Video Transcript
In this lesson, we will learn how to define water of crystallization and explain its effect on the structures and properties of crystals. It can be a nice fun activity to grow crystals of chemical compounds in the school or college lab. This can easily be achieved by making a solution of a chemical salt.
Salts are often produced by the neutralization of an acid with a base. A salt and water are formed. If the acid and base combination chosen were sulfuric acid and copper oxide, the salt produced is copper sulfate. If the solution containing the dissolved salt is left in a warm place, the water will slowly evaporate away. Some days later, there is no sign of the liquid, and some uniformly shaped dry crystals remain. The dry copper sulfate crystals will have a blue color, and they will have regular shapes whether their sizes may vary.
Are the copper sulfate crystals truly dry? They’re not wet to touch, and there’s no signs of liquid around them. However, if we would take a portion of the crystals, place them in a large dry test tube or boiling tube and heat them strongly, some rather interesting observations would be made. The blue solid crystals of copper sulfate will slowly begin to turn a white color. Condensing steam will be seen leaving the test tube. Water vapor also condenses around the top of the test tube where it’s cool and forms droplets of liquid water. After further heating, to remove all traces of these condensed water droplets in the test tube, all that remains is a white powder.
The blue copper sulfate crystals did not melt or decompose during this heating process, but they seem to have changed from a blue form to a white form. Where has this water come from? There was apparently no water in the crystals or the test tube at the start of this heating process. We would’ve discovered what is known as water of crystallization or, less commonly, water of hydration. The water molecules that were released from the crystals by heating are water molecules that are part of the chemical structure of the salt. They are present inside the crystal lattice. Remember that crystals or crystalline solids have their constituent atoms, molecules, or ions arranged in a highly regular microscopic structure, called a crystal lattice.
This crystal lattice contains repeating formula units that extend in all directions throughout the crystal. As copper sulfate is an ionic substance, its crystal lattice will contain positively charged copper ions and negatively charged sulfate ions. The blue copper sulfate crystals that were heated in the experiment were not just wet or damp crystals. They contained water molecules chemically bound into their crystal lattice. These water molecules are known as water of crystallization, and they are incorporated into the salt crystals as they crystallize out of a solution which contains mostly water.
The blue version of the copper sulfate crystals is known as the hydrated salt. Hydrated simply means that the crystals contain water of crystallization. When these crystals were heated, the heat energy was sufficient to drive these water molecules out of their positions in the crystal lattice. Some chemical bonds between the water molecules and the ions in the crystal lattice were broken. This left behind the white version of copper sulfate. These copper sulfate crystals are known as anhydrous. Anhydrous means without water of crystallization. Apart from their color, whether the crystals are hydrated or anhydrous can be identified by looking closely at the chemical formula for each compound.
Remember that hydrated copper sulfate crystals contain water of crystallization. Anhydrous copper sulfate does not contain water of crystallization. Hydrated copper sulfate crystals have the formula CuSO4.5H2O. Notice that there are two obvious parts to this formula. One is the expected formula for copper sulfate, indicating that the compound contains one copper atom, one sulfur atom, and four oxygen atoms. As this is an ionic compound, the copper is present as copper cations with a positive charge. The sulfur and oxygen atoms are chemically bonded together in sulfate anions. CuSO4 is the simple unit or building block upon which the crystal structure of copper sulfate is built. Then, there is a dot in the formula. This is a middle dot or interpoint, not a full stop or period dot as used at the end of a sentence.
After the dot, we can clearly see that there are five water molecules also present in the formula. The number of water molecules therefore present in one formula unit of copper sulfate is therefore five. In the anhydrous or white version of copper sulfate, this dot is missing. The formula of anhydrous copper sulfate is just CuSO4. So when we look at the formula for ionic salts, the water after the dot can easily be seen. It tells us that we’re dealing with the hydrated salt where water of crystallization is in fact present. Another example of a hydrated salt containing water of crystallization is iron(II) sulfate, also known as ferrous sulfate. The iron(II) sulfate contains seven molecules of water per unit of iron(II) sulfate.
In one mole of hydrated iron(II) sulfate, there is one mole of iron(II) sulfate for every seven moles of water. This stoichiometric ratio will always be seen throughout the crystal lattice. Another example is sodium carbonate, which is commonly called washing soda. Sodium carbonate decahydrate as it is known contains 10 moles of water molecules per mole of sodium carbonate in its crystal lattice. A rather more complex example is the salt chromium potassium sulfate, which is also known as chrome alum. The hydrated salt contains potassium ions, chromium ions, sulfate ions, and water of crystallization. Per mole of chromium potassium sulfate, there are 12 moles of water of crystallization.
