Previous Contents Next

Water of Hydration

(revised 13 Sept. 2024)

Introduction

Water due to its shape and charge separation is said to be highly polar compound. This polar nature means that it has a strong attraction for many compounds. Ionic compounds crystallized from aqueous solutions generally have H2O molecules incorporated in their crystal lattice in a definite proportion. These compounds are called hydrates. CuSO4 (H2O)5 is a typical example and the water is part of the formula. In some cases, heating of hydrates will drive off, all or part of, the H2O incorporated in the crystal. If all the H2O is removed, the salt is called anhydrous. Salts that are not stable at room temperature and lose water without heating are said to be efflorescent. On the other hand, those salts that readily absorb moisture are said to be hygroscopic and are used as drying agents. As the temperature is raised, molecule by molecule, the H2O is lost. If it is the salt of a transition metal, the loss of water is commonly accompanied by a color change, for example CoCl2 (H2O)6 is red CoCl2 (H2O)2 is violet and anhydrous CoCl2 is blue.

Safety

Wear your safety glasses and remember the crucible is hot after heating. Let it cool to room temperature before you weigh it.

Prelab

Procedure

Water of Hydration 

Strongly heat a dry crucible and cover for 1 to 2 minutes to drive off any absorbed moisture. Heat with lid ajar. Replace the lid and allow the crucible to cool.  Do not touch the crucible and lid with fingers at any time. Use tongs. Then weigh the crucible and lid.  Place 3 to 4 g of MgSO4 (H2O)x into the crucible, replace the lid and reweigh accurately. Place the crucible onto a clay triangle, put lid ajar and heat gently for a few minutes. After the sample has dried enough to avoid splattering (about 5 min.), heat strongly for 10 minutes. Cover completely the crucible and allow to cool for 10 to 15 minutes.  The crucible must be room temperature to weigh. Reweigh accurately. Heat the crucible containing the sample a second time, just as you did for the first sample heating. Let cool 10 to 15 minutes, and reweigh accurately. 

Your masses should now be within 0.0005 g. If not, heat, cool and reweigh. Once completed add a bit of distilled water to the crucible and observe what happens. 

From the initial weight and weight loss, calculate the number of moles of H2O per mole of MgSO4 in your sample.

Hydrates 

You will be given three (3) of the samples in Table 1 below. You are to decide whether each compound is a hydrate or not. The degree of hydration is from zero to 12. The properties of hydrates are as follows:
  1. Evolution of water upon heating (i.e. condensation in the tube)
  2. Change of color or texture ot compound (decomposition to form new crystal forms) upon heating
  3. Solubility of anhydrous residue in water
  4. Color of anhydrous residue changes back to color or original hydrate when dissolved in water
  5. Note: A compound should display all properties to be considered a hydrate
To determine if a sample is a hydrate, place about 1 g of the compound into a separate dry test tube. Observe the color and texture of the sample before heating. Next, heat the sample gently with a Bunsen burner. Observe what happens. Note the color and texture after heating.  Also look for H2O condensing on the upper end of the test tube. Finally, test the water solubility of the residue after heating by adding some distilled water with your water bottle. Observe. Note the solubility (soluble, slightly soluble, very soluble, insoluble). Also note the color of the solution or the mixture. Repeat the above steps for the two (2) other samples. 

Table 1. Names of the samples for the hydrate experiment
1.  Cobalt (II) chloride 6.  Chromium (III) chloride
2.  Copper (II) sulfate 7.  Sodium chloride
3.  Barium chloride 8.  Sodium sulfate
4.  Magnesium chloride 9.  Sodium acetate
5.  Strontium chloride 10.  Sucrose

Previous Contents Next