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.
I like this as a method for introducing it, thanks for sharing. Deliberately leading them towards negative pH calculations then making the time to explain seems like a sensible approach.
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.
Think you've hit on the key thing there — that negative pH values really aren't encountered by students in general solution chemistry. Can't even remember encountering them at university level to be honest, so probably backs up the point that there's very little need to say anything about them beyond acknowledging their existence.
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 like this as a method for introducing it, thanks for sharing. Deliberately leading them towards negative pH calculations then making the time to explain seems like a sensible approach.
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.
Think you've hit on the key thing there — that negative pH values really aren't encountered by students in general solution chemistry. Can't even remember encountering them at university level to be honest, so probably backs up the point that there's very little need to say anything about them beyond acknowledging their existence.