TEK I9A – relate the structure of
water to its function as the
universal solvent
TEK I9B – relate the concentration
of ions in a solution to physical and
chemical properties such as pH,
electrolytic behavior, and reactivity
Water
The Universal Solvent
Water’s Importance
• Water is called the
"universal solvent"
because it dissolves
more substances
than any other liquid.
•A solution is made up of a solvent (the
substance that does the dissolving) and a
solute (the substance being dissolved).
Water’s States
• Water is unique in that it is the only natural
substance that is found in all three states
at the temperatures normally found on
Earth.
Liquid
Solid (ice)
Gas (steam)
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Phase Diagram of Water
Triple
Point
Water is found on Earth in all three forms. This is
because Earth is a very special planet with just the
right range of temperatures and air pressures.
Earth is said to be at the triple point for water. Day 3: #2
Water’s States
Phase changes are physical changes – NOT chemical changes!
Sample Question
1. The diagram shows
physical changes that
occur in the water cycle.
Which of these shows
condensation?
a) Q
b) R
c) S
d) T
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Answer
Which of these shows
condensation?
d) T
Clouds are water
vapor that has
condensed back
into very small
water droplets.
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Water's Structure
• You probably know water's chemical
description is H2O. One atom of oxygen is
bound to two atoms of hydrogen.
Water is a polar
molecule. One side
is positive and the
other is negative.
The hydrogen atoms are "attached" to one side of the oxygen
atom, resulting in a water molecule having a positive charge
on the side where the hydrogen atoms are and a negative
charge on the other side, where the oxygen atom is.
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“Like Dissolves Like”
•You need to know that polar
substances, like water, dissolve
other polar substances.
•Nonpolar substances dissolve
nonpolar substances
•Put a nonpolar substance and a
polar substance together and you
will find that they don’t mix! Like oil
and water.
Very
Important!
Water is a polar
molecule. One side
is positive and the
other is negative.
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Water's Polar Structure
• Since opposite electrical
charges attract, water
molecules tend to attract
each other, making water
kind of "sticky."
• The side with the hydrogen
atoms (positive charge)
attracts the oxygen side
(negative charge) of a
different water molecule.
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Sample Question
2. Which factor makes water an effective
solvent?
a) The presence of molecular oxygen
b) Its lack of covalent bonds
c) The polar nature of its molecules
d) Its abundance on Earth’s surface
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Answer
Which factor makes water an effective solvent?
c) The polar nature of its molecules
- Polar molecules and ions are attracted to the
partial positive and negative charges within
the water molecules.
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• Why is water used as
a coolant in a car’s
radiator?
-because water holds a
lot of heat!
Instead of saying water can hold a lot
of heat, in science we say that water
has a high specific heat.
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How can some lizards and insects
walk on water?
The high surface
tension of water
allows them to walk
across the water
without breaking the
surface!
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What is Surface Tension?
• Surface tension is a
measure of how difficult it
is to stretch or break the
surface of a liquid.
• Water has a very high
surface tension and can
support some items that
have higher densities,
like the pin and insect
shown to the left.
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Most solids are heavier than
liquids. Then why does solid water
(ice) float on liquid water?
• Water is unusual in that
the solid form, ice, is less
dense than the liquid form,
which is why ice floats.
• Density of liquid water is
1g/mL
Water’s Density
• Implications…
Lakes freeze from top to bottom and thus
enables life to continue in the lake. If ice was
more dense, ice would pack to bottom. Lakes
might never completely thaw out and Earth’s
climate would be much colder!
Sample Question
3. Fish survive through severe winters because of
the property of water that allows water to —
a) form chemical bonds as it freezes, raising the
water temperature below the ice
b) increase in density while it freezes, dissolving
more oxygen from the air
c) expand when it freezes, creating a floating and
insulating layer of ice
d) precipitate vital nutrients when it freezes,
increasing the food supply
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Answer
Fish survive through severe winters because of the
property of water that allows water to —
c) expand when it freezes, creating a floating and
insulating layer of ice
- Ice is less dense than liquid water. The ice
serves as an insulator for the liquid below.
Day 3: #2
Sample Question
4. If the properties of water were to change so that
the solid form was denser than the liquid form,
organisms living in a cold pond environment
would be less likely to survive because water
would no longer —
a) dissolve enough oxygen from the air
b) produce solutions containing vital nutrients
c) remain neutral, instead becoming highly acidic
d) produce a floating insulating layer of ice
Water is a Neutral Substance
• Pure water has a neutral pH of 7, which is
neither acidic nor basic. .
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What is Ph?
• The “power of
hydrogen” (pH) scale
is from 1 to 14.
• The pH shows
whether a substance
is acid (pH 1-6),
neutral (pH 7) or
basic (pH 8-14).
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What is pH?
• That pH scale is actually a measure of the
number of H+ ions in a solution. If there are a lot
of H+ ions, the pH is very low. If there are a lot
of OH- ions, that means the number of H+ ions
is very low, so the pH is high.
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How is pH measured?
• We can measure the
pH by
-dipping pH paper into
the substance (shown
to the right)
-dipping litmus paper
into the substance (if it
turns blue it is a base)
-using a pH meter to
read the pH of the
substance
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Sample Question
5. Citric fruits, such as oranges and
grapefruit, contain citric acid, yet they are
safe to eat. What is the approximate pH of
citric fruit?
A1
B5
C7
D 10
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Sample Question
Citric fruits, such as oranges and grapefruit,
contain citric acid, yet they are safe to eat.
What is the approximate pH of citric fruit?
B5
-Citric fruits are acidic, so the pH must be
less than 7. They are safe to eat, so they
must not be that acidic
Day 3: #3
Sample Question
• 6. Two clear solutions are placed in separate
beakers. The first solution has a pH of 4, and the
pH of the second solution is unknown. If the two
solutions are mixed and the resulting pH is 5, the
second solution must have —
A fewer suspended solids
B a lower temperature
C more dissolved salt (NaCl) particles
D a higher concentration of OH– ions
Day 3: #3
Answer
• Two clear solutions are placed in separate
beakers. The first solution has a pH of 4,
and the pH of the second solution is
unknown. If the two solutions are mixed
and the resulting pH is 5, the second
solution must have —
D a higher concentration of OH– ions
Day 3: #3
Electrolyte VS Nonelectrolyte
An electrolyte is a substance that allows an electric current
to pass through. The seawater in the beaker below is an
example of an electrolyte (because the light bulb is
glowing). When water contains dissolved ions or has
dissolved a polar solute, then the solution will be
electrolytic.
Try this7. What characteristic of water remains the
same no matter what is dissolved in it?
A The ratio of hydrogen to oxygen
B The ability to refract light
C The hydroxide ion concentration
D The freezing temperature
8. Bathwater normally has electrolytic
behaviors even though distilled water does
not. This is because bathwater
(A) contains isotopes of hydrogen
(B) has been heated
(C) is separated into H+ and OH- ions
(D) contains dissolved minerals
9. Which characteristic of water best
explains its ability to dissolve a great
variety of materials?
(A) Its transparency in light
(B) Its electrical conductivity
(C) Its physical state of matter
(D) Its molecular arrangement