Last week’s stumper comes from Paul Bland who says
“We're currently revising group one metals in year 11 and have (after much staff room discussion) decided to turn to "stump the scientist" for help.
Does anybody know why, just when it has almost been used up, the sodium or potassium may suddenly give a fierce explosion and throw sparks out when put in water? Sometimes it just fizzles to nothing, but sometimes it can be fierce enough to throw out pieces that burn the bench.
I'm sure it's just a matter of time before a pupil asks me the same question!”
"Surely it's just to do with the surface area to volume ratio of the metal. As the piece of metal decreases to a very small size, there is a large increase in the S.A./vol. ratio, so a lot of metal surface is exposed, and therefore the reaction is more vigorous?"
‘It's one or both of two things:
- Super reactive oxides (sorry don't know names) that develop over time - I have just been advised to ALWAYS cut the edges off the lump before dropping it.
- A dirty trough - the lump moves to the side, stays in one place instead of buzzing around and superheats’
"Someone once told me that the final burst from Potassium is from the superoxide that builds up on the surface even when it is kept under the oil! It still reacts with air through the oil. Try this: cut off the crusty bit from the potassium metal, and keep it separate on a filter paper ready for a few minutes.
Drop the shiny bit in water, it doesn't really pop. It does self ignite.
Drop the crusty bit in the water, its much more exciting! SAFETY SCREENS and distance. . .
Have a close look through the screen at the 'metal'. Just before it pops, it is not metal at all, but purely a 'glassy' bead, floating on the water and whizzing around. And then it is gone!
I do this CLOSE to a safety screen. SO THAT . . . any exploding bits shoot UPWARDS, and can't readily shoot at any angle towards the kids. And NO peeping round the corner!
See the website (we’ve put it in our Recommended Websites of the Week section – Ed) for 'Sodium party' for film clips of 'games' with big Sodium lumps, that we can't ever possibly do in school. (In paddling pools, buckets, ponds, from boats etc . . .) Then don't be goaded by your kids to 'do a bigger bit Sir"
The last word goes to Nigel Bowen who states
"Larger pieces of sodium melt under the heat of the reaction (sodium has a fairly low melting point of 98 degrees celsius), and the molten ball of metal is floats on a cushion of hydrogen and may appear to be reacting calmly with water, until splashing of the water covers more of the sodium, causing thermal runaway and an explosion which scatters molten sodium, lye (sodium hydroxide) solution, and sometimes flame.
This behaviour is unpredictable, and among the alkali metals it is usually sodium which does this, because lithium is not reactive enough to do it, and potassium is so reactive that chemistry teachers are not tempted to try the reaction with larger potassium pieces!"
We seem to recall that it’s sights like these that remain in the minds of pupils and now hopefully we can give them a good explanation of exactly what is happening and why.
So once again the scientist was NOT stumped!
If you can help or have a burning question of your own then send us an email with STUMP THE SCIENTIST in the subject line to planet-science.news@nesta.org.uk
It’s the end of the month again! That must mean it’s time for another bit of Astronomy of Astrology. So over to Alison Begley to explain…
Virgos who suffer from vertigo should hold on to their seats, you'll be concerned with matters very far from home this month.
Try this trick. You might need to cover your ears!
Can he fix it? Yes he can!! It’s Build it with Bob live at the Science Museum!
