Salts and Ions Explained Simply
When chemistry lessons talk about salt, they almost never mean the stuff on your chips. Table salt is just one of many thousands of salts. Limescale, plaster, fertiliser, baking powder, road grit, the minerals in your drinking water - all of them are salts. What they have in common is not the taste but the structure: a salt is made of ions, electrically charged particles that stack into a regular lattice.
Every sodium ion has six chloride ions as neighbours and the other way round - drawn flat here, but in reality the pattern continues in all three directions. The chloride ions are bigger because they have taken on an electron.
An ion forms when an atom gives electrons away or takes them on. Give some away and it is left with more protons than electrons, so it carries a positive charge - such a particle is called a cation. Take some on and it is negatively charged, which is called an anion. How many electrons move is written in the periodic table: elements in group 1 give one away and become singly positive, group 2 gives two away, group 13 three. Read from the other side, group 17 takes one on and becomes singly negative, group 16 takes two, group 15 three. So the number of outer electrons decides the charge directly, and both sides are heading for the same full outer shell.
That lets you work out the formula of any salt from a single rule: the whole thing has to be neutral on the outside, so positive and negative charges must cancel exactly. For sodium and chlorine one positive charge meets one negative charge, so it is one to one - NaCl. For calcium and chlorine, the calcium ion brings two positive charges while each chloride ion only carries one negative, so you need two of them: CaCl2. For aluminium and oxygen, three positive charges face two negative ones; the lowest common multiple is six, which lands you at two aluminium ions and three oxide ions: Al2O3. The other way round, sodium oxide needs two sodium ions for one oxide ion, so Na2O.
The names follow a fixed pattern. The metal comes first, the non-metal second with the ending -ide: sodium chloride, calcium oxide, iron sulfide. Alongside these there are ions made of several atoms that still travel as one unit. The most important are sulfate (SO42−), nitrate (NO3−), carbonate (CO32−), hydroxide (OH−) and, as the only positive one, ammonium (NH4+). They end in -ate or -ide rather than an element name, and you treat them as a single particle when balancing charges - which is why calcium nitrate needs brackets: Ca(NO3)2.
The water molecules turn so that their negative end faces the positive ion. This wrapping is called a hydration shell, and it is the reason salts dissolve in water at all.
That is exactly what dissolving is. Water is a dipole, so it has a slightly negative and a slightly positive end. The negative end pulls on the cations, the positive end on the anions. Together these forces are strong enough to prise individual ions out of the lattice. Every freed ion is immediately surrounded by water molecules - the hydration shell - and then floats through the solution on its own. This is why salt water tastes equally salty everywhere: the ions spread out evenly, there are no grains of salt left.
From that mobility follows a property you can measure easily. A dry salt crystal does not conduct electricity, even though it is full of charged particles - they are simply locked in place. Dissolve it in water or melt it at high temperature and the ions become free to move, at which point the liquid conducts very well. Salt solutions and molten salts are therefore electrolytes. The reason salts have such high melting points in the first place - table salt only melts at 801°C - is how strongly the charges attract each other inside the lattice. Why a crystal shatters despite that hardness is covered under chemical bonding.
💬 Mia asks Grandpa Theo
Why does calcium nitrate need brackets when calcium chloride does not?
Because nitrate is made of four atoms and is still a single ion. You need two of them, and the brackets say: the two applies to the whole group, so twice NO3. Without brackets you would have written CaNO32, which would mean thirty-two oxygen atoms. With chloride the ion is a single atom, so the little two can sit straight after it.
If salt water is made of ions, why do you not get a shock when you swim in the sea?
You would, in principle - sea water actually conducts rather well. What is missing is a voltage. Current only flows when there is a difference in potential, and the sea has none. Add a source, say a faulty appliance on a jetty, and it becomes dangerous exactly then - and considerably more so in salt water than in fresh water.
Is sugar a salt too, then? It dissolves just the same.
Good observation, but no. Sugar is made of molecules, not ions. When it dissolves, whole sugar molecules get wrapped in water; they do not break into charged pieces. That is why sugar water conducts no electricity while salt water does. It is the simplest test to tell the two apart.
Road salt does not warm the road - it lowers the freezing point. Pure water freezes at 0°C because at that temperature the molecules can settle into a solid lattice. With ions dissolved in the water, that settling is obstructed: the water molecules are busy with their hydration shells and find it harder to come together. So it has to get colder before the lattice forms anyway. With table salt this works down to about −21°C; below that, more salt does not help, because the salt itself starts crystallising out too. This is exactly why road maintenance crews switch to calcium chloride in hard frost, which works down to around −50°C - it supplies three ions per formula unit instead of two and disrupts freezing accordingly more. The same effect explains why the sea only freezes at about −1.9°C, and why you add salt to ice water when you want ice lollies to set faster.
Where the charges come from and why the group tells you them is covered in atomic structure and the periodic table. What holds the ions together in the lattice is explained by chemical bonding. And how ions form when electrons travel from one substance to another is shown in redox reactions.
The sentence to remember: a salt is not a molecule but a lattice. That is why the formula only gives a ratio - and that ratio always comes from the charges having to cancel exactly.
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