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.
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.
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
· 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 beaker / Liquid in bagFresh water / Salt water
Fresh water / Predicted / Predicted
Actual / Actual
Salt water / Predicted / Predicted
Actual / Actual
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).
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.
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?
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—up-and-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.