The hydrated version of this salt exists as rather beautiful dark purple cubic crystals. These can be grown with a high degree of success in a school or college lab. It’s a greyish brown powder without the 12 moles of water of crystallization. Rather strangely, some salts can lose or gain water of crystallization. This can depend upon how they’re stored and the environment that they’re stored in. If sodium carbonate decahydrate, the hydrated salt, is stored in a dry place, it loses water of crystallization to the atmosphere and becomes the monohydrate. This process is known as efflorescence, and it can result in salty deposits on stonework and brickwork in buildings.
Some salts absorb water from the atmosphere. These are known as hygroscopic salts. Notice the spelling here which does not contain hydro- as in hydroelectric power, a word stem often associated with water. Sodium hydroxide, commonly known as lye, is a hygroscopic substance. When crystallized from water, it forms a monohydrate NaOH.H2O. Sodium hydroxide readily absorbs water from air, and it may even form a solution in this absorbed water. This process is known as deliquescence. It is possible to find how much water of crystallization there is in a formula of a salt by doing an experiment where some simple mass measurements are taken. The mass of a sample of a hydrated salt in a crucible with its lid would be found before a heating process took place.
We would also need to find the mass of the empty crucible with its lid beforehand. We would subtract the mass of the empty crucible with its lid from the mass of the crucible and the lid and the sample before the heating process took place in order to find the mass of the sample. After this heating process, the mass of the anhydrous salt plus the crucible plus the lid would be determined. We would need to be convinced that all the water of crystallization had left the sample. We could repeat the process and see if the mass changes. Repeating the heating and weighing processes until a constant mass is achieved is known as heating to constant mass. The apparent loss in mass is equivalent to the mass of water of crystallization removed from the sample.
This is found by simply subtracting the mass of the sample after heating from the mass of the sample before heating. We would need to know more about the exact formula of the salt concerned to find the precise number of moles of water of crystallization that this salt contains. So let us now look at a question to test our understanding in a quantitative setting. In this question, mass measurements have been taken before and after the heating of a hydrated salt.
A student is attempting to determine how many water molecules there are in the hydrate CoSO4.𝑥H2O, where 𝑥 is an integer. The student weighs a sample of the compound and heats it until the mass remains constant. Using the experiment results below, determine the value of 𝑥. Mass of sample before heating, 4.97 grams; mass of sample after heating, 2.74 grams.
In this question, we are starting with 4.97 grams of a hydrated salt sample. The hydrated salt sample is based upon cobalt sulfate. As it stands, the formula of this hydrated cobalt sulfate suggests that it contains one mole of cobalt sulfate for every 𝑥 moles of water. To find the value of 𝑥 in this formula, we need to establish the moles of cobalt sulfate to the moles of water in the terms of one to 𝑥. In the question, it states that 𝑥 is an integer. This means that it will have a whole number value. We can establish the value of 𝑥 by finding the moles of cobalt sulfate in the sample and the moles of water in the sample. We can do this by using the masses quoted in the experiment.
The mass of the sample before heating includes the mass of the cobalt sulfate plus the water of crystallization. The mass of the sample after heating represents the anhydrous sample. This contains only cobalt sulfate. The loss in mass seen in this experiment therefore represents the water of crystallization that this sample contained. The mass of water that this hydrated salt contained is therefore 4.97 grams subtract 2.74 grams, which equals 2.23 grams of water of crystallization. The molar mass for water is 18 grams per mole. This is found by adding together the atomic masses for hydrogen and oxygen as per the formula for water. The moles of water contained in this sample is therefore 2.23 grams of water divided by 18 grams per mole. So this hydrated salt contains about 0.124 moles of water.
Remember that the sample after heating contains only cobalt sulfate. The mass of cobalt sulfate in the hydrated salt — the hydrate, that is — is therefore 2.74 grams. The molar mass of cobalt sulfate adds up to 155 grams per mole. Again, we use the atomic masses for cobalt sulfur and oxygen and the formula for cobalt sulfate to find this out. To find the moles of cobalt sulfate in the sample, we take the mass of cobalt sulfate, which is 2.74 grams, and we divide it by the molar mass, which is 155 grams per mole. This gives us 0.0177 moles of cobalt sulfate in this sample.
The moles of water in the hydrate is clearly greater than the moles of cobalt sulfate in the hydrate. To simplify these two mole quantities into the form one to 𝑥, we simply divide both by the smallest number of moles. When we do this, we find that cobalt sulfate and water are in the ratio of one mole of cobalt sulfate to approximately seven moles of water. Although seven moles of water is not the precise value here, we are looking for an integer value and seven is the closest whole number. The value of 𝑥 in the formula is therefore seven.
Let us now summarize by reviewing the key points from this lesson. Hydrated salts contain water of crystallization. Anhydrous salts do not contain water of crystallization. Water of crystallization are water molecules that are chemically bonded to the crystal lattice. The number of moles of water of crystallization in a compound is indicated by the chemical formula. It is seen as a number before H2O and after a dot in the formula.
