Solubility Rules - Online Tutor, Practice Problems & Exam Prep

Solubility is the ability of a solute to dissolve in a solvent, forming a solution. Soluble compounds break into aqueous ions, while insoluble compounds remain intact. The solubility rules, summarized by "Gana Cash" for soluble and "COPS" for insoluble compounds, help determine solubility. Key exceptions include sulfates and halogens forming precipitates with certain ions like silver and lead. Understanding these rules is essential for predicting chemical behavior in solutions, particularly in reactions involving ionic compounds.

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. Soluble refers to a solute that can be dissolved into aqueous ions when placed in a solvent. For example, 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 one aluminum, so it's going to break up into one aluminum. Aluminum is in group 3A so its charge is 3 plus. 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, this, 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 three bromide ions. Bromine is in group 7A so its charge is minus 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 2 sodiums and 1 sulfate ion. So you would say Na₂SO₄ breaks up into 2 sodium ions. Sodium's in group 1A, so it's going to be plus 1. When it breaks up into its ions they're aqueous, so we put an (aq), plus, we have one sulfate ion SO₄^2- aqueous. So 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 up into 2 sodium ions and 1 sulfate ion for a total of 3 ions. So 3 ions would be our final answer.

Solubility Rules

Now we're going to say that the solubility rules are a 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 is, the bank robber was going to cash his loot, but the cops stopped him. So we're going to use "going to 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 ganache. Ganache is used for soluble ionic solutes. Now, with the exceptions creating an insoluble solute called a precipitate. So basically, if we're breaking one of our rules for ganache, we're going to make a solid called a precipitate. Now the exceptions are, shh, keep quiet about the ganache. So let's just go through ganache and see how it works. Alright.

Ganache stands for, Ghana g stands for group 1a. So we're talking about group 1a elements, hydrogen, lithium, sodium, potassium, etcetera. 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 automatically soluble. N is for nitrate which is NO3-, also no exceptions. The next a is for ammonium ion, which is NH4+. Part of cash c stands for chlorate, and by extension it stands for perchlorate. These have no exceptions. So far we've gone through ganache and see no exceptions.

The next a is just and. We have sulfate, which is SO42- and halogens. 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 HAPI. 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 of silver, or lead. Halogens are soluble unless they're connected to HAP. So if they're connected to mercury, silver, or lead, they form a precipitate, a solid. So just remember, Ganache 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, nitrate 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 sulfate is 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. These exceptions will now create soluble aqueous ionic compounds. So we know that soluble aqueous ionic compounds will break up into ions. How do we remember the exceptions? Here we're just going to say, "Oh snap, it's the cops." When we're looking at the term COPS, exceptions can arise when we get to O and when we get to S.

Here, if we take a look at this chart, we have C. C here represents carbonate ion and also chromate ion. So, carbonate and chromate. If we're just looking at COPS 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. 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 as CBS, when we talked about going to cash. 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 PO4 3-. Here, we're just looking at COPS itself. We have no exceptions. It'll form a precipitate. And then finally, we have S, which stands for sulfide, which is S2-. It also has the same exceptions as O. So here calcium, barium, and strontium. Keep this in mind, we're going to use this and look at Ghana and COPS 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's connected to CBS, calcium, barium, or strontium. Here, aluminum is not part of CBS; aluminum is not part of Ganna 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 above. Also, Ganna Cash is not involved here, because, remember, we have to take into consideration Ganna Cash 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 above. Silver is not part of Ganna Cash, so this will stay as solid.

Next we have the sulfide ion here, it's connected to calcium. Remember, sulfide has exceptions, CBS. If it is connected to calcium, barium, or strontium, it can be soluble. This would be soluble then.

And finally, we have magnesium with the chromate ion. So chromate doesn't have exceptions from the chart above, Magnesium is not part of Ganna 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 for common ionic compounds?

The solubility rules for common ionic compounds help determine whether a compound will dissolve in water. The mnemonic 'Gana Cash' is used for soluble compounds: Group 1A elements (e.g., Na⁺, K⁺), acetate (CH₃COO^-), nitrate (NO₃^-), ammonium (NH₄⁺), chlorate (ClO₃^-), and perchlorate (ClO₄^-) are always soluble. Sulfates (SO₄^2-) and halogens (Cl^-, Br^-, I^-) are generally soluble, but have exceptions with ions like Pb²⁺, Ag⁺, and Hg₂²⁺. 'COPS' is used for insoluble compounds: Carbonates (CO₃^2-), oxides (O^2-), phosphates (PO₄^3-), and sulfides (S^2-) are generally insoluble, with exceptions for compounds containing Group 1A elements and NH₄⁺.

How do you determine if a compound is soluble or insoluble in water?

To determine if a compound is soluble or insoluble in water, you can use the solubility rules. For soluble compounds, remember 'Gana Cash': Group 1A elements, acetate, nitrate, ammonium, chlorate, and perchlorate are always soluble. Sulfates and halogens are generally soluble but have exceptions with ions like Pb²⁺, Ag⁺, and Hg₂²⁺. For insoluble compounds, use 'COPS': Carbonates, oxides, phosphates, and sulfides are generally insoluble, with exceptions for compounds containing Group 1A elements and NH₄⁺. By applying these rules, you can predict the solubility of most ionic compounds in water.

What are the exceptions to the solubility rules for sulfates and halogens?

Sulfates (SO₄^2-) and halogens (Cl^-, Br^-, I^-) are generally soluble, but there are notable exceptions. Sulfates form insoluble compounds with Ca²⁺, Sr²⁺, Ba²⁺, Pb²⁺, and Hg₂²⁺. Halogens form insoluble compounds with Ag⁺, Pb²⁺, and Hg₂²⁺. These exceptions are crucial for predicting whether a compound will dissolve in water or form a precipitate.

What is the significance of the mnemonic 'Gana Cash' in solubility rules?

The mnemonic 'Gana Cash' is significant because it helps students remember the solubility rules for common ionic compounds. 'Gana' stands for Group 1A elements, acetate, nitrate, and ammonium, which are always soluble. 'Cash' stands for chlorate and perchlorate, which are also always soluble. Additionally, sulfates and halogens are generally soluble but have specific exceptions. This mnemonic simplifies the process of determining whether a compound will dissolve in water, making it easier to predict chemical behavior in solutions.

How do the solubility rules apply to predicting precipitation reactions?

The solubility rules are essential for predicting precipitation reactions. When two aqueous solutions are mixed, the solubility rules help determine if any of the resulting ionic compounds are insoluble and will form a precipitate. For example, if mixing solutions containing Ba²⁺ and SO₄^2-, the solubility rules indicate that BaSO₄ is insoluble and will precipitate out of the solution. By knowing which compounds are soluble and which are not, you can predict the formation of precipitates in chemical reactions.