How to interpret a solubility table

The solubility table is one of the most useful tools in SPM Chemistry, yet many students never learn to read it properly. Once you can, you can instantly say whether any salt dissolves, predict when mixing two solutions gives a precipitate, and choose the right method for salt preparation and qualitative analysis. This guide teaches you how to read the table and, more importantly, how to use it.

What the table actually tells you

A solubility table summarises a set of general rules about which salts dissolve in water. It is usually organised by the anion (the negative ion, chloride, sulfate, carbonate and so on), with notes on the cations (positive ions) that are exceptions. Reading it is a matter of finding the anion, then checking whether your cation is one of the listed exceptions. The full set lives on our solubility rules reference; the core rules to hold in memory are:

• All nitrates are soluble.
• All sodium, potassium and ammonium salts are soluble.
• All chlorides are soluble, except silver chloride and lead(II) chloride.
• All sulfates are soluble, except barium, lead(II) and calcium sulfate.
• All carbonates are insoluble, except sodium, potassium and ammonium carbonate.
• All oxides and hydroxides are insoluble, except sodium, potassium and ammonium (calcium hydroxide is slightly soluble).

How to look up a single salt

Work in two steps. First find the anion rule, then check the cation exceptions.

• Potassium sulfate. The anion is sulfate, soluble except barium, lead(II), calcium. Potassium is not an exception, so potassium sulfate is soluble. (The potassium rule confirms it too.)
• Barium sulfate. Sulfate is soluble except barium, and barium is exactly the exception. So barium sulfate is insoluble.
• Silver chloride. Chloride is soluble except silver and lead(II). Silver is an exception, so silver chloride is insoluble.
• Copper(II) carbonate. Carbonates are insoluble except sodium, potassium, ammonium. Copper is not on that short list, so copper(II) carbonate is insoluble.

Notice the pattern: nitrates and the sodium/potassium/ammonium salts are always soluble, so if either ion is present, the salt dissolves without further checking. Those are your quickest wins.

Reading the “slightly soluble” cases

A good table marks a few salts as slightly soluble rather than a clean soluble or insoluble. The ones the syllabus expects you to know are calcium hydroxide (limewater is its dilute solution), calcium sulfate and lead(II) chloride (which dissolves in hot water but comes out again on cooling). Treat “slightly soluble” as a signal to read the question carefully, it often explains an observation, such as a faint cloudiness rather than a heavy precipitate.

Using the table to predict a precipitate

This is where the table earns its keep. When two solutions are mixed, the ions swap partners. A precipitate forms only if one of the new combinations is insoluble. So the method is: list the possible new salts, and check each against the table.

Example: mixing barium chloride and sodium sulfate. The ions present are Ba²⁺, Cl⁻, Na⁺, SO₄²⁻. The two new combinations are barium sulfate and sodium chloride. Sodium chloride is soluble, but barium sulfate is insoluble, so a white precipitate of barium sulfate forms. Sodium chloride stays dissolved.

Example: mixing sodium nitrate and potassium chloride. The new combinations are sodium chloride and potassium nitrate, both soluble. No precipitate forms; nothing appears to happen. The table tells you this before you ever touch a test tube. You can see many worked cases in the precipitation of insoluble salts reference.

Writing the ionic equation from what you found

Once the table tells you which salt is insoluble, the ionic equation follows immediately: keep only the ions that actually form the precipitate, and leave out the spectator ions that stay in solution.

• Barium sulfate: Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)
• Silver chloride: Ag⁺(aq) + Cl⁻(aq) → AgCl(s)
• Lead(II) iodide: Pb²⁺(aq) + 2I⁻(aq) → PbI₂(s)

The state symbol (s) confirms the solid you predicted from the table, and (aq) marks the spectator ions you correctly discarded.

Why this skill pays off across the paper

Reading the solubility table underpins three different exam skills at once. It decides whether a salt must be made by crystallisation or by precipitation, the core choice in the acids, bases and salts chapter. It predicts the observations in qualitative analysis, where adding a reagent and seeing (or not seeing) a precipitate identifies an unknown ion. And it lets you write correct ionic equations quickly in Paper 2. One table, three sets of marks.

Common mistakes when reading the table

• Forgetting the exceptions. “All sulfates are soluble” is only half the rule; barium, lead and calcium sulfate are not.
• Missing a precipitate because you did not swap partners. Always list both new combinations before deciding nothing happens.
• Treating slightly soluble as fully insoluble. Calcium hydroxide does dissolve a little, that is why limewater exists.
• Writing spectator ions into the ionic equation. Only the ions that form the solid belong there.

Turning the table into instinct

The fastest way to internalise the table is to test yourself: name ten random salts and classify each, then predict the result of five mixing experiments and check them against the rules. Look up any unfamiliar ion as you go. If the exceptions keep slipping, they are the part everyone forgets, a teacher can drill them against real qualitative-analysis questions until reading the table is instant. Our online one-to-one lessons run in English from RM50 an hour, with a paid one-hour trial, so the skill is built directly on the questions that trouble you most.