In 1884, a twenty-five-year-old Swede submitted a 150-page thesis on why salt water conducts electricity. The examiners at his own university graded it fourth class, which is close to failing.
It was upgraded after he defended it. Nobody at the university took it seriously, so he sent copies to the physicists building the new science of physical chemistry in Germany and Switzerland, who were astonished by it.
Nineteen years later he was awarded the Nobel Prize in Chemistry for it.
The idea was simple and, at the time, strange. Salt dissolved in water does not need an electric current to break into charged particles. The charged particles are already there, and reactions in solution are reactions between them.
The earlier version of this post was about 650 words long. It gave the year as 1887, it had no sources at all, and it left out the claim that was actually revolutionary.
What Arrhenius actually claimed
The name "ion" was not his. Michael Faraday had coined it decades earlier, and he had a different idea about where ions come from.
Faraday thought electrolysis created them. His picture was that you run a current through a melt or a solution, and that is what makes the charged particles appear.
Arrhenius disagreed, and this is the whole of it. He said that if you dissolve a salt in water with nothing connected to anything, the solution still contains the same ions. The current is not what makes them. It only reveals them.
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| Svante Arrhenius in a period photogravure. His thesis on electrolyte conductivity was graded fourth class by the examiners and upgraded only after he defended it. He was awarded the Nobel Prize in Chemistry nineteen years later. Photogravure Meisenbach Riffar, public domain, via Wikimedia Commons |
If that is true, then everything happening in a beaker of salt water is a chemical reaction between ions that were present before anyone looked. Which means chemistry in solution can be understood by looking at what the ions are, not only at what went in.
His dissertation contained 56 separate claims. Most of them are still accepted today, unchanged or with minor edits, which is a better record than most work that wins prizes.
A name collision worth knowing
"Arrhenius equation" almost always refers to something else: the formula for how fast a reaction runs, worked out by the same man in 1889.
That equation is not about ions. It is about activation energy, and confusing the two is a common and expensive mistake.
What an ion is, and why water makes one
An ion is an atom or a group of atoms that has gained or lost electrons. That single change of electron count is the entire difference, and it is enough to make the particle behave like a different substance.
Sodium metal is a soft, reactive metal that reacts violently with water. The sodium ion in table salt is stable, and table salt is something people eat every day.
The difference between those two sodiums is ten electrons. One is neutral metal with its outer shell full and reactive; the other has lost that outer electron, has a complete shell, and has nothing left to react with.
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| Sodium chloride crystal beside water. Dissolving does not just shatter the lattice: water molecules orient themselves around each ion and hold it apart, and they stay apart with no current applied anywhere. Smokefoot, CC BY-SA 4.0, via Wikimedia Commons |
In the solid, a salt is a lattice. Every sodium is surrounded by chlorides and every chloride by sodiums, and the electrostatic attraction between opposite charges is what holds the crystal together.
Water is polar, so it is good at electrostatically hugging charged things. When it gets in among the lattice, each ion gathers a shell of water molecules oriented toward its charge, and that shell is what pulls the ions apart.
So the salt does not shatter. It is pried apart, ion by ion, by solvent. What is left in solution is sodium ions, chloride ions, and a great many water molecules that have temporarily given up some of their freedom to point at them.
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| Salt going into water. The lattice is held together by attraction between opposite charges, and water wins that attraction by surrounding each ion with its own oriented shell of polar molecules. Materialscientist, CC BY-SA 3.0, via Wikimedia Commons |
The dissolved form is written NaCl(s) going to Na+(aq) plus Cl-(aq). The (aq) means the ion is hydrated, not that it is floating loose.
Why the solution stays neutral
Every sodium ion that appears brings a positive charge with it, and every chloride brings a negative one. The salt was neutral before it dissolved and it is neutral afterwards.
So a solution of salt water carries ions everywhere and has no net charge at all. That is not a coincidence to be noted and moved past. It is why you can put a current through the solution at all, and why what flows is charge rather than electron.
Electrolyte solutions conduct through the movement of ions, and not through the movement of electrons. A wire carries electrons; salt water carries charged atoms.
