Water Basics
H 2O
(This is a study guide for some basic information about water. Some online quiz questions and
questions on next exam will come from this study guide.)
Some facts and figures
The water footprint of China is about 700 cubic meter per year per capita. Only about 7% of
the Chinese water footprint falls outside China.
Japan with a footprint of 1150 cubic meter per year per capita, has about 65% of its total
water footprint outside the borders of the country.
The USA water footprint is 2500 cubic meter per year per capita. (that’s over 1800 gallons
per day) Per capita water consumption is simply calculated by taking the total US water use
from all activity divided by the total population.
A typical US household uses about 100 gallons of water per person directly.
The facts above came from http://www.waterfootprint.org/?page=files/home which also allows you to
calculated you water footprint.
For fun, take the Water Basics T/F quiz from http://ga.water.usgs.gov/edu/mwater.html as a pre-test
After taking this quiz the answers are given and explained. Do not feel bad if you miss a lot of these
questions many are in the water trivia category but interesting anyway.
Properties (go to link for answers to next 2 questions)
http://ga.water.usgs.gov/edu/waterproperties.html
How many Hydrogen atoms are in a water molecule?
How many Oxygen atoms are in a water molecule?
How do fish survive the winter? The density of water is 1.0 gram/cc at 39.2 oF and 0.96 gram/cc at 212
o
F. (cc stands for cubic centimeter) At room temperatures the density of water is about 8.3 pounds per
gallon (8.3 lb/gal)
The density of water is greater for cold water compared to warmer water is clear from the above facts,
but interestingly this is not always true. Normally as water cools it contracts, but right before it freezes
it expands a little. The density of water right before it freezes (32.0 oF)is actually 0.9999 gram/cc making
it slightly less dense that water at 39.2F. This is lucky for fish because the 39.2 oF water sinks to the
bottom of a lake and the 32 oF rises to the surface. Thus the surface freezes over first providing an
insulating blanket of ice on top to help keep the rest of the lake from freezing solid. Water is one of the
few substances has this strange dance of density variation with temperature.
The density of ice is about 0.91 gram/cc. This means that ice floats with about 9 % of the ice berg
sticking up above the fresh water and 91 % below (aka the tip of the iceberg). Since the density of sea
water is higher than freshwater, slightly more than 9% of the ice will appear above the salty ocean.
It takes 1.0 calorie of heat energy to increase the temperature of 1.0 gram of water by 1 oC.
Alternatively, It takes 1.0 BTU of heat energy to increase the temperature of 1.0 pound of water by 1 oF.
If 1.0 calorie of heat energy were added to 1.0 gram of steel it would heat up by about 10.0 oC.
We say that water has a relatively high specific heat capacity since it can hold a lot of heat energy before
getting to hot. This make it an excellent medium for storing solar energy. When something gets hot it
is much harder to keep it hot. So if water can hold a lot of heat energy before getting very hot it is
easier to save that energy for later use.
The picture below shows that: 1) to melt ice, energy must be added to the ice; 2) to evaporate liquid
water energy must be added to the water; and 3) to sublimate ice (turn it directly from frost to invisible
vapor) energy has to be added to the ice (frost).
We say evaporation is a cooling process since energy is taken from the environment to turn liquid into
invisible water vapor. As an example: when you jump out of a pool on a sunny slightly breezy day you
get goose bumps until the water has evaporated from your skin.
Important distinction: visible clouds are actually liquid water droplets not water vapor. When you
breathe out you exhale not only carbon dioxide but also lots of water vapor. The water vapor is
invisible. As the exhaled water vapor from your breath condenses into tiny water droplets on a cold day
it becomes visible.
The picture above also shows that 1) to freeze water, energy must be removed from the liquid water
and released into the surrounding environment; 2) to condense water vapor into liquid energy must be
removed from the vapor and released into the surrounding environment; and 3) to deposit ice directly
from vapor (formation of frost is referred to as deposition) energy has to be energy must be removed
from the vapor and released into the surrounding environment.
For example during a hurricane water evaporates from the ocean (cooling the ocean a little) and then
condenses into clouds. The release of the heat energy from condensation into the atmosphere provided
the driving force of hurricanes.
As another example, it warms up when it rains because heat energy is released to the atmosphere as
water vapor turns into rain drops.
The numbers below are not to be memorized but are presented to give you a relative feeling of the
energies involved during the different phase changes shown in the figure above.
If we started with 1 g of ice at -100 oC it would take:
50 calories to raise its temperature to 0 oC so it could start melting,
80 calories to melt it,
100 calorie to raise its temperature to 100 oC so it could boil,
and 540 calories for it all to boil away,
then another 50 calories if we wanted to heat the water vapor up to 200 oC.
As ice melts its temperature stays at 0 oC until all of the ice turns into liquid. The is also true as the
water vaporizes during the boiling process (the temperature stay at 100 oC). The energy require to melt
or vaporize water is called latent (hidden) heat because there is no temperature as this heat is added to
the water.
The point of this is that the energy needed to vaporize water is huge compared to the other processes
(see the graphical comparison below). As a result evaporation is very effective at cooling. The reverse is
also true. Condensation is very effective at heating things up. Steam burns are so severe because of this
release of latent heat into your skin as the vapor condenses. This release of latent heat energy as
clouds form is also what makes hurricanes and thunderstorms so violent.