Discussions from the magazine, blog, website and social media

pH 7 is neutral

Or rather, it isn’t. Eric Scerri’s new series on the EiC blog ‘Five ideas in chemical education that must die’ launched with a campaign to dispel the notion that pH 7 is always neutral and that neutral is always pH 7.

Eric points out that once Kw is introduced, students discover pH is temperature dependant and what defines a neutral solution is the presence of an equal number of moles of H+ and OH- ions, a situation that may occur at any pH value.

Some teachers, like Helen Rogerson, were shocked:

What? I have been lying to my students.

Some, like Blair H were more stoic:

I have to say as a high school chemistry teacher this is utter nonsense. For many learners the pH scale is as far as they will go. So to introduce ion concentration or even ions at all to some learners makes it too challenging.

Some picked up on another of Eric’s concerns, the idea that pH is a range of possible values from 1 to 14. Harry Keller from the Greater Los Angeles Area, US, said:

While there are practical limits to pH, they are not arbitrary ones such as 1 and 14. This fact is so obvious that I find it hard to believe that any textbook author would write such tripe! Yet this is what we see repeatedly.

But it’s not just numbers that are the problem. Philip Dobson pointed out:

In the UK this is in the A-level specification. But the problem is that by then the students have had at least six years of believing pH 7 is neutrality. They also are of the opinion that pH 7 = green!

Many teachers talked about appropriate information at different stages of learning, for example @curiousiguana:

Utter nonsense, impossible to teach at year 7. All of science education is adding a new layer of complexity when ready.

@MagneticFlea replied:

I have always gone with ‘this is the model we’re using for now’ – prevents the ‘you lied to me!’ at each new layer.

And @curiousiguana agreed:

‘You learned this last year. Well there’s a bit more to it.’ They understand that. We do it for atoms.

Richard Boohan commented on this idea in more detail:

It’s pretty easy to take any idea at any level and to say ‘it’s more complicated than that’. What’s hard is to take a difficult idea and create a simplification that is both accessible and correct, and which can support a learner in moving on to the next stage. Not all simplifications are misconceptions (and indeed in any case not even all misconceptions are highly resistant to change and a barrier to further learning!). One could think of two extremes:

(A) Simplifications that capture important aspects of a concept, [which] enable learners to begin a process of familiarity with the concept and provide a foundation on which later ideas can be built.

(B) Simplifications that misrepresent a concept, distract learners from its real meaning and inhibit further learning.

The suggestion of starting out with ‘a pH of 7 is neutral if the temperature is 25°C’ seems very sensible and seems to fit (A) perfectly. For the learner it does not matter whether they understand the significance of any qualifications that make what they are studying a special case or not, unless this provides a barrier to further learning.

Take a look at the three other blog posts Eric has written so far, tackling Le Châtelier’s principle, the atomic orbital conundrum and chromium’s anomalous configuration.

Reorganising organic chemistry

In our July issue, David Read reported on research from the University of Ottawa, Canada, where the concept of mechanism is taught before students learn a single reaction. Reactions are organised by their governing mechanism and initial teaching aims to develop the skills needed to explain and predict mechanisms.

A number of teachers and lecturers commented that they already do something similar. For example, David Smith:

We teach mechanisms before reactions here at York (UK) – and have done so for around the last 10 years. This provides students with the ability to understand organic chemistry and get an intuitive feel for it, rather than just learn it one fact at a time.

Our first lecture course is Organic Reactivity and Mechanism – an introductory course that provides the students with the toolkit to predict all simple organic mechanisms. We then have further courses that work through compound types in more depth and detail. So for example, my introductory course would enable the students to spot, for example, a nucleophilic substitution: later courses would then look at the more subtle details of SN 1 vs SN 2.

Kuda Murombedzi extended the discussion to school teaching:

Organic chemistry definitely is a challenge for many students, particularly at the lower levels of learning. The article and the approach appears to be largely suited to higher level learners. I would like to hear opinions from educators working with GCSE level and UK A-level equivalent learners. At GCSE level the concept of the mechanism is very difficult to introduce because many of the other essential concepts such as for example bond polarity will not have been introduced to the learners. In such a situation the student is being conditioned towards rote learning, which they then carry over into the higher A-level. How best can we introduce organic chemistry at this lower level? I can see the applicability of the mechanistic approach to A-level. Having read some of the comments so far the approach certainly does have its merits. In my case I would very much take that approach now with my A-level  students.

Simon Nance replied with his own experiences:

Teaching A-level chemistry I always do a large section on the principles of organic mechanisms and basic ‘arrow pushing’ to try and ground students before moving on to specific reactions and their mechanisms. The problem is that if you don’t, many students learn the mechanisms as ‘pictures’ rather than understand them as processes, and have to rote memorise them with no flexibility for reactions that are slightly different.

Whereas teaching mechanistic principles allows students to work out basic mechanisms from first principles and develops their reasoning and problem solving skills – not to mention making the subject more engaging and interesting and allowing them to see the whole rather than treating organic chem as a bunch of unconnected reactions. Even giving them the basics of arrow pushing, leaving group stability and electronegativity builds a foundation that can be built on at university level.

Formula triangles

Jenny Koenig blogged for us about one of her pet hates: students using formula triangles to avoid rearranging equations.

Teachers everywhere vented their spleens. Some agreed with Jenny, like Diane Sanders

My view: Bad. Very bad. Memorising a triangle is only as good as long as you can remember where the letters go and what to do with them. Memorising a construct like that, with no context or application, is usually only good for the short term. With triangles, concept and meaning are totally lost.

And Joe W:

Fails as a process when more complicated formulae come along.

But many, like Andy, disagreed, explaining that formula triangles have an important role to play:

Give a mechanism for rearranging if conventional maths has let you down previously. A mental safety net/support.

And also @chemknuts:

They provide a very useful framework to support the less mathematically confident.

Eugenije Jelacic Janezic is a teacher who disagrees with the use of formula triangles, but he has a suggestion to help students:

I strongly discourage the use of triangles. Once the students get used to triangles, there is no more effort in understanding its meaning, the origin of physical phenomenon. I prefer my students to understand how and why quantities depend on each other. If pressure or density are logically explained and well understood, there is no need for a triangle.

Instead, I always suggest dimensional analysis in order to check whether the formula is correct or not. I always require students to write units along with numbers. If you need to get at the end the number of moles and instead there are joules, your universal gas law formula was wrong!

Finally, some teachers have a mixed approach and use them as a stepping stone, like ChemBones:

I agree using these triangles alone are useless, but for students who don’t understand dimensional analysis at first, it’s a great spring board. I can’t tell you how many ‘aha!’ moments I’ve witnessed when using a triangle to explain dimensional analysis. And then the student is sold on dimensional analysis – the true test of understanding what they’re calculating!