Overview: In this module you
will learn the vocabulary that chemists use in describing solutions as well
as how to calculate solution concentrations.
Let's start with a few definitions.
A mixture is matter that contains more than one pure substance and can be separated into its components
by using physical techniques. The composition is variable and its properties are related to its individual components.
In a mixture, the components retain their own chemical properties. An example of a mixture is the combination of rice and salt.
Compare this to a compound.
A compound is a substance that contains two or more different elements with their atoms in a definite ratio.
Compounds cannot be separated by physical techniques such as filtering. The composition is the same throughout.
Water will always have two hydrogen atoms and one oxygen atom. If instead the ratio were two hydrogen atoms to two oxygen atoms,
then the compound is no longer water (H2O), it is now hydrogen peroxide (H2O2). Elements
in a compound are not just mixed together; they are bonded together in a specific way. The properties of a compound are
usually very distinct from the properties of the individual elements that make the compound. For instance, sulfur,
a yellow solid, combines with oxygen, an oderless gas, to form sulfur dioxide (SO2), which is a poisonous,
colorless, pungent gas.
A solution is a special type of mixture that is homogeneous throughout.
This means that the molecules or ions involved are so well mixed that the composition
is uniform throughout the mixture. Think of salt-water. You cannot see salt
within the water when it is fully dissolved, not even with the aid of a microscope.
(This contrasts with a heterogeneous mixture in which you can identify the separate
components. For example, a mixture of salt and sand is heterogeneous.)
A solvent is the component in a solution that is present in the largest amount. In a NaCl solution
(salt-water), the solvent is water.
A solute is the component in a solution in the lesser amount. In a NaCl
solution, the salt is the solute. A solution may contain more than one solute.
There are different types of solutions. The one you are probably most familiar with is the aqueous solution.
An aqueous solution is a solution in which water is the solvent. A NaCl solution is an aqueous solution.
A non-aqueous solution is a solution in which water is not the solvent. Examples of non-aqueous solutions
are solutions used in dry cleaning (a solution of ethene in the solvent dichloromethane).
A solid solution is a solution in which a solid is the solvent. An example is a brass solution that is formed by
dissolving copper in zinc.
So what happens when you drop salt into a glass of water? The water before and after does not look different
(assuming that all of the salt is dissolved). However, if you took a drink of it, it certainly tastes different.
It boils at a higher temperature than pure water and it conducts electricity. What happened?
Your everyday table salt consists of NaCl. Water is made of H2O
molecules. When these two are combined together in a solution, NaCl actually
separates into ions. [In solid NaCl, Na+ and Cl- ions are arranged in an ordered
three dimensional array called a crystal lattice, as depicted in the figure below.] Thus NaCl (s) becomes Na+ and Cl-
ions in solution (i.e., NaCl dissolves in water). Why does NaCl dissolve?
NaCl becomes solvated ions because of favorable electrostatic interactions (you will learn about this in
Chemistry 111) and favorable entropy conditions (you will learn about this in Chemistry 112). In order for the
solute to dissolve, these two effects must be stronger than the interactions within the NaCl crystal and the solvent
molecules with themselves. In other words, NaCl dissolves in water because the electrostatic interactions and the
entropic effects are stronger for the ions and water than they are for the NaCl crystal and the H2O
by itself. It is okay if you do not fully understand this concept yet, it will become clearer throughout
Even though NaCl dissolves to become ions in a solvent, the overall charge
remains neutral. Remember that it is NaCl, a neutral compound, that forms the
Na+ and Cl- ions. There will be an equal number of positive
and negative charges; therefore, the solution will be neutral.
The diagrams above show the dissolution of NaCl solid. The water molecules surround the ions. These ions
are now free to move about in solution since they are no longer in a crystal lattice. This means that the
ionic bonds between Na+ and Cl- are broken.
Notice that the oxygen of H2O surrounds the Na+ and the
hydrogens of H2O surround the Cl-. This has to do with
the polar properties of water and the charges on the ions. The partial negative
charge on oxygen is attracted to the positive charge on the Na+ ion
(opposites attract). The same is true for the hydrogen; the partial positive
charge on hydrogen is attracted to the negative Cl- ion. You will
learn about polarity and intermolecular interactions in Chemistry 111.
When discussing solutions, we typically talk about the solution's concentration. In chemistry, we use molarity
to calculate the concentration. Other important terms are the molality and mole fraction of a solution.
The molarity is the number of moles of solute per liter of solution.
This is a specific concentration measurement. Molarity is defined as the number
of moles of solute per unit volume. The molarity is reported as M (read molar),
which is mol of solute/L of solution. Molarity is temperature dependent as the
volume of the density of a solution typically changes with temperature.
The molality is the number of moles of solute per kilogram of solvent.
This measurement is not temperature dependent, as the mass does not change with
temperature. The units are denoted by m, which is read as molal and is mol of
solute/kg of solvent.
A mole fraction, as the name implies, is a comparison of the number
of moles in solution. It is found by taking the number of moles of solutes (or
solvent) divided by the total number of moles (solutes + solvent). Since this
is a fraction, there are no units. The mole fractions of a solution must add
up to one.
Let's look at a simple solution made of two components, 1 and 2. X is the mole fraction and n is the number
A solution is prepared by dissolving 34.2 g of MgCl2 in 0.430 L of H2O. Calculate the
molarity, molality and mole fraction of MgCl2 if the density of water is 1.00 g/cm3 and
the density of the solution is 1.089 g/cm3.
For the solution in Example 1, 34.2 g of MgCl2 in 0.430 L of H2O
(r = 1.089 g/cm3), calculate the molarity, molality and mole fraction of
the Cl- ion in solution.
Now remember what is happening. When the MgCl2 is placed in water, it dissolves into
Mg2+ and Cl- ions.
In molecular solutions, bonds are not broken as they are in non-molecular
solutions (also sometimes called ionic solutions). NaCl (aq) is an example of a non-molecular
solution. Recall that in non-molecular solutions the ionic bonds were broken within the compound. For molecular solutions,
Glucose, a sugar molecule, is an example of a compound that forms a molecular
solution in water.
Now you should have a good understanding of solutions and the solvation process for ions in aqueous solutions.
You need to be comfortable with using molarity, molality and mole fractions. Also you should know the definitions
pertaining to solutions.