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| A single nitrate ion, drawn as an electrostatic potential surface. This is what an ion turns out to be once Arrhenius is done with it: not a label but a charged molecular object, with a definite charge sitting in a definite place among its neighbors. Benjah-bmm27, public domain, via Wikimedia Commons |
How much charge, and why "valency" is not enough
The earlier version of this post gave a rule: the charge on an ion is equal to the valency of its parent atom or group. It used calcium as the example, and calcium works.
Calcium has two valence electrons and loses both, so its ion carries a charge of two positive. Sodium loses one, so it carries one. On the main group elements the rule is reliable.
It stops being reliable as soon as you leave the main group. Many transition metals form more than one ion, and iron is the standard example: iron two and iron three are both ordinary, both common, and both real.
There is no single number for iron that predicts which one you get. What decides it is the chemistry of whatever the iron is sitting in, not the element on its own.
This is why modern chemistry talks about charge and oxidation state rather than valency. Valency was a nineteenth-century idea about how many atoms an element would combine with. It is a reasonable rule for the parts of the periodic table where it works, and the original post stated it as though it worked everywhere.
The general statement that does survive is about electron counting. An atom tends toward a configuration with a filled outer shell. The charge on a simple ion is the number of electrons it gained or lost on the way there.
Strong and weak electrolytes
Here is the distinction the original post never made, and it is the most useful thing in the whole subject.
Two substances can both dissolve in water, both split into ions, and both conduct. They can still behave completely differently, because one splits almost completely and the other barely splits at all.
Sodium chloride is a strong electrolyte. It dissociates essentially completely in water, which means nearly every molecule ends up as separate sodium and chloride ions.
Acetic acid, the acid in vinegar, is a weak electrolyte. Its dissociation constant is about 1.8 x 10 to the minus 5. That is a very small number, and it means only a tiny fraction of the acid exists as ions at any moment.
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| Acetic acid. Most of it stays as intact molecules and only a small fraction splits into ions at a time, which is what makes it a weak electrolyte while salt is a strong one. Ben Mills, public domain, via Wikimedia Commons |
At 0.1 mol per liter, roughly one molecule in seventy-five is ionized. That is the arithmetic: for a weak acid the fraction ionized is close to the square root of the dissociation constant divided by the concentration.
The square root of 1.8 x 10 to the minus 5 divided by 0.1 is 0.013. So about 1.3 percent, which is a little under one in seventy-five.
Two consequences follow immediately. Vinegar tastes mildly acidic because its acid is weak. And diluting a weak electrolyte raises the fraction ionized, which is why vinegar seems sharper when diluted and why the arithmetic above depends on concentration at all.
Sodium chloride shows the opposite pattern. Diluting it makes the solution conduct worse, because there are fewer ions per unit volume, even though the proportion ionized does not change.
One more fact explains a great deal of what happens in a cell. The hydrogen ion is the fastest moving ion in water, with a limiting molar conductivity of about 350 siemens per square centimeter per mole. That single fact drives fast acid-base chemistry, fast nerve conduction, and the buffering behavior of every cell in a body.
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| The pH scale, a logarithmic measure of hydrogen ion concentration. This is the one place Arrhenius has it exactly right: his definition is literally about hydrogen ions in water, and pH measures that quantity directly. Alvy16, CC BY 4.0, via Wikimedia Commons |
Where the theory stops working
Arrhenius used his idea of ions to define acids and bases as well. An acid is a substance that produces hydrogen ions in water. A base is a substance that produces hydroxide ions in water.
That definition of an acid is excellent and is still used.
The definition of a base does not survive contact with ammonia.
Ammonia, NH3, is unmistakably a base. Dissolve it in water and the solution turns alkaline and turns litmus blue. But look at the molecule: there is no hydroxide group in it, and no oxygen at all. It cannot possibly release hydroxide, because it does not contain any.
What actually happens is that ammonia reacts with water to make hydroxide, rather than simply providing it.
The two statements are not the same. Arrhenius asked which substances contain the ion, so he could describe ionic compounds like sodium hydroxide perfectly. Ammonia is not that kind of compound, and his definition has no room for it.
