Lesson 8:
The Chemistry of Solutions




In this lesson we will answer the following question:


Reading Assignment

Along with the online lecture, read chapter 7 in Basic Chemistry for Water and Wastewater Operators.



Solutions are homogenous mixtures with contain two or more substances. A solution has uniform composition. Exampes are sugar and table salt in water. The dissolving medium (water) is the solvent and the dissolved substances (sugar or salt) is the solute. If both components in a solution are 50%, the term solute can be assigned to either component. When gas or solid material dissolves in a liquid, the gas or solid material is called the solute. When two liquids dissolve each other, the major component is called the solvent and the minor component is called the solute.

A Solution


Many chemical reactions are carried out in solutions, and solutions are also closely related to our everyday lives. The air we breathe, the liquids we drink, and the fluids in our body are all solutions. Futhermore, we are surrounded bysolutions such as the air and waters.

A heterogenous mixture of two or more substances is called a suspension. For example, clay in water. In this case, suspended particles of the dispersed substance, clay, are distributed nonuniformly in the medium. The bottom part of the mixture has more particles than the upper. When suspended particles are too small to settle by the force of gravity and too large to dissolve to form a solution, the suspension is known as a colloid. Particle size in a colloidal suspension is 1-100 nm. A nanometer is equal to 1/1,000,000 mm or 10-6 mm. These particles show peculiar dancing movements known as Brownian motions due to their continuous collisions with water molecules. Suspended particles of a colloid cause turbidity, and will be discussed later in the course.

Solutions of ionic compounds and acids in water conduct electricity because they produce ions. These substances are known as electrolytes. Most of the covalent compounds stay as molecules in their solutions and do not conduct electricity. Such substances are known as nonelectrolytes. Sodium chloride and hydrochloric acid are electrolytes, and sugar is an example of a nonelectrolyte.




Types of Solutions

At the molecular level, molecules and ions of a solute are completely mixed with and interact with those of the solvent when a solute dissolves in a solvent. This type of mixing is homogenous because no boundary is visible in the entire solution. In a mixture, differences may exist between regions or parts of the whole system.

Material exists in three states: solid, liquid, and gas. Solutions exist in all of these states:



Forming a Solution

Not every combination of two substances becomes a solution.  When you pour milk on your cereal in the morning, the resulting mixture is not a solution because the cereal does not become dissolved in the milk.  Similarly, when mud in water causes turbidity, the mud is not part of a solution.  We use the term insoluble to describe two substances which do not form a solution when they are mixed together. 

A solution is only formed when two substances mix homogeneously, meaning that any portion of the solution will contain a specific amount of both the solute and the solvent.  In the cases of cereal and milk or of muddy water, the two substances in the mixture will settle apart into two layers over time, so they are not homogeneously mixed.  When you are dealing with a true solution, the two (or more) substances found in the solution will never settle apart.

The homogeneous mixture found in a solution is the result of a chemical interaction between the solute and the solvent.  In a solution, chemical bonds between the solute and solvent hold the solute in the solution and prevent it from settling out.  The chemical bonds found between solute and solvent typically include the weak intermolecular forces introduced in lesson 5 - van der Waal's forces and hydrogen bonds.  For example, when acetone is added to water, hydrogen bonds form between the two substances as shown below:

Acetone and water solution.
Two different views of the interaction between acetone and water.

These bonds keep the acetone dissolved in the solution.  Once the acetone is fully dissolved and mixed into the water, each milliliter of the solution will contain the same amount of acetone and the same amount of water. 

Solutes and Solvents

Although most of the solutions we will deal with in this course are liquids, you should be aware that solutions can be made up of any combination of gases, liquids, and solids.  For example, the air you breathe is an example of a gaseous solution consisting primarily of nitrogen and oxygen while steel is a solid solution in which iron is the solvent and carbon and manganese are the solutes.  

In each solution, the solvent is the substance which determines the state of the finished solution.  For example, when you add salt (a solid) to water (a liquid), you can tell that the water is the solvent since the resulting solution is a liquid.  If both the solute and the solvent have the same state, the solvent is typically the part of the solution which is present in the highest concentration. 

Another distinction between solutes and solvents is that solutes are sometimes changed when they become part of a solution while solvents are typically unchanged.  For example, ionic compounds such as calcium carbonate and table salt break apart into their constituent ions when they become part of a solution. 

Like Dissolves Like

Not every solvent will be able to dissolve every type of solute.  Chemists use the phrase "like dissolves like" to summarize the idea that solvents are best able to dissolve solutes which have a similar chemical composition and which form the same types of bonds. 

Although there are many characteristics which may affect how a solute and solvent relate, the simplest distinction is whether the molecule can form hydrogen bonds.  Molecules such as water, alcohols, and acetone which can form hydrogen bonds tend to be soluble in each other.  On the other hand, molecules such as oil, gasoline, and grease which cannot form hydrogen bonds and instead are attracted to each other only by van der Waal's forces tend to be soluble in each other but are not soluble in water. 

Water and oil don't mix.

When insoluble substances are mixed together, they settle out into two separate layers, as shown above.


Universal Solvents

Water is sometimes called the universal solvent because of its ability to dissolve a diverse array of substances.  Acetone is also occasionally referred to as a universal solvent since it is able to dissolve oils, alcohols, and water.  In truth, however, there is no such thing as a universal solvent because no one substance is capable of dissolving every possible solute. 

Even though there is no true universal solvent, you use one very good solvent every day - soap.  Soap is made up of molecules which act like an oil on one end and like an alcohol on the other, so they are soluble in oils, in water, and in alcohol. 

Soap attracts both oils and water.

When you add soap to your dishwater, the "head" of the soap molecule makes the soap dissolve easily in the water.  The "tail" of the soap molecule attracts oils and grease which are not usually soluble in water and which would not be removed by washing with water alone.  When you rinse the soapy water off your dishes, the oils and greases are washed away with the soap. 

Part 2: Solubility and Saturation