Download Water: The Key to Life Notes

Water and Its Properties: The Keys to Life
Formation of Water on Earth
Scientific evidence indicates that the Earth formed 4.5-4.6 billion years ago from dust and debris orbiting
the sun. Due to gravity, this debris became compacted and grew quite hot, creating hot gases, including
water vapor and carbon dioxide. Over millions of years, the Earth and its gases cooled, and it is
believed that seas formed when the Earth cooled enough for water vapor in the atmosphere to
condense. Earth is a unique planet in our solar system in that it is the only planet that we are aware of
having liquid water. As a result, Earth is the only planet that can support life as we know it. Water’s
unique properties help keep the planet from getting too hot or too cold. Water is the only compound that
commonly exists in all three states (solid, liquid, gas) on Earth. The unique properties of water are a
major factor in the ability of Earth to sustain life.
Explain how water formed on planet Earth. ______________________________________________
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What makes Earth unique in being able to support life? ___________________________________
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Most of Earth’s water (97%) is salt water found in the oceans. Only 3% of Earth’s water is freshwater.
Most of this (about 75%) is unavailable for use as it is frozen is ice caps, etc. About 23% of the
freshwater is found underground in aquifers. An aquifer is an underground layer of rock or soil that
holds water. Water that fills the cracks and spaces in underground soil and rock layers is called
groundwater. Water is also found in the bodies of living things. Less than 1% of all water is freshwater
that is available for human use. Water is also found in the bodies of organisms. About two-thirds of our
body weight is water. Water helps control body temperature. Water also lubricates our muscles so they
can move. People can live a few weeks without food but not without water. Everyday we need about 2
liters of water to make up for the water we lose when we sweat, breathe, and urinate.
How much of all water is salt water? freshwater? How much is freshwater that is available for
human use? _______________________________________________________________________
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What is an aquifer? What do we call the water found in an aquifer? _________________________
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Water and Civilizations/Settlements
The first human settlements were established near springs, rivers, and lakes. Reliable fresh water
sources and irrigation systems allowed civilizations to grow and flourish. As cities grew, different
strategies (tunnels, aqueducts, wells, cisterns, pumps, reservoirs) were employed to collect and store
water. Water is essential for agriculture. Crops watered by reliable irrigation systems are more
productive, and harvests more dependable. All foods contain some water. Even dry seeds are between
5 and 10 percent water. Milk is 87 percent water. Fruits and vegetables contain the most water.
The Water Cycle
Water is naturally recycled through the Earth in the water cycle. The water cycle is the continuous
process by which water moves through living and nonliving parts of the environment. In the water cycle,
water moves from bodies of water, land, and living things on Earth’s surface to the atmosphere and back
to Earth’s surface. There is no real beginning or end to the water cycle. Water sitting on the surface of
the ocean absorbs energy from the sun and changes to water vapor. Evaporation is the process by
which molecules at the surface of a liquid absorb enough energy to change to the gaseous. The water
that evaporates is freshwater as salt is left behind in the ocean. Other water evaporates from puddles,
lakes, rivers, streams, and plants through transpiration. Transpiration is the process by which plants
give off water from their leaves in the form of water vapor.
The evaporated water, or water vapor, moves up into the atmosphere. As it moves higher, the water
begins to cool. Cold air holds less water vapor than warm air. Some of the water vapor cools and
condenses into liquid water. Condensed droplets of water clump together around tiny dust particles
forming clouds. When the clouds contain so much water that the cloud becomes heavy, the water
droplets fall back to Earth as precipitation. Water that falls to the Earth as rain, snow, hail, or sleet is
known as precipitation. Most precipitation will fall back into the ocean (as it covers 71% of the Earth’s
surface). Several things can happen to water that falls back onto the land. Some of it evaporates
immediately. Some runs off into lakes, streams, etc. where it may evaporate or end up in the ocean.
Some water trickles down into the ground forming groundwater. Groundwater may move underground
until it finds its way back to the surface or another body of water. Precipitation is the source of all fresh
water on and below the Earth’s surface. The water cycle renews the usable supply of fresh water on
Earth. For millions of years, the amount of water on Earth has remained fairly constant because the
rates of evaporation and precipitation are balanced.
Directions: Use the diagram on p. 195 to help you draw a diagram of the water cycle. Be sure to
label evaporation, condensation, precipitation, runoff, and groundwater.
Properties of Water
The unique properties of water are a major factor in the ability of our planet to sustain life.
Although a water molecule has an overall neutral charge, one of water’s unique properties is that one
side of the molecule is slightly negative (oxygen) and the other is slightly positive (hydrogen). This
polarity of water molecules causes them to attract each other like little magnets: the slightly positive
side is attracted to the slightly negative side of an adjacent water molecule. This attraction of one water
molecule to another is called cohesion, which is the reason water molecules “stick together” and form a
“skin” at the surface known as surface tension. Surface tension enables water to support small objects,
such as water bugs, and it also allows water to form drops and bubbles. A drop of water falling from a
faucet will stretch itself very thin before it finally falls. Once it falls, it immediately forms the shape of a
sphere.
