Solubility Rules - Online Tutor, Practice Problems & Exam Prep

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 Rules

As we begin to talk about the solubility rules, we first have to familiarize ourselves with the term solubility. Now solubility is just a chemical property that deals with the ability of a solute to become dissolved in a solvent. Remember, solutes get dissolved in solvents to create solutions. Now with the term solubility, we're introduced to two other terms, soluble versus insoluble.

Now we're going to say soluble refers to a solute that can be dissolved into aqueous ions when placed in a solvent. Here we have aluminum bromide solid. We're going to throw it into water. We're saying that it's soluble, so it's going to break up into its ions. Aluminum bromide possesses 1 aluminum, so it's going to break up into one aluminum. Aluminum is in Group 3A, so its charge is 3+ it is an ion. And when you have an ion within water, water actually surrounds the ion and in that state we call it aqueous.

So a soluble ionic solute breaks up into an aqueous ion, but that's not the only aqueous ion that we have. What else do we have within this formula? Aluminum bromide has three bromines in it, so we'd have 3 bromines. Bromine is in Group 7A, so its charge is -1. Since it's an ion as well, it's aqueous within the solution. So here we break up into one aluminum ion and three bromide ions.

Now if you're insoluble, it refers to a solute that cannot be dissolved when placed in a solvent. Here we have silver bromide. Silver bromide, we're told, is insoluble, so even when I throw it into water, it will not break up into ions. It stays together, so it doesn't change in any way, and that's how we are able to distinguish soluble versus insoluble. Soluble compounds break up into ions, aqueous ions, insoluble ones tend not to.

Solubility Rules Example 1

Here it says how many ions will the following soluble compound produce. Here we have sodium sulfate. They're telling us it's soluble. So you need to predict how many ions it's composed of two sodiums and one sulfate ion.

So you would say Na2SO4 breaks up into two sodiums. Sodium's in Group 1A, so it's going to be plus one. When it breaks up into its ions, they're aqueous, so we put AQ plus. We have one sulfate ion SO4-2 aqueous O.

Remember, sulfate ion is a polyatomic ion. If you don't remember that, make sure you go back and take a look at my videos on the polyatomic ions. Here we would say you break U into two sodium ions and one sulfate ion, for a total of 3 ions. SO3 ions would be our final answer.

Solubility Rules

Now we're going to say that the solubility rules are convenient set of guidelines to help us determine if a compound will be soluble or insoluble. Our memory tool here to remind us of the solubility rules are the bank robber was going to cash his loot, but the cops stopped him.

So we're going to use gonna Cash and cops to help us learn the solubility rules and determine if an ionic compound is indeed soluble or insoluble.

Solubility Rules

So we're going to start out with Ghana Cash. Ghana Cash is used for soluble ionic solutes now with the exceptions creating an insoluble solute called a precipitate. So basically, if we're breaking our one of our rules for Ghana Cache, we're going to make a solid called a precipitate. Now the exceptions are, shh, keep quiet about the cash.

So let's just go through Ghana Cache and see how it works, All right. So Ghana Cache stands for Ghana G stands for Group 1A. So we're talking about group 1A elements, Hydrogen, Lithium, sodium, potassium, etc. They have no exceptions, meaning if they're part of our ionic compound, the ionic compound overall is automatically soluble.

Next, A. A stands for acetate ion it too. If it's connected or part of an ionic compound, it's automatically soluble. N is for nitrate, which is NO3-, also no exceptions. The next A is for ammonium ion, which is NH4+. Art of cache. C stands for chlorate and by extension it stands for perchlorate. These have no exceptions, so so far we've gone through Ghana cache and seen no exception.

The next A is just and we have sulfate, which is SO42- and halogen. So remember, these are your group 7A elements, fluorine, chlorine, bromine and iodine. They have exceptions. So let's talk about it for sulfate to remember its exception. Just remember CBS happy here. We're going to say if sulfate is connected to CBS or HAP, it's going to form a precipitate. It's going to be an exception to the solubility rule O here. It creates a solid when connected to calcium, barium, strontium, mercury, silver or lead.

