Ternary salts - ones with more than one anion or cation

Question

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1. Restatement of the problem

A student is asking what solid products are obtained when two (or more) simple ionic salts are mixed in solution and then the water is removed.

Do the different anions/cations arrange themselves into a single, ordered crystal that contains both species (a “ternary” or mixed‑anion/cation salt)?
Or do the salts simply recrystallise as separate pure phases that are just mixed together (an incoherent physical mixture)?

The question is illustrated with the following examples

Mixture in solution	What might the solid be after evaporation?
NaCl + NaF	NaCl/NaF mixed crystal or separate NaCl + NaF?
NaCl + KCl	NaCl/KCl mixed crystal or separate NaCl + KCl?
NaCl + NaF + KCl + KF	A single ternary solid or a physical mixture of several salts?

We need to explain, using solid‑state chemistry concepts, which outcome actually occurs and why.

2. Detailed solution

2.1. Definitions

Term	Meaning
Simple (binary) salt	An ionic solid that contains one type of cation and one type of anion (e.g., NaCl).
Double salt	A crystalline compound that contains two different cations and/or two different anions in a fixed stoichiometric ratio, e.g. KAl(SO₄)₂·12H₂O (alum). The composition is definite and the solid is a single phase.
Solid solution (or mixed crystal)	A single crystalline phase in which one ion (or a group of ions) is randomly substituted by another ion of similar size/charge, preserving the original crystal lattice. The composition can vary continuously (e.g., NaCl₁₋ₓFₓ).
Mechanical mixture	A physical mixture of two (or more) distinct crystalline phases that coexist side‑by‑side, each retaining its own lattice. No new chemical entity is formed.

Whether a mixture of salts yields a double salt, a solid solution, or a mechanical mixture depends on (i) crystal‑structure compatibility, (ii) ionic size/charge similarity, and (iii) thermodynamic favorability of mixing in the solid state.

2.2. The NaCl–NaF system

Both NaCl and NaF crystallise in the rock‑salt (NaCl) structure (space group Fm3̅m). The lattice is built from a cubic close‑packed array of anions with the cations occupying all octahedral holes.

Property	Na⁺	Cl⁻	F⁻
Ionic radius (Shannon, CN=6)	102 pm	181 pm	133 pm
Charge	+1	–1	–1

Size considerations

The cation (Na⁺) is identical in both salts, so no problem on the cation sublattice.
The anion sizes differ by ~48 pm, but both fit comfortably on the anion sublattice of the rock‑salt structure; the lattice can expand or contract slightly to accommodate the mixture.

Thermodynamic data
The Gibbs free energy of mixing for the NaCl–NaF system is negative over the whole composition range at room temperature (experimental phase‑diagram data). This means the two anions can substitute for each other continuously in the same lattice, forming a solid solution.

Result

When an aqueous solution containing Na⁺, Cl⁻, and F⁻ is evaporated, the solid that crystallises is NaCl₁₋ₓFₓ (0 ≤ x ≤ 1). The actual value of x is set by the relative amounts of Cl⁻ and F⁻ present in the mother liquor and by any kinetic factors (e.g., rate of evaporation). The crystals are single‑phase; they are not a pile of separate NaCl and NaF crystals, although under rapid crystallisation or very high supersaturation the mixture may sometimes give a small amount of separate NaCl and NaF grains (a “segregated” solid solution). In practice, well‑controlled slow evaporation yields a homogeneous mixed crystal.

2.3. The NaCl–KCl system

Both NaCl and KCl also adopt the rock‑salt structure, but now the cations differ while the anion (Cl⁻) is the same.

Property	Na⁺	K⁺
Ionic radius (CN=6)	102 pm	138 pm
Charge	+1	+1

Because the cation sizes differ by ~36 pm, the lattice can still accommodate both species by a modest change in the lattice parameter. Indeed, the NaCl–KCl binary system is a classic example of a continuous solid solution at temperatures above ≈ 700 °C. At lower temperatures the solid solution range shrinks but does not disappear completely; at ambient temperature the solid solution extends roughly from Na₀.₈K₀.₂Cl to Na₀.₂K₀.₈Cl (the exact limits depend on the experimental method).

Thus, after evaporating a room‑temperature solution of Na⁺, K⁺, and Cl⁻, you obtain a single crystalline phase of composition Na₁₋ᵧKᵧCl (0 ≤ y ≤ 1, within the solubility limits). The crystal lattice is still the rock‑salt type, just with a lattice constant that lies between those of pure NaCl (5.64 Å) and pure KCl (6.29 Å).

