Does CO2 dissolve in water?

Question

First of all not a homework question, but one day it suddenly popped into my head while opening a bottle of soda and accidentally leaving a glass out for a while. I get that $\ce{CO2}$ in water is not the same as Carbonic acid, however this also raises the question if $\ce{CO2}$ can combine with water (It does so in acid rain in the atmosphere(I would guess normal temperature and pressure)), then shouldn't dissolved $\ce{CO2}$ just form carbonic acid and hence become unusable for aquatic plants? And also if Carbonic acid does form in the atmosphere ( Again,I would guess normal temperature and pressure (feel free to correct me!)), why does the fizzing happen in soda bottles when opened in the first place, should Carbonic acid be stable at normal conditions?

Please do note that I talk about $\ce{CO2}$ dissolving under normal temperature and pressure

I feel like I'm missing something super basic and obvious here and I just can't put a finger on it. Thanks in advance!

Answer 1

I want to extend Maurice's comment:

The amount of $\ce{CO2}$ dissolved in water is proportional to the outer pressure. At $\pu{20 °C}$, 1 liter water dissolves about $\pu{1.7 g}$ $\ce{CO2}$ at normal pressure (1 atm). If the pressure is twice as large, the amount of dissolved $\ce{CO2}$ is twice as much, $\pu{3.4 g}$.

To talk about solubility of gases in liquids, we take the help of Henry's Law which states that:

The amount of dissolved gas in a liquid is proportional to its partial pressure above the liquid.

Mathematically, $$\pu{S_g=k_HP_g^\circ}$$

where $\pu{S_g}$ is the solubility of the gas, $\pu{k_H}$ is the Henry's law constant which is different for different gases and $\pu{P_g^\circ}$ is the partial pressure of the gas.

For $\ce{CO2}$ the value of $\pu{k_H}$ in water is $\pu{1.67}$ when the pressure is taken in atm at $\pu{20^\circ C}$. Also note that generally the solubility of gases decrease with increase in temperature and increase with increase in pressure.

There is another reason why $\ce{CO2}$ can be more soluble in water:

$$\ce{H2O + CO2 <=> H2CO3}$$

$\ce{CO2}$ reacts with water to form carbonic acid which then further dissociates to form $\ce{H+}$ ions and hence the overall solubility increases.

For your first observation, the amount of $\ce{CO2}$ released by fish(es) is so small that it is almost impossible to observe the formation of bubbles.

For your second observation, the pressure while fizzing the soda bottles is very high which leads to higher solubility of $\ce{CO2}$ and when the bottle cap is opened, the pressure inside the bottle is decreased and hence again following Henry's law, excess $\ce{CO2}$ comes out in the form of bubbles since now its amount is not so less.

Hope this answer cleared some of your doubts :).

Answer 2

When I used to make beer, I would use CO2 and put pressure in the keg of beer that needed to be carbonated. Just putting pressure in the keg would cause the CO2 gas regulator to hiss as the pressure in the keg came up. In order to get more gas into the keg, one could either put the keg and CO2 bottle in the refrigerator with pressure in the keg OR put a nominal low pressure from the CO2 bottle and agitate the keg and the CO2 regulator could be heard injecting gas as long as the agitation was continuous. For our atmosphere, during rain events, the atmosphere is active (roiling, updrafts, etc) which is the proper environment for dissolving CO2 gas into the water that is precipitating out of the atmosphere. This means that CO2 gets scrubbed from the atmosphere during rainstorms and is carried into the ground by the water it is dissolved in. So, the earth indeed has a built-in mechanism for scrubbing CO2 from the atmosphere.