Table salt looks like a powder. Drop it in water and it disappears. Where did it go? The ionic theory says it did not disappear. It broke into charged particles, and those particles are the reason the water now carries electricity.
This post explains what that theory actually claims, where it came from, and why the answer is stranger than "the salt dissolved".
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| Svante Arrhenius, who proposed the theory in 1884 at the age of 25 — Image: Meisenbach Riffar, public domain, via Wikimedia Commons |
Who Proposed It, and What Happened Next
Svante Arrhenius was a Swedish scientist born in 1859. He started as a physicist, and is usually called a chemist because of what he did next. He is one of the founders of physical chemistry.
In 1881 he left Uppsala University, unhappy with the only man there who could supervise him in chemistry. He went to Stockholm and worked on the conductivity of electrolytes, which is simply how well a liquid carries a current.
In 1884 he wrote up that work as a doctoral dissertation of about 155 pages. The professors were not impressed. He was given a fourth-class grade, the lowest kind. When he defended the work it was reclassified as third class. It contained 56 separate claims, and most of them have survived.
Nineteen years later, in 1903, the Nobel Prize in Chemistry went to him for that body of work. He became the first Swedish Nobel laureate and, from 1905, ran the Nobel Institute. The theory he proposed as a rejected student thesis is now taught in every chemistry course that covers solutions.
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| The institute in Stockholm that Arrhenius directed from 1905 until his death — Image: Hans A. Rosbach, CC BY-SA 3.0, via Wikimedia Commons |
What the Theory Actually Says
The claim has three parts, and the third is the one students forget.
First, an electrolyte in water or in the molten state separates into charged particles. These are called ions. Positively charged ones are cations. Negatively charged ones are anions. Sodium chloride gives up sodium ions and chloride ions when it meets water.
Second, those two charges always balance. However many positive and negative ions are floating about, the solution as a whole stays electrically neutral. The total positive charge equals the total negative charge.
Third, an ion does not behave like the atom it came from. This is the part with real consequences. A sodium ion is stable and unreactive in solution. The metal it came from reacts violently with water. The element has been transformed by losing an electron, and it is no longer the same substance.
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| Metallic sodium, which reacts violently with water. Its ions, in solution, do not — Image: Dnn8, CC BY-SA 3.0, via Wikimedia Commons |
One Correction Worth Making
Many students picture salt as little two-atom molecules, a sodium stuck to a chloride. That picture is wrong. In a solid salt, each ion has many near neighbors and is not part of a molecule at all. It is part of a continuous three-dimensional network, which is why salts form crystals with a repeating shape.
Ionisation and dissociation are not the same word
Arrhenius used the word ionisation. Modern usage is more careful. When energy breaks a neutral molecule into ions, that is ionisation. When an existing ionic lattice simply comes apart in water, that is dissociation. Salt dissociates. It does not ionise. The theory is about dissociation in both solids and solutions, which is why it covers molten salt as well as salt water.
Why Solid Salt Does Not Conduct but Melted Salt Does
Here is where the theory proves itself. Take a solid salt and put a wire through it. Nothing happens. The ions are locked in place in the lattice, so although they are charged, they cannot move to carry charge.
Now melt it, or dissolve it in water. The ions are free, and it conducts well. The charge was always there. What changed was mobility, and mobility is what an electric current needs.
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| The sodium chloride lattice. Every ion sits among many neighbors, not paired into two-atom molecules — Image: Solid State, public domain, via Wikimedia Commons |
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| Rock salt. The cube shape comes from the repeating order of the lattice, not from any molecule — Image: Robert M. Lavinsky, CC BY-SA 3.0, via Wikimedia Commons |
Ions in solution do not sit still. They move constantly in every direction, bumping into one another. Apply a voltage and that random motion is biased. Now they all drift one way or the other, and the solution carries current.
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| Salt crystals before and after water is added. Dry, the ions sit in a lattice. Wet, they are loose — Image: Ben Mills, public domain, via Wikimedia Commons |
How much a solution conducts depends on how many ions are in it and how easily they move. That is why a concentrated brine carries a current far better than distilled water with a pinch of salt dissolved in it.
Which Way the Ions Go
Put the solution between two electrodes and the random drift turns into a steady flow. Cations move toward the cathode, the negative electrode. Anions move toward the anode, the positive one. The two streams run opposite ways, and together they make the current flow through the liquid.
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| An electrolytic cell. Cations drift to the cathode and anions to the anode — Image: Bigspace4, CC BY-SA 4.0, via Wikimedia Commons |
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| A battery is the same idea run the other way, with the reaction driving the ions — Image: Bigspace4, CC BY-SA 4.0, via Wikimedia Commons |
This is why electroplating works. Hang a metal object at the cathode and the dissolved metal ions in the bath migrate onto it and plate over. It is also why electrolysis can break down substances that no amount of stirring would separate, because the electrodes pull the products apart instead of leaving them mixed.
