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Dehydration Reaction: Definition & Examples

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  • 0:01 Definition of Dehydration
  • 1:26 Examples of…
  • 3:05 Dehydration vs. Condensation
  • 3:48 Lesson Summary
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Lesson Transcript
Instructor: Nancy Devino
In this lesson, you'll learn how chemists define dehydration reactions and explore some specific examples from both organic and inorganic chemistry. You'll also see how dehydration reactions differ from a broader class of reactions called condensation reactions.

Definition of Dehydration

One of the hardest parts of learning science is keeping up with new terms and definitions. But before you throw up your hands in frustration, remember: lots of new terms are related to words you already know! Consider the term dehydration, which describes what happens to your body when it doesn't have enough water to carry out its everyday functions. Let's look a little deeper at the word itself. The prefix, 'de-', means removal of or from something specified, while 'hydr-' comes from the Greek word for water. Put them together, and the definition matches our experience: 'removal of water' means dehydration.

In chemistry, a dehydration reaction is a chemical reaction that involves the loss of a water molecule from the reactant. Note that the word reactant is singular: we're talking about one molecule of reactant losing one molecule of water. There's another type of reaction, called a condensation reaction, which is more broadly defined as a reaction that results in loss of a water molecule. Just so you know, some books and courses treat dehydration and condensation as interchangeable terms, but that's not technically correct.

This diagram on screen helps explain the difference. Dehydration reactions are a subset of condensation reactions, which will make more sense when we look at some examples of dehydration reactions.

Dehydration reactions are a subset of condensation reactions.
dehydration vs. condensation

Examples of Dehydration Reactions

The one thing that dehydration reactions have in common is that they start with a single molecule and produce a new molecule, plus a molecule of water. These reactions occur frequently in organic chemistry. One of the most common dehydration reactions involves the conversion of an alcohol to an alkene, as shown here, for the dehydration of ethanol to produce ethylene, also known as ethane. Notice that one hydrogen atom in the water molecule came from one carbon in ethanol, and the oxygen atom and the second hydrogen atom come from a different carbon.

Ethanol undergoes a dehydration reaction to form ethylene and a molecule of water.
ethanol dehydration to ethene

There's another kind of dehydration reaction you'll see in inorganic chemistry. Many metal salts exist as hydrates, which means that their crystal structure includes water molecules. One of the most colorful examples of this is copper (II) sulfate pentahydrate, which is sometimes known as blue vitriol. When heated above 100 degrees Celsius, the water molecules are released from the crystal structure. The resulting anhydrous product, copper sulfate, is white.

Here's the chemical equation that describes this reaction:

Copper sulfate pentahydrate undergoes dehydration to produce anhydrous copper sulfate and five molecules of water.
copper sulfate pentahydrate dehydration

Now, as if new vocabulary isn't hard enough, sometimes the reactions that remove water from hydrates aren't classified as dehydration reactions. You might see them referred to as decomposition, but they really don't decompose. In the dehydration reaction, the metal salt's crystal structure and its color may change, but it still has the same chemical formula as before. As the reaction involves the same formula and same compound, there's no decomposition.

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