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Hydrates in Chemistry: Definition, Types, and Uses

Linda Crampton has an honors degree in biology. She has taught high school biology, chemistry, and physics as well as middle school science.

Two inorganic hydrates—magnesium sulphate heptahydrate (Epsom salts) and copper sulphate pentahydrate

Two inorganic hydrates—magnesium sulphate heptahydrate (Epsom salts) and copper sulphate pentahydrate

Types of Hydrates

In chemistry, a hydrate is a compound that absorbs water molecules from its environment and includes them as part of its structure. The water molecules either stay intact inside the compound or partially break up into their elements. Three major categories of hydrates are inorganic hydrates, organic hydrates, and gas (or clathrate) hydrates.

The water molecules inside inorganic hydrates are generally released when the compound is heated. In organic hydrates, however, the water chemically reacts with the compound. A ”building block” of a gas hydrate consists of a molecule of gas—which is often methane—surrounded by a cage of water molecules. Gas hydrates have been found in ocean sediments and in polar regions. They offer the exciting possibility of acting as an energy source in the near future.

Crystals of chalcanthite (blue) and limonite (brown) minerals; chalcanthite is hydrated copper sulphate while limonite is a mixture of hydrated iron oxides

Crystals of chalcanthite (blue) and limonite (brown) minerals; chalcanthite is hydrated copper sulphate while limonite is a mixture of hydrated iron oxides


This article discusses chemicals from a scientific point of view. Anyone who uses the chemicals or who comes in contact with them should consider safety concerns. No substances with a history of medicinal use should be ingested without receiving a doctor’s advice about safety and concentration.

Inorganic Hydrates

An inorganic hydrate can release its water molecules, becoming anhydrous. The anhydrous form of the substance can absorb water, becoming hydrated. The water is known as the water of hydration or the water of crystallization.

A common inorganic hydrate is sodium carbonate decahydrate (washing soda). The first part of a hydrate's name—sodium carbonate in this example—is the name of the anhydrous compound. This is followed by the word "hydrate" preceded by a prefix that indicates the number of water molecules present in the hydrated compound. The word "decahydrate" means that one molecule of sodium carbonate has ten water molecules attached to it when it's hydrated. The table below shows the number prefixes used in chemistry and their meanings.

Number Prefixes Used in Chemistry

Number of Atoms or MoleculesPrefix





















Cobalt (ll) chloride hexahydrate is known as cobaltous chloride in an older naming system.

Cobalt (ll) chloride hexahydrate is known as cobaltous chloride in an older naming system.

Some Common Inorganic Hydrates

Some other common inorganic hydrates in addition to washing soda are magnesium sulphate heptahydrate (Epsom salts), sodium tetraborate decahydrate (borax), and sodium sulphate decahydrate (Glauber's salt, or sal mirabilis). Copper sulphate and cobalt chloride also form inorganic hydrates and have attractive colours in their hydrated forms.

Epsom Salts, Borax, and Glauber's Salt

Epsom salts are named after the town of Epsom in England, where they were discovered. They are used in different formulations as bath salts and as soil fertilizer. Many people claim that magnesium from the chemical can be absorbed through the skin. There is little scientific evidence for this process, however.

Borax is used as a household cleaner and is present in low concentrations in some cosmetics. There are multiple concerns about its safety. It mustn’t be ingested and should be kept out of reach of children and pets.

Glauber’s salt is named after Johann Rudolf Glauber, a German-Dutch chemist and apothecary who lived in the seventeenth century. Glauber discovered sodium sulphate and also discovered that it acts as a laxative in humans.

Copper Sulphate

Two popular inorganic hydrates have a dramatic difference in colour between their hydrated and their anhydrous forms. Copper (ll) sulphate, also known as copper sulphate, cupric sulphate, blue vitriol, or bluestone, is blue in its hydrated form and grey-white in its anhydrous form. Heating the blue form removes the water and causes the chemical to become white. The anhydrous form becomes blue again when water is added.

Each copper sulphate unit can attach to five water molecules, so it’s sometimes called copper sulphate pentahydrate when it’s hydrated. The formula of the hydrated form is CuSO4. 5H2O. The dot after the formula for copper sulphate indicates bonds with water molecules. Research suggests that the nature of these bonds is not as simple as was once thought.

