I just read about non-metallic oxides in my textbook which says that non-metallic oxides are a compound formed of an oxide ion and a non-metal. It also said that they are acidic in nature. It gave examples of carbon dioxide and sulfur dioxide.

But then wouldn't hydrogen oxide (water) also be a non-metallic oxide and hence, acidic in nature. But most textbooks say that water is neutral. So how's that possible?

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    $\begingroup$ Water is neutral by convention. Life on an ammonia planet with would consider water an acid. $\endgroup$ – Kevin Kostlan Oct 27 '15 at 2:13

When textbooks say that water is "neutral," textbooks mean that the water meets this condition:

$\ce{[H_3O^+]} = [\ce{HO^-}]$.

That's all.

Water can be neutral, in that its hydronium ion concentration equals its hydroxide ion concentration. Water can also be acidic if there is carbon dioxide dissolved in the water. Carbon dioxide can dissolve in water simply by letting the water come into contact with the atmosphere. Water can be basic too if you put a basic solute in water.

Water itself can also act as a weak Brønsted acid by donating a proton. This however is not a favorable process.

Water can also act as a Lewis acid because the oxygen in water withdraws electron density from the hydrogens, giving each hydrogen a partial positive charge. This in turns makes the hydrogens electrophilic - fitting the definition of Lewis acidity.

I think what your textbook is getting at is Lewis acidity. Consider carbon dioxide. This doesn't fit the Brønsted definition of acidity because it has no hydrogen protons to donate. So consider the partial charges on the carbon dioxide molecule. There is a positive partial charge on the carbon because the oxygens are more electronegative than carbon and have withdrawn electron density from the carbon. This makes the carbon electrophilic. And hence carbon dioxide as a whole can be considered a Lewis acid (but do understand that it's the carbon specifically in carbon dioxide acting as a Lewis acid).

So in general, your textbook says that non-metallic oxides tend to be acidic (in the Lewis sense) because the oxygens withdraw electron density from the non-metal (remember that oxygen is one of the most electronegative elements). This in turns creates partial positive charge(s) and makes the centers of these partial positive charges electrophilic.

Also, you could say that non-metallic oxides are precursors to Brønsted acids; consider this Lewis acid/base reaction. Remember that the central conceit of Lewis acid/base chemistry (coordination chemistry) is electron flow; nucleophiles attack electrophiles (not the other way around; that would be the fatal conceit).

So consider this reaction:

$\ce{SO_3 + H_2O -> H_2SO_4}$

We have sulfur with an electronegativity (EN) of 2.6. Oxygen has an electronegativity of 3.5. The difference is 0.9. The bond is clearly polar covalent. In addition, we have not one but three oxygens isolating electron density from the sulfur. Therefore, the sulfur most definitely has a partial positive charge and the oxygens partial negative charges. We say the oxygens are nucleophilic. And back to water, we have already concluded the hydrogen atoms in water are electrophilic due to their positive partial chage. The sulfur trioxide will attack the hydrogens and form the sulfuric acid molecule - a strong Brønsted acid (its first ionization is complete in water solution).


Hydroxonium(H3O+)=hydroxide(OH-) one acts as a strong acid and the other a strong base- the instant they come to be, we have water, which results in a neutral PH in isolation; the PH is altered, in solution, by changing relative concentrations of these ions caused by exposure to molecules such as atmospheric carbon dioxide(distilled water can absorb more CO2 and hence tends to be slightly acidic) which creates carbonic acid in solution with water. This equilibrium is also part of the fundamental nature of pure water. H2O (the molecule) is an oxide of hydrogen mathematically. Water is bound H2O plus a relatively tiny amount ofhydrogen; hydroxide ions(oxides are binary compounds of oxygen with another element). Additionally "water", under various circumstances can have oxygen dissolved at varying levels(from very low dissolved oxygen (fish breathe) in the open underwater areas of the sea to relatively high oxygen in babbling brooks and rivers) and carbon dioxide (added by contact with air)- so there are many forms of "water" but "pure water in isolation" is without these common additions. Water is in a category of it's own. There is much yet to be learned about this wonderful substance, with all of it's unique properties, including long held beliefs regarding bonds and bond strength in my opinion, but that is another discussion. The bound molecule, H2O, has been categorized as a neutral, non-metal oxide albeit with significant unique aspects as water which have not been fully explored. The molecule of H2O is existant, in our reality, in the form of water. Water is not H2O; H2O is not water. H2O, should it be isolated, is not water and does not have the properties of water. Additionally, all H2O molecules are not the same, as a small percentage of H2O molecules, in water, are deuterium, as opposed to the aneutronic hydrogen(protium), which is far more common(with the definition that "pure water" is not "heavy water"(water with a high amount of deuterium)). Both protium (most H2O) and deuterium are stable non-radioactive isotopes of hydrogen.

