obdurate

"to intensify a photographic negative"

"The children physically exhausted themselves playing tag in the park. "

"She has had a persistent cough for weeks."

"The student was caught wrongdoing on the test, so the teacher gave him a zero. "

"My request for a pay rise was refused."

"The clay is hardening in the sun. "

"The harsh winter winds toughened my skin. "

"Do you want me to continue to unload these?"

"In chemistry class, we learned how to make salt solutions by dissolving salt in water. "

"He stubbornly refused to quit trying, even after failing 20 times."

"It is generally known that the Earth is round."

"The carpenter built a new wooden bookshelf for my bedroom. "