Oceanography Page 1 of 6 Salty Water! Description: This lab is designed to demonstrate the formation of the world s oceans and why the oceans are salty, as well as the changes that take place in density with increased salinity. Background: Salinity At the simplest level, salinity is the total amount of dissolved material in grams in one kilogram of seawater. Thus, salinity is a dimensionless quantity. It has no units. The variability of dissolved salts is very small so we must be very careful to define salinity in ways that are accurate and precise. For example, the range of salinity for most of the world s oceans is from 34.60 to 34.80 parts per thousand (g/kg). The variability in a given ocean basin is even smaller. So if we want to classify the salinity of different water masses we need definitions and measurement techniques accurate to better than 1 ppm (mg/kg). Summary of units for dissolved solids weight ratio (grams solute/grams solvent or solution) parts per million - ppm = mg/kg parts per billion - ppb = μg/kg The average salinity of seawater is S = 35 which means that SW is 3.5% salt and 96.5% H2O by weight. What controls the salinity of surface seawater? Surface seawater salinity is determined by the balance between evaporation and precipitation, which in turn is controlled by solar heating. Insolation decreases with latitude and thus temperatures are highest in the tropics and decrease towards the poles. Evaporation is highest near the equator but this is not the location with highest salinity because rainfall is also high. Figure 1 below shows surface salinity for the world s oceans. The highest surface salinities for the open ocean are located at about 25 N and S in the center of the subtropical gyres. Salinities can reach higher values in relatively isolated waters like the Red Sea (S = 39). Density: Density is defined as the mass of water per unit volume and has units of grams per cubic centimeter (g /cm 3 ), kilograms per liter (kg/ L) or kilograms per cubic meter (kg/m3). The density of freshwater at 4 C is 1.0000 g/cm 3 or 1.000 kg/liter or 1000 kg/m 3. The range of density in the oceans is from about 1.020 to 1.070 g/cm 3. The changes in density are caused mainly by variations in pressure, salinity and temperature: colder water more dense saltier water more dense higher pressure causes density increase - pressure increases with depth due to the mass of water above
Oceanography Page 2 of 6 Figure 1: Surface salinity values around the world. Activity 1: I) You will create 3 different solutions of salt and water, with the salt content not to exceed 3.5g per liter of solution. Add one or two drops of food coloring to each solution and then layer the solutions with the greatest density on the bottom and the least dense on top. II) Vary the order of the solutions; i.e. add a drop or two of a solution of greater density, to a solution of lesser density and record the results. What you are demonstrating is the concept of a pycnocline. Pycnocline is a layer where there is a rapid change in water density with depth. In freshwater environments such as lakes this density change is primarily caused by water temperature, while in
Oceanography Page 3 of 6 seawater environments such as oceans the density change may be caused by changes in water temperature and/or salinity. Figure 2: Each of these graphs shows how seawater layers with depth. The middle image, the pycnocline, demonstrates the layering of seawater as a function of depth. Density and Water Movement The density of seawater determines its tendency to move vertically. If density of water at surface is higher than below, the water will sink to a level of its own density. In this situation the water column is unstable. If density of water at surface is lower than below, the water will not sink. In this situation the water column is stable. In this situation it takes energy input (usually from the wind) to "push" water downward -e.g., like submerging a rubber duck in bathtub (you supply energy). Sinking of surface water generally occurs where there is cold air to cool water at surface. This situation found at high latitudes near the poles. At these polar sites, surface waters cool and become dense enough to sink thousands of meters. Sinking of surface waters is a very important mechanism to replenish waters in the deep sea. In contrast, for most of the ocean (within ~50 of the equator) the surface waters are much warmer and less dense than the cold waters found at depth. Under these conditions surface waters do not sink and thus there is no direct contact with waters in the deep sea.
