Solubility Rules Chemistry Tutorial

Key Concepts

Solubility rules are qualitative rules to determine whether an ionic compound will, or will not, dissolve in water at 25°C. ¹

An ionic compound that does dissolve in water is said to be soluble.²
The result is an aqueous solution.
Aqueous solution of a salt is indicated by placing the letters aq in round brackets, (aq), to the right of the formula for the ionic compound.

An ionic substance that does not dissolve in water is said to be insoluble.³
The result is a precipitate, an insoluble solid.
Precipitate of a salt is indicated by placing the letter s in round brackets, (s), to the right of the formula for the ionic compound.

Solubility Rules can be given as a list, a table, or a chart.

Solubility Rules can be used to decide if a precipitate (an insoluble substance) will form from an aqueous solution at 25°C:

List the anions and cations in the solution

List the possible ionic compounds that could be produced

Use the solubility rules (list, table or chart) to decide if either of the ionic compounds are insoluble and will therefore form a precipitate.

Solubility Rules as a List

The solubility of ionic compounds in water at 25°C, in general:

All compounds of the ammonium ion (NH₄⁺) are soluble.

All compounds of Alkali metal (Group 1, or, Group IA) cations, are soluble.

All nitrates are soluble.

All acetates (ethanoates) are soluble.

All chlorides, bromides and iodides are soluble

EXCEPT those of silver, lead and mercury(I)

All sulfates (sulphates) are soluble

EXCEPT those of silver, lead, mercury(I), barium, strontium and calcium

All carbonates, sulfites (sulphites) and phosphates are insoluble

EXCEPT those of ammonium (NH₄⁺) and Alkali metal (Group 1, or, Group IA) cations.

All hydroxides are insoluble

EXCEPT those of ammonium (NH₄⁺), barium and alkali metal (Group 1, or, Group IA) cations.

All sulfides are insoluble

EXCEPT those of ammonium, Alkali metal (Group 1, or, Group IA) cations and Alkali earth metal (Group 2, or, Group IIA) cations.

All oxides are insoluble

EXCEPT those of calcium, barium and Alkali metal (Group 1, or, Group IA) cations; these soluble ones actually react with the water to form hydroxides (they are said to hydrolyse, and the reaction with water is known as a hydrolysis reaction ).

Worked Example Using the List of Solubility Rules

Question: A student adds an aqueous solution of silver nitrate, AgNO_3(aq), to dilute hydrochloric acid, HCl_(aq), at 25°C.

Will a precipitate form?

Response:

List the anions and cations in the solution

		ions from AgNO_3(aq)
		silver ion Ag⁺_(aq)	nitrate ion NO₃^-_(aq)
ions from HCl_(aq)	hydrogen ion H⁺_(aq)
ions from HCl_(aq)	chloride ion Cl^-_(aq)

List the possible ionic compounds that could be produced (remember that an ionic compound is composed of anions and cations so there will be 2 possibilities)

		ions from AgNO_3(aq)
		silver ion Ag⁺_(aq)	nitrate ion NO₃^-_(aq)
ions from HCl_(aq)	hydrogen ion H⁺_(aq)	X	hydrogen nitrate (nitric acid) HNO₃
ions from HCl_(aq)	chloride ion Cl^-_(aq)	silver chloride AgCl	X

Use the solubility rules listed to decide if either of the ionic compounds are insoluble and will therefore form a precipitate.

(i) All nitrates are soluble, so hydrogen nitrate (nitric acid) is soluble and will not form a precipitate, HNO₃_(aq).

(ii) All chlorides are soluble EXCEPT those of silver, lead and mercury(I), so silver chloride is insoluble and will form a precipitate, AgCl_(s)

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Solubility Rules as a Table

If you need to memorise the solubility rules for ionic compounds in water at 25°C, then the list above is useful.
However, all that information can be placed in a table as shown below, which makes it easier to locate solubility information for a particular ionic compound made up of a particular cation and a particular anion.
This table is designed to answer the question,

"If I have a compound made up of this cation and this anion, will it dissolve in water at 25°C to form an aqueous solution, or will it form a solid (precipitate)?"

Solubility Rules for Aqueous Solutions at 25°C
Negative Ions (Anions)	+	Positive Ions (Cations)	=	Solubility of compounds in water	Example
any anion	+	alkali ions (Li⁺,Na⁺,K⁺,Rb⁺,Cs⁺,Fr⁺)	=	soluble	sodium fluoride is soluble, NaF_(aq)
any anion	+	hydrogen ion [H⁺_(aq)]	=	soluble	hydrogen chloride is soluble, HCl_(aq)
any anion	+	ammonium ion (NH₄⁺)	=	soluble	ammonium chloride is soluble, NH₄Cl_(aq)
nitrate (NO₃^-)	+	any cation	=	soluble	potassium nitrate is soluble, KNO₃_(aq)
acetate (CH₃COO^-)	+	any cation	=	soluble	sodium acetate is soluble, CH₃COONa_(aq)
chloride (Cl^-) bromide (Br^-) iodide (I^-)	+	silver (Ag⁺) lead (Pb²⁺) mercury (Hg₂²⁺) copper (Cu⁺) thallium (Tl⁺)	=	low solubility (insoluble)	silver chloride forms a white precipitate (a white solid), AgCl_(s)
chloride (Cl^-) bromide (Br^-) iodide (I^-)	+	any other cation	=	soluble	potassium bromide is soluble, KBr_(aq)
sulfate (SO₄^2-)	+	calcium (Ca²⁺) strontium (Sr²⁺) barium (Ba²⁺) silver (Ag⁺) lead (Pb²⁺) radium (Ra²⁺)	=	low solubility (insoluble)	barium sulfate forms a white precipitate (a white solid), BaSO₄_(s)

