Water: The Universal Solvent! Powerpoint

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Easiest way to remember where all
water ends up…..
• “And though the course may change
sometimes, rivers always reach the sea..”
• Led Zeppelin, from “Ten Years Gone”
Water, the universal
Solvent!
Why?
Water as a solvent
• If you fill a glass with tap water and leave
it, notice that after the water has
evaporated a stain is left on the glass
• If a glass of ocean water is left sitting, the
water will evaporate and what will be left?
Salt!
• These leftovers, called residue, were
chemical substances that were dissolved
in the water
Water as a solvent
• Any liquid, including water, that contains
dissolved substances is called a solvent
• Water is a good solvent because it can
dissolve so many substances, and ocean
water contains numerous dissolved
substances, salt being only one of them
• The study of chemical substances found in
ocean water is called chemical
oceanography
Ocean Water
• Made up of 96.5% water molecules
• The remaining 3.5% is mostly salt, which
is dissolved in the water
• Any substance that is dissolved in a liquid
(like water) is called a solute
• In ocean water, salt is a solute because it’s
dissolved in water, the solvent
• This mixture between dissolved salt and
water is called a solution
Salt (solute)+Water(solvent)=salt
water(solution)
Ok, time to make YOUR solutions!
•
•
•
•
•
Groups of 4 please
Grab a beaker
Fill the beaker about 1/4 with tap water
Pass around the salt
Try to stir in at least 2 to 3 tablespoons of
salt
• Then, we wait!
Substances in Sea Water
• Many solutions, like ocean water are
called mixtures
• A mixture contains 2 or more substances
that can be separated by ordinary physical
means
• For example, if salt water is left to sit – the
water will evaporate and the salt will be
left!
Substances in Sea Water
• The salts dissolved in ocean water are called
sea salts, and sea salt is a compound
• A compound is a substance that contains 2 or
more kinds of atoms that are chemically bonded
• Salt=sodium+chlorine (Na+Cl)
• When salt is added to water, the compound
breaks apart, creating charged particles called
ions
• Na is + charged, while the chlorine becomes a
negatively charged chloride ion
• Opposites attract!
Substances in Sea Water
• Another compound found in ocean water is
calcium carbonate
• This is also called limestone
• Calcium carbonate is the main component of
seashells and of coral reefs
• Again, the wearing down of seashells and coral
reefs releases calcium carb into the water,
resulting in ions
• Calcium is + while carbonate is –
• How are ions joined? Electrostatic force!
Sea Salts found in Sea Water
•
•
•
•
•
•
•
Sodium Chloride – 67%
Magnesium Chloride – 14.6%
Sodium Sulfate – 11.6%
Calcium Chloride – 3.5%
Potassium Chloride – 2.2%
Other salts – 1.1%
Remember, when sea water evaporates,
all of these salts are left behind as residue!
pH
• pH is the degree of acidity or alkalinity of a
solution
• Water molecules break up into charged particles
called ions
• Water=H+ + OH• In this case, the hydrogen ion is positive, and the
negative ion is called the hydroxyl ion
• Since opposite charges attract, these ions bond
to form water!
Moving right along…
• When a solution is an acid, it means that it
contains a larger amount of hydrogen atoms
than hydroxyl atoms
• A solution that has a larger number of hydroxyl
atoms than hydrogen atoms is called a base, or
an alkaline solution
• So technically, pH refers to the concentration of
hydrogen atoms in a solution
• The pH scale! 0-14
Acid Rain
• Acid rain is formed when chemicals released by the
burning of fossil fuels are absorbed by moisture in the air
• Many bodies of water can become acidic due to this acid
rain, such as rivers and streams – and many have
• In extreme cases, these freshwater surface waters can
have a pH of 4-5, and can result in massive fish kills
• The oceans, however, have a much more stable pH and
are less affected by acid rain
• Why?
• The oceans have much more surface area to dilute the
acid rain!
Buffers
• Furthermore, the oceans have chemicals called buffers
that can help maintain a stable pH
• Buffers are substances that lessen the acidity or
alkalinity of a solution
• One of the main buffers in ocean water is the carbonate
buffer
• Limestone is made from carbonate, and limestone will
accept hydrogen ions – so the water will become less
acidic and more basic
• It can also release hydrogen ions, causing the water to
be MORE acidic and less basic
• Since it can do both, pH is pretty much kept in check and
a balance is maintained
Oxygen in the Water
• There is oxygen present in ocean water, as well
as freshwater surface sources
• This is called dissolved oxygen, or DO
• Surprisingly, most of Earth’s oxygen supply
comes from photosynthesis carried out by algae
and plants that live in the photic zones of the
water
• After oxygen leaves the plants, some dissolves
in the water and some is released into the
atmosphere
Soooo…..
• Every time we breathe, we are inhaling
some of the oxygen that marine plants and
algae release!
Oxygen Distribution in Water
• Oxygen is not very soluble in water, so the
actual quantity is small
• Air can hold about 200ppm of oxygen, while
water can only hold about 1-12ppm
• There is definitely more DO present at the
ocean’s surface than in the depths. Why?
• Because the photic zone (the zone where light
can penetrate) is at the surface, so that’s where
photosynthesis by marine plants and algae
takes place!
Salinity
• Obviously, ocean water has salt in it!
• Does salinity vary in different ocean waters on
Earth?
• It varies only slightly
• In ppt, salinity levels in the Pacific are about
35ppt, while the Atlantic levels are about 37ppt
• However, areas like the Red Sea and the
Mediterranean are even higher. Why?
• They are in areas with hot climate, so there’s
more evaporation and less precipitation
• So, salinity varies with climate
Salinity
• Salinity also varies with latitude
• Salinity is higher at 20 degrees N and S latitude
than at the equator. Why?
• Regions at the equator have a lot more rain!
• A lot of rain will dilute the ocean water, rendering
it less salty
• Also, on coasts where freshwater rivers or
streams enter the oceans salinity is also less
Salinity and Depth
• Salinity also varies with depth
• Salinity in the deeper areas is higher than near
the surface
• At a depth between 100-200m is a layer of water
called the halocline
• This layer shows a rapid increase in salinity, and
it continues to increase as depth increases
• This is because colder water causes salt ions to
move closer together, so it can hold more salt
Sources of Salt
• Freshwater rivers and streams that flow into the
ocean
• This is because these waters erode streambeds
and riverbeds that contain natural salts, and also
salts from adjacent land areas enter the streams
and rivers in the form of surface runoff
• Salt also comes from the ocean floor from
hydrothermal vents that emit hot, mineralized
water into the oceans
• The minerals present in the water include
sodium and chloride!
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