LAB 12
Lab Handout
Lab 12. Cycling of Water on Earth: Why Do the Temperature and the Surface Area to Volume
Ratio of a Sample of Water Affect Its Rate of Evaporation?
Introduction
Water can be found as a liquid, solid, or gas on Earth. Lakes, rivers, and oceans contain liquid
water. For example, Lake Tahoe (shown in Figure L12.1a), which straddles the border
between California and Nevada, contains 36.15 cubic miles of liquid water. Polar ice caps
and glaciers contain solid water. For example, the Perito Moreno glacier (see Figure L12.1b),
located in western Patagonia, Argentina, is a huge ice formation that is 30 km in length,
5 km wide, and has an average height of 74 m. The atmosphere contains gaseous water
vapor. Unlike the other two forms of water, gaseous water vapor is invisible. We can see
water vapor only when it condenses to form visible clouds of water droplets such as in Figure
L12.1c. Water vapor is responsible for humidity.
FIGURE L12.1
Water can be found in all three states on Earth. (a) Lake Tahoe contains liquid water. (b) The
Perito Moreno glacier contains solid water. (c) Gaseous water is invisible unless it condenses to
form clouds of water droplets, as seen in the Owakudani volcanic valley in Japan.
All water is made up of molecules that are composed of two hydrogen atoms and one
oxygen atom. This type of molecule is called a water molecule. All water molecules have
the same mass. Water molecules are also constantly in motion. Because water molecules
have mass and are constantly in motion, they have kinetic energy. Temperature is a way we
can measure the average kinetic energy of the molecules in any sample of water. A high
temperature means that the molecules in the sample have high average kinetic energy and
are moving quickly, while a low temperature means the molecules have low kinetic energy
and are moving slowly. The three states of water are determined by temperature. At low
temperatures (less than 0°C), water is a solid (ice); at room temperature, water is a liquid;
and at high temperatures (more than 100°C), water is a gas.
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The water cycle, shown in Figure L12.2, is a model that scientists use to explain how
water molecules move into, out of, and within Earth’s systems. This process is driven by
energy from the Sun and the force of gravity. When energy from the Sun heats liquid
water, some of it transforms into gaseous water vapor and enters the atmosphere. This
process is called evaporation. The water vapor rises into the air, where cooler temperatures
cause it to condense into tiny liquid water droplets. A huge concentration of these droplets
becomes visible to us as a cloud. Air currents move clouds around the globe. The water
droplets in clouds collide, grow, and eventually fall to the ground as precipitation. Some
precipitation falls as snow and can accumulate as ice caps and glaciers, which can store frozen
water for thousands of years. Snowpacks in warmer climates often thaw and melt in the spring,
and the melted water flows overland as snowmelt. Most precipitation, however, falls back into
the oceans or onto land. On land, the water will flow over the ground as surface runoff because
of the force of gravity.
A portion of the runoff will enter rivers in valleys in the landscape and move toward
an ocean. Some of the runoff accumulates and forms freshwater lakes. Some of the water
will also soak into the ground. This water is called groundwater. Some of the groundwater will
move deep into the ground and create aquifers, which are underground stores of freshwater,
and some will stay close to the surface. The groundwater that remains close to the surface will
seep back into lakes or rivers as groundwater discharge or will create freshwater springs. Over
time, though, all of this water keeps moving, and it eventually reaches an ocean or returns to the
atmosphere through the process of evaporation.
Water can evaporate and enter the atmosphere from sources as vast as the ocean and
as small as your pet’s water dish. At any place where the surface of the water meets with the
air, water molecules are able to leave the liquid water and enter the atmosphere. It might seem
like evaporation makes liquid water disappear, but recall that the law of conservation of matter
states that matter can never be created nor destroyed, but it can change form. When
evaporation happens, the molecules are simply changing from a liquid phase, which is visible to
us, to a gaseous phase, which we cannot see. Keeping this in mind, we can tell how much
evaporation has happened by measuring changes in the mass or volume of the
liquid water.
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You may have noticed that bodies of water evaporate at different rates. For example,
a rain puddle on the street can evaporate in a few hours, but water in a glass may take
days or weeks to evaporate. There are many factors that may affect the rate that water
evaporates:
• The amount of energy that water absorbs from the Sun
• The temperature of the water
• The surface area to volume ratio of the water; or the amount of the water’s surface
that is exposed to the air compared with its total volume.
