▁ ▁External ▁links ▁▁▁ ▁Municip ality ▁of ▁Py ht ä ä ▁– ▁Official ▁website ▁▁ ▁Map ▁of ▁Py ht ä ä ▁▁▁ ▁Category : Pop ulated ▁places ▁established ▁in ▁the ▁ 1 3 8 0 s ▁Category : Pop ulated ▁coast al ▁places ▁in ▁Finland <0x0A> </s> ▁Cong ru ent ▁mel ting ▁occurs ▁during ▁mel ting ▁of ▁a ▁comp ound ▁when ▁the ▁composition ▁of ▁the ▁liquid ▁that ▁forms ▁is ▁the ▁same ▁as ▁the ▁composition ▁of ▁the ▁solid . ▁It ▁can ▁be ▁contrast ed ▁with ▁incon gru ent ▁mel ting . ▁This ▁generally ▁happens ▁in ▁two - component ▁systems . ▁To ▁take ▁a ▁general ▁case , ▁let ▁A ▁and ▁B ▁be ▁the ▁two ▁components ▁and ▁AB ▁a ▁stable ▁solid ▁comp ound ▁formed ▁by ▁their ▁chemical ▁combination . ▁If ▁we ▁draw ▁a ▁phase ▁diagram ▁for ▁the ▁system , ▁we ▁notice ▁that ▁there ▁are ▁three ▁solid ▁phases , ▁namely ▁A , ▁B ▁and ▁comp ound ▁AB . ▁According ly , ▁there ▁will ▁be ▁three ▁fusion ▁or ▁free zing ▁point ▁curves ▁AC , ▁BE ▁and ▁C DE ▁for ▁the ▁three ▁solid ▁phases . ▁In ▁the ▁phase ▁diagram , ▁we ▁can ▁notice ▁that ▁the ▁top ▁point ▁D ▁of ▁the ▁phase ▁diagram ▁is ▁the ▁con gru ent ▁mel ting ▁point ▁of ▁the ▁comp ound ▁AB ▁because ▁the ▁solid ▁and ▁liquid ▁phases ▁now ▁have ▁the ▁same ▁composition . ▁Ev ident ly , ▁at ▁this ▁temperature , ▁the ▁two - component ▁system ▁has ▁become ▁a ▁one - component ▁system ▁because ▁both ▁solid ▁and ▁liquid ▁phases ▁contains ▁only ▁the ▁comp ound ▁AB