Solubility Rules: Videos & Practice Problems

Solubility is a chemical property describing the ability of a solute to dissolve in a solvent, forming a solution. Soluble compounds dissociate into aqueous ions in a solvent, while insoluble ones do not. Solubility rules, encapsulated by the mnemonic "gonna cash" and "cops," guide us in predicting whether an ionic compound is soluble or insoluble. Group 1A elements, acetate, nitrate, ammonium, chlorate, and perchlorate ions are always soluble, with no exceptions. Sulfates and halogens are generally soluble, except when paired with specific ions like calcium, barium, strontium, mercury, silver, or lead, forming precipitates. Conversely, "cops" represents insoluble compounds, such as carbonates, chromates, oxides, hydroxides, phosphates, and sulfides. Exceptions occur when these are combined with calcium, barium, or strontium, resulting in soluble compounds. Understanding these rules is crucial for predicting the outcome of chemical reactions and the formation of precipitates.

Solubility and Insolubility

Understanding solubility is essential in chemistry, as it describes the ability of a solute to dissolve in a solvent, forming a solution. The terms "soluble" and "insoluble" are crucial in this context. A solute is considered soluble if it can dissociate into aqueous ions when mixed with a solvent, such as water. For example, when aluminum bromide (AlBr₃) is added to water, it dissolves, breaking apart into its constituent ions: one aluminum ion (Al³⁺) and three bromide ions (Br^-). The aluminum ion, derived from group 3A of the periodic table, carries a +3 charge, while each bromide ion, from group 7A, carries a -1 charge. In solution, these ions are surrounded by water molecules, which stabilizes them in an aqueous state.

Conversely, a solute is classified as insoluble if it does not dissolve in a solvent. For instance, silver bromide (AgBr) is known to be insoluble; when introduced to water, it remains intact and does not dissociate into ions. This distinction between soluble and insoluble compounds is fundamental in predicting the behavior of substances in various chemical reactions and processes, particularly in aqueous environments.

Solubility and Ions Example

When considering the soluble compound sodium sulfate, represented by the formula Na₂SO₄, it is essential to determine how many ions it produces when dissolved in water. Sodium sulfate dissociates into its constituent ions, which include two sodium ions and one sulfate ion. The dissociation can be expressed as:

Na₂SO₄ (s) → 2 Na⁺ (aq) + SO₄^2- (aq)

Here, each sodium ion (Na⁺) carries a +1 charge, and the sulfate ion (SO₄^2-) is a polyatomic ion with a -2 charge. Since sodium is in Group 1A of the periodic table, it readily forms +1 ions. Therefore, when sodium sulfate dissolves, it produces a total of three ions: two sodium ions and one sulfate ion. This results in a final count of three ions in solution.

Solubility Rules

Solubility rules are essential guidelines that assist in predicting whether a compound will dissolve in water, which is crucial in various chemical reactions and processes. A helpful mnemonic to remember these rules is: "The bank robber was gonna cash his loot, but the cops stopped him." This phrase can be broken down to highlight key solubility principles.

In this mnemonic, "gonna" represents compounds that are generally soluble, while "cash" and "cops" refer to specific ions and compounds that influence solubility. For instance, compounds containing alkali metal ions (like sodium, potassium) and ammonium ions are typically soluble. Additionally, nitrates (NO₃^-) and acetates (C₂H₃O₂^-) are also soluble in water.

On the other hand, certain compounds are generally insoluble, which can be remembered through the "cops" part of the mnemonic. For example, most carbonates (CO₃^2-), phosphates (PO₄^3-), and sulfides (S_2-) are insoluble, except when paired with alkali metals or ammonium.

Understanding these solubility rules is vital for predicting the outcomes of chemical reactions, especially in aqueous solutions, and helps in determining whether a precipitate will form when two solutions are mixed.

Soluble Ionic Solutes

Ganache is a useful mnemonic for identifying soluble ionic compounds, particularly in the context of solubility rules. It highlights the general solubility of certain ions while also noting exceptions that lead to the formation of precipitates, which are insoluble solids. The acronym stands for various groups and ions that are typically soluble.

The first letter, G, represents Group 1A elements, which include hydrogen, lithium, sodium, potassium, and others. Compounds containing these elements are always soluble, with no exceptions. The next letter, A, stands for the acetate ion (C₂H₃O₂⁻), which is also soluble in ionic compounds. Following this, N denotes the nitrate ion (NO₃⁻), known for its universal solubility without exceptions.

