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Section 9.1 Solutes, Solvents and Water
9.1 Solutes, Solvents and Water
Definition
In Chapter 2 we talked about solutions and you have worked with them extensively in the
laboratory. This chapter will look at solutions more closely and develop some of their
properties. A true solution is homogenous on the molecular level. That means there are
no clumps bigger than a molecule.
An example
solution
Sixteen grams of copper sulfate (CuSO4) in 100 mL makes a deep blue solution when
every particle is dissolved. This solution is 1 molar because there is 1 mole of copper
sulfate per liter of solution.
Solvent and
solute
A solution always has a solvent and at least one solute. The solvent is the substance that
makes up the biggest percentage of the mixture, or is liquid. The copper sulfate solution
has water as the solvent. The solutes are the other substances in the solution. Copper
sulfate is the solute in the example.
Dissolving
When the solute particles are evenly distributed throughout the solvent, we say that the
solute has dissolved. The copper sulfate starts as a solid powder. Water is added and the
mixture is carefully stirred until all the solid powder has dissolved. Once the copper
sulfate has dissolved, the solution is transparent again.
All true solutions are transparent
Why solutions
are transparent
True solutions are transparent because all the particles are single molecules. Milk is not a
true solution because each of the tiny fat particles in milk are thousands, or tens of
thousands, of molecules. Particles this size are large enough to block light and scatter it
to the side. The scattering of light is the reason milk is opaque.
solvent - the substance that makes up the biggest percentage of the mixture, or is
liquid.
solute - any substance in a solution other than the solvent.
dissolved - when molecules of solute are completely separated from each other and
dispersed into a solution.
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The water molecule
Water is critical
to life
Your body is about 60% water by weight. In 1 hour of exercise, you may lose as much as
a half-gallon of water sweating and breathing! You also lose small amounts of dissolved
salts. If the lost water and dissolved salts are not replaced, your body stops working. The
unique physical and chemical properties of water explain why it is so important.
Water is a polar
molecule
The water molecule is polar. That means the electric charge is unevenly distributed in a
water molecule. The two shared electrons from the hydrogen atoms are attracted to the
oxygen atom. This makes the oxygen side of the molecule slightly negative and the
hydrogen side slightly positive. The diagram on the right shows the surface of the
molecule shaded red where it is more positive and blue where it is more negative.
Hydrogen
bonding
Water molecules tend to stick to each because of
the polarity. Positive hydrogen atoms from one
water molecule are attracted to the negative
oxygen atoms in another molecule. This
attraction is not as strong as a chemical bond.
BUT, it is strong enough that we give it its own
name: hydrogen bonding. Hydrogen bonding
explains some curious properties of water.
Why ice floats
When water freezes, it becomes LESS
dense! This is unusual, most materials are
more dense as solids than as liquids. Ice is
less dense than liquid water because
hydrogen bonds force water molecules to
align in a crystal structure where molecules
are farther apart than they are in a liquid.
polar - a molecule is polar when there is a charge separation that makes one side (or
end) of the molecule more positive or negative than the other side (or end).
hydrogen bond - an intermolecular bond that forms between a hydrogen atom in
one molecule and the negatively charged portion of another molecule (or another part
of the same molecule).
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Section 9.1 Solutes, Solvents and Water
The importance of hydrogen bonding
Comparing
water and
methane
Consider two molecules with similar molecular
masses. Methane (CH4) has a molecular mass of
16 g/mole and water (H2O) has a molecular mass
of 18 g/mole. Methane boils at -161°C, so at room
temperature, methane is a gas. Water has a similar
mass, but boils at an astounding +100°C! This is
far hotter than almost any other low molecularweight hydrogen compound. What is the reason?
The answer has to do with hydrogen bonding and
the shape of the two molecules.
Hydrogen
bonding causes
water’s high
boiling point
Due to its symmetrical shape, a methane molecule is non-polar! Hydrogen bonding does
not occur between neighboring methane molecules. Without hydrogen bonding, it is
easier for a methane molecule to escape its neighbors and become a gas. By comparison,
water has strong hydrogen bonding between neighboring molecules. The hydrogen
bonding is the reason for water’s unusually high boiling point. The high boiling point of
water is of critical importance to life on earth. Life needs liquid water. If water was a gas
at room temperature, it would be impossible for life to exist as we know it.
Making solid
steel float!
