Ions cannot usually be seen directly, so chemists identify them from observable behavior. The characteristic properties of ionic compounds and the movement of charged particles in an electric field both reveal ionic particles.

Using observable behavior as evidence

Chemists often rely on indirect evidence. For ionic compounds, the evidence comes from a repeated pattern of physical properties and electrical behavior that is best explained by the presence of charged particles.

The idea becomes clearer once the term ion is defined.

Ions are not just symbols in formulas such as sodium chloride or magnesium oxide. Their charge affects how substances melt, conduct electricity, and behave in an electric field.

Physical properties of ionic compounds

High melting and boiling temperatures

Ionic compounds are usually solids at room temperature and often have high melting and boiling temperatures.

This diagram shows the sodium chloride crystal structure as a repeating 3D lattice of alternating $\mathrm{Na^+}$ and $\mathrm{Cl^-}$ ions. The regular arrangement emphasizes that each ion is strongly attracted to many oppositely charged neighbors, helping explain why large amounts of energy are required to melt ionic solids. Source

This suggests that the particles are held together by very strong attractions throughout the substance.

If a substance were made of neutral particles with only weak forces between them, much less energy would be needed to separate those particles. The large amount of energy needed to melt or boil an ionic compound supports the idea that the particles are strongly attracted because they carry opposite charges.

This is evidence for ions because:

• opposite charges attract strongly

• the attraction acts throughout the solid

• a large amount of energy is needed to separate the particles

A high melting temperature does not prove on its own that a substance is ionic, but it strongly fits the model of a solid made from positive and negative ions.

Electrical conductivity in different states

One of the clearest pieces of evidence is the way ionic compounds conduct electricity. A solid ionic compound does not conduct electricity, but when molten or dissolved in water, it usually does conduct.

This pattern is very important:

• in a solid, the ions are fixed in place, so charge cannot move

• in a molten compound, the ions are free to move

• in aqueous solution, the ions can move through the liquid

The key idea is that electrical conduction requires mobile charged particles. Since conduction begins only when the particles are able to move, this provides strong evidence that the substance contains ions.

The term electrolyte is useful here.

A substance that behaves as an electrolyte is showing evidence of ions in motion. This is especially convincing because the same substance changes from non-conducting to conducting when the ions become mobile.

Brittle behavior

Many ionic solids are brittle. Instead of bending, they tend to shatter when enough force is applied. This also supports the idea of a regular arrangement of charged particles.

If layers in the solid are forced to shift, ions with the same charge may be brought close together. Strong repulsion then causes the structure to break apart. This behavior is consistent with a solid built from alternating positive and negative ions.

Brittleness is useful supporting evidence because it matches what would be expected from a structure containing charged particles in fixed positions.

Migration of ions

Movement in an electric field

Even stronger evidence comes from the migration of ions during electrolysis or any experiment in which an ionic liquid or solution is placed in an electric field.

If the particles in the liquid are truly charged:

This labeled electrolytic-cell diagram shows ion migration in an electric field: cations move toward the cathode (negative electrode) and anions move toward the anode (positive electrode). It visually links the observed direction of movement to the sign of charge, which is strong evidence that the charge carriers in the liquid are ions rather than neutral particles. Source

• cations move toward the negative electrode

• anions move toward the positive electrode

That is exactly what is observed. The direction of movement depends on the sign of the charge, which is strong evidence that the moving particles are real charged species.

When the ions are colored, their movement can sometimes be seen directly. This makes the evidence especially convincing, because the charged particles do not stay randomly mixed; they separate and move in opposite directions under the influence of the electric field.

Why migration is such strong evidence

Migration shows more than simple electrical conduction. It shows what kind of particles are carrying the charge.

If a liquid conducts because two different types of particles move to opposite electrodes, then:

• the particles must be charged

• some must be positive

• some must be negative

This explains why different products are formed at different electrodes during electrolysis. Positive ions move one way and negative ions move the other way before being discharged. The observed separation of products supports the ionic model very strongly.

Using the evidence together

No single observation is as powerful as the full pattern. The strongest case for ions comes when a substance:

• has a high melting temperature

• is a brittle crystalline solid

• does not conduct when solid

• does conduct when molten or in solution

• shows movement of positive and negative particles to opposite electrodes

Together, these observations are difficult to explain unless the substance contains ions. The physical properties show strong attractions between charged particles, and ion migration shows that those charged particles can move and carry electrical charge.