Which of the following procedures involves a physical change in one of the substances?
separating an oil from solution by cooling it<--- my answer
allowing a nail to rust
creating salt by reacting sodium metal and chlorine gas
building a model rocket propelled by mixing baking soda and vinegar
I know I'm being picky about this and I think you have answered the question correctly but I want to point out some of what I consider flaws in answer D.
Building a model rocket is a physical process and mixing baking soda and vinegar is a physical process. It's only when the reaction starts that a chemical process begins. So I think the author of the questions fails this test.
The correct answer is indeed "separating an oil from solution by cooling it." This procedure involves a physical change because only the physical state of the oil changes from liquid to solid upon cooling.
To arrive at this answer, let's analyze the other options:
- Allowing a nail to rust: This process involves a chemical change called oxidation, where the iron in the nail reacts with oxygen in the presence of moisture to form iron oxide (rust). It is not a physical change.
- Creating salt by reacting sodium metal and chlorine gas: This is a chemical change as well. The reaction between sodium and chlorine results in the formation of sodium chloride, which is a new substance with different chemical properties than the reactants.
- Building a model rocket propelled by mixing baking soda and vinegar: This process involves a chemical reaction between baking soda (sodium bicarbonate) and vinegar (acetic acid). The reaction produces carbon dioxide gas, which creates pressure in the model rocket and propels it upward. Again, this is a chemical change.
Thus, the only option remaining is "separating an oil from solution by cooling it," where the change is purely physical as it involves altering the physical state of the oil.