Chemistry/Chapter 6: Salts
6.4 Soluble and Insoluble Salts
**Soluble Salts:** Soluble salts are salts that dissolve in water
Explanation level
Introduction
Not all salts dissolve in water. Some salts are soluble — they dissolve in water at room temperature. Others are insoluble — they do not dissolve in water at room temperature.
Understanding which salts are soluble and which are insoluble is important in chemistry. It helps chemists:
- Predict whether a precipitate will form when two solutions are mixed.
- Choose the correct method for preparing a salt.
- Separate and purify salts.
This section explains the general rules for the solubility of salts in water.
Definition
Soluble Salts: Soluble salts are salts that dissolve in water at room temperature.
Insoluble Salts: Insoluble salts are salts that do not dissolve in water at room temperature.
In simple words:
- Soluble salts = dissolve in water.
- Insoluble salts = do not dissolve in water.
Main Idea
General rules for the solubility of salts help predict the solubility of various ionic compounds in water. These rules are necessary to understand the reactions of aqueous solutions.
The general solubility rules are:
- All sodium, potassium, and ammonium salts are soluble in water.
- All nitrate salts are soluble in water.
- Most chloride salts are soluble in water, except lead(II) chloride and silver chloride.
- Most carbonate salts are insoluble in water, except sodium, potassium, and ammonium carbonates.
- Most common sulphates are soluble, except lead(II) sulphate, barium sulphate, and calcium sulphate.
Core Concept
| Rule | Statement |
|---|---|
| 1. Sodium, potassium, ammonium | All salts are soluble |
| 2. Nitrates | All salts are soluble |
| 3. Chlorides | Soluble, except lead(II) chloride and silver chloride |
| 4. Carbonates | Insoluble, except sodium, potassium, and ammonium carbonates |
| 5. Sulphates | Soluble, except lead(II) sulphate, barium sulphate, and calcium sulphate |
Key point: These rules help predict whether a salt will dissolve in water or form a precipitate.
Detailed Explanation
Rule 1: All Sodium, Potassium, and Ammonium Salts Are Soluble
Statement: All sodium, potassium, and ammonium salts are soluble in water.
Explanation: This includes common compounds such as:
- Sodium chloride (NaCl)
- Sodium sulphate (Na2SO4)
- Potassium chloride (KCl)
- Potassium bromide (KBr)
- Ammonium chloride (NH4Cl)
- Ammonium sulphate (NH4)2SO4
Key point: If a salt contains sodium, potassium, or ammonium ions, it is always soluble.
Rule 2: All Nitrate Salts Are Soluble
Statement: All nitrate salts are soluble in water.
Explanation: This rule applies to all nitrate compounds such as:
- Potassium nitrate (KNO3)
- Calcium nitrate Ca(NO3)2
Key point: If a salt contains nitrate ions (NO3-), it is always soluble.
Rule 3: Most Chloride Salts Are Soluble, Except Lead(II) Chloride and Silver Chloride
Statement: Most chloride (Cl-) salts are soluble in water, except:
- Lead(II) chloride (PbCl2)
- Silver chloride (AgCl)
Explanation: This means that chlorides like NaCl and KCl are soluble, but PbCl2 and AgCl are insoluble in water.
Key point: Chlorides are generally soluble, but lead(II) chloride and silver chloride are the exceptions.
Rule 4: Most Carbonate Salts Are Insoluble, Except Sodium, Potassium, and Ammonium Carbonates
Statement: Most carbonate salts (CO32-) are insoluble in water, except:
- Sodium carbonate (Na2CO3)
- Potassium carbonate (K2CO3)
- Ammonium carbonate (NH4)2CO3
Explanation: Examples of insoluble carbonates include:
- Calcium carbonate (CaCO3)
- Magnesium carbonate (MgCO3)
Key point: Carbonates are generally insoluble, but sodium, potassium, and ammonium carbonates are the exceptions.