In this investigation, you will have an opportunity to determine how water temperature
and the surface area to volume ratio of a sample of water contribute to the rate that water
evaporates. Once you understand how these two factors affect how quickly a sample
of water will evaporate, you will then develop a conceptual model that you can use to
explain your observations and predict how quickly water will evaporate under different
Conditions.
Your Task
Use what you know about the properties of water, rates of change, and the importance of
tracking the movement of matter into, out of, and within systems during an investigation
to plan and carry out an experiment to determine how changes in the temperature and the
surface area to volume ratio of a sample of water affect how quickly it will evaporate. Then
develop a conceptual model that can be used to explain why these factors affect the rate that
water evaporates. Once you have developed your conceptual model, you will need to test it
using different water samples to determine if it allows you to predict how much liquid water will
be lost due to evaporation under different conditions.
The guiding question of this investigation is, Why do the temperature and the surface
area to volume ratio of a sample of water affect its rate of evaporation?
Materials
Water (800mL)
The Sun
A measuring cup (Beaker mL)
Markers (two different colors)
Two different size bowls (Small bowl and Large bowl)
Safety Precautions
Follow all normal lab safety rules. In addition, take the following safety precautions:
Getting Started
The first step in developing your model is to plan and carry out at least two experiments.
Figure L12.3 (p. 300) shows how you can use a heat lamp to warm your different samples of
water. The heat lamp will serve as the source of energy for each experiment. Your teacher
may also allow you to set your water samples outside in direct sunlight depending on the
time of year. Figure L12.3 also shows how you can use containers of different sizes and shapes
to manipulate the surface area to volume ratio of a sample of water. You can use a hot plate to
heat your water samples to different temperatures or to maintain the temperature of a water
sample.Before you begin to design your two experiments using this equipment, be sure
to think about what type of data you need to collect, how you
will collect the data, and how you will analyze the data. To
determine what type of data you need to collect, think about the
following questions:
● How will you track the flow of energy into each water
sample?
● How will you track the amount of water loss from a
sample?
● How will you measure the rate of water evaporation
(change over time)?
To determine how you will collect the data, think
about the following questions:
• What will be the independent and dependent variables for each experiment?
• What conditions will you need to set up for each experiment?
• How will you make sure you are only testing one variable at a time?
• How often will you need to take measurements during each experiment?
• What measurement scale or scales should you use to collect data?
• How will you make sure that your data are of high quality (i.e., how will you
reduce error)?
• How will you keep track of and organize the data you collect?
To determine how you will analyze the data, think about the following questions:
• What type of calculations will you need to make?
• How will you determine if rates of change are the same or different?
• How could you use mathematics to document a difference between conditions?
• What type of table or graph could you create to help make sense of your data?
Once you have carried out your two experiments, you will need to develop a conceptual
model. Your model needs to be able to explain why temperature and the surface area to
volume ratio of a sample of water affect the amount of water lost due to evaporation in
the way that these two factors do. The model also needs to account for the kinetic
energy of water molecules and the conservation of matter.
The last step in this investigation is to test your model. To accomplish this goal, you can
set up a third experiment to determine if your model leads to accurate predictions about
the amount of water that will be lost from different containers of water under specific
conditions (e.g., cold water, high surface area to volume ratio). If you are able to use
your model to make accurate predictions about the way water evaporates under
different conditions, then you will be able to generate the evidence you need to
convince others that the conceptual model you developed is valid or acceptable.
ADI 12- Cycling of water on Earth
Initial Argument Form
Guiding question : Why do the temperature and surface area to volume ratio of a sample of water affect its rate of evaporation. |
Claim: |
Evidence: |
Reasoning: |
Investigation Proposal
© Argument-Driven Inquiry
For classroom use only. Do not distribute without permission.
PLAN
Plan Your Investigation
Prepare a plan for your investigation by filling out the chart below.
I approve of this investigation plan
Teacher’s Signature Date
If… If… If…
Question
Hypothesis A Hypothesis B Hypothesis C
The Test
And…
Predicted Result A Predicted Result B Predicted Result C
Then… Then… Then…