Solubility Rules Checker

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How to Use the Solubility Rules Checker

1. 1. Pick a mode. “Is a compound soluble?” takes one formula. “Does mixing two solutions make a precipitate?” takes two. “Ksp and molar solubility” takes a formula and, optionally, your own Ksp.
2. 2. Type formulas the usual way. BaSO4, Pb(NO3)2 and (NH4)3PO4 all work, as do subscripts like Ca₃(PO₄)₂ and hydrates such as CaSO4·2H2O. The tool splits each formula into its cation and anion and names the rule it used.
3. 3. Read the verdict and the exceptions. The result says soluble, slightly soluble, insoluble, or reacts with water, and says when the compound is one of the rule's named exceptions.
4. 4. For mixtures, check the quantities. With concentrations and volumes entered, the tool dilutes the ions on mixing and compares the ion product Q with Ksp. Clear those boxes to see the equations only.

The Solubility Rules

In general chemistry a compound is called soluble when roughly 0.1 mol/L or more dissolves, and insoluble when a precipitate forms at ordinary concentrations. Every insoluble salt still dissolves a little; its Ksp says how much. The rules are generalisations, so the borderline cases are listed as “slightly soluble”.

Predicting a Precipitate

When two soluble salt solutions are mixed, the ions can swap partners. Write AX + BY as AY + BX, check each new compound against the rules, and if either is insoluble that is the precipitate. The other product, if soluble, stays as ions.

For lead(II) nitrate and potassium iodide, the products are PbI₂ (insoluble, from the halide rule) and KNO₃ (soluble). The tool writes three equations:

• Molecular: Pb(NO₃)₂(aq) + 2 KI(aq) → PbI₂(s) + 2 KNO₃(aq)
• Complete ionic: Pb²⁺(aq) + 2 NO₃⁻(aq) + 2 K⁺(aq) + 2 I⁻(aq) → 2 K⁺(aq) + 2 NO₃⁻(aq) + PbI₂(s)
• Net ionic: Pb²⁺(aq) + 2 I⁻(aq) → PbI₂(s)

The K⁺ and NO₃⁻ ions appear unchanged on both sides, so they are spectator ions and drop out of the net ionic equation, leaving only the species that actually react. Mixing 50 mL of 0.10 M of each solution gives 0.050 M of each of Pb²⁺ and I⁻ after dilution, so Q = 0.050 × 0.050² = 1.3 × 10⁻⁴, which is far above the Ksp of PbI₂ (8.7 × 10⁻⁹): the precipitate forms.

Ksp and Molar Solubility

For a salt AₘBₙ, the solubility product is Ksp = [A]ᵐ[B]ⁿ, and if x mol/L dissolves, [A] = m·x and [B] = n·x, so Ksp = mᵐ·nⁿ·x^(m+n). Solving for the molar solubility gives x = (Ksp ÷ (mᵐ·nⁿ))^(1/(m+n)).

• AgCl (1:1): x = √(1.8 × 10⁻¹⁰) = 1.3 × 10⁻⁵ mol/L, or 0.0019 g/L.
• Ag₂CrO₄ (2:1): x = (9.0 × 10⁻¹² ÷ 4)^(1/3) = 1.3 × 10⁻⁴ mol/L. Its Ksp is smaller than AgCl's, yet more of it dissolves, because the two salts have different ion ratios. Compare Ksp values directly only for salts of the same formula type.
• Common ion: in 0.10 M NaCl, AgCl solves (x)(x + 0.10) = 1.8 × 10⁻¹⁰, so x = 1.8 × 10⁻⁹ mol/L, about 7,500 times lower. The tool solves this exactly instead of assuming x is negligible.

Limits of the Rules

• The rules are binary; solubility is not. Slightly soluble compounds may or may not precipitate depending on concentration, which is what the Q check is for.
• Temperature matters. The Ksp values are for 25 °C, and PbCl₂ dissolves far more in hot water than in cold.
• Acid, base and complexing agents change the picture. Hydroxides, carbonates and sulfides dissolve in acid, and AgCl dissolves in ammonia because Ag⁺ forms Ag(NH₃)₂⁺. The rules assume plain water.
• Ksp tables disagree. Handbooks differ, sometimes by a factor of two or more. This tool uses one stated table and lets you type your own value.
• Not every compound is covered. The tool recognises salts of common ions. A complex, an organic salt or a mixed-valence compound such as Fe₃O₄ is reported as not recognised.

Frequently Asked Questions

What are the solubility rules?

The main rules are that all Group 1 and ammonium salts are soluble, as are all nitrates and acetates; chlorides, bromides and iodides are soluble except with silver, lead(II) and mercury(I); sulfates are soluble except with barium, strontium and lead; and hydroxides, carbonates, phosphates and sulfides are insoluble except with Group 1 and ammonium (plus Ba(OH)₂ and Sr(OH)₂).

How do you tell if a precipitate will form when two solutions are mixed?

Swap the cations and anions to get the two possible products, then check each against the solubility rules. If either is insoluble, it precipitates. For a numerical answer, calculate the ion product Q after mixing and compare it with Ksp: Q greater than Ksp means a precipitate forms.

What is a net ionic equation and what are spectator ions?

A net ionic equation shows only the species that change. Write the complete ionic equation with every dissolved ionic compound split into ions, then cancel the ions that appear unchanged on both sides. Those cancelled ions, such as K⁺ and NO₃⁻ in the lead iodide example, are the spectator ions.

Is lead(II) chloride soluble?

Slightly. PbCl₂ has a Ksp of 1.7 × 10⁻⁵, which gives a solubility of about 0.016 mol/L at 25 °C, and it dissolves much more in hot water. Many tables list it as an exception to the chloride rule, which is why this tool marks it slightly soluble rather than soluble or insoluble.

How do you calculate molar solubility from Ksp?

Write the Ksp expression for the dissolving equation, substitute m·x and n·x for the ion concentrations, and solve for x. For a 1:1 salt that is x = √Ksp; for a 2:1 or 1:2 salt, x = (Ksp ÷ 4)^(1/3). Multiply by the molar mass to convert to grams per litre.

Does a smaller Ksp always mean a less soluble salt?

Only when the salts give the same number of ions. Ag₂CrO₄, with Ksp 9.0 × 10⁻¹², dissolves to 1.3 × 10⁻⁴ mol/L, which is more than AgCl's 1.3 × 10⁻⁵ mol/L at Ksp 1.8 × 10⁻¹⁰, because the exponent in the solution depends on the ion ratio.

What is the common ion effect?

Adding an ion that the salt also produces pushes the dissolving equilibrium back toward the solid, so less salt dissolves. AgCl is about 7,500 times less soluble in 0.10 M chloride than in pure water. The effect is strongest for sparingly soluble salts.