I’m thinking of like when you bottle wine that’s still fermenting, and it explodes. I understand that the fermentation process raises the air pressure inside the bottle. What I *don’t* understand is how? When you pump air into a balloon, the air pressure rises because there are *more* air molecules entering the balloon.
With wine in a bottle, you have a fixed amount of matter at a certain temperature. I don’t understand how fermentation could increase air pressure in a closed system?
In: Physics
Volume and mass are two different things. Fermentation creates gas which has a larger volume than the base materials.
To put it another way, you have the same amount of “particles” but the particles spread out.
It’s like freezing water in a water bottle. You can put a fixed amount of water in a bottle but as it freezes it expands and it will blow the lid off the bottle.
Sugar is densely packed. Carbon Dioxide is not. The fermentation is basically the yeast unpacking the dense sugar into “loose” carbon dioxide which bounces around causing the pressure to rise.
the ideal gas law is PV = nRT, where P = pressure and n = number of moles of gas. In fermentation, if solid reactants are being converted into gaseous products, the number of moles of gas in the system increases, thereby increasing pressure.
Fermentation happens when yeast eats sugar. Just like in a person, digestion produces energy for the organism and waste. Alcohol and gas are produced as a waste byproduct of the yeast digesting the sugar. Without going into too much chemistry, the alcohol molecules are smaller and less complex than the sugar ones they’re made from. So the extra parts of the sugar molecule that don’t get used to make alcohol molecules turn into gas molecules. So there’s still the same amount of matter in there, it just changes form. As fermentation progresses, the yeast eats the sugar, uses the energy to reproduce, and poops out alcohol and gas. Over time there’s less sugar because it gets eaten, but more yeast, because it reproduces, more alcohol, and more gas. Eventually 1 of 3 things happens.
1) the yeast eats all the sugar so fermentation stops and the yeast dies.
2) the yeast produces too much alcohol and it can no longer survive in the alcohol rich environment and it dies.
3) the yeast eats sugar until the gas creates pressure too strong for the container and it breaks.
4) possible outcome is that the pressure gets too high for the yeast to survive before the container breaks, but that’s pure speculation. I have no idea how yeast reacts to pressure.
There may not be more matter, but the contents change density. When the chemical bonds in the sugar break into CO2, water, and alcohol, that takes up more space than the sugar previously had.
Like when you freeze a bottle of water, no matter entered or escaped, but the density changed.
How can a grenade explode, when pulling the pin doesn’t increase the amount of matter in the grenade? It’s because the explosive molecules (e.g. TNT) are more energy dense than the gases they become during the explosive chemical reaction.
Fermentation is doing an analogous thing (much less quickly). Biochemical reactions inside bacteria are turning sugars into alcohol and carbon dioxide. This unleashes some of the potential energy which had been confining the CO2 into compact, dense sugar molecules. That *binding energy* is ultimately what allowed the bottle to contain so much matter in the first place; as the sugar degrades, its binding energy converts into *kinetic* energy in the CO2 gas.
Basically, chemical energy stored in the form of sugar is turning into physical energy in the form of gas pressure.
ELI5: fill a glass bottle with water and close. Doesn’t break, doesn’t explode. Now freeze it. It will break. Same amount of water molecules, same mass, if you put it on a scale you’ll see it weights the same, but the water changed structure and takes more volume. Similar thing with fermentation. The stuff inside goes through some chemical reactions and some matter becomes a gas that takes more volume than before. And the glass is not strong enough to resist.
When yeast ferments sugar it is producing more molecules of gas, and those gas molecules take up far more space than when the atoms that make it up were all bonded with other atoms as sugar.
18 ml of water takes up 22,400ml at room temp and standard air pressure when it is water vapor. Gases take up way more space per molecule than liquids. So if you use a closed container and convert liquid to gas, the pressure goes way up, equivalent to the pressure of having to squeeze a much larger volume of gas down to fit inside the container. You can create gas from liquid a couple of ways. You can heat the liquid, like when you boil water and turn the liquid water into steam, or you can use a chemical reaction that converts the liquid to some other chemical that is a gas. Fermentation does the second one.
The fermentation of sugar by yeast converts a molecule of dissolved sugar into two molecules of alcohol, specifically ethanol, a liquid, and two molecules of carbon dioxide, a gas. That carbon dioxide gas is what is responsible for the increased pressure, because it is essentially being compressed as it is forms so it all fits in the bottle.
If I ball up a fist and you wrap a piece of paper around my fist, I can open my fist to expand my hand and tear the paper, even though there was no increase of matter in my hand.
Not all matter takes up the same amount of space 100% of the time.
I think dry ice works for this question, and I’m sure I’ll mess up some basics. You take a bottle that was some water in it, drop dry ice into it, and seal it. The chemical reaction is similar to boiling water and trying to freeze it at the same time. Gases are created, there is no room for them to go. Eventually the container reaches its breaking point.
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