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Demonstration of the Affect of Evaporation on the Salinity of Ocean Water

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Author: Janice VanCleave

How Does Evaporation Affect the Salinity of Ocean Water?

Category: Earth Science—Oceanography

Project Idea by: Donald Van Velzen

Salinity is the salt concentration in a salt and water solution. The average salinity of seawater is 35 parts per thousand. This is written as 35 ppt, and it means that 35 parts of salt are in every 1,000 parts of seawater. While most samples of seawater have a salinity of 35 ppt, the salinity does vary from place to place. The salinity of seawater is usually between 32 ppt and 38 ppt.

Density is the ratio of mass to volume of a material. This property of matter allows you to compare materials of the same size and determine which is heavier.

Specific gravity is the ratio of the density of a material to the density of water. Specific gravity has no units. It compares the heaviness of a material to the same volume of water. No matter what units the densities are expressed in, specific gravity is the same. For example, mercury is 13.6 times as heavy as an equal volume of water. Thus, the specific gravity of mercury is 13.6. If the temperature is such that the density of water is 1 g/mL, the density of mercury would be 13.6 × 1 g/mL, or 13.6 g/mL.

Specific gravity can be used to determine the salinity of a liquid. Salt water has a greater density than fresh water, so it also has a higher specific gravity. The higher the salinity of an aqueous salt solution, the greater its specific gravity. A hydrometer is an instrument used to measure the specific gravity of a liquid. A hydrometer floats in a liquid. The higher it floats in the liquid, the greater the specific gravity of the liquid. A project question might be, "How does evaporation affect the salinity of ocean water?"

How Does Evaporation Affect the Salinity of Ocean Water?

Clues for Your Investigation

Place a liquid made of a measured amount of salt and distilled water in an open container. Design a way to measure evaporation rate such as placing a strip of tape down the side of the container and marking the surface level of the water. At predetermined intervals, use a hydrometer to measure the specific gravity of the liquid. The hydrometer can be purchased or homemade. You can make your own hydrometer using a straw, BBs, and clay. In the figure, the scale printed on the straw shows the specific gravity higher and lower than the water, which is 1.0. The weight of the straw, the BBs, and the clay should make the hydrometer stand upright in fresh water, with 1.0 at water level.

Independent Variable: Evaporation rate

Dependent Variable: Salinity of water (determined by specific gravity)

Controlled Variables: Type of salt, distilled water, containers, amount of liquid, method of measuring, hydrometer, environmental conditions

Control: Specific gravity of distilled water

Other Questions to Explore

  1. Does the type of solute affect the solution's specific gravity?
  2. What effect does temperature have on specific gravity?
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