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Wize University Chemistry Textbook > Chemical Reactions

Solubility Rules and Precipitation

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Solubility Chart

The following table tells us if certain combinations of ions are soluble or insoluble!

Wize Concept
If a combination of ions is soluble, it means that no precipitate will form.
If a combination of ions is insoluble, it means a precipitate will form!

You can expect to see a question on your exam that asks you if a precipitate will form or not!

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Photo by OpenStax / CC BY

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Decide whether the following salts would form a precipate or not:

1) NaCl

Na+ is a group 1 ion. When these are part of a compound, it will always be soluble.
Therefore, no precipitate will form!
NaCl → Na+ + Cl-

2) AgCl

Cl- is typically soluble when in a compound, BUT when it is paired with Ag+, that is an exception so the combination of these ions will actually be insoluble.
Insoluble means they won't dissolve into their ions and so we will form a precipitate

3) AgI

I- is typically soluble when in a compound, BUT when it is paired with Ag+, that is an exception so the combination of these ions will actually be insoluble.
Insoluble means they won't dissolve into their ions and so we will form a precipitate

4) NaNO3

Na+ is a group 1 ion. All group 1 ions are always soluble
NO3 is also an ion that is always soluble.
This combination will not form a precipitate.
NaNO3 → Na+ + NO3-

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Example: Does a Precipitate Form?

In the following example, do you think a precipitate is formed? If yes, what is the formula of the precipitate?

Pb(NO3)2 + 2KI

1) Write out all ions involved
Pb2+, NO3-, K+, I-

2) Consider different combinations of the ions that were not already done:

Pb2+ & I-, NO3- & K+

3) Determine if each combination would be soluble or insoluble:

We know that I- is generally soluble, but Pb2+ is one of the exceptions so this compound is insoluble
For our second set of ions, anything with NO3- is soluble!

4) Since we do have a precipitate we must now give the formula for the precipitate:
Pb2+ and I-
PbI2

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Write the complete chemical reaction:

Pb(NO3)2
(aq)
+ 2KI
(aq)
→
PbI2(s)
+
2KNO3(aq)


Wize Tip
In a chemical reaction, (aq) means that the substance is dissolved in aqueous solution (aka it is soluble),
whereas (s) means that the substance is in solid form and is insoluble in solution (aka forms a precipitate)
This reaction can be referred to as a precipitation reaction.

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Example: Salt Solubility Practice

1 a) Saturated solutions of Na2S, CuS, SnS2, and Al2S3 are prepared in a lab at 25oC. Which salt is the most soluble in water?

a) Na2S

b) CuS

c) SnS2

d) Al2S3

Think of the salt solubility rules. Rule #1 is any group 1 cation will be soluble.
a) has Na+ (a group 1 ion)
b) Cu and S, we know S has a charge of -2, since we see no charges, Cu must have a charge of 2+
c) Sn has a charge of 4+ here (S has 2-, divide both charges by 2 to get the compound as written in c))
d) Al3+ and S2-

Therefore Na+ is the only 1+ ion so a) will have the highest solubility!

1 b) Which of the options above would produce the highest [S2-]?

This question is asking us the same thing a) did.
If we have the highest [S2-], it means that the salt was most soluble!
Therefore a) is your answer.

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2) Which of the following salts has the lowest solubility?

a) copper (I) perchlorate

b) ammonium sulphide

c) potassium hydroxide

d) mercury(II) sulphate


a) Perchlorate (ClO4-) with any compound is always soluble.
b) Ammonium is NH4+. This compound with anything is always soluble.
c) K+ is a group 1 ion. This salt is soluble.
d) Sulphate (SO42-) is generally soluble but it has a lot of exceptions. Hg2+ is one of them and so the compound is insoluble.

The question is asking us which would have the lowest solubility, so we would choose the insoluble option: d)

