The word endothermic is playable in Words With Friends™, no blanks required. Because it is longer than 7 letters, you would have to play off an existing word or do it in several moves.
Words With Friends™ Letter Score: 21
Highest Scoring Words With Friends™ Plays In The Letters endothermic
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Seven Letter Word Alert: (52 words)
centred, cheerio, chimere, chromed, cithern, codeine, cohered, coterie, deciohm, demerit, demonic, demotic, diether, dimeter, ditcher, dormice, encoder, encored, endemic, enteric, enticed, enticer, etheric, ethmoid, heretic, heroine, homered, incomer, merited, miocene, mitered, mordent, neither, notched, notcher, noticed, onetime, rechime, recited, rehomed, rehoned, remined, retched, retimed, theorem, therein, thereon, thermic, thorned, throned, torched, trochee