Forging strong understanding of metals

From gleaming jewellery to robust construction materials, and electrical wires to sophisticated mobile phone batteries, materials with metals extracted from the Earth play a pivotal role in our daily lives. At 14–16, students need to understand metal extraction processes. They need to both comprehend and articulate various reaction types, such as oxidation and reduction. But they often grapple with misconceptions when applying this knowledge to unfamiliar situations.

What students need to know

In the initial years of secondary education, students are introduced to the reactivity series of metals. They study the sequence of metals and recognise that a more reactive metal can displace a less reactive metal from its compound. At 14–16, it becomes crucial for students to understand that the reactivity of a metal correlates with its likelihood to form a metal ion. I find it’s important to emphasise that hydrogen and carbon are incorporated into the series to identify the chemical method employed for metal extraction.

Where do metals come from?

Despite practical lessons that feature metals like iron and aluminium, it’s common for students to misunderstand the origin of metals. They often believe that all metals are found in nature exactly as we see them in everyday life. Only a handful of the 93 metals in the periodic table exist in an uncombined (native) state in the Earth’s crust. Introducing the terms ore and mineral early in this teaching process helps learners understand that, typically, we need to extract the more reactive metals from chemical compounds.

How are metals extracted?

Students need to know the positions of carbon and hydrogen in the reactivity series because this will inform their understanding of the extraction method. Reactive metals at the top of the series are extracted through electrolysis and those below carbon undergo extraction through reduction reactions. The metals below hydrogen, such as gold, are found as pure metals. Although copper lies at the bottom of many exam board’s reactivity series, its main ore is chalcopyrite and we can extract copper from this ore.

The industrial extraction of metals from their ores is taught in most exam specifications, and students need to know that copper extraction is achieved by heating with carbon. Although this topic may seem a little tedious, a flipped learning approach like assigning a video task before a lesson can help spark initial engagement. Platforms like BBC Teach offer insightful videos that not only illustrate the workings of the blast furnace but also provide broader understanding of the iron ore extraction process.

How does electrolysis work?

The metals at the top of the series are very reactive, so they are never found in their elemental form in nature, only as compounds in metal ores. In electrolysis, a direct current is passed through a solution of the metal or molten compound. Get students to observe a molten metal forming on the cathode and test the product at the anode to enhance student engagement and foster a deeper understanding of electrolysis. The more confident 14–16 students are at explaining electrolysis and writing correct half-equations, the less they will struggle at post-16 when studying the electrochemical series and predicting whether a reaction is feasible or not.

Clearing up misconceptions

At the most basic level, we can explain oxidation and reduction in terms of oxygen and hydrogen. I like to display a balanced symbol equation, such as the reaction of iron(III) oxide with carbon monoxide, allowing students to identify oxidised and reduced species based on oxygen/hydrogen loss and gain.

As students move on to post-16, they will explain these reactions in terms of oxidation state and numbers. Oxidation states are easy to assign for simple ions, being equivalent to the ionic charge. So, in iron(III) oxide, iron has an oxidation state of +3, and oxygen of –2.

We can assign oxidation states in more complex substances by thinking about where the electrons would go if we pulled the atoms apart. More electronegative elements (those closer to the top right of the periodic table) have a stronger pull on the electrons and so end up with negative oxidation states. Although 14–16 students don’t need to understand redox, it can be useful for them to know what they will move on to.

It’s helpful for students to practise identifying electrodes and processes in electrolysis. Teach students acronyms like PANIC – positive anode negative is cathode – and OILRIG – oxidation is loss (of electrons), reduction is gain. Some exam boards use the anode/cathode terminology, others use positive/negative electrode. It becomes more difficult to label electrodes when comparing the two types of electrochemical cells: galvanic and electrolytic.

At post-16, students develop a deeper understanding of electrochemistry. They make a variety of galvanic cells from different half-cells and use the electrochemical series to predict cell potentials (voltages). At this stage, they will achieve a complete understanding of the chemical story. They will have started metals chemistry at 11–14, with simple displacement reactions of magnesium and copper sulfate, for example. They can then conclude their post-16 chemistry by explaining why the reaction is so vigorous.