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| Measuring conductivity means putting a known geometry around the sample, so the result means the same thing in every laboratory — Image: Omegatron, CC BY-SA 3.0, via Wikimedia Commons |
The Rules in One Table
| Rule | What it means |
| Dissociation | A salt splits into mobile ions in water or when molten |
| Electrical neutrality | Total positive charge always equals total negative charge |
| Cations and anions | Metals and hydrogen give up electrons and become positive; non-metals take them and become negative |
| Charge matches valency | Calcium forms a calcium ion with a charge of plus two, because its combining capacity is two |
| Ions differ from their atoms | An ion has different chemistry from the element it came from |
| Random motion | Ions drift in all directions until a voltage biases them |
| Direction under a field | Cations go to the cathode, anions go to the anode |
Why This One Idea Runs Through All of Chemistry
Once you accept that solutions are full of moving charged particles, a great deal follows without any new theory.
- Electrochemistry — batteries and electrolysis are driven by moving ions between electrodes.
- Analytical chemistry — acid-base titrations work because the hydrogen ions in the solution can be counted.
- Biochemistry — nerve signals, muscle contraction and fluid balance all depend on ions moving across membranes.
- Industry — refining metals and purifying chemicals are done by making ions move on purpose.
- Everyday chemistry — why salt water conducts and pure water barely does, and why wet roads corrode faster than dry ones.
A limit worth knowing
Perfectly clean ionic bonding does not exist. Every ionic compound has some covalent character, because electrons are never transferred perfectly cleanly. Chemists call a bond ionic when the ionic side dominates, which in practice means a large gap in how strongly the two atoms pull on electrons. Bonds with both characters are called polar covalent.
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 here |
| Ionic theory | the idea that an electrolyte breaks into charged particles when it dissolves or melts, which is why its solution conducts electricity |
| Ion | an atom or group of atoms carrying an electric charge |
| Cation | a positive ion, which drifts towards the cathode |
| Anion | a negative ion, which drifts towards the anode |
| Electrolyte | a substance whose solution conducts because it contains mobile ions |
| Dissociation | an ionic solid coming apart into the ions it already contained |
| Ionisation | a neutral molecule reacting so that ions appear. A different process from dissociation |
| Lattice | the regular arrangement of ions in a solid. Solid salt does not conduct because the ions in it cannot move |
Terms used in this article, defined as they are used above.
Key Takeaways
- Ionic theory says electrolytes dissociate into mobile ions in water or when molten.
- Cations are positive, anions are negative, and the solution stays neutral overall.
- An ion behaves nothing like the atom it came from. Sodium metal is dangerous; sodium ions are not.
- Solid salt does not conduct because the ions cannot move. Mobility is the whole story.
- Ions in a solid form a lattice, not a collection of two-atom molecules.
- Dissociation and ionisation are different words for different processes.
- Under a voltage, cations drift to the cathode and anions to the anode.
- Arrhenius proposed this in 1884 as a rejected doctoral thesis, and won the 1903 Nobel Prize for it.
Frequently Asked Questions
Where did ionic theory come from?
Svante Arrhenius, in an 1884 doctoral dissertation on the conductivity of electrolytes, at the age of 25. The examiners graded it fourth class. He won the 1903 Nobel Prize in Chemistry for extensions of the same work.
Does salt dissolve, or dissociate?
It dissociates. The lattice comes apart into free ions, and each ion ends up surrounded by water molecules. Nothing is destroyed, and the ions are the same before and after.
Why does a solid salt not conduct electricity?
The ions are charged but they are locked into fixed positions in the crystal. Charge only flows when charge can move. Melt the salt or dissolve it and the ions are free, and it conducts immediately.
Is pure water a good conductor?
No. Pure water has almost no ions in it, so it carries very little current. It is a poor conductor even though it is a famously good solvent.
What is the difference between an ion and an atom?
An atom is neutral. An ion has gained or lost electrons, so it carries a net charge. That charge is why it behaves so differently from the neutral element it came from.
Are all ionic compounds made of metal and non-metal?
Usually, but not always. Many ions are groups of atoms rather than single atoms, such as the ammonium ion or the sulfate ion. The rule still holds: a positive ion next to a negative one, held together by attraction.
Why is ionic bonding never perfectly ionic?
Because electrons are never transferred completely. There is always some sharing between the atoms, which is the covalent part. A bond is called ionic only when that sharing is the smaller effect.
Sources: Wikipedia articles on ionic theory and electrolytic dissociation after Svante Arrhenius, ionic bond, ionic compound, ion, sodium, sodium chloride, electrolyte, electrolysis, electrochemical cell, cathode, anode, galvanic cell and electrical conductivity. Images: Wikimedia Commons, with authors and licenses noted in each caption.
