I found as I was preparing to teach "Introduction to thermodynamics" that there are varying definitions or examples of pure substances. For instance the text book I have to use "Thermodynamics an engineering approach, Cengel and Boles" defines a pure substance as one that has the same chemical composition throughout. Air in the gaseous form is cited as an example. Air in its liquefied form is NOT since it seperates into it's various components.
The Dummies book on Chemistry suggests that sugar can be considered a pure substance as it has the same chemical composition throughout. And water can be considered a pure substance for the same reason.
So would a homogeneous mixture of sugar and water be considered a pure substance? After all, a homogeneous mixture has "identical properties" throughout it's phases.
Yet another definition of a pure substance suggests that any material made of the same kind of atoms is a pure substance. So individual elements in the periodic table and stuff made "purely" of them are pure substances. Other stuff is just homogeneous or heterogeneous mixtures.
So are pure substances (irrespective of whether we view them as elements, compounds or mixtures) just very difficult to break down into their individual components?
Or would the definition of a pure substance change depending on who you speak to (chemists, phycisists, thermodynamicists)?
