Ever wondered why some salts dissolve in water and others don’t? In this lesson you will learn about some simple salt solubility rules. Using these rules, you will be able to predict the products of many chemical reactions.
Introduction to Solubility
Let me introduce you to Shelley. Shelley is a really ace chemist; she can mix two colorless solutions and produce a bright yellow solid.
Cool, it’s magic!Actually it isn’t really magic at all, although it is still cool. You see, Shelley understands all about the solubility of common salts. She knew exactly what two colorless solutions to mix together to produce a yellow solid.
Not only that, but she also knew just what was going on in the reaction. And by the end of this lesson, you will too.
What is a Salt?
Before we get to reactions, let’s take a small step back and define a few important terms for you. Firstly, this lesson will be all about salts.
A salt is an ionic compound and it is made up of two ions — a positive cation and a negative anion.Here you can see a number of different salts of sodium.
|(aq) + Cl-(aq).
The equation tells us that solid sodium chloride has broken down into aqueous ions of Na+ and Cl-. These little aq symbols here are very important, as you will see later in the lesson.It turns out that not all salts are soluble in water, and sometimes when solutions of two different salts are mixed together, an insoluble solid is formed.
This is just like the magic Shelley showed us at the beginning of the lesson. The precipitate that forms is an insoluble solid salt separating from the solution.What is really awesome is that we can use our understanding of salt solubility to predict what will happen in a reaction between two different salt solutions. And we will look at this next.
Now, there are some pretty neat rules we can use to predict solubility. For this lesson, we are going to simply say that something is soluble or insoluble. As you go to higher levels of chemistry, you will also consider salts that are only slightly soluble. But for now, let’s keep it simple.Water Solubility of Common Salts
So, here we can see a table divided into groups of ions.
You’ll be pleased to know that you do not need to remember this table, just how to use it. It is important to notice the exceptions. We can say most chlorides and iodides are insoluble, but there are exceptions to this rule.It is important you realize this table only shows a few ions as an example. In your studies, you may be given a table that contains other ions as well. Don’t worry about this because the way you use it will be just the same.
This table comes in really useful if we want to predict whether a precipitate will form during an ionic reaction between two different salt solutions. Recall that when ionic compounds react together, we call this a double replacement reaction. In other words, two elements in different compounds replace each other. It is just like swapping dance partners in a dance.
So, let us have a go at predicting the outcome of two reactions of salt solutions.
Predicting Reaction Outcomes
Let us predict what will happen when Shelley mixes together ammonium phosphate and potassium carbonate. Do we think a precipitate will form?The first thing we will do is identify all of the ions present. We have an ammonium cation with a phosphate anion, and a potassium cation with a carbonate anion. Now the ions will switch partners, so our possible precipitation products are ammonium carbonate and potassium phosphate.
Let’s take a look at the table. This time we’ll start with lead iodide.
Our table doesn’t show lead specifically but it does tell us that salts containing iodide are mostly soluble, except when combined with lead. Always read the exceptions! We now know that lead iodide is insoluble and will precipitate.So let’s also check the other salt, potassium nitrate. This one is an easy one to answer, because all salts containing potassium are soluble and all salts containing nitrate are soluble. No question — or exception — there.
Potassium nitrate is soluble and will not precipitate.In this case, we did have a precipitate of lead iodide. In fact, this is Shelley’s magic reaction from earlier in the lesson. Potassium iodide is a very beautiful yellow solid.The chemical equation for this reaction is here: Pb(NO3)2 (aq) + 2KI(aq)–> PbI2(s) + 2KNO3(aq). Notice the solid and aqueous symbols telling us that lead iodide is a precipitate and potassium nitrate is still in solution.
In this lesson you have learned that a salt is an ionic compound that breaks down into cations and anions when dissolved in water. You have also learned that not all salts are soluble. A precipitate is an insoluble solid salt separating from the solution. To help us figure out which salts are soluble in water, we can use a salt solubility table.
This table tells us which ions are soluble and which ions are insoluble in water. This information can be used to figure out whether a precipitate will form when salt solutions are mixed together.
Once you are finished you should be able to: