MODULE 6: Solutions
Module 6 Learning Objectives
After successfully completing this module, you will be able to:
• explain what a solution is.
• explain what a hypertonic, hypotonic, and an isotonic solution is.
• explain the concepts of diffusion and osmosis.
• determine the direction water will move based on differences in solute concentration.
• explain the difference between saturated and unsaturated solutions.
• determine mass %, volume %, index, and dilution quantities.
• explain the role water plays in chemical reactions.
A solution is a mixture of two or more substances. The amount present in greater quantity is called the
solvent. The amount in lesser quantity is the solute. Solutions are found in all states. Brass is a “solution”
of copper and zinc, air is a solution of nitrogen, oxygen, argon and numerous other gases in small
quantities, gasoline is a mixture of hydrocarbons and additives. There are mixtures between states as
well. Carbonated drinks have dissolved carbon dioxide in water. Sugar dissolved in water (for tea) is an
example of a solid dissolved in a liquid. Liquid mercury is dissolved in solid silver to make dental fillings.
Figure 19: Solubility of sugar in tea. ©MRU
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In addition to true solutions, two other solution types can be formed based on the size of the solute
particles. True solutions have a particle size less than 1 nm. Suspensions have a particle size greater than
100 nm. Any solution with particle size between 1 nm and 100 nm is called a colloid. Colloids can be
separated from a solution by a process of selective diffusion called dialysis.
a) Water
Water is the most common solvent used in industry. Water is a polar molecule, meaning that one end
(the oxygen end) has slight negative charge and the other end (the hydrogen end) has a slight positive
charge. This creates attraction between the oxygen of one water molecule and the hydrogens of
another, creating intermolecular hydrogen bonds. Because of this, water has a much higher boiling point
than is expected for a molecule of its size. It can also hold heat well (ocean areas maintain higher winter
temperatures than land locked areas), requires a high amount of energy to evaporate and has high
surface tension. Surface tension is the ability of a liquid to “contract” its outer molecules to create a
barrier. The surface tension is high enough to support the weight of many species of insects.
Diffusion is the movement of particles from areas of high concentration to areas of low concentration.
Although extra energy input is not required for this to happen, diffusion tends to occur quicker at high
temperatures. Embalming liquids move from capillaries into tissues but the diffusion rate can be
impaired by surface tension (Gee-Mascarello, 2022, p. 104 - 105). Detergents or wetting agents are
used to counteract this (Gee-Mascarello, 2022, p. 104).
Water moves passively through a special kind of diffusion called osmosis. As long as there is a passage,
water will move from areas of low solute concentration (or high water concentration) to areas of high
solute concentration (or low water concentration). A way to remember this is to think that “solutes
suck water”. Water is drawn to where there are more solutes than water.
There are 3 ways to describe the relative concentration of solute, or tonicity, between two areas:
1. Hypertonic- if an area or a solution is hypertonic, it means that it has a higher concentration of
solutes than another solution or area. As long as there is a passage for water to move through,
water will move from the area of low solute concentration to the hypertonic area.
2. Hypotonic- if an area or a solution is hypotonic, it means that it has a lower concentration of
solutes than another solution or area. Again, as long as there is a passage for water to move
through, water will move from the hypotonic area to the hypertonic area.
3. Isotonic- if two areas or solutions have equal concentrations, the movement of water between
the two areas is equal; there is no net movement of water.
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Formaldehyde reacts with proteins to form water. Body tissues will be hypotonic (have more water and
less solute) so water will flow into the capillaries from the tissues. Because water always moves from an
area of high concentration to an area of low concentration, the choice of embalming fluid is based on
this osmotic principle. If a body is edematous, a hypertonic embalming fluid is required to draw the fluid
out of the tissues. Conversely if a body is dehydrated, a hypotonic fluid should be used (adapted from
Dorn & Hopkins, 1999; p. 117).
Figure 20: In this diagram, there is a higher concentration of solute than water inside of the cell.