There is a matching failure on the acid side. Carbon dioxide gas dissolved in water gives a solution that is slightly acidic, producing hydronium, carbonate and hydrogen carbonate ions. There is no hydrogen ion being released from a bond in CO2, because there is no hydrogen in the molecule to release.
The replacement theories did not throw Arrhenius away. They widened the frame.
Bronsted and Lowry said an acid is a proton donor and a base is a proton acceptor. Ammonia then works perfectly: it accepts a proton from water, which is precisely why the solution turns alkaline.
Lewis went further still and described an acid as an electron pair acceptor, which covers reactions with no proton anywhere in them.
Each is more general than the one before. That is what being superseded by a theory looks like: not being wrong, being narrower than the evidence.
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| Zinc in hydrochloric acid. An Arrhenius acid doing exactly what the definition says: supplying hydrogen ions, which are consumed at the metal surface, so the reaction runs to completion. Chemicalinterest, public domain, via Wikimedia Commons |
What it is still used for
None of that retires the original. Arrhenius is still the shortest correct description for most everyday aqueous chemistry, and the surest way to predict whether a solution will conduct.
Whenever the question is simply "does this thing conduct, and what carries the current", ions are the answer, and they are the answer because of him.
Electrolysis, and where it runs out of water
Put a potential across a solution and the ions migrate: cations toward the cathode, which is the negative electrode, and anions toward the anode, which is positive.
The two kinds move in opposite directions and both carry charge, and that opposite migration is what a current through a solution physically is.
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| An electrochemical cell. The electrodes, the electrolyte between them, and the direction each kind of ion travels when a potential is applied. Alksub, CC BY-SA 3.0, via Wikimedia Commons |
There is a place where this stops working, and the original post described it wrongly. Electrolysis does not break down impurities in water. It breaks down the water.
Two molecules of water become two of hydrogen gas and one of oxygen gas. Dissolved impurities are swept along with the products, and separating them properly takes a different process.
Molten salt, and aluminium
Water is not required. Heat a salt past its melting point and the lattice bonds are gone but the ions are still there and still free to move, so the liquid conducts.
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| Molten salt being poured. The water is gone but the conduction is not: there are no longer solvent molecules holding the ions apart, but there are no longer lattice bonds holding them together either. Firetech117, CC BY-SA 3.0, via Wikimedia Commons |
This is not a curiosity. Every gram of aluminium ever made came from alumina dissolved in a bath of molten cryolite.
The bath runs at roughly 950 degrees. Aluminium is too reactive to plate out of any aqueous solution, so it has to be done neat.
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| Molten aluminium tapped off in the Hall-Heroult process. Ion migration is the whole trick, run at about 950 degrees, because aluminium cannot be recovered from water. Kashkhan, CC BY-SA 3.0, via Wikimedia Commons |
A solution of an acid is the easy case. Every ordinary battery, every nerve impulse, and every measurement of blood salt depends on what Arrhenius described.
The numbers worth keeping
| Figure | What it is |
| 1884 | year the dissertation was submitted |
| 150 pages | length of that dissertation |
| 4th class | the grade the examiners gave it |
| 56 | separate claims it contained |
| 1903 | year of the Nobel Prize in Chemistry |
| 1.8 x 10^-5 | dissociation constant of acetic acid |
| about 1.3% | of acetic acid ionized at 0.1 mol/l |
| about 350 | limiting molar conductivity of H+, in siemens cm2/mol |
| about 950 degrees | temperature of the Hall-Heroult aluminium bath |
| 100 degrees | ceiling defining an ionic liquid by melting point |
| above 3 mol/l | concentration counted as a concentrated electrolyte |
Figures from the sources listed at the foot of this article.
