Mixing Ionic Materials with Water
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Mixing Ionic Materials with Water

Solubility of Ionic Compounds in Water

Next, let's test the solubility of three calcium compounds in water by mixing a small amount of each with about 1 ml of water. Record the results of the solubility tests shown below (or you can test them yourself in the lab) in exercise 6 in your workbook.

Ionic crytals in test tubes. [mixion1.JPG] Two solutions and a suspension.[mixion2.JPG]
Crystals of calcium nitrate (Ca(NO3)2), calcium chloride (CaCl2), and calcium sulfate (CaSO4) are placed in separate test tubes. Water is added to each to form two solutions of Ca(NO3)2 and CaCl2, and a suspension of CaSO4.

The purpose of doing this particular experiment is to give you some firsthand experience in realizing that some ionic compounds are soluble in water and some are not.

Mixing Ionic Materials with Water

As you know, table salt (as well as calcium nitrate and calcium chloride) will dissolve in water. Since salt is not a polar molecular material, there must be more to solubility and mixing than matching intermolecular bonds.

This should help you to picture what happens to salt as it dissolves in water. Salt is made up of sodium and chloride ions held together by ionic bonds. Models of salt and water, not yet mixed. [mixion3.JPG]
When sodium chloride breaks up, ionic bonds are broken. As the sodium and chloride ions move between the water molecules, the hydrogen bonds holding the water molecules together must also be broken. Because water molecules are polar, they have a positive end (H) and a negative end (O). Models of salt and water, mixing. [mixion4.JPG]
The positive ends of the water molecules are attracted to the negative chloride ions and the negative ends of the water molecules are attracted to the positive sodium ions. These attractions between ions and polar molecules are called ion-dipole bonds. They are comparable in strength to hydrogen bonds or maybe even a bit stronger. When an ionic material, like salt, dissolves in water, both ionic bonds and hydrogen bonds are broken and ion-dipole bonds are formed. Models of salt and water, mixed. [mixion5.JPG]

 

Salt is soluble in water. However, not all ionic compounds are soluble in water. Many are, many are not. How soluble an ionic compound is in water depends on the strength of the ionic bonds that have to be broken and on the number and strength of the ion-dipole bonds that are formed. Later we will deal with some of the consequences of these bond changes.

How to Determine Solubility

How can you tell whether or not an ionic material is going to be soluble in water? With ionic compounds, the most direct way to tell whether or not the material is soluble is to try it. We don't have a single rule such as "like dissolves like" for ionic compounds. If you want to find out if calcium nitrate is soluble in water, you get some calcium nitrate and put it in water and you see if it dissolves.

Another way is to realize that someone else has probably already tested that compound and written down the results in the "Handbook of Chemistry and Physics" or some other books. So you can look up that particular compound to see whether it is soluble in water.

A third way, one that you need to become familiar with, involves the use of solubility rules, which are presented on the next page in this section.

 

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