Water Ph, in solution, changes up or down depending upon the relative concentrations of the two ions, hydroxonium (acid) and hydroxide(base). When the acid ions outnumber the base there is an acid and when the base ions outnumber the acid there is base. That is, when there are more hydroxonium ions than hydroxide ions, we have an acid and when there is more hydroxide we have a base- pure water has an equal number of these 2 ions and hence is neutral, by default. The confusing part of the process is the value of "neutral PH" which changes as the temperature of water changes, matching the value of the PH of water at a given temperature. Water, on it's own, always maintains a perfectly neutral PH value, even upon change in temperature, albeit it water can react amphoterically- as an acid or a base when in solution with other substances. The equilibrium point of water shifts when the temperature is raised (an effort, at reattaining balance, by effectively moving against change). Thus, water responds with an opposite reaction to the change in temperature. The counterbalance, to heating, is an effort to move in the direction of cooling. Heating pure water also causes both of water's ions to increase in numbers(equally); this causes a seeming decrease in the numerical value of the PH of water. However, this doesn't cause the water to become any more acidic as temperature increases. Water, in solution, is only acidic with an excess of hydrogen ions, as i have already said above. Pure H2O, is PH neutral, even if it's PH value changes due to the shifting of it's equilibrium point by increasing temperature because the new PH value instantly becomes the neutral PH value for water at that particular temperature. As another way of looking at it, the neutral PH value of water at room temperature, PH 7, would actually be alkaline for water tested at higher temperatures. Effectively, the neutral PH value, of water, lowers, as water raises in temperature. The "real PH" effectively remains unchanged within the instantly stable equalibria of water at a given temperature. The same result is seen, but in the other direction, as the temperature of water is lowered from room temperature. The PH value of water rises as the temperature of water falls from room temperature but so to does the neutral PH value of water at the new temperature. The unaffected PH state of pure water, at any temperature, is a neutral PH- meaning that water is not acidic and not base in addition to meaning an equal number of hydroxonium and hydroxide ions.

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    $\begingroup$ Hydrogen(H+)=hydroxide(OH-) results in a neutral PH of 7- the PH is altered by changing their relative concentrations. This is wrong. pH is solely dependent on the concentration of hydrogen ions. Water has a pH of 7 at 25$^\circ$C but if you heat it then it's pH will decrease because of increasing self ionization. Water is neither a compound nor an oxide Water is definitely a compound, and most people would consider it an oxide as well (dihydrogen oxide). $\endgroup$ – bon Oct 26 '15 at 14:55
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    $\begingroup$ I am only pointing out the technical aspects of the post. 0. Please take some time and learn about this site by taking the tour. Also consult How to Answer 1. Please do not excessively edit your post. Even in its current stage, after 16 edits, the wall of text is barely readable. Consult this page to learn more. 2. Do not respond to comments directly. Use comments to make your post more clear, but address comments by commenting. You can even reply to a comment with typing @ and the name of the commenter you want to reply to. $\endgroup$ – Martin - マーチン Oct 28 '15 at 6:10
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    $\begingroup$ 3. It is also not a good practice to repost the same answer, after it has been deleted with good reason. 4. Your answer reads like an opinion only, it would be appropriate to back up your claims with scientific facts and not only mere statements. $\endgroup$ – Martin - マーチン Oct 28 '15 at 6:19
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    $\begingroup$ ‘H2O, should it be isolated, is not water’ stopped reading there. $\endgroup$ – Jan Nov 3 '15 at 19:49
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    $\begingroup$ Let me just say that books follow the rules of typography and punctuation — they alone facilitate reading a lot. $\endgroup$ – Jan Nov 3 '15 at 20:07

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