Key Terms in Plain English
These are the words in this article that would send you to a dictionary. Each one is given here the way it is actually used above.
| Term | What it means |
| Ion | An atom or group that has gained or lost electrons, and so carries a charge. |
| Cation | A positively charged ion. Goes to the cathode. |
| Anion | A negatively charged ion. Goes to the anode. |
| Electrolyte | A substance that conducts in solution or when molten, by moving ions rather than electrons. |
| Strong electrolyte | One that dissociates almost completely in water. |
| Weak electrolyte | One that dissociates only partly. Acetic acid is one. |
| Dissociation constant | A number describing how far an acid or base splits into ions. Small means weak. |
| Valency | An older idea: how many atoms an element combines with. A useful rule for main-group elements, unreliable elsewhere. |
| Oxidation state | The modern term for the charge bookkeeping on an ion. |
| Solvation | The shell of solvent molecules that surrounds a dissolved ion and holds it apart from its neighbors. |
| Hydrated | Marked (aq). Meaning the ion is wrapped in water molecules, not floating free. |
| Cathode | The negative electrode in electrolysis. Where cations go. |
| Anode | The positive electrode in electrolysis. Where anions go. |
| pH | A logarithmic measure of hydrogen ion concentration. |
Terms used above, in roughly the order they first appear.
Key Takeaways
- Arrhenius claimed that a salt dissolved in water contains ions with no current flowing. Faraday had thought the current created them. That disagreement was the Nobel Prize.
- His dissertation was 150 pages, contained 56 claims, and was graded fourth class by his own university before being upgraded on defense.
- Water dissolves salt by orienting around each ion and prising the lattice apart. The solution stays neutral because the charges cancel exactly.
- Ions and electrons are not the same. Electrolyte solutions conduct by moving charged atoms, not electrons.
- "Charge equals valency" is a good rule for main-group elements and a bad one in general. Iron forms both Fe2+ and Fe3+.
- Strong and weak electrolytes differ only in how far they dissociate. Acetic acid at 0.1 mol per liter is about 1.3 percent ionized.
- Hydrogen ions are the fastest moving ions in water, at about 350 siemens per square centimeter per mole. That one fact drives fast acid-base chemistry everywhere.
- Arrhenius defined a base as something giving hydroxide in water, which cannot describe ammonia. That is the failure Bronsted-Lowry fixed.
- Electrolysis splits water, not the impurities in it.
- The Arrhenius equation for reaction rates is a different thing by the same man, worked out in 1889.
Frequently Asked Questions
Is sodium metal in salt the same as a sodium ion?
No, and this is the clearest case in chemistry of two things sharing a name and having almost nothing in common.
Sodium metal is a soft reactive solid that reacts violently with water. Sodium ions in table salt are stable, and table salt is eaten every day.
The difference is ten electrons. Losing that outer electron leaves the ion with a complete outer shell and nothing to react with.
If ions exist without a current, what does the current do?
It reveals them. Applying a potential gives the ions a reason to move, and their movement in opposite directions is what we measure as a current through the solution.
Why is acetic acid called weak if it does ionize?
Weak does not mean not at all. It means the equilibrium between ionized and un-ionized forms sits heavily toward un-ionized.
For acetic acid, the dissociation constant is 1.8 x 10 to the minus 5, which works out at about 1.3 percent ionized in a 0.1 mol per liter solution. That is roughly one molecule in seventy-five.
Why does diluting vinegar make it taste sharper?
Because diluting a weak electrolyte increases the fraction that is ionized, even though the total amount of acid is unchanged.
Do all metal ions have one fixed charge?
No, and this is where the valency rule runs out. Iron gives both iron two and iron three, both common.
What decides which one forms depends on what the iron is bonded to and what else is in the solution. The element alone does not settle it, which is why modern chemistry uses oxidation state rather than valency.
What is the difference between the two Arrhenius ideas?
The Arrhenius theory of electrolytic dissociation, from 1884, says solutions contain ions and reactions in solution are reactions between them.
The Arrhenius equation, from 1889, says reaction rate rises sharply with temperature because of activation energy. Same man, five years apart, unrelated ideas.
Does electrolysis clean the water?
Not in the way it is usually described. Electrolysis splits the water itself, into hydrogen and oxygen gases.
Dissolved impurities are carried along with the products rather than being broken down by the process.
Key references
• Svante Arrhenius, 1859-1927
• Electrolytes and electrolyte dissociation
• Theories of acids and bases
• Brønsted-Lowry acids and bases, and the limits of Arrhenius
• Electrolysis
Images: Wikimedia Commons, with the author and license named in each caption. Figures follow the sources above.
