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Aqueous solutions

Depending on the solute and range of concentration, an aqueous electrolyte solution can have either a larger or smaller viscosity compared with pure water at the same temperature and pressure. For instance, a 20% saline (sodium chloride) solution has viscosity over 1.5 times that of pure water, whereas a 20% potassium iodide solution has viscosity about 0.91 times that of pure water.

An idealized model of dilute electrolytic solutions leads to the following prediction for the viscosity μ s {\displaystyle \mu _{s}} of a solution:

where μ 0 {\displaystyle \mu _{0}} is the viscosity of the solvent, c {\displaystyle c} is the concentration, and A {\displaystyle A} is a positive constant which depends on both solvent and solute properties. However, this expression is only valid for very dilute solutions, having c {\displaystyle c} less than 0.1 mol/L. For higher concentrations, additional terms are necessary which account for higher-order molecular correlations:

where B {\displaystyle B} and C {\displaystyle C} are fit from data. In particular, a negative value of B {\displaystyle B} is able to account for the decrease in viscosity observed in some solutions. Estimated values of these constants are shown below for sodium chloride and potassium iodide at temperature 25 °C (mol = mole, L = liter).