Oceanography Page 4 of 6 What controls surface salinity? Mainly the relative rates of evaporation versus precipitation. When the rate of evaporation is greater than the precipitation, then surface ocean salinity increases; when the rate of precipitation is greater than the rate of evaporation, then the surface ocean salinity decreases. Surface heating and precipitation promote water column stability by lowering the density of surface seawater. Cooling and evaporation diminish stability by increasing surface density. Analysis: Draw a diagram of your initial column of solutions and include the salinity and density of each solution. Answer the questions: 1. Why did the solutions layer as they did? 2. When you changed the order of solutions, what happened, and why?
Oceanography Page 5 of 6 Activity 2: Materials You will need: Salt Potting soil Coffee filter Paper cup Metric ruler Black paper Procedure: 1. Make a mixture of half salt and half soil by mixing an equal amount of salt with an equal amount of potting soil. 2. Take a paper cup and using a pencil, punch five small holes through the bottom of the cup. 3. Place a coffee filter inside a paper cup and put a couple of tablespoons of the salt soil mixture inside the coffee filter. 4. Hold the cup 2 cm above a sheet of black paper, add 100ml of water and allow the water to drip onto the paper. 5. When all the water has dripped out onto the paper, allow the paper to sit in the sun to dry. Data & Observations: Record qualitative and quantitative observations during your experiment. Describe how the water fell from the cup. Include descriptions of color and smell. Using the drawing tools, draw a picture of the black paper during and after the experiment. Analysis & Conclusions: 1. What was your hypothesis? 2. What was your independent variable? 3. What was your dependent variable? 4. Was this experiment a controlled experiment? 5. Why did you use black paper as opposed to white paper? 6. Explain the results. What does the dried paper suggest? 7. How can this experiment be used as a model for the formation of the world ocean? 8. The salinity of ocean water is measured in parts per thousand. Therefore, if the salinity of 500g of saltwater is 30ppt, there are 15g of dissolved salts in the seawater (1/2 of 30ppt = 15g). If the average salinity of seawater is 35ppt, how many grams of dissolved salts will 500g of seawater contain? 9. If the average salinity of seawater is 29ppt, how many grams of dissolved salts will 500g of seawater contain? 10. If the average salinity of seawater is 30ppt, how many grams of dissolved salts will 1000g of seawater contain? 11. Was your hypothesis supported or refuted? 12. If you had to perform this experiment again, what might you do differently? 13. How might this lab provide useful information for the study of oceanography?
Oceanography Page 6 of 6 Activity 3: As you now know, the ocean contains many dissolved salts. How do you think this may affect the objects within the ocean? Have you seen some objects float, while others sink to the bottom? This is directly related to density, which is a measurement of the mass per volume of an object. Density determines whether an object will float or sink. But how are salinity and density related? In this activity, you will investigate the effects of salinity on density by observing whether an object sinks or floats in water of varying salinity. 1. First, determine the problem: How does salinity affect whether a potato slice will float or sink? 2. Next, develop a hypothesis: Based on what you know about density, do you know enough to formulate a hypothesis? A hypothesis is an educated guess, so you must gather information on salinity, why things float or sink, and the specific density of a potato. Include this information in the introduction portion of your lab report. 3. Now, design an experiment: a. Develop a controlled experiment that will identify how increasing salinity influences the ability of a potato to float in water. b. You might want to use the following materials: i. small uncooked potato ii. teaspoon iii. salt iv. large bowl v. water measuring cup vi. measuring spoons vii. metric ruler 4. List the steps that you need to take to test your hypothesis. Be specific and describe exactly what you will do for each of your steps. a. How will you determine the density of the potato and the different water samples? Be sure to be specific enough so that someone else can duplicate your exact experiment. 5. Record your results in an appropriate data table and construct a corresponding graph. a. Include digital pictures or drawings with your qualitative and quantitative observations. When constructing your graph based on your data, remember, a line graph is used to demonstrate change over time, and a bar graph is used to illustrate comparisons. 6. Determine the analysis and conclusions: a. Discuss the execution of your lab, your findings, and their validity. Include a statement of response to your hypothesis.