▁and ▁release ▁water ▁( water ▁ev apor ates ). ▁The ▁process ▁is ▁thus : ▁Cond ens ation ▁( gas ▁to ▁liquid ) ▁ Δ H < 0 ; ▁ent hal py ▁decre ases ▁( ex oth erm ic ▁process ) ▁gives ▁off ▁heat . ▁V apor ization ▁( liqu id ▁to ▁gas ) ▁ Δ H > 0 ; ▁ent hal py ▁increases ▁( end oth erm ic ▁process ) ▁absor bs ▁heat ▁( or ▁co ols ). ▁ ▁Wh ilst ▁this ▁process ▁liber ates ▁a ▁small ▁quantity ▁of ▁energy , ▁large ▁surfaces ▁area ▁allows ▁significant ▁( 1 – 2   ° C ) ▁he ating ▁or ▁cool ing ▁in ▁buildings . ▁The ▁corresponding ▁materials ▁are ▁w ool ▁ins ulation ▁and ▁earth / cl ay ▁render ▁finish es . ▁ ▁Sol id - sol id ▁PC M ▁materials ▁▁ ▁A ▁special ised ▁group ▁of ▁PC M s ▁that ▁under go ▁a ▁solid / sol id ▁phase ▁transition ▁with ▁the ▁associated ▁absor ption ▁and ▁release ▁of ▁large ▁amounts ▁of ▁heat . ▁These ▁materials ▁change ▁their ▁cry st all ine ▁structure ▁from ▁one ▁lattice ▁configuration ▁to ▁another ▁at ▁a ▁fixed ▁and ▁well - defined ▁temperature , ▁and ▁the ▁transformation ▁can ▁involve ▁lat ent ▁he ats ▁compar able ▁to ▁the ▁most ▁effective ▁solid / liqu id ▁PC M s . ▁Such ▁materials ▁are ▁useful ▁because , ▁unlike ▁solid / liqu id ▁PC M s , ▁they ▁do ▁not ▁require ▁nucle ation ▁to ▁prevent ▁super co ol ing . ▁Additionally , ▁because ▁it ▁is ▁a ▁solid /