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Cobalt Chloride

Cobalt (ll) chloride is sky blue in its anhydrous form and purple in its hydrated form (cobalt (ll) chloride hexahydrate). Cobalt chloride paper is useful for indicating whether moisture is present. It's sold in vials containing thin strips of paper coated with cobalt chloride. The paper is blue when no moisture is present and turns pink in the presence of water. It's useful for detecting relative humidity.

Anhydrous cobalt (ll) chloride (or cobaltous chloride anhydrous according to the older naming system)

Anhydrous cobalt (ll) chloride (or cobaltous chloride anhydrous according to the older naming system)

Efflorescent, Hygroscopic, and Deliquescent Substances


Certain inorganic hydrates can lose at least some of their water when they are at room temperature. These hydrates are said to be efflorescent. Washing soda and Glauber's salt are examples of efflorescent substances. They become less crystalline and more powdery as they give up water. In order for the water to be lost, however, the partial pressure of the water vapour at the surface of the hydrate must be greater than the partial pressure of the water vapour in the surrounding air. Copper sulphate will effloresce only if the surrounding air is very dry.


Some hydrates absorb water from the air or from a liquid without human intervention and are said to be hygroscopic. Hygroscopic solids can be used as desiccants—substances that absorb water from the environment. This is helpful when the air in a package has to be kept dry. Anhydrous calcium chloride is an example of a hygroscopic substance that is used as a desiccant.


Some solids absorb so much water from their surroundings that they can actually form liquid solutions. These solids are known as deliquescent substances. Calcium chloride is both hygroscopic and deliquescent. It absorbs water as it becomes hydrated and then may continue to absorb water to form a solution.

The video below explores the water of crystallization in copper (ll) sulphate.

Aldehydes and Ketones


Chemicals that belong to the aldehyde or ketone family may form organic hydrates. The general formula of an aldehyde is RCHO. The R group represents the "remainder" of the molecule and is different in each aldehyde. The carbon atom is joined to the oxygen atom by a double bond. The carbon atom and its attached oxygen are known as a carbonyl group.


The general formula of a ketone is similar to the formula of an aldehyde, except in place of the H is a second R group. This may be the same as the first R group or may be different. Like aldehydes, ketones contain a carbonyl group. In the illustration below, it's understood that there is a carbon atom at the base of the double bond.

Carbonyl Hydrates

A water molecule can react with the carbonyl group of an aldehyde or a ketone to form a substance known as a carbonyl hydrate, as shown in the first reaction below. The carbonyl hydrates usually form a very small percentage of the molecules in a sample of a specific aldehyde or ketone. There are a few notable exceptions to this rule, however.

One exception is a solution of formaldehyde. The solution consists almost entirely of molecules in the carbonyl hydrate form (and its derivatives), with only a small proportion of the molecules in the aldehyde form. This is shown by the large value of the equilibrium constant (K) for formaldehyde in the illustration below. K is found by dividing the concentration of the products of a reaction by the concentration of the reactants (although some additional rules are required for determining its value).

Extent of hydration of some carbonyl compounds

Extent of hydration of some carbonyl compounds

Formaldehyde and Ethanol

Formaldehyde, also called methanal, is the simplest member of the aldehyde family. Its "R" group consists of a single hydrogen atom. A hydrate is formed from formaldehyde by the reaction of its carbonyl group with water. An H2O molecule splits up into an H and an OH as the hydrate is formed.

A solution of formaldehyde in water is known as formalin. Formaldehyde is a preservative for animal tissues and bodies, including those sent to schools for dissections in biology classes. However, it's strongly suspected of being a human carcinogen (a chemical that causes cancer). Some companies that supply preserved animals now remove the formaldehyde before shipping the animals.

Another example of organic hydrate production is the conversion of ethene (also called ethylene) to ethanol. Phosphoric acid is used as a catalyst. The formula of ethene is CH2=CH2. The formula of ethanol is CH3CH2OH. The water molecule splits up into H and OH as it reacts with ethene.

Gas Hydrates and Their Potential Uses

Chunks of gas hydrates look like lumps of ice and appear to be crystalline solids. The building blocks of the hydrates are made at low temperature and high pressure when water molecules surround a gas molecule, forming a frozen mesh or cage. The gas is often methane, in which case the name methane hydrate may be used for the hydrate, but it may also be carbon dioxide or another gas. The methane is produced by bacterial decay of dead plants and animals. Methane has the formula CH4.

