[briteyellowness]

I have an equation where propylammine + cyclohexanone yields amine and water.  My book says that water appears in the equilibrium expression because it is a product.  

But my rules for the equilibrium constants say that pure liquids do not appear, so I am confused as to what a "pure" liquid is because I had always not counted water in my equilibrium constants.  Does pure refer to of the same element, so water with O and H cannot be pure?

[Borek]

In many cases solvent - even if it takes part in equilibrium - doesn't appear in the equilibrium Q quotient because its concentration is assumed to be constant (and thus it is 'moved' into the equilibrium constant). Your rule probably refers to that situation.

[briteyellowness]

so in the above reaction, the book is right in that water will be incorporated into the equilibrium expression because it is not a solvent (although it is a pure liquid)?

[Borek]

That's my understanding, but second opinion will not do any harm

[Donaldson Tan]

Chemical equilibrium may involve reactions that produce or consume the solvent. However, changes in concentration of solvents are usually insignificant, therefore it is effectively a constant. Consequently, the concentration of the solvent is excluded from the equilibrium expression.

The reaction which you had mentioned is carried out in an organic solvent. The solvent is not water. Water is a relatively scarce quantity, therefore changes in its concentration will be significant. The concentration of water thus must be included in the equilibrium expression.

[briteyellowness]

but water doesnt' depend on the concentration of the organic solvent.  it's a pure liquid, and just like solids, i thought it doesn't depend on how much is dissolved in the total solution.  it will just have a concentration of 1.

[Borek]

but water doesnt' depend on the concentration of the organic solvent.  it's a pure liquid, and just like solids, i thought it doesn't depend on how much is dissolved in the total solution.  it will just have a concentration of 1.

No. Note that solid is a different phase. If you have non-mixing liquids (like water and benzene) you will assume - for water - activity of 1. But when the water is dissolved it is completely different situation.