why does soda go flat in a closed bottle after it has been opened but not before?

Answer 1

When a bottle has yet to be opened, it is full and there's not much room for the bubbles to come out of solution (which causes it to go flat) and mix with the air in the volume devoid of soda. (It has nothing to do with how tight the cap is.)

When it's partially empty, the carbon dioxide will come out of solution and mix with the air that occupies the rest of the bottle, increasing the pressure in the bottle (it tries to reach equilibrium). Each time the bottle is opened, it loses more and more of that precious pressure which is required to keep the carbonation in solution.

When you shake a bottle of it (regardless of how full it is), the agitation is enough to cause some of the CO2 to come out, which will cause it to run over (or explode if not opened).

When I was younger, I used to think that if you squeezed the bottle to get as much air out as possible before capping it, then the CO2 would have nowhere to go, but that's not true. It'll just expand more to force the bottle to its original shape. This actually causes it to get flatter more quickly!

The only way to prevent it from going flat for a long period of time would be to pressurize it to keep the CO2 in solution.

This all came from the top of my head. For a more explanatory answer, read up on vapor pressures, preferably on Wikipedia.

I hope this helped!

Answer 2

Because there is room for the carbon dioxide to expand into.

When the bottle is still full, there is only a little air in it, and any carbon dioxide that escapes the solution must dissolve into the air, or go back into the solution. Any carbon dioxide that escapes the solution quickly builds up pressure that makes it more difficult for the remaining carbon dioxide in the solution to leave.

It must reach what is called equilibrium, where the pressure in the solution equals the pressure in the air in the bottle.

Once some soda is poured out, several things happen.

One is that the bottle is warmed. (Usually, the room temperature is warmer than the refrigerator.) This causes some of the carbon dioxide to leave the solution. You seee this hapening when a warm bottle fizzes more than a cold bottle does. The carbon dioxide cannot remain in solution when it is warm.

Secondly, the bottle is agitated. This also causes the carbon dioxide to leave the solution. (Even if a can is ice cold, if you shake it up, you are in for a bath when you crack it open!)

Third, the remaining soda is left with warm air in the bottle. This continues to warm the remaining soda.

Fourth, the pressure is reduced. The air that was let in is at a much lower pressure than the small amount of air that was in the bottle when it was nearly full. As the carbon dioxide escapes the solution, it once again builds up the pressure to the point where no more carbon dioxide can leave the solution. But there is less solution in the bottle now and more air. Thus, the bottle becomes pressurized to a lower pressure.

Finally, some of the gas may leak out through the top. The factory seal was much tighter than you are able to re-seal it by hand.

So, instead of buying the (bulk, cheap) 3 liter bottles, get the little individual serving cans. Your soda will stay fizzy longer!

Answer 3

The way carbon dioxide is put into solution to make soda is to add it under pressure- about 3-4 atmospheres' worth, or around 40-50 pounds per square inch. The reaction looks like this:

CO2 + H2O <--> H2CO3

Carbon dioxide + water forms carbonic acid. This reaction is reversible, as the double arrow indicates; this means that it can go both ways.

In the case of this reaction, one side (the left side) has a gas. Because of the way reactions work, they like to achieve a finishing point, called "equilibrium." In this case, the equilibrium state of the reaction is to have some gas in solution (as carbonic acid), and some in the headspace over the liquid in the bottle.

If you add more carbon dioxide or you increase the pressure (or both), more carbon dioxide will dissolve. If you release the pressure or somehow remove the carbon dioxide from the top of the bottle, then more gas will come out.

It is interesting to note that the plastic that is commonly used for soda bottles (PETE, or polyethylene terephthalate) is permeable to carbon dioxide. If they were not lined with another type of plastic, all the carbon dioxide would leak out.

Answer 4

well i appreciate those who have given a detailed explanation, but well the reason is simple, it has been explained as Henry's Law, it states that the solubility of a gas in a liquid increases as the pressure increases,. also it is known that gases which are easily liquified(Dry Ice( solid is after liquid remmbr)) are more easily soluble in a liquid. So when the can is opened, the liquid is exposed to the pressure of atmosphere which is actually quite less than the pressure in which the soda was actually there,

You must have noticed that if the soda is chilled the gas takes some time to fizz out , but hotter it gets, the blander it is. This is because gas expands on heating and easily gets out.

Answer 5

soda is water that has been injected gas at very high pressure. When the bottle is full, the pressure of the gas in the liquid is lost until the pressure in the top empty bit is the same.

As you drink from the bottle, the empty space grows. so more and more gas from the liquid must be used to fill the empty space until the equilibrium is reached again and no more gas is lost from the liquid.

Eventually the empty space is too big and ends up sucking all the co2 from the liquid, that becomes flat.

Can please anyone invent a 2l plastic bottle that shrinks as it loses liquid? This way we will enjoy fizzy drinks all the way from the beggining to the end.

A

Answer 6

It is very air tight, there is nowhere for the carbonation to be except where it is in the drink. After it is opened there is space for the carbonation to leave.

Answer 7

once you open the bottle, the seal's gone and the CO2 can leak out freely.

Answer 8

Something about the carbonation being exposed to air I believe.

Answer 9

The seal isn't as tight & the CO2 escapes - no more fizz

Answer 10

Because CO2 already escaped.