Gas hydrates have been located around the world. They form in sediments at the bottom of deep oceans and lakes and are also found on land in permafrost. Methane hydrates have the potential to be an excellent source of energy. In fact, some researchers estimate that the total amount of energy trapped in the world's gas hydrates may be greater than the total energy present in all known fossil fuels on Earth. If a gas hydrate is lit by a match or another flame, it will burn like a candle.

Possible Dangers of Gas Hydrates

Not everyone is excited by the discovery of gas hydrates. Some people think that they could be a natural hazard rather than a natural resource. Researchers are currently trying to find the most effective way to extract methane molecules from their water cages. Some people worry that as a result of the extraction methane will enter the atmosphere and affect the Earth's climate. It's thought that methane in the atmosphere contributes to global warming.

Gas hydrates can block natural gas pipelines and may sometimes be a drilling hazard. Another problem could result from the fact that the hydrates cement ocean sediments together. If the hydrates in a large area melt, the sediments could move. This might produce a landslide that may cause a tsunami.

Interesting and Important Chemicals

Hydrates are interesting chemicals that are often very useful. Exploring their nature, properties, and behaviour is important. Gas hydrates are particularly interesting and are attracting the attention of many researchers. They could become very helpful in our future. There is much to learn about the best ways to use them and about safety procedures, however. Hopefully, their effects on our lives will be beneficial instead of harmful.

A Hydrate Quiz for Review and Fun

For each question, choose the best answer. The answer key is below.

  1. How many water molecules are joined to each molecule of Epsom salts?
    • four
    • five
    • six
    • seven
  2. What prefix is used in chemistry to represent the presence of five atoms or molecules?
    • hexa
    • nona
    • tetra
    • penta
  3. The chemical name for washing soda is sodium sulphate decahydrate.
    • True
    • False
  4. What colour is cobalt (ll) chloride is its anhydrous form?
    • blue
    • red
    • purple
    • white
  5. An efflorescent substance releases water at room temperature.
    • True
    • False
  6. Which chemical is often used as a desiccant?
    • sodium sulfate
    • sodium carbonate
    • calcium chloride
    • magnesium sulfate
  7. Most aldehydes exist in their carbonyl hydrate form.
    • True
    • False
  8. Gas hydrates are found on land in warm habitats.
    • True
    • False
  9. The Earth's gas hydrates contain a lot of energy, but not as much as known fossil fuels.
    • True
    • False

Answer Key

  1. seven
  2. penta
  3. False
  4. blue
  5. True
  6. calcium chloride
  7. False
  8. False
  9. False


This content is accurate and true to the best of the author’s knowledge and does not substitute for diagnosis, prognosis, treatment, prescription, and/or dietary advice from a licensed health professional. Drugs, supplements, and natural remedies may have dangerous side effects. If pregnant or nursing, consult with a qualified provider on an individual basis. Seek immediate help if you are experiencing a medical emergency.

Questions & Answers

Question: What may happen when a container of cadmium chloride hydrate is left open?

Answer: Cadmium chloride should be stored carefully. It's a hygroscopic substance. It absorbs water from its environment, is soluble in water, and forms hydrates. It’s a potentially dangerous substance in all of its forms. The MSDS (Material Safety Data Sheet) for cadmium chloride says that it’s very hazardous in case of ingestion and hazardous in case of skin and eye contact and after inhalation. It’s also a probable carcinogen. First aid and/or medical treatment may be needed if a person doesn't take precautions when dealing with the chemical.

© 2012 Linda Crampton


Linda Crampton (author) from British Columbia, Canada on February 13, 2019:

Thank you for the comment, Manas. I hope your project is successful.

Manas on February 13, 2019:

you saved my life because my project submission is tomorrow and you have made me understand hydrates in simple words.........:)

Linda Crampton (author) from British Columbia, Canada on January 20, 2018:

You're welcome, narada.

narada on January 20, 2018:


Linda Crampton (author) from British Columbia, Canada on December 19, 2017:

Thank you for the comment, Maryam.

