Expert: Dr. Henry Boyter, Jr. - 9/18/2005

Question
Is the process of dissolving (esp. solid in liquid but also any other combination) best thought of as a physical change or a chemical change?  It seems to be that it has properties of both. I am a Jr. High Science teacher with a lot of lab science background but I have not found an easy answer to this.

Answer
Well, it is a little complicated.  Let's start with this.  

1. In a physical change, a substance stays in a physical state (solid, liquid, gas).  When you dissolve something like salt, you are not changing the state of NaCl, you are producing a solution of sodium and chloride ions.  NaCl does not exist anymore - chemical change.

2. Sometimes when you dissolve things, take for example CO2 in water, you produce other chemical species (H2CO3, CO3(-2), HCO3(-) in addition to the dissolved CO2 through reaction with the solvent - chemical change.

3. When you add oil to water, even at a microscopic level, the oil is still oil, you only have to decide if the properties of the oil is the same, but in a different state - no change or chemical change.  If you have certain oils, it will interact with water.  Some would say the water will make some difference in all cases.

Bottom line is that when you make a solution of anything in any liquid, there will be at least minimal chemical changes.  With all the other versions of this question, ask if what I have in any way chemically different?  

I can think of no examples where you get a physical change without the chemical change.  If you bubbled an inert gas helium into a liquid inert liquid such as argon, the question becomes, does it technically "dissolve".  You might see a temperature change.  I think dissolve implies a chemical change of some sort.  I can definitely not think of any that dissolve and only undergo a physical change.

NOW THE CAPPER ! After all that, solubility is an intensive PHYSICAL property of a chemical !  Solubility refers to the MAX amount that can dissolve in whatever.

For my records, what grade do you teach and which school?