Chemistry Grades and Chemistry Help

In almost every chemistry lecture the nearly magical molecule of water is present. For acid-base titrations, electrochemistry, thermodynamics, hydrogen bonding, covalent bonding, solvation, boiling….the list goes on nearly forever.

In my videos I will frequently use H2O as the example. It can be a gas, a liquid, a solid, a solvent, a solute, a reactant, and a product. It has so few electrons it’s reasonable to count all of them. It has so few atoms we can calculate where they are.

Here is one example, what happens when you open the cap on a bottle of soda water? What is dissolved in the water, what is in the gas above the water, and why does opening the cap change the liquid?

The answer is that when you release the pressure of the cap, a lot of excess CO2 leaves the bottle. This CO2 is used to keep the dissolved CO2 in equilibrium. When the cap is removed the partial pressure of CO2 above the water drops to nearly zero, and LeChatlier’s principle pushes the dissolved CO2 to leave.  But it does not simply evaporate from the surface, because the solution is now supersaturated with CO2. If this bottle held perfectly pure water and the inside walls of the bottle were perfectly smooth then we might never see bubbles. But nothing is perfect, the water has tiny particles floating around which act as nucleation sites for CO2 bubbles. The CO2 “knows” it is now supersaturated and it wants to leave. It aggregates on the particle surface as a gas, and soon a bubble lifts off from the particle.

Now, why does shaking the bottle make more bubbles? The number of particles in the liquid isn’t changing. Or is it? (probably not).

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