2.4. The four‑component mixture NaCl + NaF + KCl + KF

All four salts share the same rock‑salt lattice. The anion sublattice can be occupied by either Cl⁻ or F⁻, and the cation sublattice by Na⁺ or K⁺. Because each substitution (Cl ↔ F, Na ↔ K) is individually allowed, a quaternary solid solution is possible:

[ \text{Na}{1-y}\text{K}{y}\,\text{Cl}{1-x}\text{F}{x}\qquad (0\le x\le 1,\;0\le y\le 1) ]

In practice, the composition of the solid that precipitates will be the overall composition of the solution, unless the mixture passes through composition ranges where the solid solution is limited. For the Na–K–Cl–F system, experimental phase diagrams show that the solid solution is essentially continuous at ambient temperature; only extreme compositions (very close to pure NaF or pure KF) may start to separate into two phases on cooling.

Therefore, after evaporating a solution containing all four ions, the product is one homogeneous mixed crystal whose lattice parameter is somewhere between that of pure NaCl and pure KF, reflecting the averaged ionic sizes.

2.5. Why we do not obtain a simple mechanical mixture in most cases

Common lattice type – All the salts share the same crystal structure (rock‑salt). When ions of similar charge and comparable size replace each other, the lattice can deform slightly but remains intact, favoring a single phase.
Thermodynamics – The Gibbs free energy of mixing (ΔG_mix = ΔH_mix – TΔS_mix) is negative because the enthalpy of mixing is only slightly endothermic (size mismatch) while the configurational entropy (random distribution of two kinds of ions) is large. The net result drives formation of a solid solution.
Kinetics – During slow evaporation, the system has enough time for ions to rearrange into the lowest‑energy lattice. Rapid precipitation can trap the ions in separate nuclei, giving a partial mechanical mixture, but this is a kinetic artifact, not the equilibrium state.

2.6. Summary of outcomes

Starting mixture (aqueous)	Expected solid after slow evaporation
NaCl + NaF	NaCl₁₋ₓFₓ solid solution (mixed crystal)
NaCl + KCl	Na₁₋ᵧKᵧCl solid solution (mixed crystal)
NaCl + NaF + KCl + KF	Na₁₋ᵧKᵧCl₁₋ₓFₓ quaternary solid solution (single phase)
Any two salts with different structures (e.g., Na₂SO₄ + NaCl)	Usually a mechanical mixture or a double salt if a specific stoichiometric compound exists (e.g., NaCl·2H₂O).

3. Final answer

When two (or more) simple ionic salts that share the same crystal structure are mixed in solution and the solvent is removed, the ions usually form a single‑phase solid solution (a mixed crystal) rather than a random physical mixture. In the specific cases asked about:

NaCl + NaF → NaCl₁₋ₓFₓ (a continuous solid solution, composition set by the solution).
NaCl + KCl → Na₁₋ᵧKᵧCl (continuous solid solution, composition set by the solution).
NaCl + NaF + KCl + KF → Na₁₋ᵧKᵧCl₁₋ₓFₓ (a quaternary solid solution, again a single homogeneous crystal).

Only if the component salts have different lattice types or if a specific double‑salt compound is thermodynamically favored will a distinct new compound form; otherwise the mixture remains a single-phase solid solution.

4. Common mistakes

Mistake	Why it’s wrong	How to avoid it
Assuming the salts always stay as separate crystals	Ignores the possibility of ion substitution in a common lattice; the free energy of mixing is often negative.	Check whether the salts share the same structure and whether ion sizes/charges are comparable.
Confusing “double salts” with “solid solutions”	Double salts have a fixed stoichiometry (e.g., Mohr’s salt, K₂SO₄·MgSO₄·6H₂O). Solid solutions have variable composition.	Look up phase diagrams: a line of constant composition = double salt; a continuous region = solid solution.
Neglecting temperature effects	Some systems are solid solutions only at high temperature; cooling can cause segregation.	Consult the temperature‑composition phase diagram for the system.
Thinking that any two ions can substitute for each other	Large size or charge differences can make substitution energetically prohibitive, leading to phase separation.	Compare ionic radii (± 15 % rule) and charges; large mismatches usually give separate phases.
Assuming rapid precipitation always yields the equilibrium product	Fast nucleation can trap a metastable mixture of phases.	For equilibrium predictions, assume slow evaporation or annealing, which allows the system to reach the lowest‑energy state.

By keeping these points in mind, you can correctly predict whether a mixture of salts will give a mixed crystal, a double salt, or simply a mechanical mixture.

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