Rule 5: Most Common Sulphates Are Soluble, Except Lead(II) Sulphate, Barium Sulphate, and Calcium Sulphate
Statement: Most common sulphates are soluble, except:
- Lead(II) sulphate (PbSO4)
- Barium sulphate (BaSO4)
- Calcium sulphate (CaSO4)
Explanation: This means that sulphates like Na2SO4 and CuSO4 are soluble, but PbSO4, BaSO4, and CaSO4 are insoluble (or only partially soluble, in the case of CaSO4).
Key point: Sulphates are generally soluble, but lead(II) sulphate, barium sulphate, and calcium sulphate are the exceptions.
Summary Table
| Salt Type | Solubility | Exceptions |
|---|---|---|
| Sodium, potassium, ammonium | Soluble | None |
| Nitrates | Soluble | None |
| Chlorides | Soluble | PbCl2, AgCl |
| Carbonates | Insoluble | Na2CO3\), \(K2CO3\), \((NH4)2CO3 |
| Sulphates | Soluble | PbSO4\), \(BaSO4\), \(CaSO4 |
Intermediate Level
Why solubility rules are important:
- They help predict whether a precipitate will form when two solutions are mixed.
- They help choose the correct method for preparing a salt.
- They help separate and purify salts.
Key rules:
| Salt Type | Solubility | Exceptions |
|---|---|---|
| Sodium, potassium, ammonium | Soluble | None |
| Nitrates | Soluble | None |
| Chlorides | Soluble | PbCl2, AgCl |
| Carbonates | Insoluble | Na2CO3\), \(K2CO3\), \((NH4)2CO3 |
| Sulphates | Soluble | PbSO4\), \(BaSO4\), \(CaSO4 |
Key point: These rules help predict whether a salt will dissolve in water or form a precipitate.
Solved Example
Example: Which of the Following Salts Are Insoluble?
Question: Which of the following salts are insoluble?
- Silver chloride (AgCl)
- Barium sulphate (BaSO4)
- Sodium nitrate (NaNO3)
- Ammonium carbonate (NH4)2CO3
- Lead nitrate (Pb(NO3)2)
Solution:
- AgCl → insoluble (exception to the chloride rule)
- BaSO4 → insoluble (exception to the sulphate rule)
- NaNO3 → soluble (sodium salt and nitrate salt)
- (NH4)2CO3 → soluble (ammonium salt and carbonate exception)
- Pb(NO3)2 → soluble (nitrate salt)
Final Answer: The insoluble salts are AgCl and BaSO4.
Concept Assessment Exercise 3.10 (from Section 3.10)
Which of the following salts are insoluble?
Silver chloride, barium sulphate, sodium nitrate, ammonium carbonate, and lead nitrate.
Solution:
- Silver chloride (AgCl) → insoluble
- Barium sulphate (BaSO4) → insoluble
- Sodium nitrate (NaNO3) → soluble
- Ammonium carbonate ((NH4)2CO3) → soluble
- Lead nitrate (Pb(NO3)2) → soluble
Final Answer: The insoluble salts are silver chloride and barium sulphate.
Important Points
- - Soluble salts = dissolve in water at room temperature.
- - Insoluble salts = do not dissolve in water at room temperature.
- - Rule 1: All sodium, potassium, and ammonium salts are soluble.
- - Rule 2: All nitrate salts are soluble.
- - Rule 3: Most chlorides are soluble, except lead(II) chloride and silver chloride.
- - Rule 4: Most carbonates are insoluble, except sodium, potassium, and ammonium carbonates.
- - Rule 5: Most sulphates are soluble, except lead(II) sulphate, barium sulphate, and calcium sulphate.
- - These rules help predict the solubility of salts in water.
Remember
- - Sodium, potassium, ammonium → soluble.
- - Nitrates → soluble.
- - Chlorides → soluble, except PbCl2 and AgCl.
- - Carbonates → insoluble, except Na2CO3, K2CO3, (NH4)2CO3.
