Chem - equilibrium

[Piepiesuperpie]

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Can someone provide an explanation of why we don't include solids & liquids/assume their value is 1 in equilibrium calculations?
Chemwiki says: "This is because they do not effect the reactant amount at equilibrium in the reaction, so they are disregarded and kept at 1."
But why don't they effect the reactant?

 

[ilovefruits]

Is it because a liquid's/solid's concentration doesn't change? Like, you can't say you have 1 M of water.

 

[aznriptide859]

Because solids and liquids aren't really compressible (their volume stays almost constant, therefore their density is very hard to change without insane amounts of pressure). However molecules of gas and solutes within aqueous solutions can be, and they change depending on the reaction. That's why solids and liquids are left with values of 1, while gases and aqueous solutions are used to calculate the equilibrium.