Gently place a pin on the surface of
water and the surface of alcohol. The
pin sinks immediately in the alcohol.
The pin floats on the water! Pins are
made of steel, which is dense enough
to sink in water. Why can a pin float
on water but not on alcohol?
Surface tension
The answer is a property called surface tension. Surface tension is a force that pulls
molecules together along a liquid surface. The high surface tension of water is a direct
consequence of hydrogen bonding. The pin floats on water because it has to separate
water molecules on the surface to break through and sink. The hydrogen bonds between
water molecules are held tightly enough that a gently placed pin will float. The fact that
water drops pull themselves into spheres is also caused by surface tension. Alcohol has a
lower surface tension, because it has fewer hydrogen bonds than water and the pin easily
drops through the surface to sink.
surface tension - a force created by intermolecular attraction in liquids, such as
hydrogen bonding in water. Surface tension acts to pull a liquid surface into the
smallest possible area, for example, pulling a droplet of water into a sphere.
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Water as a solvent
The universal
solvent
Water is often called the “universal solvent.” While water doesn’t dissolve everything, it
does dissolve many substances such as salts and sugars. Water is a good solvent for two
reasons:
1.
2.
How water
dissolves salt
A water molecule is small and light;
Water molecules are polar.
The polar molecules of water are especially
good at dissolving ionic compounds like salt
(NaCl). When salt is mixed with water, the
polar water molecules are attracted to sodium
and chlorine ions in a solid salt crystal. When
they hit, they transfer some energy which
causes the ions in the salt crystal to separate.
The negative side of the water molecules are
attracted to the Na+ ions and the positive ends
are attracted to the Cl- ions. Once a Na+ or Clion has broken off the crystal, it instantly
attracts the oppositely charged sides of nearby
water molecules. This process is called
hydration. The hydrated sodium and chlorine
ions stay in solution because of their
accompanying polar water molecules.
Water dissolves
many
molecular
compounds
Glucose is a covalent molecule,
but it dissolves easily in water. The
glucose molecule has areas of
positive and negative charge (red
and blue shading). When glucose
is mixed with water, the charged
areas
attract
many
water
molecules. Molecules of glucose
are not dissociated into ions, but
remain together (and neutral) in
solution.
Hydration - the process of molecules, with any charge separation, to collect water
molecules around them. While not “chemically bonded”, a hydrated molecule does
hold fairly tightly to its “private collection” of water molecules.
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Section 9.1 Solutes, Solvents and Water
Distilled water vs. tap water
Distilled water
is used to make
solutions
When making solutions with water in the chemistry lab we often use
distilled water. Distilled water is water that has been purified so that
it contains no dissolved substances. The distillation process heats water
to it’s boiling point of 100oC. The steam that forms is collected in a
separate clean container. Any ions and solid contaminants are left
behind as “residue” in the original container.
Distilled water is
sometimes incorrectly referred to as deionized water however, this is
incorrect. Deionized water is water that has had its ions removed
using a specific filtration process. Deionized water is sometimes used
instead of distilled water, because it is cheaper to produce and, for most
purposes, yields sufficiently pure water.
In contrast, tap water often contains dissolved salts and minerals from the ground water
supply and even from the pipes that carry it from the reservoir. Many minerals, such as
Fe2+, Ca2+ and Mg2+, that may be present in water are important for our health. Other
sources of ions in tap water come from water supplies that are treated with chloride ions
(Cl-) for purification purposes. Chloride ions kill bacteria that may be present in the
water supply. Fluoride (F- ion) is also added at 4 mg/L to help with dental health.
Evidence?
How do we know that tap water contains ions and distilled water does not?
Why do we use
distilled water?
When making solutions in the laboratory we need to know there are no other chemicals
in the solution, besides the ones that we add. Contaminating ions and minerals can
interfere with the chemical reaction we are trying to perform. For this reason we use
distilled or deionized water when we make our solutions in the chemistry laboratory.
Distilled water - water purified by heating water to steam and condensing the steam
into a clean or sterilized container.
Deionized water - water without ions, purified using filtration methods.
Tap water - drinking water from the faucet or “tap.”
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Reactions in aqueous solutions
Aqueous
means
dissolved in
water
A solution with water as the solvent is called an aqueous solution. Aqueous solutions
are so important that chemists consider being dissolved in water to be almost a fourth
state of matter! In writing reactions, we use the symbols (s), (l), (g) and (aq) to show
what state of matter the reactants and products are in.
