DRAFT (V3) – September 2012
TSI Aquatic Module 1 Physical
Unit 2: Density
2.1 DENSITY, TEMPERATURE, AND SALINITY
Density is a measure of how much matter is in a given space. Density differences
in water masses are determined by differences in salinity and temperature.
Density
Density is a measure of how much mass there is in a given amount of space (volume).
The density of any substance is calculated by dividing the mass of the matter by the
volume of the matter.
In Fig. 2.2, volume is represented by boxes and individual particles of matter are
represented by colored shapes.
 Box A has 5 circles.
 Box B is the same size, and has the same volume as box A, but box B has 10
circles.
 Box C has the same mass as box A, with 5 circles, but box C has a larger volume
than boxes A and B.
 Box D has the same volume and number of green circles as part A, but also
includes other types of matter than the rest of the boxes—red circles and blue
squares.
If the amount of matter is increased without changing the volume, then the density
increases (Fig. 2.2 A to 2.2 B). If volume increases without an increase in mass, then the
density decreases (Fig. 2.2 A to 2.2 C). Adding additional matter to the same volume also
increases density (even if the matter added is a different type of matter, Fig. 2.2 A to 2.2
D).
Fig. 2.2. The boxes and colored shapes in this figure demonstrate the effects of changing
mass and volume on density.
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DRAFT (V3) – September 2012
TSI Aquatic Module 1 Physical
Unit 2: Density
Salinity Affects Density
When salt is dissolved in fresh water the density of the water increases because the
mass of the water increases. This is represented by the addition of red circles and blue
squares to the box from Fig. 2.2 A to Fig. 2.2 D. Salinity describes how much salt is
dissolved in a sample of water. The more salt dissolved in the water, the greater its
salinity. When comparing two samples of water with the same volume, the water sample
with higher salinity will have greater mass, and it will therefore be more dense.
Temperature Affects Density
Cold water takes up less volume than warm water. When the temperature drops, water
contracts. This means that the same amount of water is squeezed into a smaller volume,
thereby decreasing its density. When the water is heated, it expands, increasing in
volume. This is represented by the increase in the size of the box from Fig. 2.2 A to 2.2 C.
The warmer the water, the more space it takes up, the lower its density. When comparing
two samples of water with the same salinity, or mass, the water sample with the higher
temperature will have more volume, and it will therefore be less dense.
Water Layers
If water masses have salinity or temperature differences, they will form water layers
because they have different densities. Water layers can be felt when swimming. For
example, on hot days the sun’s heat can make water at the surface noticeably warmer
than the deeper, cooler water. Whether a layer of water floats or sinks is determined by
the relative density of one water mass in relation to another.
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DRAFT (V3) – September 2012
TSI Aquatic Module 1 Physical
Unit 2: Density
Activity: Density Bags
Test the effects of salinity and temperature on the floating and sinking of liquid
samples in bags.
In Fig. 2.4, the beaker of liquid simulates a body of water like the ocean or a lake. The
bag of liquid simulates a layer of water. The relative density of the liquid in the bag
compared to the liquid in the beaker can be determined by observing whether the bag
sinks or floats.
 In Fig. 2.4 A, the bag rose to the top of the beaker and is now floating on the
surface. The yellow liquid and the bag are less dense than the liquid in the beaker.
 In Fig. 2.4 B, the bag is floating in mid-water (subsurface floating). The orange
liquid and the bag are equal in density to the liquid in the beaker.
 In Fig. 2.4 C, the bag sank to the bottom of the beaker. The green liquid and the
bag are more dense than the liquid in the beaker.
In this activity, you will compare liquids of different salinities and temperatures by
determining the relative density of liquids in bags compared to the liquid in larger
containers.
Fig. 2.4. (A) The bag filled with yellow liquid is floating on the surface. (B) The bag
filled with orange liquid is floating in mid-water (subsurface floating). (C) The bag
filled with green liquid has sunk to the bottom of the beaker.
Materials for Parts A-C
 Small plastic bags
 Scissors
 Permanent marker
 Labeling tape
 Fresh water
 Four beakers or large cups, ~1000 mL each (or wide enough for plastic
bags to float unobstructed)
 Small cup
 Tray
 Towels
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DRAFT (V3) – September 2012
TSI Aquatic Module 1 Physical
Unit 2: Density

Food coloring (optional)
Additional Materials for Parts A and C
 Salt water
Additional Materials for Parts B and C
 Heat source
 Ice bath (container in a tray of ice cubes and water, see Fig. 2.5)
 Thermometer
 Tongs
Procedure
A. Test the effect of salinity on the rising or sinking of bags of liquid.
1. For each combination of fresh water and salt water, make a prediction as to
whether the bag will
a. sink (the bag and liquid are more dense than the liquid in the
beaker),
b. float (the bag and liquid are less dense than the liquid in the beaker),
or
c. subsurface float (the bag and liquid are the same density as the
liquid in the beaker).
Record your predictions in Table 2.1.
Table 2.1. Predictions and results of the effect of salinity on the relative density of
liquids in bags compared with liquids in beakers.
