Chemistry H2
Unit: Solutions
Unit Notes
1. What is a solution?
A solution is a homogeneous mixture of two or more substances in a single
physical state. A solution appears uniform throughout and the substances that make up a
solution can be separated by different chemical methods such as distillation and
chromatography. Examples of solutions are: lemonade mix and water, the mixture of
gases that make up our atmosphere appear as one colorless gas, but are actually many
colorless gases, and alloys of metals (2 or more metals melted down then allowed to cool
and fuse together).
2. Properties of Solutions
a) The particles in a solution are extremely small and are spread out evenly in the
mixture.
b) The particles in a solution will not settle out of the solution mixture.
c) The substance that is dissolved in a solution is called the solute. The solute is
dissolved by another substance in the solution called the solvent. The most widely used
solvent on our planet is water.
d) If a solute can dissolve in a particular solvent, then that solute is considered soluble or
miscible in that solvent. If a solute cannot dissolve in a particular solvent then the solute
is considered insoluble or immiscible. In order to be miscible, a solute must have the
same polarity (polar or nonpolar) as the solvent that it is mixed with.
3. Phases of Solutions
a) Solutions can exist in all three physical states of gas, liquid, and solid.
b) An example of a solid solution is as mentioned previously the mixture of 2 or more
different metals called an alloy. An alloy is produced by trying to mix the appropriate
ratios of different metals to get strong, rust resistant, and lightweight materials. Most
solutions exist as liquid solutions where gases, liquids, and even solids can be dissolved
in a liquid solvent. And finally solutions can be in the gas phase by a mixture of different
gases mixing together. Any solution where water is the solvent are referred to as aqueous
solutions coming from the Latin word aqueus meaning water-based.
4. Concentrations of Solutions
a) The amount of solute per amount of solvent is called the concentration of a solution.
Solutions that have a high amount of solute per solvent are said to have a high
concentration and solutions with low amounts of solute per solvent are said to have a low
concentration. For example, when maple syrup is first tapped from trees the sugar
concentration is low with a lot of water as the solvent and some sugar as the solute. The
syrup is boiled (which allows the water to evaporate) and the solution gains a higher
concentration of sugar to make the sweet finished product you put on your waffles and
pancakes. Solution concentrations are usually expressed in measurements called Molarity
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(M), molality (m) and mole fraction. All three types of solution concentration
measurements use the concept of the chemistry mole.
b) Molarity-molarity is expressed in moles of solute dissolved in Liter of solution (both
the volume of the solute and the solvent). Solutions used in chemistry with specific
molarities are typically measured and mixed in a piece of glassware called a volumetric
flask. An electronic balance is used to measure the solid amount (usually in grams) of
solute that will be mixed to the solution then water is added to the flask to the final
volume achieving a specific molarity. For example, to make a 1M (one molar) solution of
NaCl and water a chemist would mass 58 g of NaCl (the molar mass of table salt) on a
scale and add it to a volumetric flask. Then the chemist would add enough distilled water
to achieve a total volume of 1 Liter of solution. The result would be 1 mol NaCl (the
solute) / 1 Liter of solution (NaCl and the distilled water poured) = 1M solution.
A diagram of a volumetric flask on the left. There is usually a line that
marks the 1 Liter mark on the top of the neck of the flask.
How would you make a 0.5 M NaCl/Water solution? Think about it, how many moles of
NaCl would you have per 1 Liter of solution?
c) Molality-molality is another way chemists measure the concentration of a solution. Be
careful it sounds a lot like molarity which is symbolized by a capital M whereas molality
is represented by a lowercase m. Molality by definition is the moles solute dissolved in
kg of solvent. For example going back to our 1M NaCl water solution to make a 1 m (1
molal solution) of NaCl and water you would still need 58 grams of NaCl (1 mole of
table salt) then add 1 kg (1000g or 1000 ml) of water to make a 1 m solution.
**Remember the density of water is 1 g per 1 ml. Molarity compares the volume of
solution to moles of solute and molality compares mass of solvent to moles of solute that
is the difference and it is a subtle one.
