Concentration
Solutions and Concentration
In lab, we will often deal with solutions. This lesson introduces two related concepts the concentration of a solution and the process of diluting a solution.
A solution
A solution consists of a liquid (the solvent) with a substance (the solute) dissolved in it.
You are probably familiar with many solutions from your everyday life. Milk is a
solution consisting of water (the solvent) with lactose and salts dissolved in it. Ocean
water is another type of solution. Both of the examples given above, along with many of
the solutions we work with in lab, are known as aqueous solutions because the solvent is
water.
Have you ever mixed up orange juice from concentrate and added too many cans of
water? I'm sure you could taste the difference between the watery orange juice and the
properly prepared orange juice. The amount of solute (orange juice concentrate in the
case of this example) in a solution is known as the solution's concentration. As you will
find, solutions with different concentrations act differently in the lab, just as different
concentrations of orange juice taste different in your kitchen. The rest of this page will
be devoted to the terminology and math we use to determine the concentration of
solutions.
Types of solutions
Combination of materials to create a solution includes liquids, solids and gases that
dissolve in a liquid or in a gas. The main requirement is that items must be both be
either polar or both non-polar for them to readily mix and form a solution. It is
possible for polar and non-polar materials to form a solution in some special cases.
(See Polar and Non-Polar Molecules for more information.)
Liquid in a liquid
Alcohol and water molecules will mix evenly together to form a solution. This is
because they are both polar molecules. The molecules are spread throughout the
solution, but yet they do not chemically combine to form another substance.
One plus one less than two
It is interesting to note that you can combine a liter of water with a liter of alcohol,
but you do not get 2 liters of the mixture. Instead you only get about 1 1/2 liters.
(The same would hold for measuring in quarts or pints). The reason that 1 + 1 is not
equal to 2 in this case is that the molecules mix in between each other and fill in
spaces that would normally be empty in the liquid by itself.
Solid in a liquid
Some solids can dissolve in a liquid to form a solution. Common examples are mixing
salt in water or sugar in water.
Sugar water
Sugar water still tastes sweet, although you cannot see the sugar crystals, which
have dissolved into the water.
Salt water
When salt dissolves in water, the NaCl molecules split into ions, with positive Na+
ions and negative Cl- ions. This is because H2O is a polar molecule and Na has only
one electron in its outer orbit and would be "more comfortable" with a complete
outer shell, and Cl is missing one electron from completing its outer shell.
Gas in a liquid
A gas can dissolve in a liquid. Since ammonia (NH3) is a polar gas, it will readily
dissolve in water (H2O) to form a solution.
Although Carbon Dioxide (CO2) is a non-polar molecule, it does dissolve in water to
form a solution under special circumstances. Carbon Dioxide is dissolved in water
under pressure in carbonated drinks. When the pressure is released by opening the
bottle or can, the CO2 gas quickly separates from the water in the form of bubbles.
Liquid in a gas
An example of a liquid dissolving in a gas is water dissolving into air to form
humidity.
Gas in a gas
The major gases in air, Nitrogen (N2), Oxygen (O2) and Carbon Dioxide (CO2) are
non-polar molecules and thus will combine to form a solution. Air is a solution where
molecules are evenly spread throughout.
Dissolving
The amount of a material and the rate at which is will dissolve in a solvent depends
on temperature and pressure.
Saturation point
Once the amount of material reaches its maximum for a given temperature and
pressure, it has reached its saturation point. After that, no more of the material
will dissolve in the solvent. For example, sugar will dissolve in water until you reach
the saturation point for that temperature. After that any sugar you add will not
dissolve but simply collect at the bottom of the container.
Dissolving liquids and solids
Liquids and solids are generally more soluble at higher temperatures.
Salt in water
That means that more salt will dissolve faster in warm water than in cold. Less salt
will dissolve in colder water. If you just about reach the saturation point for salt in
water at a given temperature and then lower the temperature of the water, the
excess salt will then precipitate or "un-dissolve" and appear at the bottom of the
container.
Water in air
Likewise, a certain amount of water can dissolve in air at a given temperature. The
percentage allowed at a given temperature is called the relative humidity. The
saturation point for adding water to air at a given temperature means the relative
humidity is 100%.
If the relative humidity is very high and temperature decreases or cools down, less
water vapor can be dissolved in the air, so then the water will precipitate in the form
of rain. The dew point is the temperature at which rain will occur for a given
relative humidity.
Your body cools off through perspiration. If the humidity and temperature are high,
this liquid will not readily dissolve in the air and you will be sweaty and
uncomfortable. If the temperature is high but the humidity is low, your perspiration
will evaporate into the air and you won't feel as uncomfortable.
Dissolving gases
Gases are more soluble as the pressure increases. This means that Carbon Dioxide
will readily dissolve in water or a sugar-water solution under high pressure. Once
that pressure is released--such as when you pop the top on your soft drink bottle-less CO2 can be dissolved in the liquid, so your carbonated bubbles appear.
Other characteristics
Other characteristics of solutions include how they separate, their conduction of
electricity and their concentration.
Concentration of a solution
The concentration of a solution is the percentage of one molecule with respect to the
other. When the amounts are equal, the concentration is 50%.
Liquor
By law, a liquor bottle must state the concentration or percentage of the amount of
alcohol to water. They use the term proof, which is twice the concentration
percentage, such that 200-proof is the maximum. 80-proof liquor has 40% alcohol in
it.
Separation
Methods to separate solutions include exceed the saturation point, evaporation and
boiling.
Exceed saturation point
The material will separate from the solvent when the solution is more than
supersaturated. Thus was previously seen with lowering the temperature of a saltwater solution to precipitate out excess salt.
Evaporation
When water evaporates from a solution, the concentration of the other material
becomes greater until it reaches the saturation point, at which it will precipitate out.
Distillation
Distillation is heating a solution of two liquids with different boiling points. As you
reach the lower boiling point of two, the more volatile material will vaporize first,
leaving the other material. Some of the less volatile material will also vaporize, so
you don't get a 100% pure substance.
One example is boiling an alcohol-water solution to vaporize the alcohol before the
water. The boiling point of water is 100° C (212° F) and that of alcohol is 78.5° C
(173° F). The vapor will be richer in alcohol. Repeated distillations can result in a
95% alcohol solution.
Boiling
Boiling a solution of a solid dissolved in a liquid can result is the solid precipitating
out. For example, boiling off the water in a salt solution will soon result in the
concentration of salt being so great that the it will precipitate out and settle at the
bottom.
Electrolytes
Some water solutions conduct electricity and are called electrolytes. This is usually
the case when the atoms break into ions, because it allows for the free movement of
electrons. A common example of this is a salt-water solution.
Summary
A solution is the result of dissolving a substance in a fluid solvent, where the
materials do not react chemically and are either both polar or both non-polar
molecules. The solution's molecules or ions mix homogeneously and do not separate
by mechanical means. Instead, changing the temperature usually separates them.