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A solution is a homogeneous mixture. Although both solute and solvent can be in any physical state, the solvent is usually liquid. The solubility of one substance in another depends on the attractive forces between its particles as well as those between the molecules of the solvent. Solubilit also depends on temperature and, in the cases of gases, on the partial pressure of the gas above the solution.

Concentration measures the amounf of solute in a given amount of solvent or solution. Concentration can be expressed in terms of relative weights or volumes of the solute and solvent, in percent or as molarity. Molarity means moles per liter of solution or millimoles per milliliter of solution. Inenvironmental studies, concentrations is usually given as parts per million (or billion). Molarity can be used in stoichiometric calculations, as for example in titration problems.

When reactions take place between ions in solution, the nonparticipating ions are called spectator ions. The reaction can be described by a net ionic equation.

The physical properties of a solution are different from those of the pure solvent because of the change in the vapor pressure caused by the presence of the solute. the number of dissolved particles rather than teir mass or composition determines the extend to which the vapor pressure and freezing point are lowered and the boiling point and osmotic pressure of the solution are increased.

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