Halogens are soluble unless they're connected to hemp. So if they're connected to mercury, silver or lead, they form a precipitate a solid. So just remember Ghana Cash helps us to identify our soluble ionic compounds. If there are exceptions, they will form a precipitate known which is our solid. So just keep this in mind when looking at different types of ionic compounds.

Solubility Rules Example 2

Here it says according to the solubility rules, which of the following ionic compounds will be insoluble. So if we start out with the first one, it's composed of sodium which is a group 1A element. Remember, anything connected to group 1A is automatically soluble, so we don't even need to look at the fact that nitrate is involved. If we did, Nitro would be the same thing. If it's present automatically, the compound is soluble. You don't need both to be present. As long as one of them is present, the whole ionic compound is soluble.

Next, we have calcium acetate. Acetate is one of the other ions that we discussed earlier. If it's part of the ionic compound, the ionic compound overall is soluble by default. Next we have barium sulfate. Remember, sulfate has exceptions. CBS Happy and here sulfates connected to barium. Barium is the B. In CBS we said that if sulfate is connected to CBS or HAP, it will be insoluble and form a precipitate. So C here is our answer.

Now if we look at the other choices, ammonium ion is part of this ionic compound, so by default it's soluble overall. And then here perchlorate is part of this ionic compound, so it too is soluble overall as default. So here the only option that would make an insoluble compound known as a precipitate would have to be option C.

Solubility Rules

Now COPS is used for insoluble ionic solutes, and we're going to say it also has its exceptions now. These exceptions will now create soluble aqueous ionic compounds, and we know that cyber aqueous ionic compounds will break up into ions. Now how do we remember the exceptions here? We're just going to say, oh snap, it's the cops.

So when we're looking at the term comps, exceptions can arise when we get to O and when we get to S. So here if we take a look here at this chart we have CC here represents carbonate ion and also chromate ion. So carbonate and chromate, if we're just looking at comps and not thinking about Ghana Cash, we're going to say there are no exceptions. So we're going to form a precipitate.

In this case, OH, O stands for oxides, which is O2- and hydroxide which is OH-. Now here this is when we have our exceptions. So here the exception is something we remember CBS when we talked about Ghana cache. Now here this means that if oxide or hydroxide are connected to calcium, barium or strontium, they're going to create a soluble aqueous compound which can break up into ions.

Next we have P, which stands for phosphate. Remember, phosphate ion is PO43-. Here, if we're just looking at coms itself, we have no exceptions. It'll form a precipitate and then finally we have which stands for sulfide, which is S2-. It also has the same exceptions as O. So here calcium, barium and strontium.

So keep this in mind, we're going to use a, we're going to look at Ghana cache and comps overall to tell if a compound at the end of the day is soluble or insoluble.

Solutions Example 3

In this example question, it says based on the chart shown above, determine which of the following substances will be soluble in water. So in the first one we have hydroxide. Hydroxide can be soluble if it is connected to CBS, Calcium, Barium or Strontium. Here aluminum is not part of CBS. Aluminum is not part of Ghana Cash. Therefore, this is going to stay a solid.

Next we have phosphate and zinc. Now phosphate doesn't have any exceptions from the chart up above. Also Ghana Cash is not involved here because remember we have to take into consideration Ghana cache. Depending on what they're connected to, they could have made this soluble. So here this is not going to be soluble.

Next we have silver with carbonate. Carbonate doesn't have any exceptions. From the chart up above, silver is not part of Ghana Cash, so this will stay as solid. Next we have sulfide ion. Here it's connected to calcium. Remember sulfide has exception CBS. If it is connected to calcium, barium or strontium, it can be soluble. This would be soluble then.

And then finally we have Magnesium with Chromate ion. So chromate doesn't have exceptions from the chart up above. Magnesium is not part of Ghana Cash, so this would not be soluble. It would stay insoluble. So out of the choices given, only option D would create a soluble ionic solute.

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

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

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.