Things never exist in isolation. I posted earlier this week about the hole in the ozone layer. I came across this article today:
(reproduced from http://www.sciam.com/article.cfm?id=updates-jul-08&sc=rss)
Ozone Recovery, Warmer Antarctica
The Antarctic ozone hole that forms every spring has kept that continent's interior cold even as the rest of the world has warmed over the past few decades [see "A Push from Above"; SciAm, August 2002]. Thanks to the global ban on chlorofluorocarbons, stratospheric ozone levels there are slowly recovering. A repaired hole, however, could speed Antarctic ice melting and change weather patterns, according to a computer model by Judith Perlwitz of the University of Colorado at Boulder and her colleagues. With more ozone, the lower stratosphere would absorb more ultraviolet light and warm up by as much as nine degrees Celsius. That in turn would break down circulation patterns that trap cold air over Antarctica's interior, making the continent heat up. The changed patterns would also make Australia warmer and drier, and South America could get wetter. Such ozone details may need to be worked into global climate models, most of which have neither incorporated such effects nor included enough of the stratosphere. The journal Geophysical Research Letters published the study on April 26.
Friday, 27 June 2008
Wednesday, 25 June 2008
Mentos-Soda Fountain
If you haven't seen a mentos-soda fountain yet, have a look here:
http://www.youtube.com/watch?v=quYx0uJtzLo
(or simply search youtube for mentos and diet coke*)
And now we know how it works:
http://www.sciam.com/podcast/episode.cfm?id=A0F42D0D-E721-EABE-745EA5C44B66F777
It turns out not to be a chemical reaction at all, but a physical one. In fact, it is nucleation, the same process that I talked about here. This time, instead of bubbles of steam, they are bubbles of carbon dioxide that had been dissolved in the soda. The rough surface of the mentos provides lots of nucleation points at which the bubbles form and the bubbles then form nucleation points for more bubbles, etc. It all adds up to a whole lot of bubbles!! Because of the rapid expansion (bubbles take up a lot more space than liquid), a lot of the bubbly mix gets forced out the small neck of the bottle and voila! A mentos-soda fountain.
*Note that although youtube has mostly mentos and diet coke, it should work with any type of soda. If you are experimenting, it would be worth trying other sweets as well
http://www.youtube.com/watch?v=quYx0uJtzLo
(or simply search youtube for mentos and diet coke*)
And now we know how it works:
http://www.sciam.com/podcast/episode.cfm?id=A0F42D0D-E721-EABE-745EA5C44B66F777
It turns out not to be a chemical reaction at all, but a physical one. In fact, it is nucleation, the same process that I talked about here. This time, instead of bubbles of steam, they are bubbles of carbon dioxide that had been dissolved in the soda. The rough surface of the mentos provides lots of nucleation points at which the bubbles form and the bubbles then form nucleation points for more bubbles, etc. It all adds up to a whole lot of bubbles!! Because of the rapid expansion (bubbles take up a lot more space than liquid), a lot of the bubbly mix gets forced out the small neck of the bottle and voila! A mentos-soda fountain.
*Note that although youtube has mostly mentos and diet coke, it should work with any type of soda. If you are experimenting, it would be worth trying other sweets as well
Tuesday, 24 June 2008
The Hole in the Ozone Layer
I was out in the sun with some of my friends the other day and the topic of the hole in the ozone layer came up (thanks to one of my Aussie friends). The hole in the ozone layer is more important to Australians than it is to people in the northern hemisphere, but the question is, why?
To explain why the hole in the ozone layer is mostly over the Antarctic (from where it somtimes spreads over Australia), I’ll start with a bit about ozone itself. Ozone is a molecule of three oxygen atoms. Mostly, oxygen atoms form pairs, which is what we breathe, but high up in the atmosphere short wave length radiation from the sun splits up these oxygen pairs leaving two free oxygen atoms. Oxygen atoms by themselves are not very stable, so they react with a nearby oxygen pair to form ozone.
One of the quirks of ozone is that it is very good at absorbing ultraviolet radiation (240-320nm). When ultraviolet rays are hit by an ultraviolet photon, they split up into a pair and a single atom again, absorbing the energy, and then the single atom reacts with a pair again to form a new ozone molecule, with the UV photon totally used up in the process. This is called the “ozone cycle”. When the free oxygen atoms end up together as a pair, then the cycle is broken. In the ozone layer, the recombining of single oxygen atoms into pairs is balanced by the splitting of pairs, and there is an abundance of ozone left to form the layer.
