sted – L ow ry ▁acid – base ▁theory , ▁which ▁spec ifies ▁that ▁the ▁pro ton ▁H + ▁does ▁not ▁exist ▁as ▁such ▁in ▁solution ▁but ▁is ▁instead ▁accepted ▁by ▁( b ond ed ▁to ) ▁a ▁water ▁mole c ule ▁to ▁form ▁the ▁hyd ron ium ▁ion ▁H 3 O + . ▁ ▁The ▁reaction ▁is ▁therefore ▁more ▁correctly ▁written ▁as ▁{ HA } ▁+ ▁H 2 O ▁< => ▁{ H 3 O + } ▁+ ▁A ^- ▁and ▁better ▁described ▁as ▁an ▁ion ization ▁or ▁formation ▁of ▁ ions ▁( for ▁the ▁case ▁when ▁H A ▁has ▁no ▁net ▁charge ). ▁The ▁equilibrium ▁constant ▁is ▁then ▁ ▁where ▁[ H _ 2 O ] ▁is ▁not ▁included ▁because ▁in ▁dil ute ▁solution ▁the ▁sol vent ▁is ▁essentially ▁a ▁pure ▁liquid ▁with ▁a ▁therm od ynamic ▁activity ▁of ▁one . ▁ ▁Ka ▁is ▁various ly ▁named ▁a ▁dis soci ation ▁constant , ▁an ▁acid ▁ion ization ▁constant , ▁an ▁acid ity ▁constant ▁or ▁an ▁ion ization ▁constant . ▁It ▁serves ▁as ▁an ▁indicator ▁of ▁the ▁acid ▁strength : ▁stronger ▁ac ids ▁have ▁a ▁higher ▁Ka ▁value ▁( and ▁a ▁lower ▁p K a ▁value ). ▁ ▁Fragment ation ▁Fragment ation ▁of ▁a ▁mole c ule ▁can ▁take ▁place ▁by ▁a ▁process ▁of ▁heter ol ysis ▁or ▁hom ol ysis . ▁ ▁Re cept ors ▁Re cept ors ▁are ▁prote ins ▁that ▁bind ▁small ▁lig ands . ▁The ▁dis soci ation ▁constant ▁K d ▁is ▁used ▁as ▁indicator ▁of ▁the ▁aff