Why are all ionic compounds electrolytes

Because all ionic compounds are formed when anions and cations are attracted to one another, ionic compounds frequently have similar characteristics.

Índice Show

Ionic Compounds Form Crystals
Ionic Compounds Often Have High Melting and Boiling Points
Ionic Compounds Are Hard and Brittle
Ionic Compounds Conduct Electricity When Dissolved in Water or Melted

Ionic Compounds Form Crystals

Ionic compounds consist of cations and anions that stick next to each other because of their opposite charges. Imagine a single lithium cation stuck next to a single chlorine anion to form lithium chloride. Now, it's unlikely that only one lithium ion and one chloride ion will be present in this location—generally, when we speak of chemical reactions, we're talking about a huge number of atoms undergoing a reaction in a very small place (one teaspoon of salt contains approximately 1022 atoms). As a result, if our single LiCl pair were to come close to another LiCl pair, the following would take place:

Because oppositely charged ions attract one another, the LiCl pairs will tend to form larger groups. These larger groups, in turn, will form even larger groups of ions, as shown in the following figure:

Figure 8.3The positive charge on the lithium cation of one pair will be attracted to the negative charge on the chloride ion of the other pair.

Figure 8.4This process, where stacks of LiCl ions combine with one another, will continue until there are no more lithium or chloride ions.

Crystals are large arrangements of ions or atoms that are stacked in regular patterns. Many ionic compounds form very large crystals.

These large arrangements of ions are referred to as crystals. Though crystals are frequently formed from ionic compounds, they also exist in some other chemical compounds, such as diamonds. We'll talk about this in much greater detail in Solids.

Hydrates are formed when one or more molecules of water attach themselves to ionic compounds. These compounds are interesting because they appear dry but give off water when heated. Particularly interesting is Epsom salt, or magnesium sulfate heptahydrate (MgSO4·7H2O). When heated, enough water is given off that it actually dissolves the magnesium sulfate!

Ionic Compounds Often Have High Melting and Boiling Points

What happens when you heat something up in your kitchen? You may have discovered while cooking (or while microwaving random things while bored) that most of the foods we eat either melt or burn when heated. Some foods even do both! As you can probably guess, I'm an expert when it comes to putting out house fires.

Ionic compounds, on the other hand, frequently melt and boil at much higher temperatures than other materials. In order for ionic compounds to melt, enough energy must be added to make the cations and anions move away from one another. Because these attractions are so strong, it takes a lot of energy to pull these ions apart. Adding this much energy to ionic compounds requires a great deal of heat, which is why ionic compounds have very high melting and boiling points.

Ionic Compounds Are Hard and Brittle

Imagine bashing a big chunk of lithium chloride against your head. What do you suppose that might feel like? If you guessed that it would hurt like crazy, you were right. Like many ionic compounds, lithium chloride is as hard as a rock.

Ionic compounds are extremely hard because it is difficult to make the ions move apart from each other in a crystal. Even if you apply a great deal of force on the crystal (imagine running headlong into a giant wall of lithium chloride), the attraction between the cations and anions will frequently continue to hold the crystal together.

Let's say, though, that you really want to break apart an ionic compound. While very hard, ionic compounds are also frequently very brittle, meaning that they break apart when the right kind of force is applied. As the following figure shows, where you apply the force is just as important as how much force you use.

Figure 8.5By applying force in a way that pries the cations and anions apart from each other: you can cause a crystal to completely break apart.

As you can see from this diagram, ionic crystals align themselves such that there are regions where a small force can break apart the crystal. These regions are sometimes referred to as "cleavage planes" because they are the locations where the crystal is weakest and can most easily be broken.

Ionic Compounds Conduct Electricity When Dissolved in Water or Melted

Once upon a time, there was an inventor who came up with a device for drying hair. This "hairdryer" as he called it, heated air with electricity and blew it across the hair of the person holding it. Because water evaporates when heated, the hair dried more quickly. This inventor's legacy lives on to this day in a household appliance loved by millions.

Electrolytes are compounds that conduct electricity when dissolved in water. Many ionic compounds are considered to be electrolytes. However, some ionic compounds don't dissolve in water. As a result, they don't share this property.

