Learning Lesson: That Sinking Feeling

Overview

Ocean water is salty and, in general, cold. Differences in the water density associated with temperature and salinity are vital in shaping the Great Ocean Conveyor. This lesson plan demonstrates how salinity and temperature affect density of water.

TOTAL TIME	10 minutes
SUPPLIES	Fish bowl (or large, deep glass container such as fish tank, bowl or pitcher); Food coloring; Table salt; Ice cube tray
PRINTED/AV MATERIAL	None
TEACHER PREPARATION	Make the ice cubes the evening before. Into one-half of the ice cube tray pour fresh, dark food-colored water. Using a different color, pour dark-colored, very salty water into the other half of the tray. Freeze the solutions.
SAFETY FOCUS	Thunderstorm safety rules

Procedure

1. Fill the fish bowl nearly full with tap water. Allow the water to settle for a minute or two.
2. Gently add one cube of each color into the water.
3. Have the students describe and explain what occurs.

Discussion

In both cases, the colored waters will begin to sink to the bottom of the bowl. This is due to the temperature of water from the melting cube is denser than the surrounding tap water and therefore sinks. However, the colored water from the salty cube should sink faster than the fresh water cube because the addition of salt.

Each winter, in the far North Atlantic Ocean as new sea ice forms, the salt that is left behind in the ocean makes the water very dense. This dense water sinks to the ocean floor and is the "engine" for Great Ocean Conveyor's motion.