Molarity = moles solute/ Liter of solution
M (capital M)
molality = moles solute/ kg solvent
m (lowercase m)
d) Mole Fraction-The concept of mole fraction uses the variable X with a subscript
indicating whether the mole fraction of the solute or the mole fraction of the solvent is
being calculated divided by the whole mole fraction of the solution. There are no units on
mole fraction it is simply the proportion of the solution occupied by the solute and
solvent. The mole fraction of the sum of the solute and the solvent always equals 1!!!
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Example:
X solute = moles of solute/ moles of solution
X solvent = moles of solvent/moles of solution
X solute + X solvent / moles of solution = 1
For example: A mixture of NaCl and water would be if you had 1 mole of NaCl and 9
moles of water the mole fraction would be:
X solute (NaCl) = 1 mole NaCl/10 moles of solution = 0.1
X solvent (Water) = 9 moles water/ 10 moles of solution = 0.9
Basically a tenth (0.1) of the solution by mole amounts is NaCl and 9 tenths (0.9) of the
solution by moles is water.
The mole fraction of NaCl 0.1 + the mole fraction of water 0.9 = 1.0. (Always)
5. Saturationa)The amount of solute added to a solvent does have a limit. You may have
experienced this while adding too much sugar to your coffee or tea. The sugar eventually
settles to the bottom of the coffee in solution. When the maximum amount of solute is
dissolved in an amount of solvent the solution is said to be at its saturation point.
Solutions at their saturation point are said to be saturated solutions. Those solutions
under their saturation point are referred to as unsaturated solutions (more solute can be
added to the solution). Finally, solutions above their saturated points are refereed to as
supersaturated solutions (no more solute can be added to the solution)
Temperature and pressure have major effects on the saturated points of solutions.
b) Conceptual understanding: Think of a solution this way, suppose you wanted to
find the saturation point of sugar in your coffee. The coffee is almost entirely made up of
water with a minute amount of caffeine and other chemical extracted from the coffee
bean. There are on the molecular level, extremely small spaces between the coffee
molecules (mainly water molecules). As sugar is added to the coffee, the sugar molecules
are small enough to settle within the spaces between the coffee molecules. Once there is
no more space, the sugar has no where to fit and becomes supersaturated and begins to
form on the bottom of your coffee cup. Now, if you heated your coffee, the coffee
molecules would move around faster and expand, thus increasing the size of the spaces
between adjacent coffee molecules allowing more sugar to be added. This is why hot
coffee dissolves more sugar than cold coffee!!
Below is a diagram displaying the fitting in the spaces between solvent and solute
particles.
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6. How Does a Solution Form?
a) A solution forms by the attraction between the molecules of the solute and the solvent
that it is mixed with. For example, NaCl is held together by strong chemical bonds. Water
a solvent, is also held together by chemical bonds. When a salt molecule is mixed with
water molecules the attraction between the water molecules and the salt causes the water
to pull the salt apart ion by ion. The oxygen atom (negative charge) on water orientates
itself towards the Na atom (positive charge) on the salt molecule and literally pulls it
away from the Chloride ion. The process of dissolving takes place at the surface of the
solid. The interaction between solute and solvent particles is called solvation. The process
by which ions are pulled away from each other is called dissociation.
In a solution, solute and solvent particles are intermingled This means that the
water molecules must also separate form one another to make room for the solute
particles. Thus the solvation of a solution of NaCl in water involves the breaking of
attractions among solute particles., the breaking of attractions among solvent particles
and the formation of attractions between solute and solvent particles.
Anytime those attractions are broken, energy is required. So the separation of
solute particles from one another and of solvent particles from one another is both
endothermic (energy absorbing process). The formation of attractions between solute and
solvent particles is an exothermic (energy releasing process). Whether energy, in the form
of heat, is absorbed or given off in the overall process of solution formation depends on
the balance between the two processes. If breaking attractions requires more energy than
is released in forming attractions, heat will be absorbed in the overall process. If breaking
attractions requires less energy than is released in forming attractions, heat will be given
off in the overall process.
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7. Solubility Continued
a) The amount of solute that can dissolve in a given amount of solvent is called solubility.
Solubility is usually expressed in grams of solute per 100 grams of solvent at a specified
temperature and pressure.
b) The solubility rule states that “like dissolves like” meaning that polar solvents will
dissolve polar solutes and that likewise nonpolar solvents will not dissolve polar solutes
but will dissolve nonpolar solutes. Grease which is nonpolar cannot be washed away with
water (polar) but grease could be cleaned from your hands using turpentine which is
nonpolar.
c) The solubility of solids in a liquid solvent increase with increasing temperature
because the intermolecular attractions between the solvent and the solid solute increase.
