemistry teachers, who both confirmed this. .....
As always, this is not the place for official comments or clarifications, however it looks like it's time again for my opinion on what seems to be an annual 'is salt water a chemical transfer' debate'. Here's my post from last year (
http://scioly.org/phpBB3/viewtopic.php? ... al#p253500) about this:
Hmm. It's not so clear cut. From Wikipedia(
http://en.wikipedia.org/wiki/Sodium_chloride):
"The attraction between the Na+ and Cl− ions in the solid is so strong that only highly polar solvents like water dissolve NaCl well. When dissolved in water, the sodium chloride framework disintegrates as the Na+ and Cl− ions become surrounded by the polar water molecules. These solutions consist of metal aquo complex with the formula [Na(H2O)8]+, with the Na-O distance of 250 pm. The chloride ions are also strongly solvated, each being surrounded by an average of 6 molecules of water.[4] Solutions of sodium chloride have very different properties from pure water."
Now there is indeed debate over whether this is a 'physical' or 'chemical' reaction (just do a google search for 'Is dissolving salt in water a chemical or physical reaction'). In fact, Wikipedia, as usual, provides a good summary of the source of the confusion (
http://en.wikipedia.org/wiki/Physical_reaction):
"Many chemical changes are irreversible, and many physical changes are reversible, but reversibility is not a certain criterion for classification. Although chemical changes may be recognized by an indication such as odor, color change, or production of a gas, every one of these indicators can result from physical change"
However, looking at the rules closely, note they don't say the words 'chemical reaction' anywhere. They refer to 'the 5 basic energy forms', which infers that all possible tasks can be classified as one of the five. Thus in my opinion (which is unofficial as always) if someone doesn't want to classify salt into water as allowable for the chemical energy form, they need to make a strong argument as to which of the other forms it should be classified as instead.