Things all begin to change when something else interrupts the ozone cycle. Some of the main culprits are chlorine and bromine atoms. For example, a chlorine atom reacts with an ozone molecule and produces ClO and an oxygen pair. The ClO then reacts with a free oxygen atom and you get another oxygen pair and the free chlorine atom back with no UV radiation absorbed by the process. And because the chlorine reactions happen faster than the ozone cycle, the whole balance has been shifted back towards oxygen pairs and there is much less ozone around to absorb the UV radiation.
So how does the chlorine get all the way up to the ozone layer? This is where CFCs (chlorofluorocarbons) come in to the picture. CFCs used to be used a lot in refrigerators, air conditioner and aerosol sprays in the 1980's, although they have now been banned. There are no natural sources of CFCs that we know about, so just about all the CFCs in the atmosphere are manmade. Further, because CFCs are generally stable and the chlorine isn’t used up in the process described above, it will take a long time before the hole goes away again.
Because the CFCs don’t react to anything in the lower atmosphere, they get caught up in the general air movement and get mixed throughout the whole lower atmosphere and are carried up to the stratosphere, where the ozone layer lives. Once there, the short wave radiation (the same radiation that breaks up the oxygen pairs) is the first thing the CFCs encounter that can break them up, releasing the chlorine atoms right in the ozone layer.
Finally I’m back to our original question; why is the hole over the Antarctic? Firstly, the cooling air in the winter and the rotation of the earth combine to form a large vortex over the South Pole. Because the vortex is so cold, clouds form (mostly from nitric acid; there’s very little water that high in the atmosphere – it all falls back to earth as rain much lower down). These clouds act as catalysts for the creation of the free chlorine atoms (i.e. they take part in the reaction, but are not used up by it, and in general make the reaction happen faster), so you have even more ozone destroying chlorine atoms then usual. This means you have much less ozone over Antarctica than elsewhere and this is what we call the hole.
There is also a small hole, sometime called a dimple, over the arctic. The vortex over the arctic isn’t as cold or as powerful as the one over the Antarctic due to the distribution of land masses. Correspondingly, there are less clouds formed and the whole (hole) problem isn’t as bad.
Image courtesy of NASA (note this image is in the public domain)
This is obviously a complex phenomenon, so if you are after more detail on the ozone layer and its hole, here are some good websites:
http://www.faqs.org/faqs/ozone-depletion/
http://en.wikipedia.org/wiki/Ozone_depletion
http://www.theozonehole.com/
To explain why the hole in the ozone layer is mostly over the Antarctic (from where it somtimes spreads over Australia), I’ll start with a bit about ozone itself. Ozone is a molecule of three oxygen atoms. Mostly, oxygen atoms form pairs, which is what we breathe, but high up in the atmosphere short wave length radiation from the sun splits up these oxygen pairs leaving two free oxygen atoms. Oxygen atoms by themselves are not very stable, so they react with a nearby oxygen pair to form ozone.
One of the quirks of ozone is that it is very good at absorbing ultraviolet radiation (240-320nm). When ultraviolet rays are hit by an ultraviolet photon, they split up into a pair and a single atom again, absorbing the energy, and then the single atom reacts with a pair again to form a new ozone molecule, with the UV photon totally used up in the process. This is called the “ozone cycle”. When the free oxygen atoms end up together as a pair, then the cycle is broken. In the ozone layer, the recombining of single oxygen atoms into pairs is balanced by the splitting of pairs, and there is an abundance of ozone left to form the layer.
Things all begin to change when something else interrupts the ozone cycle. Some of the main culprits are chlorine and bromine atoms. For example, a chlorine atom reacts with an ozone molecule and produces ClO and an oxygen pair. The ClO then reacts with a free oxygen atom and you get another oxygen pair and the free chlorine atom back with no UV radiation absorbed by the process. And because the chlorine reactions happen faster than the ozone cycle, the whole balance has been shifted back towards oxygen pairs and there is much less ozone around to absorb the UV radiation.
So how does the chlorine get all the way up to the ozone layer? This is where CFCs (chlorofluorocarbons) come in to the picture. CFCs used to be used a lot in refrigerators, air conditioner and aerosol sprays in the 1980's, although they have now been banned. There are no natural sources of CFCs that we know about, so just about all the CFCs in the atmosphere are manmade. Further, because CFCs are generally stable and the chlorine isn’t used up in the process described above, it will take a long time before the hole goes away again.
Because the CFCs don’t react to anything in the lower atmosphere, they get caught up in the general air movement and get mixed throughout the whole lower atmosphere and are carried up to the stratosphere, where the ozone layer lives. Once there, the short wave radiation (the same radiation that breaks up the oxygen pairs) is the first thing the CFCs encounter that can break them up, releasing the chlorine atoms right in the ozone layer.
