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Have you ever seen a silica gel packet? Often, you can find them in new shoes. But, I've even had them in food products, like electrolyte powders. What are they used for? They wick up moisture. Think about all the people who try on a pair of shoes. That's a lot of sweaty feet. Shoe companies want to make sure to remove the odor. In the case of food powders, excess moisture causes caking. It's better if everything stays dry. That's also true in chemistry! Sometimes, solid compounds incorporate water molecules. When this occurs, they are called hydrates.

Hydrate Compounds

Technically, hydrates are any molecule that contains water. But, the most common hydrates occur in inorganic chemistry. They are ionic salts or metal complexes with water molecules within their crystal lattice. They always happen in fixed ratios.

Historically, we didn't know the structure of hydrates. Due to this uncertainty, water molecules were indicated in the empirical formula using a dot. For example, aluminum chloride hexahydrate is $AlCl_3 \cdot\ 6H_2O$. It indicates that for every mole of aluminum chloride, there are six moles of water.

Hydrates have two structural options. Why? Water has two ways of interacting. We know water is a polar covalent molecule that hydrogen bonds. So, in some hydrated salts, the water engages in ion-dipole interactions. The water lodges itself within the crystal lattice. Then it interacts with the salt's ions. When does this happen? With metal cations whose electrons are in the s and p atomic orbitals. So, the smaller alkali and alkaline earth metals. But, the transition metals have d-orbitals. The d-orbitals allow the transition metals to form complex ions. Water's oxygen can donate its lone electron pairs. So it can act as a lewis base. When water binds directly to the metal, we know the molecular geometry and structure. When we know the molecular structure, we write the regular empirical formula. So, we'd write aluminum chloride hexahydrate's formula as $[Al(H_2O)_6]Cl_3$. This version indicates that aluminum has six water molecules bound to it through each oxygen atom.

Some compounds start off as anhydrous, meaning they don't contain water. But they can absorb moisture from the air. Molecules that absorb moisture are called hygroscopic. Hygroscopic compounds are often used as drying agents to absorb unwanted moisture. Some compounds even undergo physical changes which indicate when they are wet or dry.

Drierite is a desiccant or drying agent often used in laboratory settings. In contains gypsum or calcium sulfate ($CaSO_4$) with a small amount of copper (II) chloride ($CuCl_2$). When dry, drierite is blue due to the anhydrous state of $CuCl_2$. The compound changes from blue to pink when copper (II) chloride is hydrated. The pink color is a visual indicator that the drierite absorbed moisture.

Determining Hydrate Formula Experimentally

When compounds absorb unwanted moisture, they can be heated to release water molecules. We use the change in mass before and after heating to determine the number of water molecules.

For example, let's say we take 2.02 grams of $Mg(SO_4)_2$ and heat it. The weight decreases to 0.890 grams. What is the hydrate's empirical formula?

We are looking to solve for 'x' in this formula: $Mg(SO_4)_2 \cdot\ xH_2O$. First, we determine the mass of water:

$$Final\, mass\ - \ initial\, mass\ = 2.02\ g\ -\ 0.890\ g\ = 1.13\ g\ H_2O$$

Next, we can determine the moles of water using the molar mass, 18.02 g/mol.

$$1.13g \times \frac{1\ mol\ H_2O}{18.02\ g} = 0.0627\ mol\ H_2O$$

Next, we need to calculate our moles of magnesium sulfate (120.4 g/mol):

$$1.24g \times \frac{1\ mol\ Mg(SO_4)_2}{120.4\ g} = 0.0103 mol\ Mg(SO_4)_2$$

Last we can determine the mole ratio:

$$x\ = \frac{moles\ H_2O}{moles\ Mg(SO_4)_2} =\ \frac{0.0627\ mol\ H_2O}{0.0103\ mol\ Mg(SO_4)_2}\ =\ 6.08$$

Rounding to the whole number, which is possible due to experimental uncertainty, we get x=6. So the formula is $Mg(SO_4)_2\ \cdot\ 6H_2O$.

(Video) Nomenclature of Hydrated Salts

by Professor Dave Explains

Hydrates are unique compounds. So, they have their own unique nomenclature. In this video, Professor Dave explains how to name hydrates. It's a combination of the ionic and covalent systems. You'll also get a quick, simple definition of what a hydrate is and why they exist!