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In the borax bead test, we take a crystal of borax and then heat it.

A small loop is made in the end of a platinum or nichrome wire (as used in the flame test) and heated in a Bunsen flame until red hot. It is then dipped into powdered borax, and the adhering solid is held in the hottest part of the flame where it swells up as it loses its water of crystallization and then shrinks, forming a colourless, transparent glass-like bead (a mixture of sodium metaborate and boric anhydride)

Whenever I've done the experiment, I've noticed that the process looks like this:

  • It heats up in a second, in this time there are no visible changes
  • It starts expanding/blooming. Bubbles appear, and little pseudopodia(closest work I can think of) of borax flail about. This takes approx five seconds. At the end of this, the borax looks like whipped cream that's been...well.. whipped .
  • The thing suddenly contracts into a tiny bead.

Now, as mentioned in the quote, basically borax loses its water of crystallization, and then decomposes into $\ce{NaBO2 +B2O3}$, the small bead.

The flailing about is probably due to steam escaping.

But, the reaction proceeds in such an interesting manner, I can't help but think that there's something else going on as well.

What are the details of the mechanism of the borax bead preparation reaction?

ManishEarth
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1 Answers1

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I've never seen this reaction, but it sounds quite entertaining. And I learned a couple of new words in looking at the literature to explain this phenomenon.

  • Intumescence: The property of a material to swell when heated. Borax is intumescent.
  • "Calcination is a thermal treatment process in presence of air applied to ores and other solid materials to bring about a thermal decomposition, phase transition, or removal of a volatile fraction. The calcination process normally takes place at temperatures below the melting point of the product materials."

The mechanism for the calcination of borax quite complex and is covered in some detail in this article. Borax decahydrate (BDH), $\ce{Na2B4O7.10H2O}$ is easily dehydrated to Borax pentahydrate (BPH), $\ce{Na2B4O7.5H2O}$ without much visible change. The actual form of the borate ion in both BDH and BPH is $\ce{B4O5(OH)4^2-}$ with 2 of the water molecules actually incorporated as hydroxyl groups in the ion. (The other 2 hydroxyl groups come for the water hydrogens and 2 oxygens from the $\ce{B4O7^2-}$.) That means that the last two water molecules can only be removed by molecular decomposition. So in BPH, you actually have only 3 "free" (i.e., easy to remove) water molecules.

So here's what happens. The borax decahydrate dehydrates smoothly with minimal density changes (no swelling) until you get to borax pentahydrate. At this point, the BPH dissolves in its own water of dehydration and the "blossoming" you see is from the solution erupting through the partially dehydrated crystal surface as it's heated. The density of the borax changes dramatically as it goes from the pentahydrate (about $0.85\ \mathrm{g/cm^3}$) to the dihydrate (about $0.14\ \mathrm{g/cm^3}$) which is why it ends up looking "whipped".

To do the final transformation to a tiny bead, the remaining 2 waters of hydration (those tied up as hydroxide ions) are driven off as the $\ce{B4O5(OH)4^2-}$ decomposes to give metaborate, $\ce{BO2^2-}$ and boric anhydride, $\ce{B2O3}$. $$\ce{B4O5(OH)4^2- -> 2 BO2 + B2O3 + 2H2O}$$

Fascinating.

Janice DelMar
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  • Nice :) Try it some day, borax is common in a lab. Take a platinum loop and heat a few wet crystals in it. Fun! Any reason why the metaborate clumps together? Probably cohesion, IMO – ManishEarth Jun 06 '12 at 05:09
  • My thought was that it just melted at that point and changed to a glass...the paper talks about sintering as it dehydrates. – Janice DelMar Jun 06 '12 at 05:34
  • Yeah, possibly. The glass, being molten and sticky, would easily coalesce. – ManishEarth Jun 06 '12 at 05:59