Finally I’m back to our original question; why is the hole over the Antarctic? Firstly, the cooling air in the winter and the rotation of the earth combine to form a large vortex over the South Pole. Because the vortex is so cold, clouds form (mostly from nitric acid; there’s very little water that high in the atmosphere – it all falls back to earth as rain much lower down). These clouds act as catalysts for the creation of the free chlorine atoms (i.e. they take part in the reaction, but are not used up by it, and in general make the reaction happen faster), so you have even more ozone destroying chlorine atoms then usual. This means you have much less ozone over Antarctica than elsewhere and this is what we call the hole.
There is also a small hole, sometime called a dimple, over the arctic. The vortex over the arctic isn’t as cold or as powerful as the one over the Antarctic due to the distribution of land masses. Correspondingly, there are less clouds formed and the whole (hole) problem isn’t as bad.
Image courtesy of NASA (note this image is in the public domain)This is obviously a complex phenomenon, so if you are after more detail on the ozone layer and its hole, here are some good websites:
http://www.faqs.org/faqs/ozone-depletion/
http://en.wikipedia.org/wiki/Ozone_depletion
http://www.theozonehole.com/
Tuesday, 17 June 2008
Why boiling water in the microwave is a bad idea!
Today at work the kettle broke. Being the caffeine addicts that we are, going without our coffee was simply not an option. My friend was surprised when I was tentative about boiling the water in the microwave, so she got an impromptu lesson in super heating and nucleation :-)
When you boil water in a saucepan, you may have noticed that you initially get small bubbles forming at seemingly random points around the pan. These bubbles are forming at small defects in the pan. These tiny defects hold microscopic bubbles that make it slightly easier for the new bubbles to form. This process of forming new bubbles is called nucleation and the small defects are called nucleation sites.
What happens if there are no nucleation sites? If you have a nice new clean mug/glass that you are using in the microwave, then this might well be the case. The short answer is that bubbles don't form. This means that the microwave is continuing to add energy to the water, heating it up, but the water isn't boiling. You end up with superheated water! The water is more than 100°C but still in liquid form in your mug.
Now you take your mug out of the microwave and add your coffee. The coffee granules create nucleation points and suddenly your water is boiling very rapidly!! The massive expansion of the water as it become steam causes the water to seemingly explode out of the mug and all over you (causing nasty scalds). This could even happen just by moving your mug of superheated water, so be careful!
If you are interested in a few numbers, there is a good article on this here:
http://www.phys.unsw.edu.au/~jw/superheating.html
They also have a video:
When you boil water in a saucepan, you may have noticed that you initially get small bubbles forming at seemingly random points around the pan. These bubbles are forming at small defects in the pan. These tiny defects hold microscopic bubbles that make it slightly easier for the new bubbles to form. This process of forming new bubbles is called nucleation and the small defects are called nucleation sites.
What happens if there are no nucleation sites? If you have a nice new clean mug/glass that you are using in the microwave, then this might well be the case. The short answer is that bubbles don't form. This means that the microwave is continuing to add energy to the water, heating it up, but the water isn't boiling. You end up with superheated water! The water is more than 100°C but still in liquid form in your mug.
Now you take your mug out of the microwave and add your coffee. The coffee granules create nucleation points and suddenly your water is boiling very rapidly!! The massive expansion of the water as it become steam causes the water to seemingly explode out of the mug and all over you (causing nasty scalds). This could even happen just by moving your mug of superheated water, so be careful!
If you are interested in a few numbers, there is a good article on this here:
http://www.phys.unsw.edu.au/~jw/superheating.html
They also have a video:
This is the mpeg format video taken straight from the above website (from the physics department at UNSW) - this video is theirs and all rights remain with them. They also have a quicktime format to be found at the above link. !
We did heat our water in the microwave and we didn't get hurt: our mugs are quite old and therefore have many hairline cracks and other such nucleation points and also we didn't quite heat the water to boiling (it only needed to be hot enough to dissolve the instant coffee).
I am please to report that by lunchtime we had a new kettle :-)
Introducing Miss Geekette
Hello avid reader! Welcome to my blog. I am aiming to post at least once a week on a variety of science related topics. Whether it is science in my evey day life, interesting science articles or personal stories as a professional science geekette, I hope you find it interesting!
A bit about me:
I am an engineer by trade and an geekette by nature. I love finding out how things work and applying that knowledge to new and different situations. I have a long term ambition to be more involved in science communication, so this is my humble beginning. I firmly believe that we live in a fantastically interesting world and there is much fun to be had finding out about it. Anything that I can do to foster interest in science is worth doing. So, please feel free to ask me questions, leave comments or email me (science.geekette@gmail.com).
Today I leave you with one of my favourite images from the mandelbrot fractal :-)
Subscribe to:
Posts (Atom)
Posts
