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Activity

Water flow with temperature and salt variation

Summary: 
Layer warm, cold and salty water in a small tube, or plastic box, to find out which sink and float relative to each other. Use to model the flow of warm and cold air in the earth's atmosphere or the flow of warm, cold and salty water in the oceans.
Science content (2016 curriculum): 
Chemistry: States of Matter, Properties of Materials (K-7)
Earth/Space: Weather, Seasons, Climate Change (K, 1, 4, 7)
Earth/Space: Water cycle, Water conservation (2)
Earth/Space: Sustainable practices, Interconnectedness (2, 5, 7)
Science topic (2005 curriculum connection): 
Earth and Space Science: Air, Water and Soil (grade 2)
Earth and Space Science: Weather (grade 4)
Lessons activity is in: 
Materials: 
  • bottles that deliver single drops e.g. small food dye dispensers
  • salt water, 1 teaspoon salt in 100ml water, plus 20 drops green food dye to colour
  • warm water, plus red food dye to colour (about 20 drops in 100ml)
  • microwave to keep warm water heated
  • iced water, plus blue food dye to colour (about 20 drops in 100ml)
  • ice in a container to keep the iced water cool
  • clear-sided box, or small clear tubes
  • tap or squeeze bottle of water to rinse small tubes after each use
  • optional: large box of water tipped at one end, to show larger scale
  • optional: red, green and blue coloured pencils for students to record their observations
Procedure: 

Free experimentation in small tubes:
Instruct students to half fill a small tube with room temperature water. Then drop in one or more of the water types (salty/cold/warm). Watch whether the drips sink or float. Use coloured pencils to show observations. Then try adding different combinations of water to see where they settle.

In general, salty water will settle lowest, then cold water, then warm water will stay on the top (note that after adding salty water, the salt mixes in and will make the whole tube salty, so that cold water will stay near the surface, sometimes even above a red layer (that is warm but now has some salt mixed in). Students will get their own unique results depending on what order and how much of each water type they add. They should be encouraged to look closely and observe water flowing in the tube when they add each type.

Discuss how this relates to world-wide ocean currents:
The heating of the surface of the ocean, and freshwater flow into the ocean changes the temperature and salinity of the ocean. Warmer water is less dense than cooler water, and saltier water is more dense than less salty water. Denser water sinks below less dense water, so the differences in temperature and salinity causes water to move, driving ocean currents.
The thermohaline circulation of ocean water (called the ocean conveyer belt) flows around the world. Warm water from the Tropics is driven North by wind. In the North Atlantic it cools. Evaporation and ice formation in the North also makes the water more salty, making it more dense. The cooler, saltier water, sinks, displacing the bottom water, which flows south beyond the equator to Antarctica. These cold bottom waters flow around the globe and eventually mix with warmer water and move to the surface in the Pacific and Indian Oceans. The cycle is completed when warm surface waters head north again.
http://www.nasa.gov/images/content/436189main_atlantic20100325a-full.jpg
https://en.wikipedia.org/wiki/Thermohaline_circulation

Discuss how this models air flow in our atmosphere:
Air is warmed by the sun, predominantly at the tropics. This warm air rises, and cooler air sinks (just as the warm water rises and the cool water sinks). This movement of air in our atmosphere creates winds.
In addition, ocean currents, caused by differences in temperature and salinity of the water, move heat around the globe.

To show larger scale flowing and layering in a clear-sided box:
Fill the box with room temperature water.
Elevate one end of the box to make a sloped bottom.
Drip each of the water types (salty/cold/warm) in turn and watch them sink (salty/cold) or float (warm) in the water, and flow along the bottom or surface.

Notes: 

Primaries at Tyee just mixed warm and cold water, whereas intermediates also used the salty water.

When this activity is done in a larger box, try measuring the temperature of the warm and cold water layers.

Grades tested: 
Gr K
Gr 1
Gr 2
Gr 3
Gr 4
Gr 5
Gr 6
Gr 7
Teacher: 
Alane Lublow
Becky Evermon
Brenda Koch
Daphne Gurney
Fiona Laporte
Heather Wallace
Ingrid
Nina Hooker
Scott Malin
Sharon Ghuman
Taz Ismail
Teaching site: 
Aboriginal Focus Elementary
ingridscience afterschool
Tyee Elementary
Activity originally developed and delivered: 

Aboriginal Focus School with the Scientist in Residence Program http://scientistinresidence.ca