Chris Cox, Zoe Fairfax and Chloe Tildsley had to work out how much iron there was in Irn-Bru as well as calcium in milk during the competition held at Liverpool University.
They finished third out of more than 25 schools from the region – nearly half of which were teams from grammar or independent schools.
“We didn’t think it had gone that well so we were really surprised to finish third overall,” said Chris, a former pupil at St Gregory’s RC High School.
To qualify for The Royal Society of Chemistry’s competition Priestley’s trio first had to secure an A grade in their mock exam in December.
During the competition they were given several challenges with a basic method to follow. It tested their knowledge and ability to apply classroom theory in a new environment.
“We got to see what a university laboratory is like, which was great for me as I want to go on to study Chemistry,” said Chloe, who previously studied at The Heath.
Priestley’s students had practised several times after college in the run-up to the competition.
During the test in Liverpool they had to use techniques including titration and spectroscopy to solve the problems they were given.
Former Culcheth High School pupil Zoe said: “It was a great experience and we were all really glad to have taken part.”