October 23, 2017

Chemistry: What Does the Word 'Saturated' Mean?

In chemistry, the word saturated can be used to describe individual molecules or entire solute/solvent mixtures, and it has a different meaning in each of those situations.

Generally speaking, something that's saturated is completely full, as in a saturated sponge (one that's filled with water) or saturation bombing (using hundreds of bombs to destroy an entire area).

Saturated and Unsaturated Molecules

When the word is applied to an individual molecule, it means that the molecule doesn't have any carbon-carbon double bonds or rings. In the molecules below, notice how changing a single bond to a double bond means that you have to remove two hydrogen atoms:

Because carbon can only have four bonds, adding C=C double bond uses up two places that hydrogens could have been bonded two instead.

The first molecule has as many hydrogen atoms as it can hold (no double bonds), so it's saturated, or full of hydrogens. Because the second molecule has a double bond, it doesn't have all the hydrogens it could potentially hold, and it's unsaturated.

Important: Fatty acids are considered saturated if the hydrocarbon chain part of the molecule contains no double bonds. The C=O in the COOH group at the left end of the molecule isn't considered.

Note that the presence of a ring will also make a molecule unsaturated. Straight-chain and branched pentane both have the formula C5H12, while the ring-shaped molecule cyclopentane has the formula C5H10:

Forming the ring shape involves joining the two carbons at the ends of the chain, using up two places to bond where hydrogen atoms would have been. Straight-chain and branched-chain hydrocarbons are saturated, while ring-shaped hydrocarbons are unsaturated.

By the way, there's no such thing as a supersaturated molecule. If you see that option on a test, you're looking at a trick question.

Saturated, Unsaturated, and Supersaturated Solutions

The word saturated has a different meaning when it's used to describe solutions.

For the definitions below, remember that the solvent is the chemical that the solute dissolves in. There's always more solvent than solute.

For example, salt water is a solution of sodium chloride (the solute) in water (the solvent).

The solvent is usually a liquid, but it doesn't have to be: air is a solution of oxygen, carbon dioxide, and water vapor (solutes) in nitrogen (the solvent). Steel is a solution of carbon (the solute) in iron (the solvent).

Saturated solutions contain as much solute as the solvent can hold.

The Dead Sea is saturated: it contains all the salt and minerals it can hold. It has so much salt that the water is more dense than the human body, and people can float easily in it. In the picture to the right, the white solid bits are the pieces of salt that aren't able to dissolve because the water is already saturated.

Unsaturated solutions contain less solute than the solvent can hold.

Unlike the Dead Sea, normal ocean water is unsaturated. Even though it's really salty, it doesn't contain as much salt as it can hold.

Supersaturated solutions contain more solute than the solvent is supposed to be able to hold.

You can make rock candy by boiling sugar water and then cooling it, leading to supersaturation: the sugar is supposed to come out of solution, but it won’t do so until you drop a seed crystal in. (The lack of a seed crystal is also one of the reasons that trees don't freeze in the winter.)

The video below shows how you can make your own supersaturated solution of sodium acetate and use it to make sculptures of "hot ice":

You'll also need to know the terms below:

Dilute solutions contain very little solute compared to the amount of solvent.

Concentrated solutions contain a lot of solute relative to the amount of solvent.

Trick Questions

A common trick question involves asking you to make the correct distinctions between the terms saturated, unsaturated, supersatured, dilute, and concentrated. 

A solution can be both saturated and dilute: When solubility is low, such as when iodine crystals are added to lake water to kill germs, the solution is saturated (full of solute) and dilute (not very concentrated) at the same time. It's a good thing that Iis nonpolar and not very water-soluble: if the solution were concentrated, you'd be drinking large quantities of an elemental halogen!

Ocean water, on the other hand, is both unsaturated and concentrated.

The Dead Sea is both saturated and concentrated.

Comment below if you have any questions!

1 comment:

  1. best simple explanation I have ever read in any article thanks