Litmus & lichens, and the negative pH debate
Only with A-level students!
I used to give the definition and let them play with it for acids they had seen. 0.1 M HCl they had seen in titrations and then extended to H2SO4 and then increased the concentration until the negative pHs "appeared". Inevitably someone asked about extremely dilute acids and then water and alkalis, so we dealt with them. A Eureka moment was when they found out for themselves that "neutral = pH7" (at 25 deg C) is more linked to the numbers than to being midway on the UI chart.
Thanks for all you do with Ci
I agree with Andy - introduce pH's above and below the typical scale, but don't push the issue. This leaves them knowing that life is more complicated than 1st year Chem so they aren't blindsided later, but doesn't belabor the point.
I agree that 0-14 is good enough for almost all chemistry, with just a mention that super strong acids and bases may give values outside that range. Full stop there - no examples, no discussion needed. I teach all levels of chemistry, from General High School, International Baccalaureate HL, AP, Freshman, and Organic, and have found that when those exceptions to pH 0-14 come up, it is easy enough to deal with the issues then. Just make sure to mention to eager chemistry students that there are extensions, but not in general solution chemistry.