(s)
indicates a solid
(l)
indicates a liquid
(g)
indicates a gas
(aq)
indicates a substance dissolved in water
A good example is the familiar baking soda and vinegar reaction. All four states of matter
appear in this reaction.
Reading the
symbols (s), (g),
(l) and (aq)
The (s) tells us the baking soda is a solid in the reactants and the (aq) tells us the acetic
acid (vinegar) is an aqueous solution. On the product side of the reaction, the first (aq)
tells us the sodium acetate is dissolved in the water that was part of the vinegar. The
water (l) is a liquid. The carbon dioxide (g) is a gas. The gas is what makes the bubbles.
If you are not careful, any product that is a gas quickly escapes, taking its mass with it! If
the CO2 is allowed to escape, we would expect to measure less mass in the products of
this reaction compared to the reactants. You can learn a lot about a reaction by knowing
how to read these special symbols!
Aqueous solution - An aqueous solution is any solution where the solvent is water.
Molecules, atoms, or ions in aqueous solutions are identified by the symbol (aq) after
a chemical formula. For example CO32-(aq) is how you would write a carbonate ion
(CO32-) that is dissolved in water.
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Section 9.1 Solutes, Solvents and Water
The importance of aqueous solutions
Reactions in
solids
In order for a chemical reaction to occur,
molecules must touch each other. This
seems obvious, but think about what it
means. In a solid, molecules cannot easily
move around. Chemical reactions DO
occur in solids, but they are slow, like
changing limestone into marble under heat
and pressure in the earth. This takes
thousands of years.
Reactions in
gases
In a gas, molecules move around quickly.
Reactions (like combustion) can happen
quickly in a gas, but there are very few
chemicals that are gases at room
temperature. Also, gases are not very
dense. One mL of a gas (like air) contains
a thousand times fewer molecules than the
same volume of a liquid, like water.
Reactions in
liquids
Liquids combine the advantages of both high density and mobility. In a liquid there are
lots of molecules, close together, and they can move around and reach each other.
Relatively few chemicals are liquid at room temperature either. Sugar is solid, salt is
solid, and virtually all proteins and carbohydrates are also solids at room temperature.
Why life
requires water
Life involves reactions of thousands of complex chemicals. This can only happen in
aqueous solutions! Only in a solution can the molecules in living things circulate and
reach each other to have reactions. For example, a sequence of reactions called the Krebs
Cycle occurs in virtually all living things on Earth. In one step of this reaction, malic acid
reacts with a chemical called NAD+ to produce oxaloacetic acid and NADH. This is part
of how energy is extracted from glucose in every cell of your body.
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Other solvents
Non-polar
solvents
Water may be the most important solvent
to living things, but lots of other solvents
exist, and you will probably use them
from time to time in your life. For
example, artists thin oil-based paints with
odorless mineral spirits. Mineral spirits is
the common name for hexane (C6H14), a
linear molecule derived from oil. Hexane
is non-polar, and good at dissolving other
non-polar solutes.
Why oil does
not dissolve in
water
Oil does not dissolve in water because
water is a polar molecule and oil
molecules are nonpolar. However, oil
dissolves easily in mineral spirits. In
general, polar solvents dissolve polar
solutes, like water dissolving salt. Nonpolar solvents dissolve non-polar solutes,
such as mineral spirits dissolving oil.
In general, like dissolves like
Polar solvents dissolve polar solutes
Non-polar solvents dissolve non-polar solutes
“Solvent”
means different
things in
everyday use
In every-day conversation the word “solvent” usually means organic liquids such as
mineral spirits, acetone (nail polish remover), turpentine, alcohol, and similar chemicals.
Many of these chemicals are flammable, and noxious to breathe. In chemistry, the term
“solvent” means something that dissolves something else. The common meaning and the
chemistry meaning are similar, so don’t get confused! Water is a solvent in chemistry, the
most important one to living things.
TABLE 9.1. Some common solvents other than water
Name
Formula
Typical uses
Hazards
Mineral spirits
C6H14
paint thinner
flammable, toxic
Methanol
CH3OH
solvent, fuel
flammable, toxic
Acetone
CH3COCH3
Tetrachloroethylene
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C2Cl4
nail polish remover
flammable, toxic, carcinogen
dry cleaning chemical
flammable, toxic, carcinogen
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