The Importance of Subject Knowledge: What Teaching Electrolysis Taught Me

Why is deep subject knowledge so critical for effective teaching? My 'electrolysis moment' taught me it’s the foundation of all pedagogy. This post explores why teacher CPD must focus on building this expertise, moving beyond generic strategies

There's a special kind of dread reserved for teachers. It's not the fear of a tough class, but the cold, creeping realisation that you don't truly understand the concept you're supposed to be an expert in. For me, that moment of impostor syndrome had a name: electrolysis. I was a brand-new teacher, confident that my recent degree had prepared me for anything. But standing in front of thirty expectant faces, I learned the hard way that knowing a fact is not the same as understanding it.

There I was, in front of a sea of expectant faces, trying to explain something I only sort of understood myself. I powered through. Unashamedly vague yet enthusiastic, I was there, dragging my poor pupils kicking and screaming along for the ride. The following year, I found myself in the same situation. Same topic. Same sense of panic. But this time, about 15 minutes in, it clicked. Having spent the previous evening wrestling with the core concepts myself and finding a new analogy, I finally saw the logic in a way I hadn't before. I actually understood electrolysis and, miraculously, because my explanation was clearer and more confident, so did the students.

You can complete a PGCE and an ECT year without ever truly grasping just how vital subject knowledge is. These days, whether I'm working with PGCE students, NQTs, or my department, I make subject knowledge the cornerstone of everything. Because once you really understand something, you can explain it to others in ways they can not only recall it, but actually understand it.

Of course, whenever I talk about subject knowledge, I begin with my go-to anecdote: teaching electrolysis to Year 10. From there, I move into the research on why subject knowledge matters so much in teaching, and you can’t talk about this without mentioning Lee Shulman. In his 1986 paper, Shulman categorised teacher knowledge into three distinct areas: Content Knowledge, Pedagogical Content Knowledge, and Curricular Knowledge.

Content knowledge is the aspect of the subject that we as teachers must know to not simply deliver the content, but to explain the underlying reasoning behind it. Content knowledge forms the bedrock of effective teaching, extending far beyond the mere transmission of facts. It encompasses a deep understanding of the subject matter, enabling teachers to not only present information but also to unravel the intricate layers of reasoning that underpin it. Shulman himself states:

“Teachers must not only be capable of defining for students the accepted truths in a domain. They must also be able to explain why a particular proposition is deemed warranted, why it is worth knowing, and how it relates to other propositions, both within the discipline and without, both in theory and in practice.”

Pedagogical Content Knowledge goes beyond knowing what to teach; it encompasses how we teach it. It involves the “most powerful analogies, illustrations, examples, explanations, and demonstrations” that enable students to truly grasp the content. Crucially, it also includes an awareness of the aspects of the subject that students typically find difficult: the common misconceptions, preconceptions, and the diverse backgrounds they bring to the classroom. A teacher’s role is to anticipate these challenges and adapt their approach to help students overcome them and develop a secure understanding.

Finally, Shulman highlights what he saw at the time as the most neglected domain: Curricular Knowledge. In the nearly 40 years since his research was published, this gap has received significant attention, with it being a major focus throughout even my own career. Yet Shulman’s definition goes further than simply knowing the sequencing and rationale of one’s subject. He argues that teachers should also understand how their subject fits within the broader context of the entire school curriculum; how it connects to and complements what students are learning elsewhere.

For example, I am currently teaching AQA Combined Science B3: Infection and Response to my Year 9 students. The content knowledge I want them to grasp is the concept of antibacterial resistance. Since this topic featured in my degree, I have a solid understanding of both the core content they need and some additional depth I can choose to include hinterland, such as the current concerns around antibiotics of last resort, for example. Without this level of understanding, it would be difficult to effectively communicate the broader societal implications or the ethical debates that surround the issue. While these may not appear directly in their exam questions, they are vital for helping students conceptualise the topic fully.

