Anything with a very low pH is acidic, while substances with a high pH are alkaline. It wasn't until later that I found out what the pH scale actually meant, and when I did, it blew my mind completely.
I'd always assumed that it either stood for two words or was named after someone. I did sometimes wonder why it was written like that small p, capital H , but never enough to seriously ask about it. For starters, that H isn't a H. The reason it's capitalised is because it's the symbol for hydrogen. The p isn't a p either, it's a letter used as a shorthand for a mathematical operation. To be specific, the operation "-log10".
What I'd been assuming were the letters pH was actually a scientific formula, -log10 of the concentration of hydrogen ions:. With that in mind, the pH scale made a lot more sense as a measure of acidity. Acids have a few different definitions, but overall they are substances that can generate hydrogen ions when in a solution. In order to be acidic then, a substance must contain hydrogen, in a form that can be released into water.
Substances such as CH4 methane are not acidic as all four hydrogens are bound very tightly to the carbon and are not going anywhere. CH4 has a neutral pH, around 7. On the other hand, substances such as hydrochloric acid, HCl, are held together by polar ionic bonds and when placed into water the hydrogen will break away to form hydrogen ions, making the liquid acidic.
HCl therefore has a very low pH and is a very strong acid. Weak acids, with pH 5 or 6 are slightly more complex. These are formed when a compound can release hydrogen ions, but only very weakly. Examples are often organic compounds, which while they have many hydrogens in their chemical structure, very few of these can break away in solution. Acetic acid HC 2 H 3 O 2 is a weak acid, as only the hydrogen at the front of the equation that can dissociate, and it is not hugely energetically favourable for it to do so.
The pH scale, with a list of substances at each pH. Acids at the top, bases at the bottom. Image credit below. I do like the fact that "pH" has an actual chemical and mathematical meaning, rather than just being a random set of letters. The views expressed are those of the author s and are not necessarily those of Scientific American. There are a large number of weak acids, but fortunately they fall into a few well-defined categories:.
All react with water in the same way as acetic acid [Eq. The strength of carboxylic acids is dependent on the electronegative strength of the atoms in the "R" group. Fluorine is the most electronegative element, while hydrogen is comparable to carbon in electronegativity. Thus, the fluorines pull electron density away from the carboxyl group. This removes electron density from the acidic oxygen-hydrogen bond, which weakens it.
This weaker bond means that the hydrogen can be removed more easily, which creates a stronger acid. This concept can be applied to any R group. The more electronegative the R group, the stronger the carboxylic acid will be. These have the same general formula H n XO m as strong oxyacids, but the number of hydrogens is equal to or one less than the number of oxygens. Some examples are:. Hydrogen fluoride HF has a very strong bond and does not donate its proton as readily as other hydrogen halides.
In the latter case, even though H is bonded to C, the electronegative N atom pulls some electron density away, and the HCN molecule is a very weak proton donor.
The positive charge of the metal ion pulls electron density away from the surrounding water molecules, weakening the hold of the oxygen atoms for the hydrogen atoms.
The latter can consequently be more easily donated as protons:. Certain other ions can donate protons. An example of this is the hydrogen sulfate ion formed when sulfuric acid donates a proton:.
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