	+	any other cation	=	soluble	copper(II) sulfate is soluble, CuSO₄_(aq)
sulfide (S^2-)	+	alkali metal ions (Li⁺,Na⁺,K⁺,Rb⁺,Cs⁺,Fr⁺) alkali earth metal ions (Be²⁺,Mg²⁺,Ca²⁺,Sr²⁺,Ba²⁺,Ra²⁺) and hydrogen ion (H⁺_(aq)) and ammonium ion (NH₄⁺)	=	soluble	magnesium sulfide is soluble, MgS_(aq)
sulfide (S^2-)	+	any other cation	=	low solubility (insoluble)	zinc sulfide is insoluble, ZnS_(s)
Hydroxide (OH^-)	+	alkali metal ions (Li⁺,Na⁺,K⁺,Rb⁺,Cs⁺,Fr⁺) hydrogen ion (H⁺_(aq)) ammonium ion (NH₄⁺) strontium ion (Sr²⁺) barium ion (Ba²⁺) radium ion (Ra²⁺) thallium ion (Tl⁺)	=	soluble	strontium hydroxide is soluble, Sr(OH)₂_(aq)

	+	any other cation	=	low solubility (insoluble)	silver hydroxide is insoluble (forms a precipitate), AgOH_(s)
phosphate (PO₄^3-) carbonate (CO₃^2-) sulfite (SO₃^2-)	+	alkali metal ions (Li⁺,Na⁺,K⁺,Rb⁺,Cs⁺,Fr⁺) hydrogen ions (H⁺_(aq)) ammonium ions (NH₄⁺)	=	soluble	ammonium phosphate is soluble, (NH₄)₃PO₄_(aq)
phosphate (PO₄^3-) carbonate (CO₃^2-) sulfite (SO₃^2-)	+	any other cation	=	low solubility (insoluble)	magnesium carbonate is insoluble, MgCO₃_(s)

Worked Example Using the Solubility Rules Table

Question: A student adds an aqueous solution of barium nitrate, Ba(NO₃)_2(aq), to dilute sulfuric acid, H₂SO_4(aq), at 25°C.

Will a precipitate form?

Response:

List the anions and cations in the solution

		ions from Ba(NO₃)_2(aq)
		barium ion Ba²⁺_(aq)	nitrate ion NO₃^-_(aq)
ions from H₂SO_4(aq)	hydrogen ion H⁺_(aq)
ions from H₂SO_4(aq)	sulfate ion SO₄^2-_(aq)

List the possible ionic compounds that could be produced (remember that an ionic compound is composed of anions and cations so there will be 2 possibilities)

		ions from Ba(NO₃)_2(aq)
		barium ion Ba²⁺_(aq)	nitrate ion NO₃^-_(aq)
ions from H₂SO_4(aq)	hydrogen ion H⁺_(aq)	X	hydrogen nitrate (nitric acid) HNO₃
ions from H₂SO_4(aq)	sulfate ion SO₄^2-_(aq)	barium sulfate BaSO₄	X

Use the solubility rules in the table to decide if either of the ionic compounds are insoluble and will therefore form a precipitate.

(i) Go to the row labelled "nitrates (NO₃^-)" under the heading "Negative Ions (anions)", fourth from the top of the table).
Go across the row to the column for "Positive Ions (Cations)".
Read, "nitrate (NO₃^-) + any cation = soluble compound"
So hydrogen nitrate (nitric acid) is soluble and will not form a precipitate, HNO₃_(aq).

(ii) Go to the row labelled "sulfate (SO₄^2-)" under the heading "Negative Ions (anions)", seventh from the top of the table).
Go across the row to the column for "Positive Ions (Cations)".
Read, "sulfate (SO₄^2-) + barium (Ba²⁺) = insoluble compound"
So barium sulfate is insoluble and will form a precipitate, BaSO₄_(s) .

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Solubility Rules as a Chart

The solubility rules for aqueous solutions at 25°C can also be used to construct a different sort of table which we will call a solubility chart.

The solubility chart below is designed to answer the question,

"Will an ionic compound made up of this cation and this anion dissolve in water at 25°C?"

The following key is used:

yes = The compound is soluble, it will all dissolve in water at 25°C

no = The compound is insoluble, there is no appreciable concentration of ions in the water at 25°C, the solid compound will precipitate out of solution.

slightly = A very small amount of the compound will dissolve in water at 25°C, that is, there is a very, very, small concentration of ions in solution.