- - Sulphates → soluble, except PbSO4, BaSO4, CaSO4.
- - Solubility rules help predict precipitate formation.
- - Solubility rules help choose the correct method for preparing a salt.
Common Mistakes
| Mistake | Correction |
|---|---|
| Thinking all chlorides are soluble | Lead(II) chloride and silver chloride are insoluble |
| Thinking all carbonates are insoluble | Sodium, potassium, and ammonium carbonates are soluble |
| Thinking all sulphates are soluble | Lead(II) sulphate, barium sulphate, and calcium sulphate are insoluble |
| Thinking all sodium salts are insoluble | All sodium salts are soluble |
| Thinking all nitrate salts are insoluble | All nitrate salts are soluble |
| Forgetting the exceptions to the solubility rules | Always remember the exceptions: PbCl2, AgCl, PbSO4, BaSO4, CaSO4 |
| Confusing soluble and insoluble salts | Soluble = dissolves in water; insoluble = does not dissolve in water |
Quick Revision
Soluble and Insoluble Salts:
Soluble salts are salts that dissolve in water at room temperature, and insoluble salts are salts that do not dissolve in water at room temperature.
General rules for the solubility of salts:
- All sodium, potassium, and ammonium salts are soluble in water. This includes common compounds such as sodium chloride (NaCl), sodium sulphate (Na2SO4), potassium chloride (KCl), potassium bromide (KBr), ammonium chloride (NH4Cl), ammonium sulphate (NH4)2SO4, etc.
- All nitrate salts are soluble in water. This rule applies to all nitrate compounds such as potassium nitrate (KNO3), calcium nitrate Ca(NO3)2, etc.
- Most chloride (Cl-) salts are soluble in water, except lead(II) chloride (PbCl2) and silver chloride (AgCl) are insoluble in water.
- Most carbonate salts (CO32-) are insoluble in water. Except sodium, potassium, and ammonium carbonates which are soluble. Examples of insoluble carbonates include calcium carbonate (CaCO3), magnesium carbonate (MgCO3), etc.
- Most common sulphates are soluble, except lead(II) sulphate, barium sulphate, and calcium sulphate.
- Concept Assessment Exercise:
- Which of the following salts are insoluble?
Silver chloride, barium sulphate, sodium nitrate, ammonium carbonate, and lead nitrate.
Solution:
- Silver chloride (AgCl) → insoluble
- Barium sulphate (BaSO4) → insoluble
- Sodium nitrate (NaNO3) → soluble
- Ammonium carbonate ((NH4)2CO3) → soluble
- Lead nitrate (Pb(NO3)2) → soluble
60-Second Revision
- Soluble = dissolves in water.
- Insoluble = does not dissolve in water.
- Sodium, potassium, ammonium → soluble.
- Nitrates → soluble.
- Chlorides → soluble, except PbCl2 and AgCl.
- Carbonates → insoluble, except Na2CO3, K2CO3, (NH4)2CO3.
- Sulphates → soluble, except PbSO4, BaSO4, CaSO4.
- Solubility rules help predict precipitate formation.
- Solubility rules help choose the correct method for preparing a salt.
- ⚠️ Warning: always remember the exceptions to the solubility rules.
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Quick revision
Revision Zone
6.4 Soluble and Insoluble Salts
Sticky Notes · Quick Revision
Key Point 1
- Soluble = dissolves in water.
Key Point 2
- Insoluble = does not dissolve in water.
Key Point 3
- Sodium, potassium, ammonium → soluble.
Key Point 4
- Nitrates → soluble.
Key Point 5
- Chlorides → soluble, except PbCl2 and AgCl.
Key Point 6
- Carbonates → insoluble, except Na2CO3, K2CO3, (NH4)2CO3.
Key Point 7
- Sulphates → soluble, except PbSO4, BaSO4, CaSO4.
Key Point 8
- Solubility rules help predict precipitate formation.
Key Point 9
- Solubility rules help choose the correct method for preparing a salt.
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