Education: Classroom Activities - Density: Sea Water Mixing and Sinking

• Sea water has characteristic properties (e.g. density) that are independent of sample size.
• There are two main factors that make ocean water more or less dense: temperature and salinity.
• Cold, salty water is denser than warm, fresher water and will sink below the less dense layer.
• Density is defined as the measure of a material's mass (e.g. grams) divided by its volume (e.g. milliliters).
• Mixing of seawater influences the density of seawater thereby affecting ocean circulation. Seawater mixing also has an effect on ocean life.

Activity 1 

Activity 2 - Sea Water Mixing and Sinking

Background
In the oceans, the salinity varies over time and from place to place.  Typical open ocean salinities vary between 33 and 36 PSU (Practical Salinity Units), equivalent to 33-36 parts per thousand. Two of the most important characteristics of ocean water are its temperature and salinity. Together they help govern the density of seawater, which is the major factor controlling the ocean's vertical movements and layered circulation. The following activity investigates the role of temperature and salinity in determining seawater density. It does so by using a Temperature-Salinity (T-S) Diagram to examine the effect of mixing on density. Such mixing can be a significant factor in causing surface seawater to sink as part of vertical circulation. The T-S Diagram is a simple, but powerful tool used in studies of seawater density, mixing, and circulation. In a T-S diagram, temperature is plotted along the vertical axis in degrees Celsius and salinity is measured along the horizontal axis in PSU.  Seawater density is illustrated in the diagram by curved lines of constant density.  Surface waters are mixed by winds and deep ocean water mixing is driven by density differences. Circulation in the depths of the ocean is referred to as thermohaline circulation. The deep ocean is layered with the densest water on bottom and the least dense water on top. Water tends to move horizontally throughout the deep ocean, moving along lines of equal density. Vertical circulation is limited because it is easier for water to move along lines of constant density (isopycnals) than across them.

Materials:

Preparation: None (although "Potato Float" is a good activity to acquaint students with the concept of densitities of liquids.)

Activity / Assessment / Questions
As a class or in small groups, examine the Temperature-Salinity (T-S) Diagram. Temperature is plotted along the vertical axis in degrees Celsius (°C). Salinity is measured along the horizontal axis in Practical Salinity Units (PSU) that is numerically equivalent to parts per thousand (‰). Seawater density, in grams per cubic centimeter (g/cm^3), is shown on the diagram by curved lines of constant density. The value of each curved line appears immediately above each line. Note that temperature and salinity together govern the density of seawater.

As shown by the T-S Diagram, the density of seawater increases with (increasing) (decreasing) temperature and with (increasing) (decreasing) salinity.

On the T-S Diagram, each seawater sample is plotted as a dot (•) at the point determined by its temperature and salinity. Find the temperature and salinity for the two surface seawater samples labeled “A” and “B” and record these values in the Water Sample Table.

The density of seawater samples must be determined to several decimal places in order to detect significant differences. Read from the T-S Diagram the densities for the two surface seawater samples labeled “A” and “B” to the fourth decimal place. Record these values in the table. Note that their densities are the same.

If surface waters of the same density are brought together, they tend to mix. The temperature and salinity of the resulting mixture are somewhere between the temperatures and salinities of the original waters prior to mixing. Record in the table, the temperature and salinity of a water sample “C” that would result if equal volumes of samples “A” and “B” were mixed together. (Hint: Mixing one liter of 10°C water with one liter of 30°C water produces two liters of water at 20°C.)

Comparison of the seawater densities recorded in the table shows that the density of sample “C” is (less than) (equal to) (greater than) the density of samples “A” and “B” prior to mixing.

On the T-S Diagram, draw a straight line between the points representing samples “A” and “B”. Any possible mixture of these seawater samples, including sample “C,” would be represented by a point falling somewhere on the straight line. Regardless of the relative volumes of seawater samples “A” and “B” mixed together, the T-S Diagram shows the resulting mixture will always be (denser) (less dense) than either “A” or “B.”

Comparison of the density of surface seawater samples “A” and “B” with the density of any resulting mixture of these original samples indicate that the mixed water will (remain at the ocean surface) (sink). This can result in broad-scale motions that play significant roles in the layered circulation of the ocean.

Have students make predictions about where high and low density water might be located in the worlds's ocean. Use the Global Ocean Salinity Map to verify predictions and discuss contradictions between the map and student predictions.

This investigation shows that mixing surface seawaters of the same density, but different temperatures and salinities, produces seawater of (greater) (equal) (lesser) density.

Original source: Adapted from the Maury Project, American Meteorological Society