The second A refers to the ammonium ion (NH₄⁺), which, like the previous ions, ensures solubility in compounds. The letter C signifies chlorate (ClO₃⁻) and perchlorate (ClO₄⁻), both of which are soluble without exceptions. The next letter, S, represents sulfate (SO₄²⁻) and halogens (Group 7A elements: fluorine, chlorine, bromine, and iodine), which generally are soluble but have specific exceptions.

For sulfates, the exceptions can be remembered using the acronym CBS HAP, indicating that when sulfate is combined with calcium (Ca²⁺), barium (Ba²⁺), strontium (Sr²⁺), or lead (Pb²⁺), it will form a precipitate. Similarly, halogens are soluble unless they are paired with mercury (Hg²⁺), silver (Ag⁺), or lead (Pb²⁺), which also results in a precipitate.

In summary, Ganache serves as a guide to determine the solubility of ionic compounds, helping to identify when exceptions lead to the formation of precipitates. Understanding these rules is essential for predicting the behavior of ionic compounds in various chemical reactions.

Soluble Ionic Solutes Example

Understanding solubility rules is essential for predicting whether ionic compounds will dissolve in water. According to these rules, certain ions confer solubility to compounds. For instance, any compound containing a Group 1A element, such as sodium, is automatically soluble. This means that sodium compounds do not require further analysis of other ions present, such as nitrate, which is also soluble on its own.

Similarly, compounds containing acetate ions are generally soluble. Therefore, calcium acetate, which includes the acetate ion, is soluble by default. However, when examining barium sulfate, we must consider the exceptions to the sulfate solubility rule. Sulfates are typically soluble, but there are exceptions, particularly with the ions represented by the acronym CBS (Calcium, Barium, and Strontium) and HAP (Lead, Silver, and Mercury). Since barium is part of this group, barium sulfate is insoluble and will form a precipitate.

In contrast, compounds containing ammonium ions or perchlorate ions are also soluble. Thus, when evaluating the solubility of the given ionic compounds, the only one that is insoluble and will form a precipitate is barium sulfate, making it the correct answer.

Solubility Rules Exceptions

In the study of solubility of ionic compounds, the acronym COPS is essential for identifying insoluble ionic solutes, while also acknowledging certain exceptions that lead to soluble aqueous ionic compounds. Understanding these exceptions is crucial, as soluble ionic compounds dissociate into ions in solution.

The components of COPS include:

• C for Carbonate (CO₃^2-) and Chromate (CrO₄^2-): These ions typically form precipitates, meaning they are insoluble in water.
• O for Oxides (O^2-) and Hydroxides (OH^-): Here, exceptions arise. When oxides or hydroxides are paired with calcium (Ca²⁺), barium (Ba²⁺), or strontium (Sr²⁺), they become soluble, forming aqueous compounds that can dissociate into ions.
• P for Phosphate (PO₄^3-): Phosphates generally do not have exceptions and will form precipitates.
• S for Sulfide (S^2-): Similar to hydroxides, sulfides also have exceptions. When sulfides are associated with calcium, barium, or strontium, they are soluble in water.

To summarize, while COPS provides a guideline for predicting the solubility of ionic compounds, it is important to remember the exceptions related to oxides and hydroxides, as well as sulfides, which can lead to the formation of soluble compounds when paired with specific alkaline earth metals. This understanding is vital for determining whether a compound will remain in solid form or dissolve in solution.

Solubility Rules Example

When determining the solubility of various substances in water, it is essential to refer to specific solubility rules and exceptions. For hydroxide ions (OH^-), they are generally insoluble unless paired with certain cations, specifically calcium (Ca²⁺), barium (Ba²⁺), or strontium (Sr²⁺). In this case, since aluminum (Al³⁺) is not one of these cations, hydroxide will remain insoluble.

Next, considering phosphate ions (PO₄^3-), they typically do not have exceptions that would make them soluble unless they are combined with alkali metals or ammonium. Since zinc (Zn²⁺) does not fall under these categories, the phosphate will also remain insoluble.

For silver (Ag⁺) combined with carbonate (CO₃^2-), the carbonate ion lacks exceptions that would allow for solubility, and silver is not part of the soluble group, leading to the conclusion that this combination will also be insoluble.

