For a few years now I have been showing a clip from Brainiac when discussing the reaction of alkali metals with water. I can remember as a boy watching the following clip and being delighted when a piece of caesium was dropped into the water (that bowl is called a pneumatic trough by the way).
I liked the sense of impending danger those rubber gloves conveyed and then the little whistle that someone makes just before the video cuts out. You can imagine my delight when I discovered that Brainiac had a similar video in which they dropped 2g of caesium into a bathtub! I saw it at a teaching conference on ways that ICT can be used to enhance lessons, using things such as YouTube videos for an example. The Brainiac clip below was one of the examples given.
Now that is an explosion! I also like the way that they have edited the music to develop the tension. It is very nicely produced.
It is also entirely faked. This is not what happens when you put 2g of caesium into a bath of water. If you watch carefully you can apparently see the wires running to the explosive charge they used to produce this effect. I was utterly and unthinkingly taken in because it did not occur to me that a program claiming to be interested in science would produce something so misleading. I am sure that I have had pupils tell me that it looked faked in class but I would have denied it. My critical thinking faculties were turned off. I stopped thinking and just believed that what I was watching was true. I suppose that this is a good lesson for me to be more sceptical and think about every claim before I inadvertently perpetuate myths.
Science Bit
The relative atomic mass of caesium is about 133. This means that one mole of caesium atoms has a mass of 133g. A mole of an element contains 6×1023 atoms of that element. One mole of hydrogen has a mass of 1g so caesium has a relative mass 133 times greater than hydrogen. 2g of caesium is about (2÷133) 0.015 of a mole. When it reacts with water, because it reacts in a 1:1 ratio, it would produce about 0.015 mole of hydrogen. Are you with me so far?
One mole of hydrogen (under normal conditions) occupies about 22 litres of space. 0.015 mole of hydrogen therefore occupies about 0.33 (or one third) of a litre. According to Richard Hammond it is the volume of expanding hydrogen that causes the damage. Hydrogen is highly explosive but the weight of water would be more than enough to quell the explosive force from this reaction.
I feel pretty stupid that I did not rumble this sooner but I am glad that at least now I will not be misleading my pupils next term (well, not on this topic at least).
For further reading I recommend Ben Goldacre (from 2006 so I am really behind on this story) and the video below from a YouTube user called Thunderf00t who has put the record straight through experimentation. He gets quite technical but listen to what he has to say and see how much of it makes sense.
Moral of the story – if something seems too good to be true, it probably is.
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