Friday, 10 January 2014

Floating raisins and sad shellfish

The floating raisin is a classic Christmas kitchen experiment. A raisin is dropped into a carbonated beverage (often champagne) and after a minute or so the raisin rises to the top of the drink and then sinks back to the bottom to repeat the process. I posted a video on Facebook of the experiment and had a few people ask me how it works.

Why is champagne bubbly? The bubbles in champagne and other carbonated drinks are bubbles of carbon dioxide gas (CO2).
These bubbles of CO2 are formed by tiny yeast microbes during a second fermentation step in producing champagne. During this step, the yeast eats the sugar breaking it down into carbon dioxide.

Why don't we see bubbles of CO2 when champagne is corked? This is due to a dynamic equilibrium inside a closed system - dynamic in that chemicals are always moving from one state to another (in this case carbon dioxide in solution and carbon dioxide gas) and an equilibrium in that the rate of change going from the liquid to the gas phase is the same as going from the gas into the liquid. No bubbles form, as that would indicate more carbon dioxide moving from the liquid to the gas and the system would be out of equilibrium.
Another time you may have come across this is when you leave your drink bottle in the car on a hot day with a little bit of water in the bottom. When you come back to open the bottle you hear a release of gas when the lid is opened. This is due to the water in the gas phase which is in equilibrium with liquid water until you open the bottle. We call this vapour pressure in chemistry. Below is a video of water at the gas-liquid interface.

So we know the bubbles come from CO2 escaping due to a non-equilibrium open system.  

Considering that CO2 is usually thought of as a gas (at room temperature) how is it stable in water? Hydrogen bonding holds the answer to CO2 stability in water. In this case, oxygen is electronegative (has higher negativity) than hydrogen, which is comparatively more positive. When oxygen comes near the small positive hydrogen, the positive and negative attract to form a very strong intermolecular (between molecules) bond. This stabilises the CO2 in water, stopping it from grouping together and forming a bubble.

The bonding H2O around CO2 [1], [2]
However, it is important to keep in mind that due to thermal energy (temperature) there is always change at a molecular level. This perfectly bonded sphere is the representation of an average of these bonds, which are themselves constantly changing.

When the cork is popped the pressure at the top is released and so there is no gas pushing on the liquid. Without this pressure, the gas at room temperature will push the water molecules apart and form a bubble.
Above you can see a bubble of CO2 with water on either side. However, water is quite heavy and will push on this bubble until the CO2 goes back into the water and no bubbles form in the liquid. [4]

If no bubbles can form in a liquid how do they form in champagne? This is due to microscopic dirt and scratches on the inside of the glass on which the bubbles can form. Only on a solid, where CO2 forms weak bonds with the surface, can enough CO2 molecules get together to form a bubble. When this bubble reaches about 1 micron (a hair being about 100 microns in diameter) it is large enough to continue to grow by itself in the liquid. [5-7]
Above is a bubble formed on a cotton fibre. It can be seen to form, grow and then detach and continue to grow as it travels through the liquid. Champagne bottles and glasses can be scratched using lasers or mechanically to tune just how quickly the champagne will stay bubbly. This video explains it with some simple experiments.

Coming back to the raisin in the champagne, it is easy to see how bubbles can form on its wrinkled surface.

Why do the bubbles grow larger than a micron on a raisin and actually stick to it? In short, the bubble of CO2 likes the sugary surface of the raisin more than it wants to float. As a result, the raisin is pulled up to the top of the glass due to the buoyancy of the bubbles. Once at the surface there is no liquid to hold the gas in bubbles so it escapes into the air and the raisin sinks again.

Super-heated water
A dangerous example of the need for impurities on which bubbles can form is super-heated water. (Don't try this at home - after this I have an experiment you can try at home.) Pure water (with a complete lack of impurities) can be heated above boiling in the microwave for a few minutes. When it comes in contact with a spoon (as my father found out while flatting to disastrous consequences) or sugar (as the video below shows) it explodes as gas is released.

Super-cooled water
Instead of this rather dangerous experiment, what you can try at home is super-cooling water. If you put purified water into the freezer it will cool below freezing until either the introduction of an impurity or a shockwave (for example, hitting the bottle) starts the ice crystal formation.

The really important research that is going on in this area is to do with CO2 storage. Due to the blanket of CO2 around the earth, our atmosphere is heating up. Something slowing down this increase in CO2 are the oceans, which are absorbing excess CO2. However, this presents two long-term problems - firstly, as the earth heats up further less CO2 can be stored in the oceans, so this is a finite solution, and secondly, it acidifies the ocean making it very difficult for shellfish to make their shells.

Sebastian is sad because his shell is slowly dissolving in the ocean's acidity.

The picture below shows pteropod shells that have already started dissolving in the Southern Ocean.

A shell placed in seawater with increased acidity slowly dissolves over 45 days.<div class='credit'><strong>Credit:</strong> A shell placed in seawater with increased acidity slowly dissolves over 45 days.</div>
A more scientific representation of a sad shellfish.

Shellfish use calcium carbonate to form their shells. The shell is made out of calcium with the carbonate ions helping to make sure the calcium isn't dissolved in water. But by flooding the oceans with CO2 we are removing carbonate and forming bicarbonate, therefore removing the molecules needed for shells to not dissolve.

This is why the study of CO2 in water and also trapping CO2 by adsorbing them onto solids is of great importance to the earth and why the chemistry of floating raisins is much more interesting and important than you could have imagined.

[1] H. Sato, N. Matubayasi, M. Nakahara and F. Hirata, Which carbon oxide is more soluble? Ab initio study on carbon monoxide and dioxide in aqueous solution, Chem. Phys. Lett. 323 (2000) 257 - 262.

[2] G. K. Anderson, Enthalpy of dissociation and hydration number of carbon dioxide hydrate from the Clapeyron equation, J. Chem. Thermodynamics 35 (2003) 1171-1183.

[3] G.A. Gallet, F. Pietrucci, W. Andreoni, J. Chem. Theory Comput. 8, 4029-4039 (2012)

[4] W. L. Ryan et al., J. Coll. Interf. Sci. 1993, 157, 312. DOI: 10.1006/jcis.1993.1191

[5] G. Liger-Belair et al., Langmuir 2004, 20, 4132. DOI: 10.1021/la049960f

[6] G. Liger-Belair et al., J. Phys. Chem B, 2005, 109, 14573. DOI: 10.1021/jp051650y

[7] G. Liger-Belair et al., J. Phys. Chem B, 2006, 110, 21145. DOI: 10.1021/jp0640427

For more of the physics behind champagne check out this great review: