Discover Water
Water is considered common place to most of us. Many times we don’t even think about
it unless we are hot or thirsty. The oceans are full of water and because of this we never
think about a world without water. However, earth is the only known planet that has
liquid water. This liquid water is what allows life on earth to exist!
Water, like all matter, is made up of tiny particles called atoms. One oxygen atom and
two hydrogen atoms form a water molecule. The oxygen atom is much larger than the
hydrogen atoms. Water is a polar molecule, meaning it is electrically charged. This
happens because the electrons spend more time orbiting around the bigger oxygen atom,
which gives it its negative charge and leaves the hydrogen atoms with a positive charge.
You know how they say opposites attract, well the same is true for electrical charges.
Because of their polarity, or opposite electrical charges, the water molecules are attracted
to other molecules, including each other. Hydrogen bonds are the attraction between
liquid water molecules. They are not very strong bonds but they are very important and
are responsible for many of the unique properties of water.
Picture of the planet from a distance so can see all the water, the ocean, and a water
molecule with the charges and other molecules being attracted to it.
Water Water Everywhere
Any substance can exist in three different physical forms; solid, liquid and gas. Water is
the only substance on earth that naturally occurs in all three states. Temperature reflects
the average speed of molecules and the faster the molecules are moving the higher their
temperature. This is just like how the faster you move, the hotter you get! When moving,
the hydrogen bonds between the molecules of water can break. The water molecules in
the liquid phase can move so fast that they break their hydrogen bonds and go from a
liquid to a gas phase. If temperatures get hot enough, the molecules all move fast enough
to break their hydrogen bonds and move into the gas phase. We see this happening when
water boils!
When liquid cools the molecules slow down and pack closer together. Volume is
decreasing without mass changing and since density = mass / volume, the water gets
denser. So as seawater gets denser, it sinks to the bottom of the ocean. This sinking of
cold water at the poles creates global circulation at the bottom of the ocean. As water
continues to cool the hydrogen bonds form and hold the water molecules in place as a
solid. This solid form of water is ice.
Fresh water is different from salt water. Ever notice that ice cubes float in your drink?
That is because fresh water gets denser as it cools until it reaches about 4oC, (39oF).
Below that temperature ice gets less dense as it cools. This property allows fish living in
lakes and ponds to survive the winter. The ice that freezes on the surface insulates the
bottom water so that it doesn’t freeze! This happens because water expands as it freezes.
If you have ever made ice cubes you have seen this happen. Since the mass remains the
same, but the volume of the solid is greater, ice is less dense than liquid water and so it
floats! It is extremely unusual for the solid phase of a substance to be less dense than its
liquid phase, but lucky for aquatic life it is!
Pictures of boiling water, ice cubes in a tray, water frozen on the top of a lake, and of the
ocean.
Melting Ice
A large amount of heat is necessary to melt ice. Water molecules in the ice begin to
vibrate as heat is added, and eventually break their hydrogen bonds and move around. Ice
requires a much warmer temperature to melt due to its hydrogen bonds. Without the
strength of the hydrogen bonds, ice would melt at -90oC (-130oF) instead of 0oC (32oF).
Ice works well to cool your drinks because any heat that enters the liquid goes into
breaking the hydrogen bonds of the ice, not into increasing the molecules’ velocity,
which would raise the temperature of your beverage. Because all of the energy does not
go into increasing the speed of the water molecules but also into breaking the hydrogen
bonds, it requires a lot of heat to raise the temperature of water. The heat capacity of a
substance is the amount of heat that must be added to raise the temperature of a substance
a given amount. Water has one of the highest heat capacities of any natural substance.
This reflects on the amount of heat water can store. Ever hear of using water as a coolant
in car engines? This is because it can absorb a lot of heat without increasing much in
temperature. You experience the effects of water’s high heat capacity when you visit the
shore. When you go to the beach in June, even though it has been warm for a few
months, the water is still really cold. And if you return in November, even though the air
temperatures have been cool for a few months, the water is still relatively warm. This is
all because of water’s high heat capacity!
Pictures of ice in a drink of water, and of the shore in the early summer and fall.
Water as a Solution
Water is called the universal solvent because it can dissolve more things than any other
natural substance. Water is especially good at dissolving salts. Salts are a combination of
particles with opposite charges, which are called ions. Table salt consists of a positively
charged sodium ion (NA+) and a negatively charged chlorine ion (Cl-) to form NaCl.
When placed in water the strongly charged ions that make up the salt attract the weak
charges of the water. Water molecules surround each of the ions and this weakens the
electrical charges that were holding the salt molecules together. The salt crystals breaking
apart is the process of the salt dissolving in water. Because the salt dissolved in water
increases the mass of the water solution without increasing the volume by much, it
increases the density of water (density = mass / volume ).
Pictures of a solution, table salt, sodium chloride molecule with the charges, and of the
water breaking the molecule apart by surrounding it.
What do you know about water?
1. Which phase of water allows life to exist on earth?
a. Liquid
b. Solid
c. Gas
d. Vapor
e. They all do
2. ( T / F ) Hydrogen bonds are the extremely strong bonds that hold water
molecules together.
3. Temperature reflects on the _______________________ of molecules.
4. What property of water allows aquatic life to live through the winter in lakes and
ponds?
a. Water gets denser as it cools
b. Water has a high heat capacity
c. Water gets denser as it cool until it reaches 4oC and then is becomes less
dense
d. Water is a universal solvent
5. Water works well as an engine coolant because…
a. It can absorb a lot of heat
b. It is a clear substance
c. It is the universal solvent
d. It can absorb a lot of heat without increasing in temperature