When examining sulfide ions (S^2-), they are generally insoluble except when paired with cations from the CBS group (calcium, barium, strontium). Since sulfide is connected to calcium in this case, it will be soluble.

Lastly, magnesium (Mg²⁺) with chromate (CrO₄^2-) also results in an insoluble compound, as chromate does not have exceptions for solubility, and magnesium is not part of the soluble group.

In summary, among the substances analyzed, only the combination of sulfide with calcium results in a soluble ionic solute, confirming that option D is the correct choice for solubility in water.

Based on your understanding of the solubility rules, which of the following ionic compounds will be insoluble? 

• A. Zinc chloride
• B. Manganese (V) chlorate
• C. Lead (II) sulfate
• D. Gallium acetate

Answer: (C)

Which pair of compounds is insoluble in water?

• A. PbSO₄ and Pb₃(PO₄)₂
• B. Na₂S and CuS
• C. (NH₄)₂SO₄ and AgI
• D. AgNO₃ and KNO₃

Answer: (A)

Here’s what students ask on this topic:

What are the solubility rules?

Solubility rules are guidelines that help predict whether a particular solute will dissolve in a solvent, especially in water. These rules are based on empirical observations and are useful in chemistry for determining the outcomes of double displacement reactions and for predicting the formation of precipitates. Here's a summary of the key solubility rules:

1. Nitrates (NO₃^-): All nitrates are soluble in water.
2. Alkali metal ions (Li⁺, Na⁺, K⁺, Cs⁺, Rb⁺) and Ammonium ion (NH₄⁺): Salts containing these ions are generally soluble.
3. Chlorides (Cl^-), Bromides (Br^-), and Iodides (I^-): Most of these salts are soluble, except those formed with silver (Ag⁺), lead (Pb₂⁺/Pb₄⁺), and mercury (Hg₂⁺).
4. Sulfates (SO₄^2-): Most sulfates are soluble, but those of calcium (Ca²⁺), strontium (Sr²⁺), barium (Ba²⁺), and lead (Pb₂⁺/Pb₄⁺) are exceptions and are poorly soluble.
5. Carbonates (CO₃^2-), Phosphates (PO₄^3-), Chromates (CrO₄^2-): These salts are generally insoluble, except when formed with alkali metals or ammonium ions.

How can I remember solubility rules?

Remembering solubility rules can be challenging, but there are strategies you can use to make it easier. One effective method is to create mnemonics, which are memory aids that help you recall larger pieces of information. For example, for the solubility of common salts, you can use the phrase 'NAG SAG' — Nitrates (NO₃^-), Acetates (CH₃COO^-), and Group 1 elements (Li⁺, Na⁺, K⁺, etc.) are Soluble Always Good. Similarly, 'CASTRO Bear' can remind you that Chlorides (Cl^-), Ammonium (NH₄⁺), Sulfates (SO₄^2-), and Thallium (Tl⁺) are generally soluble, except when paired with silver (Ag⁺), lead (Pb₂⁺/Pb₄⁺), or mercury (Hg₂⁺).

Another strategy is to create a solubility chart and practice it regularly. By repeatedly writing and reviewing the rules, you'll reinforce the information in your memory. Additionally, try to understand the underlying chemistry concepts, such as why certain ions tend to be soluble or insoluble in water, which can help you deduce the solubility of compounds that you might not have memorized. Practice problems can also help solidify your understanding and recall of solubility rules.

How can solubility rules be used?

Solubility rules are guidelines that help predict whether a particular solute will dissolve in a solvent, which is particularly useful in aqueous (water) solutions. These rules are based on empirical observations and can be used to determine the outcome of mixing different ionic compounds with water.

For example, if you are working in a chemistry lab and need to know if a reaction will produce a precipitate, solubility rules can guide you. They state that most nitrate (NO₃^-) salts are soluble, so if you mix a nitrate with another substance in water, you can expect it to dissolve. Conversely, most carbonates (CO₃^2-) are not soluble, except for those of alkali metals (like sodium and potassium), so a carbonate mixed with a non-alkali metal in water is likely to form a precipitate.

In practical terms, you would use the solubility rules to predict the products of a double displacement reaction, to understand the behavior of ions in a solution, or to design a chemical synthesis where the solubility of reactants and products is a critical factor.