Shortly afterward, there was a guy who decided that he didn't want to wait to get out of the bathtub before drying his hair. His legacy: A hairdryer warning sticker with a picture of a guy getting electrocuted.

When ionic compounds are placed in water, they cause the water to conduct electricity. Normally, water doesn't conduct electricity well at all. However, when salts dissolve in water, they break up into their constituent cations and anions and it is the presence of these ions that allows it to conduct electricity. Because salts conduct electricity when dissolved in water, they are referred to as electrolytes.

In the same way, pure salts also conduct electricity when they are melted. As a solid, the anions and cations in an ionic compound are locked in place and unable to move electrical charge. However, when the ionic compound is melted, these ions are free to move around and conduct charge.

Excerpted from The Complete Idiot's Guide to Chemistry © 2003 by Ian Guch. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.

To order this book direct from the publisher, visit the Penguin USA website or call 1-800-253-6476. You can also purchase this book at Amazon.com and Barnes & Noble.

Updated March 25, 2019

By Lee Johnson

Salt water is the most well-known example of an ionic solution that conducts electricity, but understanding why this happens isn’t as simple as performing a home experiment on the phenomenon. The reason comes down to the difference between ionic bonds and covalent bonds, as well as understanding what happens when dissociated ions are subjected to an electric field.

In short, ionic compounds conduct electricity in water because they separate into charged ions, which are then attracted to the oppositely charged electrode.

You need to know the difference between ionic and covalent bonds to get a better understanding of the electrical conductivity of ionic compounds.

Covalent bonds are formed when atoms share electrons to complete their outer (valence) shells. For example, elemental hydrogen has one “space” in its outer electron shell, so it can bond covalently with another hydrogen atom, with both sharing their electrons to fill their shells.

An ionic bond works differently. Some atoms, like sodium, have one or very few electrons in their outer shells. Other atoms, like chlorine, have outer shells that just need one more electron to have a full shell. The extra electron in that first atom can transfer to the second to fill that other shell.

However, the processes of losing and gaining elections create an imbalance between the charge in the nucleus and the charge from the electrons, giving the resultant atom a net positive charge (when an electron is lost) or a net negative charge (when one is gained). These charged atoms are called ions, and oppositely charged ions can be attracted together to form an ionic bond and an electrically neutral molecule, such as NaCl, or sodium chloride.

Note how "chlorine" changes to "chloride" when it becomes an ion.

The ionic bonds that keep molecules like common salt (sodium chloride) together can be broken apart in some circumstances. One example is when they’re dissolved in water; the molecules “dissociate” into their constituent ions, which returns them to their charged state.

The ionic bonds can also be broken if the molecules are melted under high temperature, which has the same effect when they remain in a molten state.

The fact that either of these processes leads to a collection of charged ions is central to the electrical conductivity of ionic compounds. In their bonded, solid states, molecules like salt don’t conduct electricity. But when they're dissociated in a solution or through melting, they can carry a current. This is because electrons can’t move freely through water (in the same way they do in a conductive wire), but ions can move freely.

To apply a current to a solution, two electrodes are inserted into the liquid, both attached to a battery or source of charge. The positively charged electrode is called the anode, and the negatively charged electrode is called the cathode. The battery sends charge to the electrodes (in the more traditional way involving electrons moving through a solid conductive material), and they become distinct sources of charge in the liquid, producing an electric field.

The ions in the solution respond to this electric field according to their charge. The positively charged ions (sodium in a salt solution) are attracted to the cathode and the negatively charged ions (chloride ions in a salt solution) are attracted to the anode. This movement of charged particles is an electric current, because current is simply the movement of charge.

When the ions reach their respective electrodes, they either gain or lose electrons to revert to their elemental state. For dissociated salt, the positively charged sodium ions congregate at the cathode and pick up electrons from the electrode, leaving it as elemental sodium.

At the same time, the chloride ions lose their “extra” electron at the anode, sending electrons into the electrode to complete the circuit. This process is why ionic compounds conduct electricity in water.