Changing a physical property of a metal ( 1 ) iron/steel

Chemical properties of metals- reactions with oxygen .
Galvanised iron is iron dipped in molten zinc. The coating which protects the iron from rusting is only 0.002 cm thick on each side. Yet this layer has approximately 80 000 layers of zinc atoms!!!
Give pupils a strip of galvanised iron- perhaps about 10 cm by 1 cm, cut from galvanised sheet available from plumbing merchants. Preferably use fairly thin galvanised iron- easier to cut and easier to heat strongly.

Don’t tell pupils what you are giving them, other than a piece of a commonly used metal.

Ask pupils to warm the galvanised iron strip until there is a first change, touch the surface with a spatula, then allow to cool. When galvanised iron is heated two things happen :
[1] gentle heating - the surface starts to look 'wet'

Zn (s) → Zn (l) PHYSICAL CHANGE

Then heat the strip as strongly as possible, just above the blue cone of the Bunsen flame. Pupil may find that there seems to be some burning, if not it will still get red hot. Observe, cool and observe again.
Before you explain what has happened, take a sheet of galvanised Fe, (possibly A5 size or ?), clamp this vertically, and after bringing pupils around demonstration bench, heat the galvanised Fe from behind**, gently at first :

It is possible to observe the layer running down the sheet !!

[the coating melts but the greater part of the metal does not]
**Then heat the metal strongly from behind, you need a fairly hot burner, a Bunsen alone will not provide enough heat. Propane/oxygen or acetylene/air or acetylene/oxygen work and possibly propane/air with a bigger burner will do the job.
It is observed that the coating bursts into flame, even though the burner is behind the sheet, and a white smoke and white, then yellow solid form. Once cooled the yellow solid turns white. If heated again then yellow…cooled then white again, etc. Quite impressive !!
A way to get lots of ‘mileage’ out of this :

a. clearly galvanised iron has a ‘sandwich’ structure …

coating/main metal/coating

b. when heated the coating burnt, probably to an oxygen compound…oxide!!

c. ask pupils for possible ‘coating’ metals, accept all answers

eg. Cu, Pb, Zn, Al, Mg, Ca, Ag etc etc

d. eliminate some ; Ag- too much $. Ca – reacts with water, Cu- wrong colour

e. you are left with Pb, Zn, Al, Mg

f. obtain samples of the oxides : Pb/O yell or black or red – clearly wrong, all rest white

g. you are now left with the coating being Al or Zn or Mg

h. heat all white oxides, Al₂O₃ white cold – white –hot

MgO white cold – white hot

ZnO white cold – yellow oxide !!!!!
Takes too long ??– not realy, it is another situation where pupils get to know & believe

what they have observed, rather than what they have been told by us.

[2] strong heating - the surface becomes white / yellow or burns to form a powder which is white when cold - zinc oxide

2 Zn + O₂ → 2 ZnO CHEMICAL CHANGE

[As an add on:if you have a hot enough burner you can melt the main metal, the Fe, and this

emphasises the fact that metals have different melting points.]

Galvanised Fe R.B.Jansen MGS