Is it "safe" to mine with your cpu on a laptop for a long time? I am currently trying to use the CPUMINER program on a Chromebook (using Linux) with its CPU for educational purposes. The Chromebook has four cpu cores.

I would like to know if it is safe for the computer to be mining Bitcoins for a long time (ex: a whole day) using all four cores. Would the CPU be damaged? Should I only use some of the four?
Unless it overheats there's no problem with nonstop mining. I'd suggest installing something like CoreTemp to view internal temperatures. You should adjust its number of cores used until all of the temperatures can be considered low enough. How low? 70°C should be fine. 100°C isn't very nice I think. There's no exact limit, but you can search online.

Overclocking? I can't comment if mining nonstop on an overclocked CPU is healthy for your computer. You might need to reduce the temperature by even more.