My pedagogical content knowledge comes through in the teaching strategies I use. For example, I use mini whiteboards and two coloured pens to model the survival of stronger bacteria after a patient prematurely stops taking antibiotics. I then guide students through a flow chart showing how these more resistant bacteria continue to reproduce, turning that same patient from careless to seriously ill. I deliberately simplify the process, leaving out complex ideas like avoparcin, efflux pumps, or distinctions between gram-negative and gram-positive bacteria. This ensures that the content remains accessible and relevant, while also laying a foundation for students who may choose to study the topic further in the future.

My curricular knowledge involves understanding how this topic fits within the wider curriculum. I know, for instance, that students interpret graphs showing the effects of poor healthcare in their Geography lessons. I have used Geography GCSE questions to help them practise data interpretation. I have also introduced ethical debates around antibiotic resistance, such as the refusal of treatment in high-risk cases or the use of antibiotics in livestock. These discussions often connect with content in their RE lessons. I break down the etymology of words such as vaccines and highlight how 'vaca' is Spanish for cow. In addition, I draw on mathematical concepts such as percentage increase, doubling time, and graph interpretation. In doing so, I am not only supporting learning within science but also helping students make connections across subjects and see how their learning fits together as a whole.

Some elements of subject knowledge, such as content knowledge, have long been a focus in teacher development. However, it is pedagogical content knowledge and curricular knowledge that take time to build and bring together. Because they require experience and reflection, they are often overlooked, particularly in the early stages of a teaching career. As a result, new teachers may rely on revision guides and exam questions, believing that this is enough to teach effectively. But this approach often misses the deeper, more nuanced aspects of teaching. These are the kinds of insights that typically emerge through informal conversations with colleagues when someone asks, “How do you teach X?” and suddenly hears a fresh, effective way to explain a complex idea in a way that makes sense to a 13- or 14-year-old.

Adam Boxer’s Teaching Secondary Science and Cracking Key Concepts in Science are excellent examples of resources that support teachers not only in knowing what to teach but in understanding how to teach it effectively. The clarity and practicality of these books made such an impact that I purchased a copy of each for every member of my department. 

However, despite efforts from many to improve this aspect of subject-specific CPD, much of the time spent in whole-school CPD sessions and PGCE courses is focused on general pedagogy, often disconnected from the specific content we teach. While learning about behaviour management and questioning techniques is certainly valuable, it does not automatically equip a teacher to explain complex subject matter in a way that students can truly grasp. That level of teaching expertise can only come through subject-specific CPD.

To embed this approach in our department, I began sharing a page or chapter from books and blogs each week for staff to read and reflect on during our weekly or fortnightly faculty meetings. I chose topics that aligned with what we were currently teaching, ensuring relevance and immediacy. Alongside this, I gave staff an example question and asked them to write a ‘perfect’ model answer. The goal wasn't just to check content knowledge, but to force us to confront the pedagogical choices we make. By writing a 'perfect' answer, we had to consider the precise vocabulary, the structure of the argument, and the common pitfalls we'd need to pre-empt in our teaching to get students to that point and plan accordingly.

I know I am extremely fortunate to have dedicated CPD time with my faculty every fortnight, and even though it’s sometimes cancelled, it remains one of my favourite parts of the school week. There’s something powerful about sitting with Chemistry and Physics specialists, colleagues with degrees in organic chemistry or biomedical science, and gaining insight into how best to explain a concept. They often share the kind of depth and nuance that helps all of us teach with greater confidence and clarity.

That said, this shouldn’t be a privilege reserved for the few. Those responsible for Teaching and Learning could make a significant impact on student outcomes by protecting time for departments to come together and develop their subject knowledge collaboratively. Teachers working outside their specialism, in particular, benefit enormously from sitting down with an experienced colleague who can guide them through not just the content, but the pedagogical approaches needed to deliver it effectively.

However, this time must be planned carefully and intentionally. Teaching and Learning leads should work alongside Heads of Department to map out what subject knowledge development should look like over a term or year. If there’s an area the HoD isn’t fully confident in, but another colleague is, then that person should take the lead. Yes, this takes time, but, as many will attest, the professional growth of teachers always does. If school leaders are serious about improving student outcomes, they must empower their middle leaders and protect this time. Without making a deliberate, structural space for subject-specific growth, we cannot expect teaching, and therefore learning, to truly improve.