Hydrates are ionic compounds that contain water molecules as part of their crystal structure. The bound water is called the water of hydration or water of crystallization. Some examples include minerals such as gypsum (CaSO4•2H2O), borax (Na3B4O7•10H2O) and Epsom salts (MgSO4•7H2O).
A hydrate contains a definite number of water molecules bound to each ionic compound (also called the anhydrous salt). The formula of the hydrate is represented by the formula of the anhydrous salt followed by a dot and xH2O, where x is the number of moles of water per mole of the anhydrous salt. The hydrate is named by naming the anhydrous salt followed by the word ‘hydrate’ preceded by a Greek prefix to indicate the number of water molecules. For example, CaSO4•2H2O is named calcium sulfate dihydrate.
A newer naming convention is to explicitly state the component formula and name followed by '-water (x/y)' where x is the stoichiometric number of the parent compound and y is the stoichiometric number of the water molecules bound within the compound. Thus, CaSO4•2H2O would be named 'calcium sulfate-water (1/2)'. A second example would be the compound Co2O3•nH2O (the 'n' indicates the number of water present is not fixed) which would be named 'cobalt(III) oxide-water (1/n)'.
Although both naming conventions are valid, the older naming convention is more common and will be used here. The newer naming convention should be used if an unequivocal naming of the compound is needed.
1.1 Properties of Hydrates
In general, the water of hydration can be removed by heating a hydrate sample. After heating, the anhydrous salt remains as residue and the water escapes into the atmosphere as water vapor.
Example: CuSO4•5H2O(s) ∆→ CuSO4(s) + 5H2O(g)
blue crystals white ash
The anhydrous salt will have a different structure and physical properties such as texture and color compared to the hydrate.
Most hydrates are stable at room temperature. However, some hydrates lose the water of hydration spontaneously and are said to be ‘efflorescent’. On the other hand, other compounds spontaneously absorb water from the atmosphere and are said to be ‘hygroscopic’. These hygroscopic substances are typically used to ‘dry’ other materials; they are referred to as desiccants. An anhydrous salt (without water) can absorb so much water from the atmosphere that it will spontaneously dissolve in its own water of hydration and is said to be ‘deliquescent’.
1.2 Formula of a Hydrate
As the formula indicates, water forms a definite percentage of the mass of any hydrate. For example, the mass of a mole of CaSO4•2H2O is 172 grams (this is the molar mass of CaSO4•2H2O). A mole of CaSO4•2H2O contains 2 moles of water (which corresponds to 36 grams of water) as part of its structure. Therefore, the compound CaSO4•2H2O always consists of 36/172 or 21% water by mass.
All hydrated compounds may be dehydrated by heating. A solid sample will show a decrease in mass as water escapes into the atmosphere. As part of this experiment, the amount of water lost by an unknown hydrated compound on heating will be determined.
mass of water lost=mass of hydrate-mass of anhydrous salt
The ratio of the mass of water lost to the mass of the hydrate will indicate the percentage of the water in the hydrate.
% water of hydration= mass of water lostmass of hydratex 100
References and further reading
Nomenclature of Inorganic Chemistry, IUPAC Recommendations 2005. https://old.iupac.org/publications/b..._Book_2005.pdf (accessed 07/06/2020)
Technique F Use of Crucible
2.0 SAFETY PRECAUTIONS AND WASTE DISPOSAL
3.0 CHEMICALS AND SolutionS
4.0 GLASSWARE AND APPARATUS
Part A. Hygroscopic and Efflorescent Solids
- On an analytical balance, weigh a pea-sized sample of each of the compounds below (see 6.0 Data Recording Sheet) on separate, clean and dry watch glasses. Record the initial masses of watch glasses and samples.
- Label and set them aside out of the way in one area on your work bench for observations. After one hour (or at the end of the lab period) note any changes in the physical appearance of each sample.
- Weigh the samples and record the masses as final masses.
- Calculate the change in mass for each sample. A substance is classified as efflorescent if its mass decreases by 0.005 g or more; and it is classified as hygroscopic if its mass increases by 0.005 g or more.
Part B. Reversibility of Hydration
- Grind a few crystals of CuSO4•5H2O in a mortar. Gently heat the powder in a Pyrex test tube fastened in a horizontal position by a clamp on a ring stand. Observe whether any water can be seen (as a mist or droplets) condensing inside the test tube. Note the appearance of the solid residue after heating. When the residue seems to be completely dehydrated, allow it to cool. Then add a few drops of water to the residue in the tube. What does the water seem to have done to the solid? Record your observations.
- Repeat with crystals of cobalt(II) chloride hexahydrate (CoCl2•6H2O). Do the same for iron(II) sulfate heptahydrate (FeSO4•7H2O) . Record your observations.
Note: Because of the color changes associated with hydration and dehydration of anhydrous salts, some of these compounds can be used to detect small quantities of water.
Part C. Percent Water of Hydration
- Place a clean, dry porcelain crucible and lid on a wire triangle, supported on a ring attached to a ring stand, with the lid slightly offset to allow an opening. Heat them strongly (high heat) for about 3 minutes to be sure they are perfectly dry. Allow the crucible and lid to stand in place on the triangle long enough to cool again, and then weigh them separately using an analytical balance. (Sometimes lids slip and break, so it is best to have a separate mass for each.) Record the exact mass of the crucible and lid.
- Obtain a sample of one of the hydrated salts. Transfer a sample of the unknown hydrate weighing 2 to 4 grams into the crucible and weigh the crucible with salt and lid again as accurately as possible. Record the mass on your data recording sheet. Obtain the exact mass of the sample by subtraction of the mass of the empty crucible and lid.
- Put the crucible, lid, and sample back on the wire triangle on the ring stand. Position the crucible such that it is at a slight angle on the triangle. With the lid slightly offset (allow an opening for the water to escape), heat the crucible gently for 5 minutes (slowly move the Bunsen burner back and forth across the bottom of the crucible). Then, gradually increase the heat. Continue heating for another 10-15 minutes. Do not let the bottom of the crucible turn red hot at any time.
- Remove heat and allow the crucible, lid, and contents cool completely. Reweigh and record exact mass. (Repeat this process, more than once if necessary, until the mass is within 0.01 g of the first mass in part 4.) Obtain the exact mass of the heated sample by subtraction of the mass of the empty crucible and lid.
- If time permits, perform a second determination using a different mass of hydrate sample.
6.0 DATA RECORDING SHEET
- Hygroscopic and Efflorescent Solids
- Reversibility of Hydration
- Percent Water of Hydration
Identification code for "Unknown": ____________________
After cleaning and heating empty crucible
After heating crucible with unknown
Based on my results the formula for the unknown is: ______________________
7.0 CALCULATIONS AND DATA ANALYSIS
- Calculate the percent water of hydration of your unknown hydrate sample from each determination and take the average, if you did two.
- Obtain the identity of the unknown from your instructor. Calculate the percent water of hydration from the chemical formula. How does this compare with your experimental data?
8.0 POST-LAB QUESTIONS AND CONCLUSIONS
- Write the chemical equations to illustrate the dehydration of hydrate samples used in Part B of this experiment.
- Did the compound(s) that appeared wet in Part A lose or gain water? Explain what may have happened.
- Determine the effect (too high, too low, or no effect) of each of the following procedural steps on the calculated percent of water in the hydrate sample. Explain your answer briefly.
- The hydrate sample was not heated sufficiently (for example, only 5 minutes of total heating time) to drive off all the water.
- The crucible lid was not slightly ajar while heating.
- The crucible was not cooled completely or was still warm when weighed.
- The hydrate was overheated so that may have led to the decomposition of the compound.