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Density: Seawater Mixing & Sinking

Grade level: 5-8
Theme: ocean circulation

Big Idea
Two of the most important characteristics of ocean water are its temperature and salinity. Together they help govern the density of seawater, which is a major factor controlling the ocean's vertical movements and layered circulation.

Key Concepts
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.

Density is weight divided by volume. The density of fresh water is 1 gram (mass) per cubic centimeter (volume). In other words, if you had a cube with the dimensions: 1cm x 1cm x 1cm; and filled it with pure water, that cube of water would weigh 1 gram. This density is expressed as 1 g/cm3. If you dissolve salt into the water, the salt will increase the fluid's mass, while its volume will remain the same. Thus, the liquid's density will increase. (more)
Student Outcomes 
After completing this activity, students should be able to:
Explain the effect of density on ocean circulation. (O: 5-8)

Key:  C = climate / O = ocean circulation / T = 21st century technology / W = water cycle