Ch.4 - Chemical Quantities & Aqueous ReactionsWorksheetSee all chapters
All Chapters
Ch.1 - Intro to General Chemistry
Ch.2 - Atoms & Elements
Ch.3 - Chemical Reactions
BONUS: Lab Techniques and Procedures
BONUS: Mathematical Operations and Functions
Ch.4 - Chemical Quantities & Aqueous Reactions
Ch.5 - Gases
Ch.6 - Thermochemistry
Ch.7 - Quantum Mechanics
Ch.8 - Periodic Properties of the Elements
Ch.9 - Bonding & Molecular Structure
Ch.10 - Molecular Shapes & Valence Bond Theory
Ch.11 - Liquids, Solids & Intermolecular Forces
Ch.12 - Solutions
Ch.13 - Chemical Kinetics
Ch.14 - Chemical Equilibrium
Ch.15 - Acid and Base Equilibrium
Ch.16 - Aqueous Equilibrium
Ch. 17 - Chemical Thermodynamics
Ch.18 - Electrochemistry
Ch.19 - Nuclear Chemistry
Ch.20 - Organic Chemistry
Ch.22 - Chemistry of the Nonmetals
Ch.23 - Transition Metals and Coordination Compounds
Solution Stoichiometry
Solubility Rules
Net Ionic Equations
Redox Reaction
Balancing Redox Reactions
Activity Series
Chemical Quantities Additional Problems
Additional Practice
Redox Reactions
Calculate Oxidation Number
Additional Guides
Net Ionic Equation
Oxidation Reduction (Redox) Reactions
Oxidation Number
Types of Chemical Reactions

Whether you use the term molarity, concentration or solubility they all represent the maximum amount of solute that can successfully be dissolved in a solvent. 

Understanding the Solubility Rules

Concept #1: The Rules for the Solubility of a compound


I honestly hate when we have to memorize a lot of things, polyatomic Ions is one and this would be the second. Here we have to learn our solubility rules. Here we’re going to say the solubility of a compound represents the maximum amount of solute you can dissolve in the solvent.
Solubility also has another name. Its other name is concentration. And remember, we said that another name for concentration is molarity. Solubility, concentration, molarity, all three of these terms are basically pointing at the same thing. It’s talking about how much solute do we have in our solvent and together what kind of solution forms?
Now, what we should realize here is these are our solubility rules. Now, here on the left side, we talk about soluble ionic compounds and on the right, we talk about insoluble ionic compounds. Let’s go through this really quickly just to see some of the basic types of ideas we have. Here we’re going to say Group 1A ions, so Group 1A ions: H⁺, Li⁺, Cs⁺, Na⁺, K⁺ or ammonium which is a polyatomic ion, when they’re connected to someone else, that compound will always be soluble.
For example, if I had NH₄ connected with Cl. Soluble means when I throw it into water, it’s going to break up into ions. These will break up into NH₄⁺ (aqueous) plus Cl⁻ (aqueous). It breaks up into the ions that are present before we threw it into the water.
Nitrates, acetates, and remember acetate can be written in two forms, and most perchlorates are soluble. Again, when these guys are connected to a compound, they’re going to be soluble. If we have KClO₄, this breaks up into K⁺ plus ClO₄⁻.
Cl⁻, Br⁻, and I⁻ are soluble. Meaning they break up when they’re connected with something else, except when they’re connected to silver, lead (II), copper (I) or mercury (I), when they’re with these guys they stay as solids. We’re going to say that they’re insoluble. Sulfates are soluble. Meaning they break up except when they’re with one of these guys here.
Now, insoluble ionic compounds basically means that they’re solids. Another name for solid is precipitate. You’re going to hear these words used interchangeably as well. Insoluble means it’s a solid. It doesn’t break up. Solid also means precipitate. All three words are saying the same exact thing. OH⁻, S2-, they’re insoluble except when they’re with Group 1A, ammonium, or one of these Group 2A ions.
You can see how the things fit together. Of course, they would be soluble with these guys because Group 1A ions are always soluble with everyone. Ammonium is always soluble with anyone. Calcium, strontium and barium are soluble with everyone except for sulfate. And then, carbonate and phosphate.Basically, calcium, strontium, barium are insoluble with the -ates: sulfates, carbonates, phosphates.
Then we're going to say carbonates and phosphates are insoluble except when they’re with Group 1A or ammonium. There’s a lot of overlapping of trends. Your best strategy is just to remember Group 1A elements and ammonium are soluble with everyone. Just remember that let’s see the Group 2A ions, calcium, strontium, and barium are insoluble when they’re with the –ates, so sulfate, carbonate, phosphate.
Just remember these little tricks here and there to help you remember what’s what. Other than that it’s really just memorizing and it sucks, I know. But, you're going to have to do it to be able to do a lot of questions, because they’re going to ask you to give us the formula when these two things mix together. What are the products formed? 

Certain positive ions combine with negative ions to form either a liquid, aqueous, solid or gaseous compound. The Solubility Rules help us determine which form is created. 

Concept #2: The meaning of the terms soluble and aqueous


We talked about this already, when we classify a compound as soluble, it means that the compound is aqueous when it breaks up. It means it breaks up into aqueous ions when it’s dissolved in water. It is also known as an electrolyte, which means it can conduct electricity.
Basically, we have pure water here. When we throw this ionic compound in, it breaks up into ions. Why does it break up into ions? Because remember, Na is a Group 1A metal. They’re always soluble with everyone, so they’re going to break up. Nitrates are also always soluble.Both of them are saying they're soluble when connected to anything, so, of course, they’re going to break up into ions.
These ions will be free floating in the water. And because they have a charge, they can conduct electricity. That’s what an electrolyte is. An electrolyte just means we have ions. Those ions are the electrolytes. They conduct electricity.

Saying a compound is soluble means that the compound will dissolve into aqueous ions in a solvent. These ions can conduct electricity and are called electrolytes

Concept #3: The meaning of the terms insoluble and precipitate


When we classify a compound as insoluble, it means that that compound usually is a solid or precipitate. And, we call these guys non-electrolytes. These guys do not conduct electricity because they do not break up into ions when they are in water. A good example here we have what’s called methanol, CH₃OH. Here it’s a liquid before I throw into water. When I throw into water, it becomes aqueous. All that aqueous means is that water is surrounding it. That’s all aqueous means, water surrounds it.
And, what you should realize is that the compound was methanol in the beginning. It stayed intact before I threw it in, even when it’s in the water it’s still intact. It doesn’t break up at all. It stays together. This is what a non-electrolyte does; it doesn’t break up at all.
Barium sulfate is a solid. And remember, sulfates are soluble unless they’re connected to certain ions. One of those ions is barium. That means that this compound would be insoluble, following our solubility rules. When we throw it into water, we shouldn’t expect it to break up. It stays together. Now, it becomes aqueous as well because even though it's still a solid, it surrounded by water. Because it’s surrounded by water, we say its state is now aqueous.
Now, this page really is, again, learning a few trends here and there, but for the most part, the bulk of it is just memorizing what you see for the solubility rules. 

Saying a compound is insoluble means that the compound will not dissolve into aqueous ions and forms a solid called a precipitate, which will not conduct electricity.