The movement of water is therefore into the cell. ©MRU
In solids and liquids, particles are packed closely together. It is impossible to compress solids and liquids.
These particles are held together by forces of attraction. When something dissolves, its particles are
separated from each other. Particles will only separate – overcoming forces of attraction – if enough
energy is supplied (this is what happens when you boil a liquid) or if new forces of attraction are created
that compensate for the forces of attraction that are broken. Solubility therefore depends on
the relative strength of the forces of attraction. In other words, a solution will only form if there is an
attraction between the solute and the solvent. Polar solutes are attracted to polar solvents and will
Video: Khan Academy – Surface Tension
Note: Only view the assigned video. You are not required to view related topics.
https://www.youtube.com/watch?v=_RTF0DAHBBM
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therefore dissolve in them. Non-polar solutes are attracted to non-polar solvents and will dissolve in
them. It is a case of “like dissolves like.”
Figure 21: Solids have strong forces of attraction between particles. If enough energy is supplied to the solid,
the bonds begin to break and the solid becomes a liquid. If more energy is added, the remaining forces of
attraction are broken and the liquid becomes a gas. ©MRU
b) Electrolytes and Non-electrolytes
Solutes that dissolve into ions in solution are called electrolytes. They can conduct electricity because of
their charges. There are two types of electrolytes, strong and weak.
Strong electrolytes are formed by the complete dissolution of the compound into ions. Strong acids and
bases are strong electrolytes. Sulfuric acid is used in car batteries. Table salt, NaCl, is another example of
a strong electrolyte.
Video: Khan Academy – Solubility and Intermolecular Forces
Note: Only view the assigned video. You are not required to view related topics.
https://www.youtube.com/watch?v=ccDKr4TIWfk
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Weak electrolytes form both ions and molecules when placed in solution. The ions will conduct
electricity but the molecules do not. Because not all of the particles in the solution are conducting
electricity the amount of current produced is less. Vinegar (acetic acid) is a weak acid and a weak
electrolyte.
A solute that does not form ions in solution is called a non-electrolyte. These solutions do not conduct
electricity at all. While sugar (sucrose) dissolves in water it does not conduct electricity and is therefore
a non-electrolyte. Pure water (de-ionized) and most organic compounds are non-electrolytes.
c) Acid and Base Strength
Strong acids or bases dissociate (come apart) completely in an aqueous solution to form ions and are
strong electrolytes; whereas weak acids or bases will only partially dissociate and are weak electrolytes.
Strong Acids Strong Bases
perchloric acid lithium hydroxide
hydroiodic acid sodium hydroxide
hydrobromic acid potassium hydroxide
hydrochloric acid calcium hydroxide
sulfuric acid strontium hydroxide
nitric acid barium hydroxide
Figure 22: This is a listing of all of the strong acids and some examples of strong bases.
Any cation that is attached to a hydroxide ion is considered to be a strong base if it is aqueous.
All other acids and bases are considered to be weak and will only form a weak electrolytic solution.
Video: Khan Academy – Solubility of Organic Compounds
Note: Only view the assigned video. You are not required to view related topics.
https://www.youtube.com/watch?v=_mWHWJiMKXU
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d) Solubility
Solubility is the ability of a solute to dissolve in a solvent. Solvents have a limit to how much solute can
be dissolved in them; generally, the larger the volume and/or the temperature of the solvent, the more
solute can be dissolved in it. When no more solute can be dissolved in a solvent, the solution is said to
saturated. Conversely, if more solute can be dissolved, the solution is unsaturated.
Figure 23: Solubility is a measure of how much solute a solvent can hold at a given temperature. An unsaturated
solution can dissolve more of the solute, but a saturated solution cannot dissolve any more solute,
and so the solute will precipitate out in a solid form on the bottom of the container. ©MRU
The amount of a solute a solvent can hold is called its solubility. This value varies from compound to
compound and can even change for any one compound depending on temperature.
