Why does iron spark?

Iron and materials containing iron give off sparks when ground, but other metals do not spark for grinding. Why? This is a surprisingly complex question that involved more concepts from teaching high school chemistry than I initially expected. The short answer is, of course, because it’s iron.

The physical process of grinding does a few things. It adds energy via friction. It produces small fragments, depending on the tensile strength of the metal (some metals shatter while others will deform) that are flung away due to the kinetic energy of the grinding. One potential explanation for the difference in outcomes is simply that softer metals will experience less friction (and thus a lower increase in thermal energy).

Fragments of metal (warm) contain newly exposed surface which then has the opportunity to oxidize. Oxidization is a really fundamental process- it’s the process we typically try to prevent in food in the kitchen, the process by which fire burns, and the process by which our bodies turn food into energy. Oxidation is typically a spontaneous process, a reaction that wants to happen.

Metal oxides are formed by the interaction of a metal with oxygen. Copper oxides are responsible for the greenish hue of the statue of liberty, while the lead oxides that form on batteries leads reduce conductivity over time (oxides are better insulators than the metals they are formed from).

In chemistry, when we consider the likelihood of a reaction, and the energy released, we look at values of free energy and enthalpy (heat of formation). These are experimentally determined and relatively straight forward to look up. These tell us which compounds should release the most energy when they are formed (the more negative, the more energy released to the surroundings)

Metal OxideEnthalpy (kJ/mol)
Aluminum Oxide–1676
Copper (I) Oxide-157.3
Copper (II) Oxide-168.6
Iron (III) Oxide-824.2
Iron (II,III) Oxide-1118.4
Lead (II) Oxide-218.99

All of these metal oxides are formed by exothermic reactions, that is, the formation of the oxide releases energy, giving off heat to the environment. The copper and lead oxides are relatively mildly exothermic, but aluminum oxide is substantially more exothermic than the iron oxides (though those are much more so than copper or lead).

So why doesn’t aluminum oxide spark? Heat of formation isn’t the whole story. Once we know the amount of energy generated, we still need to consider what the actual change in temperature is.

This is another value. Different substances absorb temperature differently. Water, for example, is extremely good at absorbing energy without changing temperature- it has a high specific heat capacity.

Metal OxideSolid Heat capacity (J/mol K)
Aluminum Oxide102.42
Copper (I) Oxide59.42
Copper (II) Oxide42.24
Iron (III) Oxide103.7
Iron (II,III) Oxide147.2
Lead (II) Oxide51.65

For Iron and Aluminum, even heat capacity doesn’t provide the whole story. The table compares heat capacity of solids- apples to apples, as it were. But Iron oxides and aluminum oxides also don’t have the same melting temperature. The difference is well over a thousand degrees, and a huge decrease in heat capacity.

Liquid iron oxides change temperature in response to energy much more readily, and so get hotter than do aluminum oxides. Additionally, once an aluminum oxide forms, the reaction typically stops. Aluminum oxide is an excellent insulator.

Conclusion: Copper really doesn’t generate all that much energy when it oxidizes, neither does lead, and should be pretty safe. Aluminum is a bit more complicated, and it really should be possible to get aluminum to spark under the right circumstances. Iron and iron containing compounds will certainly spark for grinding.

And of course, Aluminum and Iron powders are a very risky combination- but more on that another time.

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