Practice: Dissociation

What ions does Na2CO3 dissociate into in water?
Extra Practice
Practice: Ranking Solubilities
You have a series of beakers, each containing 100 mL of a different solution. If you added nickel (II) hydroxide (Ni(OH)2) to each of the beakers, rank them in order of the relative solubility of the Ni(OH)2 in each:
𝑁𝑖(𝑂𝐻)2(𝑠)⇌𝑁𝑖2+(𝑎𝑞)+2𝑂𝐻−(𝑎𝑞)𝐾𝑠𝑝=6.0×10−16𝑁𝑖(𝑂𝐻)_2 (𝑠) ⇌𝑁𝑖^{2+} (𝑎𝑞)+2𝑂𝐻^− (𝑎𝑞) \qquad \qquad 𝐾_{𝑠𝑝}=6.0×10^{−16}
Additional Data:
A student adds some CuBr (s) (Ksp = 6.3x10−9) to each of the flasks shown here.
Assume that any salts in each flask were fully dissociated before CuBr(s) was added.
Additional data:
Solubility
Which of the following compounds is soluble in water at 25 °C?
Solubility: Dissociation of ions
What ions does Na2CO3 dissociate into in water?
Solubility
Which of the following compounds is soluble in water at 25 °C?
Solubility
Which of the following correctly describes what happens when aqueous solutions of ammonium carbonate, (NH4)2CO3, and potassium bromide, KBr, are mixed?
Does a precipitate form if Pb(NO3)2 is mixed with NaBr?
Solubility
Which of the following correctly describes what happens when aqueous solutions of ammonium carbonate, (NH4)2CO3, and potassium bromide, KBr, are mixed?
Solubility
If we mixed KCl(aq) and Pb(ClO3)2(aq) would a precipitate form? If yes, name the precipitate.
Enter the answer as either no or yes, chemical formula of precipitate
Solubility
If we mixed 2AgNO3(aq) and MgBr2 would a precipitate form? If yes, what would the precipitate be?
Enter answer as either no or yes, chemical formula of precipitate.
Does a precipitate form if Pb(NO3)2 is mixed with NaBr?
Enter no or yes, name of formula of precipitate
For PbCl2 (Ksp = 2.4 x 10–4), will a precipitate of PbCl2 form when 0.10 L of 3.0 x 10-2 M Pb(NO3)2 is added to 400 mL of 9.0 x 10-2 M NaCl?
If 4.5g of NaCl are added to a 2.0 L saturated solution of AgCl, what mass of AgCl(s) will precipitate? Ksp for AgCl is 1.7 x 10-17
Solubility: Rules and mechanisms
You have a series of beakers, each containing 100 mL of a different solution. If you added nickel (II) hydroxide (Ni(OH)2) to each of the beakers, rank them in order of the relative solubility of the Ni(OH)2 in each:
𝑁𝑖(𝑂𝐻)2(𝑠)⇌𝑁𝑖2+(𝑎𝑞)+2𝑂𝐻−(𝑎𝑞)𝐾𝑠𝑝=6.0×10−16𝑁𝑖(𝑂𝐻)_2 (𝑠) ⇌𝑁𝑖^{2+} (𝑎𝑞)+2𝑂𝐻^− (𝑎𝑞) \qquad \qquad 𝐾_{𝑠𝑝}=6.0×10^{−16}
Additional Data:
Solubility: Will a Precipitate Form?
The Ksp for strontium fluoride SrF2 is 4.3x10−9. If 5.0 x10−6 mol of NaF (s) are added to 10.0 mL of 1.5x10−3 M Sr(NO3)2, will a precipitate form? Assume that any sodium salts or nitrate salts fully dissociate in water.
The Ksp for CuBr is 6.3x10−9. When 1.00 mL of 0.0010 M CuNO3 is mixed with 1.00 mL of 0.0010 M NaBr, will a precipitate form? Assume that any sodium salts or nitrate salts fully dissociate in water.
Which of the following would decrease the Ksp for PbI2?
For PbCl2 (Ksp = 2.4 x 10–4), will a precipitate of PbCl2 form when 0.10 L of 3.0 x 10-2 M Pb(NO3)2 is added to 400 mL of 9.0 x 10-2 M NaCl?
A student adds some CuBr (s) (Ksp = 6.3x10−9) to each of the flasks shown here.
Assume that any salts in each flask were fully dissociated before CuBr(s) was added.
Additional data:
Solubility: Practice
A student is preparing to test the molar solubility of PbHPO4 (s) (Ksp=3.5×10−12) in different aqueous solutions. When this salt dissociates, it produces Pb2+ (aq) and the amphiprotic ion HPO42− (aq). They prepare separate beakers with 100 mL of each of the following solutions to test:
water
0.1 M Pb(NO3)2 (aq) (which fully dissociates in water)
Molecular bonding: Physical Properties
Consider two liquids: pentane, C5H12, and a light mineral oil, C12H26.
a. In the table below identify which liquid is being described.
b. A student observed that a drop of water spreads on a glass surface while on a paraffin coated glass surface it beads. The student’s explanation for this is: “The interaction between water and paraffin is strong because it forms a bead.” Is the student correct or incorrect? Explain
Solubility and Equilibrium
Does a precipitate of PbBr2 form if 50.0 mL of 0.0100 M Pb(NO3)2 is mixed with 50.0 mL of 0.100 M NaBr? (Ksp PbBr2 = 2.1 x 10-6)
Solubility
A student is preparing to test the molar solubility of PbHPO4 (s) (Ksp=3.5×10−12) in different aqueous solutions. When this salt dissociates, it produces Pb2+ (aq) and the amphiprotic ion HPO42− (aq).They prepare separate beakers with 100 mL of each of the following solutions to test:
water
0.1 M Pb(NO3)2 (aq) (which fully dissociates in water)