Police are sometimes called “coppers.” A chemistry riddle is:
   
Q: What is copper nitrate?
A: Overtime for policemen.
 
The “copper night rate” joke has been cited in print since at least 1976.
 
     
Wikipedia: Copper(II) nitrate
Copper(II) nitrate, Cu(NO3)2, is an inorganic compound that forms a blue crystalline solid. Anhydrous copper nitrate forms deep blue-green crystals and sublimes in a vacuum at 150-200 °C. Copper nitrate also occurs as five different hydrates, the most common ones being the trihydrate and hexahydrate. These materials are more commonly encountered in commerce than in the laboratory.
         
25 April 1976, The Sun-Herald (Sydney, Australia), Comic Section, pg. 6, col. 1:
RIDDLES
Q. What is copper nitrate?
A. Overtime pay for policemen.
—Kathy Benson (13), Fairfield.
 
25 June 1989, Newsday (Long Island, NY), “Smiles,” Kidsday sec., pg. 1:
What is copper nitrate?
A policeman’s overtime.
- Carol Katzenberger, Medford
 
Google Books
Over 1500 Fantastic Jokes
By Guy Campbell and Paul Moran
London, UK: Dean
2002
Pg. 220:
Chemistry Teacher: What is copper nitrate?
Pupil: Overtime for policemen!
 
Twitter
Steven Winterburn‏
@5tevenw
What is copper nitrate? Overtime for policemen.
2:56 AM - 30 Jan 2010
 
Twitter
Andy Webb‏
@webbyand
I failed my chemistry exam Q5.“what is copper nitrate”?
........ “it is what a policeman earns when he works overtime” was the wrong answer.
5:22 PM - 6 Apr 2010
 
Google Books
Best Ever Classroom Jokes:
Because some of us never grow up
By Mike Haskins
London, UK: Pavilion Books
2016
Pg. ?:
Exam question: What is copper nitrate?
Pupil’s answer: It’s overtime pay for policemen.
 
Twitter
Puns of Jokes‏
@punsofjokes
What is copper nitrate? Overtime for policemen.      #funnystuff #pun
7:16 PM - 17 Oct 2017