AQUEOUS SOLUTIONS AND COLLOIDS
Solutions and colloids are essential to life. The solutions in living
systems are aqueous solutions; that is they are made with water. First, we
will identify the types of solutions; then, we will learn various ways of
expressing their concentrations. A molecular model of solutions will be
presented and will be used to explain their biologically important
properties. Finally, we will study colloids and learn how they differ from
solutions.
Types of solutions:We are all familiar with the fact that sugar dissolves in water. The
result of mixing a small amount of sugar in water is a homogenous
mixture called a solution. We define a solution: - as a homogeneous
mixture of the molecules, atoms or ions of two or more different
substances. The substances that make up a solution are called its
components. There is usually more of one component than the other
components in the solutions. The component present in excess is called
the solvent. The other components are called the solutes. In a solution of
sugar in water, water is the solvent and sugar is the solute.
The three states of matter can combine in nine different ways to form
solutions containing tow components. Parts of these combinations are
listed in table 1.
Solutions that contain liquids as solvents are the types of solutions
most familiar to us. Numerous examples of solutions containing solids in
liquids, gases in liquids and liquids in liquids are available from everyday
experience. Less familiar as solutions are those with solids as solvents,
yet alloys and amalgams are important in may commercial produces.
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Table 1. types of solutions
Solvent
Solute
Example
Liquid
Liquid
Alcoholic beverages
Liquid
Solid
Salt water
Liquid
Gas
Carbonated beverages (soda pop beer, champagne)
Solid
Liquid
An amalgam such as mercury in silver
Solid
Solid
Metal alloys such as brass or tin
A given solution has a particular composition but the composition
can be varied by adding more of either component. For example the
sweetness of a solution of sugar and water varies with the amount of
sugar dissolved. By contrast the composition of a pure substance never
varies.
Solubility:There is usually a limit to the amount of solute that can be dissolved
in a solvent at a particular temperature. When this limit is reached no
more solute will dissolve in the solvent. When this happens, we say that
the solvent is saturated with solute. We call such a solution a saturated
solution of the solute in the solvent. Solubility is defined as: the amount
of solute that dissolves in a given quantity of solvent to form a saturated
solution. The solubility of a solute in a particular solvent depends on a
number of factors, such as: - the kind of solvent, the kind of solute, the
temperature of the solvent, and the pressure above the solvent.
1. The kind of solvent: There is a general rule that like dissolves like by this we mean that a
polar solvent such as water is a good solvent for ionic compounds such as
sodium chloride (Nacl). Gasoline, a mixture of nonpolar organic
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compounds is a good solvent for other nonpolar organic compounds such
as greases and oils. It follows from this general rule that polar and non
polar will not from solutions an example is gasoline and water.
2. The kind of solute: Sometimes there is no limit to the amount of one substance that can
dissolve in another. This is particularly true for solutions of a liquid in a
liquid. Some liquids are infinitely soluble in another liquid: any amount
of one liquid will dissolve in any amount of another liquid Ethyl alcohol
and water provides an example of two liquids that are infinitely soluble in
each other. Such a pair of liquids is said to by completely miscible. Other
liquids are only slightly soluble in each other. Such liquids are said to be
partially miscible. Liquids that are insoluble in each other are said to be
immiscible thus gasoline is immiscible with water.
3. The temperature of the solvent: The temperature of the solvent affects the solubility of a solute. In
general, solute is more soluble in hot than cold solvents. The solubilities
of several solids increase greatly with increasing temperature, others
increase only slightly and some actually decrease. This is shown by the
solubilities of several solutes (solids) in water at 20°c and 100°c listed in
table 2.
Gases are other compounds whose solubilities in water decrease with
increasing temperature. A familiar example is boiling water. The bubbles
that form when water is heated are air escaping from solution because
dissolved air is less soluble in water at higher temperatures. Boiled water
has a characteristic flat taste that is due to the absence of dissolved gases.
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4. The pressure above the solvent: The solubility of a gas is greatly affected by the pressure of that gas
above the solution. In general the solubility of any gas increases as the
partial pressure of the gas above the solution is increased.
Carbonated beverages contain the gas carbon dioxide dissolved in
water and are bottled under high pressure. The amount of carbon dioxide
in solution depends directly on the partial pressure of carbon dioxide
above the liquid. When a bottle of carbonated beverage is opened the
partial pressure of carbon dioxide above the liquid decrease and the
solubility of the carbon dioxide decreases. As a result, buddles of carbon
dioxide form and escape from the liquid. This is how a bottle of soft drink
that is left open for a while goes flat. In contrast to gases the solubilites of
solids and liquids are practically unchanged with a change in pressure.
It is useful to know in general what substances dissolve in water and
what factors affect the solubilites of gases, liquids, and solids in water
however in the laboratory and in clinical work it is necessary to specify
exactly the amount of solute in a solution.
Solubility at 20c˚
Solubility at 100c˚
(g/ 100mL)
(g/ 100mL)
NH4Br
97.1
146.0
KBr
59.4
102.0
KNo3
37.8
247.0
Nacl
36.2
39.1
Li2co3
1.33
0.725
CaSo4
0.21
0.16
Compound
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Concentrations of solutions:We describe the relative amounts of solute and solvent in a solution
by means of units of concentration. There are several such units, and we
will examine the most commonly used ones.
Weight/ weight percent:One way to specify the concentration of a solute in a solution is as a
percent by weight. The concentration of the solute is given by the
following equation:Weight of solute, in g
Weight of solute, in g + weight
of solvent , in g
Percent by weight solute =
× 100…(1)
The following example shows how to express the concentration of a
solute in this unit.
