Lab 4
11
Lab 4
Properties of Seawater: Salinity and pH
Description of the Lab
This lab explores the relationship between density and buoyancy in a using two
types of soda with different densities. Students will investigate the relationship between
Outline of Investigation
Exercise 1 – Observing How the Density of Water Affects Buoyancy
Materials needed:
• Access to hot and cold tap water
• cold can of diet soda and cold can of regular soda, identical brand
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W
i
WNA
Exercise 2 – Labeling a Loading Line
Materials needed:
• Diagram to label provided in lab manual
Note: Ships on the Great Lakes will have a different pattern of load lines. Some ships
TF
F
i
T
W Salt water in winter
WNA Winter in North Atlantic
Exercise 3 – Determine pH Change as a Function of Acid Concentration
Materials needed:
• distilled water (or deionized)
• two 50 mL beakers
Note: A pH meter can be used instead of pH testing strips and color chart. Make sure
students wear gloves and eye protection when working with acid.
Exercise 4 – Determining pH Change in the Presence of a Buffer
Materials needed:
• two 50 mL beakers (rinsed and reused from exercise 3)
• labeling tape or grease pencils.
Exercise 5 – Natural Buffering Capacity of Seawater
Materials needed:
• two 50 mL beakers (rinsed and reused from exercise 4)
• labeling tape or grease pencils.
Exercise 6 – Buffering Capacity in Standing Freshwater
Materials needed:
• two 50 mL beakers (rinsed and reused from exercise 5)
• labeling tape or grease pencils.
• pens
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Laboratory Materials and Equipment for Student Use
Per
Student
Per Pair of
Students
Per Lab
Group
Per
Room
2
1
1
1
1 dropper
1 dropper
Preparation of Laboratory Materials and Reagents
Reagents
0.1M HCl
Labeling Tape
Sharpie pens
pH testing strips (or pH meter)
5
Gloves (s, m, l, xl, non latex)
Goggles
4
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0.1M sodium acetate buffer
1. Weigh 8.2 grams of sodium acetate
Answers to Questions for Lab 4
1. In the cold tap water did the diet and regular soda cans both float at the same level?
Explain your results in terms of density.
The regular soda should have sunk to the bottom of the aquarium while the diet
2. Did adding the salt to the aquarium water make a difference? If so, why? Did
changing the temperature of the aquarium water make a difference? If so, why?
Adding enough salt to the aquarium water will increase the density of the water
3. Would the Loading Line on a ship that was only traveling in the Great Lakes be the
same as an ocean going ship? Why or why not?
No, because the Great Lakes are freshwater and in general the density of the
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4. Describe the differences in the pH change in the five solutions: the distilled water, the
sodium acetate solution, the seawater, and the standing freshwater.
The distilled water had the greatest pH change, the standing freshwater would not
Which acid strength (concentrated vs. diluted) had a more significant pH difference
when added to solution?
5. Were any of your water samples good buffers? Explain why or why not for each of
the samples (seawater and standing freshwater). Consider the source of the water in
your answer.
The sodium acetate solution is strongly buffered, the seawater is mildly buffered
6. Discuss your results of buffering capacity of the seawater sample compared to the
artificial buffer.
The seawater will be weakly buffered, depending on the source, the standing
7. Based on the information you’ve gathered in the lab, discuss the implication of acid
rain and acidic water deposition on the ocean. How might this compare to your two
freshwater samples? Include what you have learned about pH, the buffering capacity
of seawater and freshwater ecosystems.
Acid rain can be responsible for necrotic lesions on plants. The change in the pH
of the water in the soil water matrix can also increase the solubility of some ions
8. Upon completion of this exercise, what have you learned about buffering capacity?