Ionic & Covalent Solutes: Definition & Difference

Instructor: Justin Wiens

Justin teaches college chemistry and has Bachelor and Doctorate degrees in chemistry.

In this lesson we will discuss what happens when we dissolve two different types of compounds in liquids: ionic and covalent solutes. The chemical differences between these solutes affect the properties of the solutions they form.

Introduction

On a warm summer's day, it's nice to mix up a batch of lemonade and lie on a hammock in the backyard. Of course, don't forget an important part of the recipe, or your lemonade will taste be too sour: sugar! You dissolve sugar in your acidic, sour-tasting lemon water to make a sweet, tangy solution. A solution is a uniform mixture of two or more solutes and a solvent. The substance in the larger amount is the solvent, and the substance in the lesser amount is the solute. In this case, water is the solvent, and sugar and lemon juice are both solutes. They contribute unique flavors and chemical properties to your lemonade. Let's discuss these different solutes and the classes of compounds they belong to, as well as the properties of solutions containing them. Throughout the lesson, we will talk about solutions made with water as the solvent, but the basic principles apply to all solvents.

Covalent Solutes

Covalent solutes are molecules or compounds that have covalent bonds, where electrons are shared between atoms. The main-group elements form the majority of existing covalent molecules or compounds. Such chemicals are found everywhere: in the air we breathe, the water we drink, and the food we eat. Sugar and lemon juice are both covalent solutes. Luckily, both of them dissolve easily in water. In other words, water molecules are able to surround these solutes to form a solution; neither solute would float on top of the water or sink to the bottom of your glass of lemonade (assuming you do enough stirring). However, only the acidic lemon juice will increase the electrical conductivity of your lemonade. Let's investigate the reason below.

Electrolytes vs. Non-electrolytes

The acidic lemon juice is an electrolyte because it forms ions in solution. Specifically, the acidic molecule in lemon juice loses H+, a proton, leaving behind a negatively charged ion. When lemon juice is added to water, the water molecules quickly seize the opportunity to surround the proton and negative ion in optimal ways: the more positive parts of the water molecules (closer to their H atoms) point toward the negative ion, whereas the more negative part of the water molecules (the O atom) points toward the protons.


Weak Acid Dissolution

Several water molecules are able to surround and dissolve each ion. However, not all of the acid molecules will break up into ions--just a small percent of them. Reorientation of the water molecules in optimal ways to dissolve a solute also happens for the acid molecules that do not ionize. Molecules or compounds that only partially ionize in a solution are called weak electrolytes. The various acids in lemon juice are therefore weak electrolytes because they dissolve fully but only partially ionize. Molecules or compounds that completely dissociate into ions are called strong electrolytes. Strong electrolytes include some covalent solutes such as strong acids (for example, hydrochloric acid) and ionic compounds, which we will talk about later on.

The presence of charged species in solution makes it easy for electricity to flow through solution (hence the name electrolyte). You can confirm this by passing a current from a battery through a strong or weak electrolyte solution, to a light bulb on the other side.


Electrolytes

Weak electrolytes will cause the light bulb to glow dimly (left panel), whereas strong ones will make the bulb glow brightly because the solution is much more conductive (right panel).

On the other hand, sugar does not break up into ions in solution at all: it is a non-electrolyte. For this reason, non-electrolyte solutions do not conduct electricity well. In other words, the light bulb would not turn on at all if you add sugar but forget the lemons for your lemonade!

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