Hydrated Ionic Compounds

Did you know that there are some solid ionic compounds that have water as part of their ionic formula?


B5.13 Desiccant pack

© Wikimedia Commons
B5.16 Hydrated copper(II) sulfate

© Wikimedia Commons
B5.17 Dehydrated copper(II) sulfate

© Wikimedia Commons
B5.18 Rehydrating the sam
ple
Desiccant packets are added to substances to absorb water; this prevents mold, mildew, and corrosion. These desiccants are anhydrous forms of hydrated ionic compounds. It is important to note that hydrated compounds are not solutions; they are actually dry solids.

In these compounds, water is bound in the hydrated crystal lattice structure with a fixed ratio of water molecules. If the water is removed, the properties of the substance change and the compound is referred to as the anhydrous form. Some anhydrates are used as drying agents because they absorb water efficiently. The water in the formulas affects the properties of the compound, such as shape, colour, or melting point. Notice that the compound in image B5.16 is blue, whereas once it is dehydrated, it is white, as in image B5.17. Adding water back to the anhydrous form will re-create the hydrated compound, as in image B5.18.

The key guidelines to remember when assigning IUPAC names for hydrates are as follows:

  1. The ionic compound is named first using the guidelines you have already learned.

  2. The second part is often referred to as the water of crystallization and is named by writing "-water (1/n)", where “n” is the coefficient in front of the water portion.

  3. When the chemical formula for a hydrated ionic compound is written, the formula for the ionic compound is separated from the waters of hydration by a centred "dot."


Examples

Each example has a video to go with it. To play the video, click on the play icon next to the example.

Naming Hydrates

Name the ionic portion of the hydrate using the rules already learned.

barium hydroxide
Place a dash followed by the word “water.”

barium hydroxide-water
In brackets, write the ratio of ionic compound to water molecules.

barium hydroxide-water (1/8)

Watch this video to see a teacher work through this example.  https://adlc.wistia.com/medias/by7w3jvghh
 


Name the ionic portion of the hydrate using the rules already learned.

calcium nitrate
Place a dash followed by the word “water.”

calcium nitrate-water
In brackets, write the ratio of ionic compound to water molecules.

calcium nitrate-water (1/4)

Watch this video to see a teacher work through this example.  https://adlc.wistia.com/medias/4kn1d6c77z
 

Writing Formulas of Hydrates

Write the formula for the ionic portion of the hydrate using the rules already learned.

Place a dot followed by the coefficient of water molecules.

FeSO4•7
Write the formula for water: H2O.

FeSO4•7H2O

Watch this video to see a teacher work through this example.  https://adlc.wistia.com/medias/cy329tvg3n
 


Write the formula for the ionic portion of the hydrate using the rules already learned.

Place a dot followed by the coefficient of water molecules.

Na2CO3•10
Write the formula for water: H2O.

Na2CO3•10H2O

Watch this video to see a teacher work through this example.  https://adlc.wistia.com/medias/gzxr95f6hp
 

  Did You Know?

The classical naming system of hydrates uses Greek prefixes.

Copper(II) sulfate-water (1/5) is classically known as copper(II) sulfate pentahydrate.
© Wikipedia
B5.19 Gypsum


B5.20 caste made from plaster of Paris


Gypsum and plaster of Paris are two separate hydrates of the same ionic compound that can be converted from one form to the other. By heating the gypsum to release steam, plaster of Paris is formed; and if water is added to plaster of Paris, gypsum is formed.

Gypsum is a mineral that is used to make drywall for construction.

Once gypsum is heated, it is converted into plaster of Paris.

CaSO4•2H2O + heat → CaSO4•0.5H2O + 1.5H2O (released as steam)

Plaster of Paris is a fine powder that when mixed with water, creates resilient lightweight solid. This is the traditional method for making orthopedic castes.

  Practice Questions

Complete the following practice questions to check your understanding of the concept you just learned. Make sure you write complete answers to the practice questions in your notes. After you have checked your answers, make corrections to your responses (where necessary) to study from.

  1. Provide the correct IUPAC name for each of the following hydrates.


    		 CoCl2•6H2O
    sodium sulfate-water (1/10)
    nickel(II) nitrate-water (1/9)

    		ZnSO4•7H2O

    cobalt(II) chloride-water (1/6)
    		 CoCl2•6H2O
    sodium sulfate-water (1/10)
    		 Na2SO4•10H2O
    nickel(II) nitrate-water (1/9) Ni(NO3)2•9H2O
    zinc sulfate-water (1/7)
    		 ZnSO4•7H2O