Background Information

Many ionic compounds naturally contain water molecules as part of the crystal lattice structure. A hydrate is a compound that has one or more water molecules bound to each formula unit. Ionic compounds that contain a transition metal are often highly colored. Interestingly, it is common for the hydrated form of a compound to be of a different color than the anhydrous form, which has no water in its structure. A hydrate can usually be converted to the anhydrous compound by heating. For example, the anhydrous compound cobalt(II) chloride is blue, while the hydrate is a distinctive magenta color (as seen below).

The hydrated form of cobalt(II) chloride contains six water molecules in each formula unit. The name of the compound is cobalt(II) chloride hexahydrate and its formula is CoCl2 • 6H2O. The formula for water is set apart at the end of the formula with a dot, followed by a coefficient that represents the number of water molecules per formula unit. These water molecules are part of the structure and hydrated chemical, therefore cannot be changed when balancing a chemical equation.

In a hydrate, water molecules are distinct parts of the compound but are joined to it by bonds that are weaker than either those forming the anhydrous salt (without water) or those forming the water molecules. Hydrates can usually be converted to the anhydrous form by heating:

Hydrated salt → anhydrous salt + water

*Usually a triangle is written above the arrow, to indicate heating.

Additional information for the curious:

Some hydrates such as CoCl2 lose water to the air in a process called efflorescence. There hydrates can absorb water from the air or other sources so strongly that they can be used to dry liquids or gases. These substances are called desiccants, and are said to hygroscopic. Some compounds, such as NaOH, can take up so much water from the air that they dissolve the water they absorb. This process is called deliquence.

Experimental Set-up

The following video provides the concept of the lab.

Please take note of the following things which you should NOT do as demonstrated in the video:

  • The copper(II) sulfate used may look different initially (not a large blue crystal). It might not be as deep blue (but it is still blue).

  • try NOT to poke around with a glass stirring rod. Every time you touch your compound some may be lost to the stirring rod, impacting your results.

Lab Handouts, Analysis, Calculations & Discussion Questions

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1. Lab Handout

2. Procedure Summary

3. Vocabulary/ Important Terms

4. Sample Data

5. Sample Analysis/ Calculations

6. Discussion Questions

7. Possible Sources of Error

8. Links to Similar Labs

9. Related Problems/ Previous AP FRQs

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Hydrate - Analysis