Alpha #:
Name:
Date:
Period:
Ocean Water Density Lab:
The density of ocean water varies. Density is affected by the salinity of the water and the
temperature of the water. Furthermore, the salinity of an area of ocean is affected by the rate
of evaporation or freezing and by the addition of fresh water and salts. The temperature of the
ocean is determined by the amount of solar radiation that reaches Earth’s surface. In this lab,
you will observe the effects of temperature and salinity and evaporation on the density of salt
water.
Objectives:
Measure the temperature and density of water.
Analyze the effects of temperature, salinity, and evaporation on the density of water.
Materials:
 Beaker (250mL)
 Clay
 Hot plate
 grease pencil
 Ruler
 Scissors
 Straw
 Table salt
 Teaspoon
 Thermometer
 Water
 Goggles
Set up Procedure:
1. Make a hydrometer by filling 1 cm of one end of the straw with modeling clay.
2. Pour 200 ml of water at room temperature into a beaker. Float the straw upright in
the beaker. If the straw does not float upright, cut off the open end at 1 cm intervals
until if floats upright.
3. Use a grease pencil to mark ½ cm intervals from the top of the straw until you reach
the clay. Number every other line on the straw starting from the top of the straw.
4. Measure the density of the water with your straw hydrometer. Record your
observation on Table 1
5. Add 4 tsp of salt to the water. Stir until all of the salt has dissolved. Measure and
record the water volume and temperature on table 1.
Part 1 Procedures:
6. Place the hydrometer in the salt water and measure the density. Realize that the
higher the hydrometer floats out of the water, the denser the water is. The numbers
on the straw are labeled so that they will indicate whether the density is increasing
or decreasing. But LOOK VERY CAREFULLY- a lot of the changes in this lab
will be small changes!
7. Turn the hot plate on low (4-5). Place the beaker of salt water on the hot plate.
You will need to wear goggles when you are heating the glass beaker.
 NOTE The hot plate is not a toy and should not be played with- do not place anything
on the hotplate that should not be there!
 DO NOT leave the hydrometer in the water when you are not measuring…
this will cause the clay to dissolve & fall out & you will have to start
over!!!!!!!
8. When the water’s temp. reaches 25 C, immediately record the density. Record the
density on Table 2. Be careful that the thermometer does not touch the sides or bottom
of the beaker.
9. Continue to raise the temp of the salt water by 5 C until a change in density is observed.
The closer you observe the hydrometer- the quicker you will complete Part 1.
10. Record your observations for Part 1 in Analysis Section. How did the density change?
Part 2 Procedures:
11. Turn the hot plate on high. Heat the salt water until it is boiling. If it starts to boil
over- turn the temp of the hotplate down a little so that it continues to boil without
boiling over.
12. When the water begins to boil (really boiling- not just a little bit of bubbles)- Use the
tongs to take the beaker off of the hotplate. Be very careful not to burn yourself or your
lab partners with the hot water. You can not measure the density if the water is boiling.
So you must wait till the water stops bubbling. Measure and record the density & temp
on Table 2.
13. Boil the water for 5 minutes and then observe the density again. (Again- take the
beaker off of the hot plate). Continue to do this until you observe a change in the
density & temp.
14. When you observe a change in density for Part 2 you are finished. Complete the
analysis questions for Part 2 and then turn your lab in.
15. Record your observations for Part 2 in Analysis Section. (How did the density Change?
Table 1:
Beginning Temp & Density of water:
Density:
&Temp:
.
Beginning Temp & Density of Salt Water
Density:
&Temp:
.
Temp C
Density
25
30
35
40
45
50
55
60
Part 1 Observations:
Table 2:
Time Boiling
First Boiling
5 minutes
10 minutes
15 minutes
20 minutes
Temp C
Density
Part 2 Observations:
Analysis Questions:
Be sure to use complete sentences and restate the questions in your answer. Make sure you
clearly explain your reasoning and use data to support your answers.
1. In which trial was the water the most dense? Why?
2. In which trial was the water the least dense? Why?
3. As the temperature of the water increases in Trial 1, does the water’s density increase or
decrease? Why? (think about what the molecules are doing)
4. Based on your observations, infer the density of polar ocean waters and compare it with the
density of equally saline water near the equator. Explain your answer:
5. Why did the amount of water in the beaker change during part 2?
6. Why was the temperature not changing during part two?
7. How did the density change during part 2? Why?
8. How would you expect the density of the water to change if the water was frozen instead of
boiled? Explain your answer.