There are hazards associated with students burning steel wool. The main danger of this experiment is the high temperatures that are created in the oxidation process, so students should not touch the hot steel wool. Consideration must also be given to the ignition method, as it is difficult to maintain the burning and collection of all of the resulting products, if this is to be a quantitative activity.
Ignition of steel wool
For the steel wool to “burn”, you are correct that it is better if it is fluffed up to allow sufficient oxygen to encourage the continued combustion of the iron into the middle of the sample. This often requires repeated ignition and rotating the ball of steel wool.
Here are a few methods of ignition that have been used.
- Using a gas lighter—the type designed for lighting barbeques, not cigarettes. There is no moving of burning steel wool and no risk of the wool sticking to the lighter. The lighter is designed to be handheld, the risk here is of the students using it to ignite other items in the room.
- Holding the iron wool with tongs in a Bunsen burner flame until it ignites, and then transferring the sample to a heatproof surface. This has the danger that the student may “flick” the sample, if they get a surprise when it ignites, the moving of the wool will also most likely lose some of the product in transit.
- Touching the steel wool with a 9 volt battery. The short circuiting of the terminals across the steel wool produces sufficient heat to ignite it. One possible risk here is that the steel wool melts onto the battery terminal.
Science ASSIST does not recommend ignition with a Bunsen burner by lifting and pointing the flame at the steel wool. Holding the Bunsen burner has its problems and is not the recommended method of use for the Bunsen.
Conducting an on-site risk assessment looking at the options outlined above will determine your preferred method.
Collection of resulting products
We suggest the use of an evaporating basin or tin lid on a non-combustible protective bench mat (e.g., a cement sheet). It is best to use a course-grade steel wool, which will burn with less sparking and has no loss from pieces flying off that may not be included in the final mass after combustion. Ensure that the steel wool is uncoated and does not contain soap.
For this experiment to give meaningful results, it is important that all the resulting material is collected. The evaporating basin or tin lid should be pre-weighed to determine its mass. The steel wool, once fluffed out, is placed in the evaporating basin or on the tin lid and the total mass is noted and the steel wool mass determined. Allow the evaporating basin or tin lid to cool before re-weighing to determine the change in mass due to the oxide formation.
Alternatives to using magnesium and steel wool
We have considered other metals, however they are either too hazardous to use in schools or do not burn. See below.
Burning magnesium: In the school science curriculum, magnesium metal is commonly used to demonstrate combustion of metals. Combustion is the reaction of substances with oxygen/air, producing heat and light. Magnesium is the relatively safer option out of the ignitable metals. The ability of magnesium to combust fully in air to form MgO helps to extend the activity to a quantifiable investigation.
2 Mg(s) + O2(g) → 2 MgO(s)
Burning steel wool: Steel wool is used as an alternative metal for demonstrating a combustion reaction in many published practical activities. However, being a less reactive metal compared to magnesium, steel/iron wool has its limitations. The main difficulty is in maintaining the burning and ensuring that all of the iron has been oxidised.
Below are two sample results, the change in mass is not large so a larger sample piece may give a more measurable change. These results give a return of about 75% yield.
Mass of steel wool
Mass of steel wool/oxide produced
Change in mass
When ignited, steel wool will burn in air, allowing students to observe the formation of rust.
4 Fe(s) + 3 O2(g) → 2 Fe2O3(s)
However, the combustion of steel wool in air will be incomplete and therefore if the aim is to attempt to determine the empirical formula, it would not be totally satisfactory. Burning iron will most likely form some Fe3O4, which is a combination of Fe2O3 and FeO.
For complete combustion, placing glowing steel wool into an atmosphere of pure oxygen is effective (see video links below). This procedure is not recommended as a student activity.
Combustion reactions of other metals
Here are some reactions of other metals with oxygen, It is not advised to perform these is your school, but search for examples on the internet to show them, if required.
Different metals react with air at very different intensities, and varying conditions are required for the reaction to occur.
- Potassium and sodium are reactive metals and are ignited by gentle heating and burn vigorously in air with a characteristic flame to form metal oxides by combining with oxygen.
4K(s) + O2(g) → 2K2O(s)
4Na(s) + O2(g) → 2Na2O(s)
- Calcium metal needs strong heating to ignite. It burns vigorously to form its oxide.
2Ca(s) + O2(g) → 2CaO(s)
- Aluminium and zinc burn moderately under strong heating. They cannot be ignited to hold a flame; form oxides.
4Al(s) + 3O2(g) → 2Al2O3(s)
2Zn(s) + O2(g) → 2ZnO(s)
- Copper and lead metals do not burn. They will glow under very strong heating and form oxides.
2Pb(s) + O2(g) → 2PbO(s)
2Cu(s) + O2(g) → 2CuO(s)
- Silver and gold have no reaction with air.
‘Combustion of metals’ University College, Cork website, https://web.archive.org/web/20170205133306/https://www.ucc.ie/academic/c... (Original page no longer exists, this copy made available through the Internet Archives Wayback Machine, November 2017)
‘Combustion of Steel Wool’, Department of Chemistry, University of Washington website
http://depts.washington.edu/chem/facilserv/lecturedemo/CombustionofSteelWool-UWDept.ofChemistry.html (Accessed December 2015)
‘Empirical Formula of Iron Oxide’, Science Curriculum by Aaron Keller, Scarborough High School Maine
http://kaffee.50webs.com/Science/labs/Chem/Lab-Empirical.Formula.Fe2O3.html (October 2015) Instructions for the determination of the empirical formula of iron oxide (Hazardous/fiddly. Might be best as a demonstration)
‘Iron and oxygen’, ChemDemos website, https://chemdemos.uoregon.edu/demos/Iron-and-Oxygen (Accessed December 2015) Instructions for burning steel wool in oxygen atmosphere.
Mak, Dr. Kendrew, (n.d) Reactions of metals, Department of Chemistry, The Chinese University of Hong Kong website, http://resources.edb.gov.hk/gifted/ijso/HKIJSO_TrainingManual/Phase3/Chemistry/Lesson5_ReactionOfMetals/PhaseIII_Chem_L5_ReactionOfMetals_ppt.pdf (Accessed December 2015)
‘Microscale Gas Chemistry: Experiments with Oxygen’, Bruce Mattson’s Home Page, Creighton University, http://mattson.creighton.edu/O2/ (Accessed December 2015) Instructions for the small scale demonstration of steel wool burning in oxygen atmosphere.
Sconzo, Penney, 2011. The Burning Question: A Conversation of Matter Experiment, Adam Equipment website, http://www.adamequipment.com/education/Documents/experiment_3_R2.pdf
‘Steel Wool’, YouTube (1:04 min) https://www.youtube.com/watch?v=bo2Ygyh21KM (August 2012) (Video clip demonstrating reaction of steel wool in liquid oxygen)
‘Steel wool in an oxygen-rich environment – Dazzling demonstrations’, YouTube (1:02 min) https://www.youtube.com/watch?v=F_1o0s4G9qA (May 2013)