# Does CO2 dissolve in water?

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!

• "it can't be dissolved in H20 on normal temperature and pressure" — says who? BTW, there is carbon dioxide dissolved in your blood right now. May 28, 2020 at 20:24
• Do you drink soda? May 28, 2020 at 20:27
• The amount of CO2 dissolved in water is proportional to the outer pressure. At 20°C, 1 liter water dissolves about 1.7 g CO2 at normal pressure (1 atm). If the pressure is twice as large, the amount of dissolved CO2 is twice as much, 3.4 g. May 28, 2020 at 20:37
• When you open that bottle, does all of the carbon dioxide escape? Observation 1 is quite good.
– Zhe
May 28, 2020 at 20:40
• But the soda will lose almost all of it's carboration if left in an open glass in a relatively short amount of time so I thought CO2 wasn't as likely to dissolve in H20.. but thanks I understand now
– Dp03
May 29, 2020 at 3:17

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.