In contrast, the solubility of gases in a liquid solution decrease as temperature increases
because the kinetic energy of the gas molecules increases causing them to escape from
solution and lose their solubility. This is the reason why warm soda goes flat because
CO2 molecules escape from the soda as it warms. Cold soda is the most carbonated
having the greatest CO2 content.
d) The pressure applied to a gas above a liquid directly influences the solubility of that
gas in the liquid. For example as pressure on a gas increases, the pressure forces gas
molecules in solution, increasing the solubility of the gas. This is called Henry’s Law
which states the solubility of a gas in a fluid is directly related to the partial pressure
exerted by that gas.
8. Factors that Affect the Rate of Dissolving
a) Surface Area- The dissolving of a solid solute in a liquid solvent takes place at the
surface of the solute. By increasing the surface area of the solute, the interactions
between solvent and solute particles increase. An example would be why powdered sugar
dissolves faster than granulated sugar which dissolves faster than a sugar cube. The sugar
cube has the smallest surface area and thus dissolves the slowest.
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b) Stirring a solution and heating a solution also increases the rate of dissolving because
it increases the rate of solute and solvent molecules interacting with one another.
An example of a solubility chart above.
9. Colligative Properties
a) Some physical properties of liquid solutions differ from those of the pure solvent. A
property that depends on the concentration of the solute particles but is independent of
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their nature is called a colligative property. Colligative means “depending on the
amount”.
Following are 4 colligative properties we will discuss in class.
b) Vapor Pressure Reduction (VPR)-Liquids have a tendency to vaporize to leave the
liquid phase and enter the gas phase above the liquid. The pressure that they exert as a
gas is called the vapor pressure of that gas. Likewise, some of the gas molecules strike
the surface of the liquid and condense back to the liquid phase. Pure solvents have higher
vapor pressures than those of solutions with nonvolatile solutes. The nonvolatile solute
(nonvolatile means that it is unlikely to vaporize) prevents the solvent molecules from
striking the surface of the liquid and jumping into a gas phase (vaporize) so hence since
the vaporization of the liquid decreases there is less gas above the liquid and less vapor
pressure. The extent to which a nonvolatile solute lowers the vapor pressure is
proportional to its concentration. Doubling the concentration of solute doubles its effect.
c) Boiling Point Elevation (BPE)
-The boiling point of a solution is defined as the temperature when the vapor pressure
above a solution is equal to the external pressure on its surface. Because the addition of a
nonvolatile solute reduces the vapor pressure of the solution, a higher temperature is
necessary to get the vapor pressure of the solution up to atmospheric pressure so that the
solution boils. The amount by which the boiling temperature is raised is the boiling point
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elevation and is directly proportional to the amount of nonvolatile solute added to a
solvent. For example, by adding salt to boiling water, the vapor pressure is reduced so the
salt water must get to a higher temperature in order to boil; This is why pasta cooks faster
in boiling salty water than pure water. Boiling point elevation is symbolized by ΔTb
which is the difference between the boiling point of the solution and the boiling point of
the pure solvent.
ΔTb = Kbm
ΔTb = boiling point elevation
Kb = molal boiling point elevation constant
(each solvent has its own value for this variable)
m- molality moles of solute / kg of solvent
Molal Boiling Point Elevation Constants
Solvent
Kb (°C/m)
Acetic acid
3.22
Benzene
2.64
Carbon tetrachloride
5.02
Chloroform
3.63
Ethanol
1.23
water
0.51
d) Freezing Point Depression (FPD)
-The ability of a nonvolatile solute to lower the freezing point of a solution is known as
freezing point depression. The freezing point of a substance is the temperature at which
the vapor pressures of the solid and liquid phases are the same.
ΔTf = Kfm
ΔTf = freezing point depression
Kf = molal freezing point constant is the specific effect a solute on a given
solvent.
m- molality moles of solute / kg of solvent
Molal Freezing Point Depression Constants
Solvent
Kf (°C/m)
Acetic acid
3.90
Benzene
5.12
Naphthalene
7.00
Chloroform
4.68
Camphor
40.0
water
1.86
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