# ¿ Al or Mg ?

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Hi.

Cannot find how to discern if an unmarked automobile wheel is magnesium or aluminium.

Any sure way to test please ?

If a drop of battery acid is applied to an alloy rim, what reaction, color, etc. is expected if aluminium or if magnesium ?

Or any other simple way to tell ?

Thanks,

Miguel

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I have very limited knowledge of the alloys they use in car wheels (I bet coquina could tell us a whole chapter about them though ), but I'd imagine that if a car part is of high magnesium content, it will react quite strongly with rather pure HCl, and vice versa only small bubbles of hydrogen are formed on an aluminum part. (Can't recall the equation for H2SO4 + Al though, but I'd imagine Mg reacts more rapidly in that case too). If you're sure the part is either aluminum or magnesium, it's rather easy to distinguish which one it is with HCl.

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By HCl he means hydrochloric acid, as opposed to hydrogen chloride, which is what HCl actually is.

Mg will react with 0.5M HCl (aq) visibly, although only just, 1M or more is more clearly visible, with 1.5M or 2M being very visible.... I did an experiment with Mg + HCl (aq) recently.

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Contact the American Society for non-destructive testing and ask them if there is an easy way to find out. http://www.asnt.org/ndt/primer5.htm

The only other suggestion I have is as follows:

Are the wheels on or off the car? If they are off it and you can drill a small hole in the back side where it won't show, and retrieve the turnings (pieces of metal removed by the drill) if they are magnesium, they will burn at a very high temperature. If you spray water on them it will cause a small explosion.

If you decide to try this experiment, you only need one or 2 pieces of turning. Put them where they can't catch anything else on fire - like on a piece of concrete. Use a long handled grill lighter, and have a bucket of sand ready to throw on the fire to douse it.

Aluminum will burn at high enough temperature, but a grill lighter won't make enough heat to ignite it.

You can also start a small fire using tinder and throw the turnings on and see if they burn - if they do, there will be a bright white flash.

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If you can remove it and if it is a pure metal (no iron or other metals attached that cannot easily be removed), you could place it into a tank of water that is completely full, and collect the amount of water that is displaced out of the tank. Then, weigh the object, and the mass divided by the volume of water it displaced will be its density, which you can compare to the densities of Aluminum (2700kg/cubic meter) and Magnesium (1738 kg/cubic meter).

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It is acutally very easy. You need to apply some drops of 30% acetic acid to clean surface. If it bubbles it is mostly Mg, if not, its Al. You can also test by differences in specific gravity.

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conc Nitric is stored in Al containers, do the same with Mg... Boom!

sodium Hydroxide will react with Al, but not with Mg.

therein lies a clue

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Hi.

Just found out that acetic acid (vinegar) applied onto recently scraped magnesium will bubble and fizz. It will not happen on aluminium.

That's a household chemical easy to try with.

¿Does it make sense? I will try later myself...

Thanks,

Miguel

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wow... people here are very smart...

Anyway, I have a questions, where would one attain Hcl (is that the thing for Hydrocloric acid?). Thx.

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Yeah hydrocloric acid is HCl gas dissolved in water. If your looking to play around with acids i would suggests sulfuric acid (H2SO4) its much easyer to come by its in car batteries at pretty high concentrations! Do a search on MSDS (material safety data sheet) on sulfuric acid before you use it, be safe.

~Scott

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if u take fairly stronh HCl and add it to aluminum, after a minute to even 5 min it will get very exothermic and fairly violent and the reacton rate will get faster and faster, while on Mg it will just fizz right away if the surface is clean

.

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if u take fairly stronh HCl and add it to aluminum, after a minute to even 5 min it will get very exothermic and fairly violent and the reacton rate will get faster and faster.

Factoid:

the reason for this is that it takes a while to penetrate the Alu Oxide layer to expose the pure metal.

hg

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actually, even with a perfectly clean pieceof aluminum it takes just as long

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try it sometime its fairly simple

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actually, even with a perfectly clean pieceof aluminum it takes just as long

That's because perfectly clean aluminum has a solid layer of aluminum oxide on it. Aluminum metal is INCREDIBLY reactive. In fact, it's right up there with sodium and potassium in terms of reactivity. Pure aluminum without any oxide coating on it will rip water apart quite rapidly. The thing is, aluminum readily reacts with oxygen and forms a layer of oxide which adheres strongly to the metal and does not want to come off. As a result, it becomes fairly non-reactive as we see it. It's just that we're not seeing "real" aluminum. The reason why mercury is able to make aluminum react like crazy is that Hg removes the oxide coating from the Al allowing the highly reactive metal to react with anything and anything around it. Sodium hydroxide attacks aluminum because it removes the oxide coating and allows the raw metal to react with the water. Acids also do the same thing by slowly removing the oxide layer and then allowing the metal to start reacting with both the acid and the water.

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Then, from your statement, if aluminium is sawed off/scraped while under water, will yield a nearly explosive or violent reaction ?

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Nope. While there may be a reaction at first, the formation of a protective barrier of the hydroxide/oxide on the surface of the aluminum will immediately cease any and all reaction. You would have to have a way to contantly remove the coating from the metal in order for it to keep reacting. With stuff like sodium and potassium, any hydroxide that is formed is very water soluble so it will immediately move away from the metal's surface and allow more metal to react. Aluminum oxides and hydroxides bind VERY tightly to the surface of the metal.

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ive thought about sanding aluminum in a beker of water with sand paper, then i would add H2SO4 to it to see the reaction, but then realized that the oxide layer will probally still form because there is always allittle air dissolved in water.

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you could use a dremmel with the sanding attatchment and a stream of Argon gas (from a welding shop).

ok, its a little costly for such an experiment, but if youre Really determined...

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i was expirementing to see wat metals i should use for a big battery i am making, so i took copper and iron, zinc, magnesium (i knew the reaction would be to fast with Mg for a battery but i wanted to see the voltage anyway) tin, lead and aluminum. I found that the best combination would be Cu and Zn. when expirementing with the Cu and Al i found that when i put the aluminum rod in the solution i got about 400 mv from the thing. I noticed the voltage was steadily climbing, until it peaked out at about 700 mv. the sulphuric acid mustve not been able to expose any more aluminum at that extent (to get it past 700 mv.) Still it was pretty cool how i could see on my voltmeter the oxide layer being broken.

If the above paragraph was realy hard to follow its because i need to go to bed realy bad.

If aluminum werent so reactive you would find it in nature pure (it took so long for humans to finaly isolate it because it binds to other elements so good.)

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why dont you try the same set of metals using a Base instead of an Acid, then compare results, Ill think you`ll be pleasantly surprised with some of them

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