Solubility rules stem from electrolytic ability. Most ionic compounds (salts) are strong electrolytes and
are generally soluble. Compounds, ionic or molecular, that are non-electrolytes form gases, liquids or
solids in solution. Ionic compounds that don’t dissolve in water are called insoluble salts. These
compounds have many positive and negative charges (cations and anions) holding the compound
together and the polar attraction of water is not strong enough to break these bonds.
Recall the solubility table from the previous module. The table can be used to determine if an ionic
compound will be soluble in water. For instance, barium phosphate has Ba+2 ions and PO4
3-
ions. The
attraction of the cations to the anions (because of the +2 and -3 charges) is so strong that the dipoles on
water do not provide a stronger bond and so barium phosphate is insoluble in water.
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Certain regions have an abundance of limestone rock that becomes eroded as water flows through as a
river or a stream. This causes water sources to have elevated levels of ions like calcium and magnesium
and makes the water, hard water. The ions that are found in hard water can decrease the effectiveness
of soaps as cleaners and they can react with clotting factors in blood and cause coagulation.
Coagulation can impair the diffusion of the embalming solution and must be treated with the use of
anticoagulants (discussed further in Module 12).
e) Concentration of a Solution
Concentration refers to the amount of solute in a standard amount of solvent.
Concentration = __solute__
solvent
There are a number of ways of calculating this.
Concentration Using Mass %
This is the mass of the solute divided by the mass of the solution, which is the mass of the solvent plus
the mass of the solute. The units for the solute must be the same as the units for the solvent. If they are
not you need to do a conversion.
mass % (m/m) = ___solute (grams)__ x 100
solution (grams)
Concentration Using Volume %
This is a useful measure for liquids and gases. It is very similar to mass % except we use volume in either
mL or L. Again, the units must be the same.
volume % = ____solute (mL or L)_____ x 100
solvent (mL or L)
Example 6-1: Calculating mass % given mass of solute and mass of solvent
What is the mass % of 5.00 grams of NaCl in 250 grams of water?
mass % = __5.00 g_______ x 100
(250 g + 5.00 g)
mass % = 1.96 % (m/m)
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Concentration Using Mass/Volume %
This is probably the most common measurement of concentration in the medical industry. The index of
a formaldehyde solution indicates how many grams (mass) of formaldehyde gas is dissolved in 100 mL
(volume) of solution (Gee-Mascarello, 2022; p. 112).
mass/volume % = ___mass (g)___ x 100
volume (mL)
f) Dilution of Solutions
It is often necessary to dilute highly concentrated solutions to a more useful concentration. Some type
of solvent, probably water, needs to be added. The equation for dilution is:
C1 x V1 = C2 x V2
C1 is the original concentration, V1 is the original volume, C2 is the new concentration and V2 is the new
volume. In Example 6-4, this equation shows how to calculate any dilution for any solution so long as
neither the type of solution nor the amount of solute changes.
Example 6-2: Calculating volume % given volume of solute and volume of solvent
What is the volume % of a solution of iodine if 10 mL of iodine is dissolved in hexane to a final
volume of 250 mL?
volume % = ____10 mL____ x 100 = 4% (v/v)
250 mL
Example 6-3: Calculating index given mass of solute and volume of solvent
What is the index (mass/volume) of a solution of 35 grams of formaldehyde gas dissolved in 250 mL
of water?
index = __35 g___ x 100 = 14 or 14% (m/v)
250 mL
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Example 6-4: Calculating final volume required for a dilution
Ammonia can be purchased in 1.0 L containers as a 30 % (w/v) solution. This is too strong to be used
as is so it needs to be diluted. What volume of water would be required for a final concentration of
1.5%?
C1 x V1 = C2 x V2
30% x 1.0 L = 1.5% x V2, rearranging the equation to solve for V2 gives
V2 = __30% x 1.0 L__ = 20 L
1.5%
It is important to note that since the final volume must be 20 L, 19 L of water must be added to the
1.0 L of the 30% ammonia.
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