Example 1:- What is the percent by weight of sugar in a solution made
by dissolving 10 g of sugar in 90 g of water?
Solution: Solvent = water
solute = sugar
Weight of solvent = 90 g
Percent by weight sugar =
weight of solute = 10g
10g
10g + 90g
×100 =
10g
10g + 90g
× 100 = 10 %
Volume / Volume percent:A convenient way of expressing the concentration of a liquid solute
dissolved in a liquid is as a percent by volume. This unit of concentration
is similar to percent by weight except that volumes in milliliters are used
instead of weights in grams. The equation is as follows:-
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Percent by volume solute =
Volume of solute, in mL
Total volume of solution, in mL
× 100 .(2)
Example2: - What is the percent by volume of ethyl alcohol in a solution
made by diluting 10 mL of Ethyl alcohol to 100 mL with water?
Solution:Volume of solute = 100 mL
Total volume of solution = 100 Ml
10 mL
100 mL
Percent by volume ethyl alcohol =
× 100 = 10 %
Weight / Volume percent: This widely used method of expressing concentrations is a
combination of weight and volume. The weight is usually that of the solid
solute and the volume is that of the total solution. This is defined as
follows:
Percent by weight / volume solute =
Weight of solute, in g
Total volume of solution, in mL
The use of this unit is shown in the following example:Example 3:- What is the percent by weight / volume of sodium chloride
in a solution made by diluting 1.5 g of sodium chloride to 100 mL with
water?
Solution:Weight of solute= 1.5 g
Total volume of solution = 100mL
1.5g
× 100 = 1.5 %
100 mL
Low concentrations of solute are often expressed in milligrams per
Percent by weight / volume NaCl =
100mL. This weight/ volume percent unit is defined as follows:-
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× 100 (3)
Milligrams per 100 mL= mg / 100 mL =
Weight of solute, in mg
100 mL of solution
..(4)
This unit is often used to express the concentrations of solute in blood
and urea as shown in the following example:Example 4:- A 1-mL sample of blood plasma is found to contain 3.3mg
of sodium ions express this concentration in mg / 100ml.
Solution: Step 1:- Express the amount of solute in mg per total volume of solution:Solute = 3.3mg
Solution = 1 mL of blood plasma
3.3mg
1mL
Step2 :- the definition of mg/100mL is the weight of solute in 100mL of
solution. We know how many mg are in 1mL of blood plasma. To find
out the number of mg in 100mL of blood plasma, we set up the following
proportion:-
3.3mg
X
=
1mL
100mL
Step 3:- rearrange and solve for X.
X=
3.3mg × 100 mL
= 330mg
1 mL
There are 330 mg of sodium ions in 100mL of blood plasma. Therefore
according to equation 4.
Milligrams per 100 mL =
330mg
100mL
This value is within the normal concentration range of sodium ions in
blood plasma (320 to 350 mg/ 100mL).
The unit mg / mL is sometimes called mg percent.
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Parts Per Million and Parts Per Billion:These units of concentration are widely used to report very small
amounts of solute in a solution. The concentration of pollutants in water
and air are usually reported in these units.
One part per million, abbreviated ppm, contains 1 part of solute per1
million (106) parts of solutions. By parts we mean any unit of measure
such as grams, liters or anything else we choose. For example the
concentration, of solid pollutants in solid food is given in ppm expressed
as mg of pollutant (the solute) in 1 million of solid food (the solution).
Because 1 million mg g is equivalent to 1 kg ppm is usually defined as
follows:-
ppm =
Weight of solute, in mg
Weight of solution, in kg = 106 mg of
solution
to express the concentrations of small quantities of solid solutes n water
the unit ppm is usually defined as mg of solute per liter of solution. This
change from weight to volume of solvent can be made because 1 million
mg (1kg) of water occupies approximately I L. This definition of ppm is
also frequently used even though the solution may weigh somewhat more
or less than 1kg.
Air pollution is measured in ppm on the basis of measurements of
volume rather than weight. Thus 1 ppm means that there is 1µL of
pollutant (the solute) per 1 million (106) µL (IL) of air (the solution).
The sensitivity of analytical methods has improved so much that
parts per billion, abbreviated ppb, has become a common unit of
concentration. Its use and definition are similar to those of ppm. Thus 1
ppb contains 1 part of solute per 1 billion (109) parts of solution. Again,
the parts refer to weight or volume, depending on whether the solution is
a gas, liquid, or solid.
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The units ppm and ppb are extremely small. For example, 1 ppm is
equivalent to 1 cent in 10,000 dollars, and 1ppb is equivalent to 1 cent in
10 million dollars. These units are convenient means to express very
small concentrations without using exponential notation. The use of these
units is shown in the following example: -
Example 5:- The maximum Food and Drug Administration (FDA)
tolerance of mercury in fish is 0.5 ppm. A 10 – g sample of fish is found
to contain 72 µg of mercury. Does the amount of mercury in the fish
exceed the FDA maximum tolerance?
Solution:Step1:- Calculate the mg of mercury in 1 kg of this fish using the
following proportion: 72µg
x
10g = 1kg
Step 2: Rearrange and solve for X:72µg
103g 1mg
X=
× 1kg ×
= 7.2mg
10g
1kg × 103µg
Step 3:- Determine ppm form its definition:-
ppm =
Weight of solute in, mg
1kg of solution
7.2 mg of mercury
=
1kg of fish
= 7.2 ppm
The amount of mercury in the fish exceeds the maximum FDA tolerance.
The fish is contaminated with mercury.
Notice that the molecular weight of the solute is not needed when we
express the concentration of the solute in any of the units of percent, ppm
or ppb. Thus we can specify the concentration of a substance without
knowing anything about its chemical composition.
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