Maryam on December 19, 2017:


Linda Crampton (author) from British Columbia, Canada on November 19, 2017:

The online version of the Merriam-Webster dictionary has basic chemical definitions for acetal, ketal, and hemiacetal. The Khan Academy has a video showing the difference between a hemiacetal and a hemiketal.

Deyaa Hassan on November 11, 2017:

what are the difference between hydrates, ketals, semiketals, acetals and hemiacetals

Linda Crampton (author) from British Columbia, Canada on May 19, 2017:

Thank you for the comment. If you do an Internet search for "list of hydrates in chemistry" you should find a list of the most common inorganic hydrates, though not of all of them.

xxkelseyxx on May 19, 2017:

Thank you for the article, it was very helpful for me. I was wondering if you know a list of examples of inorganic hydrates

Linda Crampton (author) from British Columbia, Canada on March 03, 2017:

Thank you, Jessica! I appreciate your comment.

Jessica Chelsea on March 03, 2017:

Thank you for this article! I fully understood hydrates now. This helped in my research in hydrates. :)

Jason from Indianapolis, IN. USA on August 27, 2015:

Oh sure no problem!

Linda Crampton (author) from British Columbia, Canada on August 27, 2015:

Thank you very much for the kind comment, jbosh1972!

Jason from Indianapolis, IN. USA on August 27, 2015:

Very clever! You have taken a subject that seems at first glance possessing only a short answer and expanding it to the most comprehensive article on hydrate chemistry I have ever seen!

Linda Crampton (author) from British Columbia, Canada on August 07, 2015:

Thank you very much, melbel! I appreciate your visit and kind comment.

Melanie Palen from Midwest, USA on August 07, 2015:

Wow, such an awesome hub. I remember learning about hydrates in Chem I (and I wasn't a huge fan of the concept.) Your hub really explains hydrates really well. Awesome!

Linda Crampton (author) from British Columbia, Canada on January 30, 2015:

Thank you very much for such a lovely comment, Mari!

Mari on January 30, 2015:

This is one of the best articles on hydrates I've ever read.

Linda Crampton (author) from British Columbia, Canada on February 25, 2013:

I'm glad that my article helped you with your lab report, Joel - and thank you so much for giving me credit!

Joel on February 25, 2013:

thanks so much! this helped with immensely with a chemistry lab report (don't worry, I gave you your due credit :p)

Linda Crampton (author) from British Columbia, Canada on May 26, 2012:

Thanks for the visit and the comment, Ely. There are a lot of terms to remember in science!

Ely Maverick from The Beautiful Archipelago of the Philippines on May 26, 2012:

I must review my Chemistry first, remembering those terms.

Linda Crampton (author) from British Columbia, Canada on May 16, 2012:

Hi, Nell. Don't worry - the quiz was for fun! Thank you very much for the comment and the share, Nell. I appreciate them both.

Nell Rose from England on May 16, 2012:

Hi, I have to sit the exam again I got a paltry 22 percent on your test at the bottom! lol! I read it all, made sure I understood the difference between inorganic and organic hydrates then promptly forgot! but seriously this is great for schools and university and will be really helpful, up and shared! nell

Linda Crampton (author) from British Columbia, Canada on May 14, 2012:

Thank you very much for the comment, drbj. I think that hydrates are interesting - and so is the Hydra!

drbj and sherry from south Florida on May 14, 2012:

Before I read this entertaining and extremely informative hub, Alicia, my hydrate knowledge was pathetically inadequate. Who knew? Thank you for enlarging my hydrate education. Heretofore, the only Hydra I really knew was that multiple-headed monster in Greek mythology slain by Hercules.

Linda Crampton (author) from British Columbia, Canada on May 12, 2012:

Thank you very much for the visit and the comment, CyclingFitness!!

Liam Hallam from Nottingham UK on May 12, 2012:

Nie article, to the point, well thought out and covers a lot of different visual media for the viewer. Looks like it'll be an ideal resource for chemistry students

Linda Crampton (author) from British Columbia, Canada on May 12, 2012:

Thank you for the comment, teaches! I appreciate your visit.

Dianna Mendez on May 12, 2012:

This is an interesting hub on hydrates. I can see where chemistry students would enjoy this hub. Thanks for the simple definition and for the examples of hydrate compounds.

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