Liquid in bag
Liquid in beaker
Fresh water
Salt water
Fresh water
Salt water
Predicted
Predicted
Actual
Actual
Predicted
Predicted
Actual
Actual
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DRAFT (V3) – September 2012
TSI Aquatic Module 1 Physical
Unit 2: Density
2. Fill and label one 1000 mL beaker with salt water and one 1000 mL beaker with
fresh water. Label your beakers.
3. Observe as your teacher shows you how to fill the plastic bags for your
experiment.
a. Use a permanent marker to label an empty small plastic bag with the
salinity of the water (fresh or salt) that will be poured into it.
b. (Optional) Add a drop of food coloring to the empty plastic bag to
make the liquid in the bag easier to observe.
c. Fill the small cup with the same type of water that will be poured into
the bag.
d. Overfill the bag with water using the small cup. Do this step over a
tray, spare beaker or towel.
e. Seal the bag so it does not leak or have air bubbles inside. Do this
step over a tray, if done correctly liquid will spill out of the overfilled
bag.
f. Shake the bag to distribute the food coloring.
g. Pat the plastic bag dry.
h. Cut off the excess plastic of the bag above the closure.
4. Make a bag of fresh water and a bag of salt water.
5. Place one bag into one beaker and record your observations in Table 2.1.
Repeat this step until you have tested and recorded your observations for each
combination of bag and beaker. Make sure to pat the bags dry before placing
them in the beakers or when transferring them between beakers.
6. Compare your results with those of your classmates.
B. Test the effect of temperature on the rising or sinking of bags of liquid.
7. Make a data table similar to Table 2.1, but instead of fresh water and salt water
use only fresh water. The variable will now be temperature.
a. For each combination of hot water and cold water, make a prediction
as to whether the bag will sink, float, or subsurface float.
b. Record your predictions in your data table.
8. Prepare hot water and cold water in 1000 mL beakers. Make sure to use safety
equipment and be careful with the hot water. Label your beakers.
a. To prepare hot water, place a beaker of water on a hot plate (see
Fig. 2.5 A), and heat to 50–70 degrees Celsius (˚C)
b. To prepare cold water, place a beaker of water in an ice bath until the
water in the beaker reaches a temperature of ~5 ˚C. Do not put ice
cubes in the beaker (see Fig. 2.5).
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DRAFT (V3) – September 2012
TSI Aquatic Module 1 Physical
Unit 2: Density
c. The beakers can stay on the hot plate or in the ice bath during the
experiment.
Fig. 2.5. (A) Place a beaker of water on a hot plate to create hot water. (B) Place
water and ice cubes in a tray to create an ice bath. Place a beaker of water in the
ice bath to cool the water in the beaker.
9. Make a bag of hot water and a bag of cold water (follow the steps described in
procedure 3). Place each bag into each beaker and record your observations in
your data table.
a. Pat the bags dry before placing them in the beakers or when
transferring them between beakers.
b. If the hot water is uncomfortably hot, use tongs to transfer the bags
into and out of the water.
10. Compare your results with those of your classmates.
C. Test the effects of both salinity and temperature on the rising or sinking of
bags of liquid.
11. Design an experiment to test the effects of both temperature—hot or cold—and
salinity—fresh water or salt water—on the rising and sinking of bags of liquid.
For example, you might want to determine the relative density of a hot salty
water bag in a beaker of cold fresh water.
12. Make a data table showing which combinations of liquids in bags and liquids in
beakers you plan to use.
a. For each combination of liquids, predict whether the bag will float,
sink, or subsurface float.
b. Record your predictions in your data table.
13. Prepare the beakers and bags you need to carry out your experiment. Record
your observations in your data table.
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DRAFT (V3) – September 2012
TSI Aquatic Module 1 Physical
Unit 2: Density
14. Compare your results with those of your classmates.
Activity Questions
1. How did your answers compare to your predictions?
2. Did your classmates have the same results for each of the temperature and
salinity combinations? Explain why the results were the same or why they were
different.
3. Using the term density, explain how a bag of salt water can sink in a beaker of
salt water. Where might this occur in the ocean?
4. If the temperature of the liquid in a bag and the liquid in a beaker were the
same, under what conditions would the bag float? Where might this occur in
nature?
5. Would fresh water flowing into the ocean sink or float on top of seawater?
Explain your reasoning.
6. How might you use what you have learned in this activity to help explain the
formation of layers in large bodies of water like lakes or the ocean?
7. What do you think might happen to the liquid in the bags if the bags were not
sealed? How could you test this?
8. If a sealed bag of hot salty liquid is placed into a beaker of cold fresh water for
24 hours, what will happen to the following:
a. the temperature of the two liquids?
b. the salinity of the two liquids?
c. the density of the two liquids?
9. If you used the food coloring, do you think it affected the density of the liquids in
the bags? How could you verify your answer?
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College of Education.  University of Hawai‘i, 2012. This document may be freely reproduced and
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p.7 of 11
DRAFT (V3) – September 2012
TSI Aquatic Module 1 Physical
Unit 2: Density
Density and Buoyancy
Density can be determined by measuring the mass and volume of an object. In the
Density Bags Activity, density was not calculated. Instead, relative density was
determined by observing whether a bag of one liquid floated or sank in another
liquid. A bag of liquid that sank was determined to be more dense than the liquid in
the beaker. A bag of liquid that floated was determined to be less dense than the
liquid in the beaker.
