Dante's Peak

Would an acid-filled lake really not float Pierce Brosnan's boat, or is the science of Dante's Peak off-kilter?

By: The Hollywood Science team (Programme and web teams)

  • Duration 10 mins
  • Updated Monday 22nd August 2005
  • Introductory level
  • Posted under Chemistry
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Pierce Brosnan Copyrighted image Copyright: BBC After dashing off to Grandma’s house to rescue his girlfriend, her two children (and Grandma herself) Pierce Brosnan discovers, to his horror, that a landfall has blocked his retreat. Too late! What to do when Grandma’s house is in serious danger of being atomised by that nasty erupting volcano yonder?

Our hero does the only logical thing, he loads his comrades into an aluminium dingy and sets off across the lake. Of course there’s a hitch. The dingy begins to dissolve. Why? And does this proposition measure up to our Hollywood Scientists?

In the movie, gases are bubbling up through the bottom of the lake, and it is these gases that turn the water into acid. How?

An erupting volcano will release gases, fragmented rock and heat into the atmosphere.

Volcano image Copyrighted image Copyright: Used with permission

Most of this gas is simply water vapour (H2O), carbon dioxide (CO2) and sulphur dioxide (SO2).

Other gases include: hydrogen sulphide (H2S), hydrogen (H2), carbon monoxide (CO), hydrogen chloride (HCl), hydrogen fluoride (HF), and helium (He), in smaller amounts.

Let’s take a look of the effect of the main gases released by the volcano. Water will not make any difference, CO2 will react with water to create a weak acid called carbonic acid.
C02(g) + H2O(aq) ->H2CO3(aq)

Sulphur dioxide is a colourless suffocating gas with a pungent odor. Its water solubility is 10 g/100 mL at 20 degrees celcius. Because it is twice as heavy as air it tends to fall to ground level.
In water sulphur dioxide will dissolve to form a weak solution of sulphurous acid.
SO2 (g) + H2O (l) ->H2SO3(aq)

It also reacts with oxygen to produce sulphur trioxide (SO3).
2 SO2 (g) + O2 (g) ->2 SO3 (g)

Chemical reaction graphic Copyrighted image Copyright: Used with permission

This reaction is usually very slow. However, one of the mysteries during early research on air pollution was how the sulphur dioxide produced from the combustion of sulphur-containing fuels is so rapidly converted to sulphur trioxide in the atmosphere.

It is now known that dust and other particles can act as heterogeneous catalysts, speeding up the reaction. In the case of an erupting volcano the air would be filled with such particles. So what does sulphur trioxide form when mixed with water? You guessed it, sulphuric acid! Very nasty!

SO3 (g) + H2O (l) H2SO4 (aq)

Other gases that could be contributing to the acidity of the lake include hydrogen sulphide (H2S), hydrogen chloride (HCl), hydrogen fluoride (HF). HCl and HF are very strong acids, whilst H2S is a weaker acid, but in a volcanic eruption these three would only found in small quantities. So the lake would indeed be acidic. But how acidic? We’ll come back to this critical point soon. First a bit of theory about aluminium...

As our Hollywood Scientists Jonathan Hare and Robert Llewellyn point out, aluminum is a very reactive metal; however, as soon as it is exposed to air, a thin layer of aluminum oxide forms rapidly over the surface.

What would happen to this protective oxide layer in the presence of sulphuric acid? The thin oxide layer would react with the acid to form the corresponding aluminium salt and water.

Al203(s) + H2SO4(aq) Al2(SO4)3(aq) + H2O
The aluminium metal will further react with the acid...

...to form the aluminum salt and hydrogen.

2Al(s) + 3H2SO4(aq) -> Al2(SO4)(aq) + 3H2(g)

And all of this means that the aluminium, theoretically, could dissolve in acid. But here’s the $64,000 question, how quickly? Well in the case presented in Dante’s Peak, not very. When our Hollywood Scientists did their experiment, they used a bowl of water, rocks and some of the chemicals one would find in a volcanic reaction. But lakes are very much bigger than bowls of water.

Let’s suppose our lake was 1 km square and 50 metres deep. This would amount to 50,000,000m3 or 50 x 109 litres.
To make the lake slightly acidic, e.g. pH6, you would need 10-6 mol c-1 of acid.

This means you need to add 50 x 109 x 10-6 moles of acid = 50,000 moles. Now one mole of sulphuric acid is 98g (we divide by 2) 50,000 x 98 divided by 2 = 2,500 kilograms of acid!

To put that another way, you will need 50,000 x 22 divided by 2 litres of SO2 gas, which would need 550,000 litres of gas bubbling up - not very likely!

Also, the rock acts as a buffer and would neutralise the acid, again, stopping the lake getting acidic.

Aluminium requires a strong acid to remove the outer coating, greater than 4 mol/litre. Thus you would need 4 x 50,000,000,000 mols or 10,000,000,000 kg of acid.

We here at the Hollywood Science website don’t really think that the volcano would create such an enormous amount of acid.

Jonathan Hare and Robert Llewellyn gave Dante’s Peak a Hollywood Science rating of 8/10.

However, even though the volcano will indeed make acid, it would not nearly be at a sufficient strength to strip the aluminium from the boat. So we at the Hollywood Science website feel that we can only give it a rating of:

2 out of ten Copyrighted image Copyright: Used with permission

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