Oct 15, 2026 · 5 min · 5 segments
As long as we're naming stuff, let's name these hydrated salts! You know, like epsom salts? Watch the whole General Chemistry playlist: http://bit.ly/ProfDaveGenChem…
We always name the cation first and then the anion, applying prefixes and suffixes as necessary so that the name will unambiguously refer to the compound in question.
But sometimes in the formation of an ionic compound or a salt, a number of water molecules will be associated with the ions within the crystal structure.
This is called a hydrated salt, or simply a hydrate, and these water molecules, unlike the water molecules floating around in an aqueous solution, are actually part of the formal structure of the salt.
There will always be a specific number of water molecules coordinated to each formula unit of a particular salt due to the specific geometry of the lattice, and we can report this number when we list the chemical formula of the hydrate.
For example, cobalt II chloride will form a hydrate with six water molecules for every formula unit.
To write the formula for this, we simply list the ionic compound as we normally would, followed by a dot to separate the salt from the waters of hydration, and then we list a water molecule with a stoichiometric coefficient reporting how many waters are involved per formula unit, in this case six.
If instead we wish to refer to this compound with words rather than formula notation, once again, we simply list the name of the ionic compound as we normally would, with a Roman numeral if necessary to indicate the oxidation state of the cation, and then the word "hydrate" with a prefix indicating the number of water molecules involved in one formula unit of the hydrate.
Epsom salt, which has various everyday applications, is another example of a hydrate, and it is the common name for magnesium sulfate heptahydrate, since there are seven water molecules per formula unit.
We should be familiar with the prefixes we would use in a context like this, but in case we have forgotten, here they are, from di- to deca-, or two through ten.
The prefixes can go beyond ten, but for our purposes, memorizing this list should suffice.
Sometimes a hydrate can lose its waters of hydration when exposed to the atmosphere, and this process is called efflorescence.
The result would be an anhydrous salt, one with no water molecules in the lattice, which would be written like a normal ionic solid.
Likewise, some anhydrous salts can become hydrates by absorbing water molecules from the atmosphere.
We always name the cation first and then the anion, applying prefixes and suffixes as necessary so that the name will unambiguously refer to the compound in question.
But sometimes in the formation of an ionic compound or a salt, a number of water molecules will be associated with the ions within the crystal structure.
This is called a hydrated salt, or simply a hydrate, and these water molecules, unlike the water molecules floating around in an aqueous solution, are actually part of the formal structure of the salt.
There will always be a specific number of water molecules coordinated to each formula unit of a particular salt due to the specific geometry of the lattice, and we can report this number when we list the chemical formula of the hydrate.
For example, cobalt II chloride will form a hydrate with six water molecules for every formula unit.
To write the formula for this, we simply list the ionic compound as we normally would, followed by a dot to separate the salt from the waters of hydration, and then we list a water molecule with a stoichiometric coefficient reporting how many waters are involved per formula unit, in this case six.
If instead we wish to refer to this compound with words rather than formula notation, once again, we simply list the name of the ionic compound as we normally would, with a Roman numeral if necessary to indicate the oxidation state of the cation, and then the word "hydrate" with a prefix indicating the number of water molecules involved in one formula unit of the hydrate.
Epsom salt, which has various everyday applications, is another example of a hydrate, and it is the common name for magnesium sulfate heptahydrate, since there are seven water molecules per formula unit.
We should be familiar with the prefixes we would use in a context like this, but in case we have forgotten, here they are, from di- to deca-, or two through ten.
The prefixes can go beyond ten, but for our purposes, memorizing this list should suffice.
Sometimes a hydrate can lose its waters of hydration when exposed to the atmosphere, and this process is called efflorescence.
The result would be an anhydrous salt, one with no water molecules in the lattice, which would be written like a normal ionic solid.
Likewise, some anhydrous salts can become hydrates by absorbing water molecules from the atmosphere.
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