Water molecules also stick to molecules of other substances. The attraction of water molecules to other
substances, like grass or glass, is called adhesion. It is adhesion that causes water’s surface to rise
near a container’s walls; if there were no opposing forces, the water would creep up the walls higher and
higher until it overflowed the container. However, in most cases, cohesion causes the formation of a
“bridge” in the liquid. The various forces — adhesion between water and glass, cohesion between water
molecules, and the force of gravity on the water — work in opposition until equilibrium is reached. It is
these forces that lead to the concave meniscus in a graduated cylinder or test tube.
Directions: Fill in the blanks with correct answer.
Water molecules have an attraction to each other, which is called ______________________________.
This attraction is due to the fact that each molecule has a positive side and a negative side or
__________________________. The positive, or hydrogen, side of the molecule attracts the negative,
or oxygen, side of another molecule. Because the molecules at the surface have nothing above them to
which to be attracted, they are attracted even more to the water molecules at their sides and below
them. The pulling between molecules forms a tight “skin” over the water, which is called ____________
___________________. Water molecules also stick to molecules of other substances. The attraction of
water molecules to other substances, like grass or glass, is called ____________________. This force
causes water to adhere to the walls of a glass container and causes the water’s surface to rise near the
container’s walls.
Capillarity is also the result of a combination of adhesion and cohesion, but one in which adhesion
overcomes gravity and cohesion. Capillary action is the phenomenon in which the surface of a liquid is
elevated or depressed where it comes in contact with a solid. When a glass tube is placed in water, the
water rises in the tube, just as water rises in a piece of paper when a portion of it is placed in water. In
such cases, water’s adhesion to the glass and paper overcomes gravity and its own cohesion. Water
moves through plants and to the tops of tall trees against the force of gravity due to capillary action.
Without water reaching the leaves of plants, photosynthesis could not take place and life, as we know it,
on Earth would not exist. Capillary action is also responsible for helping to carry blood throughout
organisms.
Directions: Fill in the blank with the correct answer.
Water’s ability to climb upward against the force of gravity is called ___________________________.
With _________________________ and cohesion, capillarity allows tress and other plants to absorb
water and nutrients through their roots and distribute them throughout the entire plant. When moving
through roots, water molecules cling, or adhere, to the inside of the root and move upward. As the first
molecules move up, others follow due to the attraction of one water molecule to another, or
____________________________.
Density is the mass of a substance divided by its volume (D = mass / volume). Unlike most substances,
the solid form of water (ice) is less dense than liquid water and therefore floats on it. This happens
because as water freezes, the water molecules arrange themselves in an organized crystalline pattern.
The molecules take up more space in this pattern, so the freezing water expands. Because the freezing
water occupies more space (has a greater volume), its density is less than that of liquid water. The fact
that ice floats has important consequences for organisms that live in water. If water acted as most
substances do when they freeze, ice on a pond or lake would sink to the bottom of the lake as it formed,
potentially allowing the entire lake to freeze in winter.
Because water is a polar molecule, it has the ability to dissolve many substances. For this reason, water
is sometimes referred to as a “universal solvent.” The positive and negative sides of the water molecule
are attracted to opposite charges in other substances. The minerals and nutrients found in foods are
attracted to opposite charges in the water molecule. In this way, minerals and nutrients are dissolved
and transported throughout organisms.
Directions: Fill in the blanks with the correct answer.
_____________________________ is the mass of a substance for its volume. ___________________
water has a lower density than liquid water so _________________ will float on water. This property
means that ice typically forms at the ______________ of a lake in winter, insulating the rest of the lake.
Water is called the ____________________________ because its _____________________ enables
the water molecule to dissolve so many different materials. All life is dependent on water’s ability to
dissolve substances to get minerals and _________________________ to all parts of organisms.
Water has an unusually high specific heat. Specific heat is the amount of heat needed to increase the
temperature of a certain mass of a substance by 1 degree C. Compared to other substances, water
requires a lot of heat to increase its temperature. Water’s high specific heat is due to the many
attractions among water molecules. Air and rocks/soil have fewer attractions between their molecules
so their temperatures increase more quickly as they are heated. One effect of water’s high specific heat
is that land areas located near large bodies of water experience less dramatic temperature changes than
areas far inland. In the summer, the sun’s heat warms the land more quickly than the water. The warm
land heats the air above it to a higher temperature than the air over the ocean. As a result, the air is
warmer inland than on the coast. Just the opposite effect occurs in the winter. The land loses heat to
the air more quickly than the water. The water remains warm and keeps the air above it warmer than
the air over the cold land.
Directions: Fill in the blanks with the correct answer.
The large amount of heat needed to raise the temperature of a substance one degree is known as
______________________________________. Because water has a high specific heat, it absorbs and
releases heat energy very _______________________. Large bodies of water (Atlantic Ocean,
Chesapeake Bay, etc.) help to ____________________________ temperatures on land (such as in
Tidewater) as compared to areas such as Richmond that are further inland. In the winter, the air over
the Chesapeake Bay and Atlantic Ocean is _____________ than the air over cooler land in Richmond
so that coastal areas typically have warmer winter temperatures. In the summer, ______________ air
over the Chesapeake Bay and Atlantic Ocean keep the Tidewater area cooler than warmer air over
inland areas such as Richmond.