... = no data available

reacts = The compound does not dissolve in water (which is a physical change), instead it reacts with water (which is a chemical change)

anions → cations ↓	chloride Cl^-	bromide Br^-	iodide I^-	oxide O^2-	sulfide S^2-	hydroxide OH^-	carbonate CO₃^2-	chromate CrO₄^2-	sulfate SO₄^2-	acetate CH₃COO^-	nitrate NO₃^-
ammonium NH₄⁺	yes	yes	yes	...	yes	yes	yes	yes	yes	yes	yes
sodium ion Na⁺	yes	yes	yes	reacts	yes	yes	yes	yes	yes	yes	yes
potassium ion K⁺	yes	yes	yes	reacts	yes	yes	yes	yes	yes	yes	yes
magnesium ion Mg²⁺	yes	yes	yes	no	reacts	no	slightly	...	yes	yes	yes
calcium ion Ca²⁺	yes	yes	yes	reacts	reacts	slightly	no	slightly	slightly	yes	yes
barium ion Ba²⁺	yes	yes	yes	yes	yes	yes	no	no	no	yes	yes
manganese(II) ion Mn²⁺	yes	yes	yes	no	no	no	no	...	yes	yes	yes
iron(II) ion Fe²⁺	yes	yes	yes	no	no	no	no	...	yes	yes	yes
iron(III) ion Fe³⁺	yes	yes	...	no	...	no	...	no	yes	no	yes
copper(II) ion Cu²⁺	yes	yes	...	no	no	no	no	no	yes	yes	yes
nickel(II) ion Ni²⁺	yes	yes	yes	no	no	no	no	no	yes	yes	yes
cadmium(II) ion Cd²⁺	yes	yes	yes	no	no	no	no	no	yes	yes	yes
zinc ion Zn²⁺	yes	yes	yes	no	no	no	no	yes	yes	yes	yes
tin(II) ion Sn²⁺	yes	yes	slightly	no	no	no	...	...	yes	yes	...
mercury(I) ion Hg₂²⁺	yes	slightly	no	no	no	no	no	no	reacts	yes	yes
lead(II) ion Pb²⁺	slightly	slightly	no	no	no	no	no	no	no	yes	yes
silver ion Ag⁺	no	no	no	no	no	...	no	no	slightly	slightly	yes
cations ↑ anions →	chloride Cl^-	bromide Br^-	iodide I^-	oxide O^2-	sulfide S^2-	hydroxide OH^-	carbonate CO₃^2-	chromate CrO₄^2-	sulfate SO₄^2-	acetate CH₃COO^-	nitrate NO₃^-

Worked Example Using the Solubility Rules Chart

Question: A student adds an aqueous solution of lead(II) nitrate, Pb(NO₃)_2(aq), to an aqueous solution of sodium iodide, NaI_(aq), at 25°C.

Will a precipitate form?

Response:

List the anions and cations in the solution

		ions from Pb(NO₃)_2(aq)
		lead(II) ion Pb²⁺_(aq)	nitrate ion NO₃^-_(aq)
ions from NaI_(aq)	sodium ion Na⁺_(aq)
ions from NaI_(aq)	iodide ion I^-_(aq)

List the possible ionic compounds that could be produced (remember that an ionic compound is composed of anions and cations so there will be 2 possibilities)

		ions from Pb(NO₃)_2(aq)
		lead(II) ion Pb²⁺_(aq)	nitrate ion NO₃^-_(aq)
ions from NaI_(aq)	sodium ion Na⁺_(aq)	X	sodium nitrate NaNO₃
ions from NaI_(aq)	iodide ion I^-_(aq)	lead(II) iodide PbI₂	X

Use the solubility rules in the chart to decide if either of the ionic compounds are insoluble and will therefore form a precipitate.

(i) Go to the row labelled "sodium ion Na⁺" under the heading "cations", (second from the top of the table).
Go across the row of "anions" and look at the last column labelled "nitrate NO₃^-".
Read, "yes"
So sodium nitrate is soluble and will not form a precipitate, NaNO₃_(aq).

(ii) Go to the row labelled "lead(II) ion Pb²⁺" under the heading "cations", (second from the bottom of the table).
Go across the row of "anions" to the third column labelled "iodide I^-".
Read, "no"
So lead(II) iodide is insoluble and will form a precipitate, PbI₂_(s) .

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1. Solubility Tables (tables of solubility) give a quantitative measure of how much solute dissolves in water at 25°C.

Solubility Curves (graphs) give a quantitative measure of how much solute will dissolve in water at different temperatures.

2. A solute is said to be soluble if its solubility is greater than 1 g/100 g of water (1 g/100 mL water at 25°C)

3. A solute is said to be insoluble if its solubility is less than 0.1 g/100 g of water (0.1 g/100 mL water at 25°C)

A solute is said to be slightly soluble, or sparingly soluble, if its solubility is between 0.1 g/100 g of water and 1.0 g/100 g (between 0.1 g/100 mL water and 1.0 g/100 mL at 25°C)