Solubility Rules for Salts

Introduction

Stir sodium chloride into water and it vanishes; stir in calcium carbonate and the water just turns cloudy before the powder settles out. Both are salts, yet they behave very differently. Knowing which salts dissolve is essential: it decides how a salt can be made, explains why some mixtures form sudden cloudy solids, and underpins tests for ions. Fortunately, a small set of solubility rules covers almost all the common salts you will meet.

Core explanation

What "soluble" means here. In these rules, soluble means that a useful amount of the salt dissolves in water at room temperature. Insoluble means so little dissolves that the salt appears as a solid; in reality a tiny amount dissolves, but not enough to matter for most purposes.

The rules.

Compounds Soluble? Main exceptions --- --- --- All sodium, potassium and ammonium salts Soluble none All nitrates Soluble none Most ethanoates Soluble none common at this level Most chlorides (and bromides, iodides) Soluble silver chloride, lead(II) chloride Most sulfates Soluble barium sulfate, lead(II) sulfate; calcium sulfate is only slightly soluble Most carbonates Insoluble sodium, potassium and ammonium carbonates are soluble Most hydroxides Insoluble sodium, potassium and ammonium hydroxides are soluble; calcium hydroxide is slightly soluble

How to use them. Look first at the positive ion: if it is sodium, potassium or ammonium, the salt is soluble. Otherwise, look at the negative ion. Nitrates are always soluble. Chlorides and sulfates are soluble except for the listed exceptions. Carbonates and hydroxides are insoluble unless the metal is sodium, potassium or ammonium.

Examples.

- Copper(II) sulfate: sulfates are mostly soluble, and copper is not an exception → soluble (blue solution). - Barium sulfate: an exception → insoluble (white solid). - Magnesium carbonate: most carbonates are insoluble → insoluble. - Potassium carbonate: potassium salts are all soluble → soluble. - Silver chloride: an exception → insoluble (white solid). - Lead(II) nitrate: all nitrates are soluble → soluble.

Why solubility matters for preparing salts. Solubility decides the method. A soluble salt is made by reacting an acid with a suitable base, metal or carbonate, then crystallising the salt from its solution. An insoluble salt is made by precipitation : mixing two solutions that each contain one of its ions, so the insoluble salt forms as a solid that can be filtered off. For example, mixing barium chloride solution with sodium sulfate solution gives a white precipitate of barium sulfate.

Hazard note. Solubility also affects safety. Soluble barium and lead compounds are toxic because they can be absorbed by the body, whereas very insoluble barium sulfate passes through the body almost unchanged.

Step-by-step reasoning

To predict solubility:

1. Is the cation Na⁺, K⁺ or NH₄⁺? If yes, soluble. 2. Is the anion NO₃⁻? If yes, soluble. 3. Is it a chloride or sulfate? Soluble unless it is an exception. 4. Is it a carbonate or hydroxide? Insoluble unless step 1 applied.

Visual explanation

Imagine two test tubes. In one, blue copper(II) sulfate crystals dissolve to give a clear blue solution: the ions spread out among water molecules. In the other, a clear barium chloride solution meets a clear sodium sulfate solution and a thick white cloud of barium sulfate forms instantly, then settles to the bottom.

Real-world analogy

Solubility rules are like a club's guest list. Sodium, potassium, ammonium and nitrate are "always admitted" to the solution. Chlorides and sulfates usually get in, apart from a few named exceptions. Carbonates and hydroxides are usually turned away unless they arrive with one of the always-admitted ions.

Real-world example

Hard water contains dissolved calcium and magnesium ions. When it is heated, insoluble calcium carbonate forms and builds up as limescale in kettles and pipes. Hospitals use insoluble barium sulfate as a "barium meal" to show the digestive system on X-rays, safe precisely because it does not dissolve.

Why?

Why are some salts insoluble? Dissolving requires water molecules to pull ions out of the lattice. In some salts, such as barium sulfate, the ions attract each other so strongly that water cannot separate them enough, so very little dissolves.

Common misconception

"Insoluble means none dissolves at all." Every salt dissolves slightly; "insoluble" means the amount is too small to matter in most situations. Tiny dissolved amounts can still matter, for example with toxic ions.

Worked example

Question: Predict whether zinc nitrate, calcium carbonate and ammonium sulfate are soluble.

Reasoning: All nitrates are soluble. Calcium carbonate is a carbonate without Na⁺, K⁺ or NH₄⁺. Ammonium salts are all soluble.

Answer: Zinc nitrate soluble; calcium carbonate insoluble; ammonium sulfate soluble.

Quick check

1. Is silver nitrate soluble or insoluble in water? Answer: Soluble, because all nitrates are soluble.

Exam focus

Learn the rules and the key exceptions: silver and lead chlorides, and barium, lead and (slightly) calcium sulfates. Be ready to use solubility to choose a method for preparing a salt and to predict whether a precipitate forms when two solutions are mixed.

Advanced insight

Solubility depends on a balance between the energy needed to break up the ionic lattice and the energy released when ions are surrounded by water molecules (hydration). Small, highly charged ions form strong lattices, which is partly why many carbonates and hydroxides of 2+ and 3+ metal ions are insoluble. Solubility also usually changes with temperature.

Summary

All sodium, potassium, ammonium and nitrate salts are soluble. Most chlorides are soluble except silver and lead chlorides; most sulfates are soluble except barium and lead sulfates, with calcium sulfate slightly soluble. Most carbonates and hydroxides are insoluble except those of sodium, potassium and ammonium. Solubility decides whether a salt is made by crystallisation or precipitation.

Practice questions

1. State two types of salt that are always soluble in water. Answer: Any two of: sodium salts, potassium salts, ammonium salts, nitrates. 2. Name two insoluble chlorides. Answer: Silver chloride and lead(II) chloride. 3. Predict whether copper(II) carbonate is soluble, and explain. Answer: Insoluble, because most carbonates are insoluble and copper is not sodium, potassium or ammonium. 4. Explain why barium sulfate must be made by precipitation rather than by crystallisation from solution. Answer: It is insoluble, so it cannot stay in solution; it forms as a solid when solutions containing Ba²⁺ and SO₄²⁻ are mixed, and is then filtered off.