# If carbon dioxide is non polar, why does it react with water?

I’ve read in many places that carbon dioxide forms carbonic acid with water since the carbon is partially positive and thus the oxygen bonds with it. But isn’t carbon dioxide in its entirety non polar since the 180 angle removes any dipole moment, so how would oxygen be attracted to the carbon?

Is it that individually each atom has partial charge but not collectively? Or am I wrong in some way?

• Fluorine gas is about as non-polar as you can get, but you don't want to mix it with water (or anything else...). Meanwhile, caesium fluoride is extremely polar, but happily dissolves in water with no reaction, and can be completely recovered by drying. Jun 3 '19 at 10:56
• Carbon dioxide is pretty polar. True, it has zero dipole moment, but that's irrelevant. Jun 3 '19 at 11:05
• There is need to distinguish polarity of the whole molecule and polarity of its parts. CO2 as a molecule us not polar ( but is polarizable), but CO bonds are polar. Jun 3 '19 at 12:30
• Jun 3 '19 at 13:18
• Related: Why is carbon dioxide nonpolar? Jun 3 '19 at 15:47

## 4 Answers

I can only repeat myself here: Polarity is an ill-defined concept that has a nice potential for confusion.

In most cases, when specifying a molecule as polar, one is colloquially referring to the presence of a dipole moment, i.e. one actually categorises the molecule as dipolar. As described by ron in "Why is carbon dioxide nonpolar?", $$\ce{CO2}$$ has no dipole moment, it is therefore not dipolar, or colloquially it is not polar.

However, $$\ce{CO2}$$ has two very dipolar bonds, and a significant quadrupole moment. If one were to extend the nomenclature, one would say the molecule is quadrupolar. However, this may lead to complications down the line.

On the other hand, as I have written in the linked question, toluene is often considered an unpolar/non-polar solvent, which is not really true considering it has a small dipole moment.

There are a couple of things one can predict with the concept of polarity, and fortunately, the more complex the molecules become, the better the approximation becomes. It is small highly symmetric molecules, which break these approximations.

Related reading:

Carbon dioxide actually is polar. It is not di polar, but it has a quadrupole -- a combination of two opposing dipoles. Quadrupoles interact only weakly at a distance; the electrostatic interaction energy with an external charge falls off as $$1/r^3$$. But as with a dipole, a close-up external charge or dipole can interact selectively with one of the component charges by drawing close to the favored component, like the hydrogen atoms of a water molecule drawing close to one of the negatively charged oxygen atoms in the carbon dioxide quadruple.

There is also something of a "bonus". Water is best known as a dipole, but it too has a quadrupole along the line through the two hydrogen atoms (perpendicular to the dipole). The water quadrupole has its negative charge in the middle whereas carbon dioxide has a positive charge in the middle; and the two molecules are similar in size. We therefore have a "hand in glove" situation where the water has its quadrupole aligned with the carbon dioxide quadrupole but the charges oppositely distributed, enabling every atom of the water molecule to simultaneously attract an appropriate atom from the carbon dioxide. This extra solvation interaction not only improves solubility, it gets the molecules properly arranged to combine and form (a trace of) carbonic acid.

• I would like to suggest this answer, since this also explains the carbonic acid formation. May 16 '21 at 4:25

A molecule of carbon dioxide has a slight negative charge near the oxygen and a slight positive charge near the carbon. CO2 is soluble because water molecules are attracted to these polar areas. The bond between carbon and oxygen is not as polar as the bond between hydrogen and oxygen, but it is polar enough that carbon dioxide can dissolve in water.

A molecule is said to be "non-polar" as a whole and carbon dioxide falls into this category. However, it does not mean that parts of the molecule aren't indeed polar, even if their effects cancel out when considering the whole molecule.

EDIT: So the CO2 molecule is indeed very polar and, more especially, the central carbon atom is really electrophilic. That is why it reacts with water.