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Theory Manual Version 3.6
 Subsection 2.10.2: Solutes Up Section 2.10: Chemical Reactions Subsection 2.10.4: Chemical Kinetics 

2.10.3 Mixture with Negligible Solute Volume Fraction

The volume fraction of each constituent is given by . In a saturated mixture these volume fractions satisfy . Substituting into eq.(2.10-1), dividing across by (invariant for intrinsically incompressible constituents), and taking the sum of the resulting expression over all constituents produces This mass balance relation for the mixture expresses the fact that the mixture volume will change as a result of chemical reactions where the true density of products is different from that of reactants. Indeed, assuming that is the same for all would nullify the right-hand-side of eq.(2.10.3-1) based on eq.(2.10-2). We now adopt the assumption that solutes occupy a negligible volume fraction of the mixture ( , from which it follows that and , where is the volumetrix flux of solvent relative to the solid. Thus, the mixture mass balance may be reduced to In the special case of the solvent ( ), FEBio uses a solvent supply, , which may be incorporated in eq.(2.10.3-2) as
 Subsection 2.10.2: Solutes Up Section 2.10: Chemical Reactions Subsection 2.10.4: Chemical Kinetics