Solubility Rules

MHS Chemistry
Solubility Rules

Objectives

Observe and record chemical changes involving precipitates.
Deduce basic solubility rules based on observations
Write net ionic equations for reactions that took place.

Introduction
We have already discussed some of the patterns in the periodic table of the elements, but chemistry deals with more patterns than just these! In this lab we will investigate the products formed by the combination of many different ions, and look for patterns. When we are finished, we should be able to make rough predictions about whether or not two chemicals will react to form a solid.

An example of a solubility rule involves the sodium ion (Na+). Many people have observed over the years that compounds with sodium ions are very soluble in water, so there is a solubility rule that says "all sodium salts are soluble." If we combine a sodium compound with another compound and a solid is produced, we can be pretty sure that the positive ion in the product is not sodium.

Vocabulary
The reaction of barium hydroxide with sodium sulfate can be written three different ways. First, there is the "plain reaction" which just shows reactants and products: Ba(OH)2 + Na2SO4 ® BaSO4 + 2NaOH

This only tells that a reaction takes place. In reality, these will be combined as aqueous solutions in test tubes. We can write these in terms of the ions that are involved ("aq" stands for aqueous and "s" stands for solid):
Ba²⁺(aq) + 2OH^–(aq) + 2Na⁺(aq) + SO4^2–(aq) ® BaSO4(s) + 2Na⁺(aq) + 2OH^–(aq)

This overall ionic reaction tells us more than we need to know! Notice that the sodium ions and the hydroxide ions do not participate in the reaction at all. Since they are just hanging around watching, they are called spectator ions. Barium and sulfate combine to form a solid precipitate. A more useful way to show this reaction is to ignore the stuff we don't need to deal with: Ba²⁺(aq) + SO4^2–(aq) ® BaSO4(s)

This form is the net ionic equation. It includes only the positive and negative ions (cations and anions) that are relevant. For each reaction that takes place in this experiment, you will write the net ionic equation.

Procedure
Place a copy of the data table on your lab bench, under a piece of clear plastic. Combine the ions in their correct columns and rows by adding them two drops of each onto the sheet. Record the appearance of any products on a second copy of this sheet. If no reaction takes place, write "NR" in the space, and if you decide you don't need to combine a pair, put an "X" in the space.

Anions® ¯Cations¯	CO3^2– (Na2CO3)	Cl^– (NaCl)	OH^– (NaOH)	NO3^– (NaNO3)	PO4^3– (Na3PO4)	SO4^2– (Na2SO4)
Al³⁺ (AlCl3)	.	..	..	..	..	..
NH4⁺ (NH4Cl)	.	..	..	..	..	..
Ca²⁺ (CaCl2)	.	.	.	.	.	.
Cu²⁺ (CuSO4)	.	.	.	.	.	.
Na⁺ (NaCl)	.	.	.	.	.	.
Fe³⁺ (FeCl3)	.	.	.	.	.	.
Pb²⁺ (Pb(NO3)2)	.	.	.	.	.	.
Mg²⁺ (MgSO4)	.	.	.	.	.	.
K⁺ (KI)	.	.	.	.	.	.
Ag⁺ (AgNO3)	.	.	.	.	.	.
Zn²⁺ (ZnCl2)	.	.	.	.	.	.

Before you clean up, make sure you have accurately recorded all of your observations. Clean up according to your teacher's instructions.

Analysis

Which anions generally form precipitates? What are the exceptions?

Which anions generally do not form precipitates? What are the exceptions?

Which cations generally do not form precipitates?

Write the balanced overall reactions for the precipitation reactions you observed involving aluminum and involving copper.

3(aq)

3(s)

(aq)

Write the balanced net ionic reactions for every precipitation reaction you observed.

³⁺

^2–

(aq)

3(s)

[Solubility Rules score sheet][MHS Chem page]