Solubility Rules - Online Tutor, Practice Problems & Exam Prep

Solubility is a chemical property indicating a solute's ability to dissolve in a solvent, forming an aqueous solution. Soluble compounds dissociate into ions, while insoluble ones do not. The solubility rules, summarized by "GANA CASH" for soluble compounds and "COPS" for insoluble ones, help predict solubility. Key exceptions include sulfates and halogens forming precipitates with certain ions like lead and mercury. Understanding these rules is essential for predicting reactions and solubility behavior in various chemical contexts.

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. 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. So here we have aluminum bromide solid. We're going to throw it into water. 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+. It is an ion and when you have an ion within water, the 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 we have. What else do we have within this formula? Aluminum bromide has three bromines in it. So we'd have three, Br, bromine's 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.

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. 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 sodiums. Sodium's in group 1A, so it's going to be +1. When it breaks up into its ions they're aqueous, so we put (aq), plus, we have one sulfate ion SO₄²⁻ 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 "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 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:

• G stands for 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.
• A stands for acetate ion. It too, if it's connected or part of an ionic compound, is automatically soluble.
• N is for nitrate which is NO3−, also has no exceptions.
• A is for ammonium ion, which is NH4+. Part of ganache.
• C stands for chlorate, and by extension, it stands for perchlorate. These have no exceptions.
• H is for 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 (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, 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, the 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 by 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. 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 COPS, exceptions can arise when we get to O and when we get to S.

Looking 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. The exception is something we remember as CBS, when we talked about going to cash. 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 the phosphate ion is PO₄^3-. Here, just looking at COPS itself, we have no exceptions. It'll form a precipitate. Finally, we have S, which stands for sulfide, which is S^2-. It also has the same exceptions as O. So, keep this in mind. We're going to look at Ghana Cash 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 states, based on the chart shown above, determine which of the following substances will be soluble in water. In the first case, 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 Ghana Cash, therefore, this will stay as a solid.

Next, we have phosphate and zinc. Now, phosphate doesn't have any exceptions from the chart above. Also, Ghana Cash is not involved here because we have to take into consideration Ghana 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 Ghana Cash, so this will stay as a solid.

Next, we have the sulfide ion; 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.

Finally, we have magnesium with chromate ion. Chromate doesn't have exceptions from the chart 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 the option connected to calcium 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 in chemistry?

Solubility rules are guidelines that help predict whether an ionic compound will dissolve in water. The mnemonic 'GANA CASH' helps remember the rules for soluble compounds: Group 1A elements, Acetate (C₂H₃O₂^-), Nitrate (NO₃^-), Ammonium (NH₄⁺), Chlorate (ClO₃^-), and Perchlorate (ClO₄^-) are generally soluble. Exceptions include sulfates (SO₄^2-) and halogens (Group 7A elements) when paired with certain ions like Pb²⁺, Hg₂²⁺, and Ag⁺. 'COPS' helps remember insoluble compounds: Carbonate (CO₃^2-), Chromate (CrO₄^2-), Oxide (O^2-), Hydroxide (OH^-), Phosphate (PO₄^3-), and Sulfide (S^2-), with exceptions for oxides and hydroxides when paired with Ca²⁺, Ba²⁺, and Sr²⁺.

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

To determine if a compound is soluble or insoluble, use the solubility rules. For soluble compounds, remember 'GANA CASH': Group 1A elements, Acetate, Nitrate, Ammonium, Chlorate, and Perchlorate are generally soluble. For insoluble compounds, use 'COPS': Carbonate, Chromate, Oxide, Hydroxide, Phosphate, and Sulfide are generally insoluble. Exceptions exist, such as sulfates and halogens forming precipitates with Pb²⁺, Hg₂²⁺, and Ag⁺. Oxides and hydroxides are soluble when paired with Ca²⁺, Ba²⁺, and Sr²⁺.

What are the exceptions to the solubility rules?

Exceptions to the solubility rules include: Sulfates (SO₄^2-) are insoluble when paired with Ca²⁺, Sr²⁺, Ba²⁺, Pb²⁺, and Hg₂²⁺. Halogens (Group 7A elements) are insoluble when paired with Ag⁺, Pb²⁺, and Hg₂²⁺. Oxides (O^2-) and Hydroxides (OH^-) are soluble when paired with Ca²⁺, Ba²⁺, and Sr²⁺. These exceptions help predict when a compound will form a precipitate instead of dissolving in water.

Why are solubility rules important in chemistry?

Solubility rules are important because they help predict the behavior of ionic compounds in aqueous solutions. Knowing whether a compound is soluble or insoluble allows chemists to anticipate the formation of precipitates, understand reaction mechanisms, and design experiments. These rules are essential for tasks such as predicting the outcomes of double displacement reactions, understanding solubility equilibria, and performing qualitative analysis in laboratory settings.

What is the difference between soluble and insoluble compounds?

Soluble compounds dissolve in water, breaking up into their constituent ions, forming an aqueous solution. For example, NaCl dissolves in water to form Na⁺ and Cl^- ions. Insoluble compounds do not dissolve in water and remain in their solid form. For instance, AgCl does not dissolve in water and remains as a solid precipitate. Solubility rules help determine whether a compound will be soluble or insoluble in water.