The motion of any object is due to forces, which are pushes or pulls. Vertical—upand-down—movement of water masses in the ocean can be explained in terms of
two forces. The gravitational force (G) of the earth pulls down and is proportional
to the mass of an object. The gravitational force on an object is also called weight.
The force due to gravity is greater on objects that are more massive, or weigh
more. The buoyant force (B) of water pushes up. In the third century B.C., the
Greek philosopher Archimedes was the first to describe buoyancy. He observed
that the volume of water pushed out of a tub, or displaced, by an object was equal
to the volume of the object. The buoyant force of the water is equal to the weight of
the water displaced. This concept is known as Archimedes’ Principle, and it
explains why objects sink or float.
When the forces on an object are unequal, the object accelerates. Although
acceleration is commonly used to describe an object that is speeding up, the
scientific definition of acceleration means changing speed. An accelerating object
can be speeding up or slowing down. An object will always move in the direction of
the greater force. An object may accelerate downwards (sink) or upwards (rise) in
a body of water.
 Sinking is a downward vertical movement that occurs when the
gravitational force (G) on an object is greater than the buoyant force (B)
supporting it (G > B).
 Rising is the upward vertical movement that occurs when the gravitational
force is less than the buoyant force (G < B).
If all of the forces on an object are balanced, there is no acceleration. In this case,
the object may not move (like a book sitting on a flat table) or the object may move
at a constant speed (like a car traveling at a steady fifty miles per hour). In the
water, an object might not move when it is located at the surface or within the
water column.
 Surface floating occurs when an object stays at the surface, because the
forces are balanced at the surface (G = B).
 Subsurface floating, or neutral buoyancy, occurs when an object
maintains its position in mid-water, neither sinking nor rising (G = B).
Three cubes of the same size, but having different masses and thus different
densities, are placed in three beakers of water (Fig. 2.6). Because the cubes are
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DRAFT (V3) – September 2012
TSI Aquatic Module 1 Physical
Unit 2: Density
identical in volume, they displace the same amount of water. By Archimedes’
Principle, the buoyant force (B) acting on each cube is equal. Buoyant force is
represented in Fig. 2.6 as upward pointing arrows, indicating the water is pushing
up on the cubes. These arrows are the same length for each of the cubes,
indicating that the strength of the buoyant force acting on each cube is the same.
Because the masses of the cubes are not equal, the gravitational force (G) acting
on each cube is different. Gravitational force is represented in Fig. 2.6 as
downward pointing arrows, indicating the gravitational force is pulling down on the
cubes. These arrows are different lengths for each cube, indicating that the
amount of the gravitational force is different for each cube. The downward pointing
arrow in Fig. 2.6 A is the shortest, indicating that the yellow cube has the least
mass and is the least dense. The downward pointing arrow is the longest in Fig 1.6
C, indicating that the green cube has the most mass and is the most dense.
Fig. 2.6. The yellow cube (A) is less dense than water, the red cube (B) has the
same density as water, and the green cube (C) is more dense than water. The
buoyant force is represented by the letter “B” and upward pointing arrows. The
gravitational force is represented as downward pointing arrows and the letter “G”.
The density of the cube relative to the density of water determines if the cube will
float, sink, or be neutrally buoyant.
 If the density of the cube is less than the density of the water, gravitational
force will be less than the buoyant force (G < B), and the object will rise to
the surface (Fig. 1.6 A).
 If the density of the cube is equal to the density of the water, the cube will
float in middle of the column of water (Fig. 2.6 B) because the gravitational
force and buoyant force are balanced (G = B). This cube is neutrally
buoyant.
 If the density of the cube is greater than the density of the water, the cube
will sink because the gravitational force is greater than the water’s buoyant
force (G > B, Fig. 2.6 C).
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p.9 of 11
DRAFT (V3) – September 2012
TSI Aquatic Module 1 Physical
Unit 2: Density
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p.10 of 11
DRAFT (V3) – September 2012
TSI Aquatic Module 1 Physical
Unit 2: Density
Further Investigations: Density, Temperature, and Salinity
Fig. 2.7. Create water layers by pouring water of a different temperature or salinity
slowly down a stirring rod.
1. Simulate layers of water formed by differences in temperature and salinity.
a. Prepare two 100 mL water samples, one of cold salt water and the other
of warm fresh water. Add a different color of food coloring to each.
b. Place the cold salt water into a beaker. To prevent mixing, slowly pour
about 20 mL of the warm fresh water down a stirring rod or down the
back of a spoon onto the cold salt water in the beaker (see Fig. 2.7).
Look for evidence of layering.
c. Test out different combinations of water temperature and salinity.
d. Try to create three or more water layers in your beaker.
2. What do you think the relative density of the bag is alone (without any liquid) in
the Density Bags Activity? How might the density of the bag affect the results?
Design an experiment to test your hypothesis.
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College of Education.  University of Hawai‘i, 2012. This document may be freely reproduced and
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