Demo: Mixtures & Density (grade 6)

Demonstration: Mixtures & Density

(Denée Nickel & Tessa Webb)

This demonstration is intended for grade 6, focusing on the following aspects of the curriculum:

Big Idea: Everyday materials are often mixtures.

Competencies: Make predictions about findings of their inquiry, make observations in familiar or unfamiliar contexts, identify environmental implications of the findings (connect the demonstration to ocean environment)

Content:

• Heterogeneous mixtures
• Mixtures: separated using a difference in component properties (in this demonstration, differences in density between salt/fresh water and hot/cold water)

Materials:

• 2 x Bins (large, rectangular, transparent, somewhat deep)
• Roll of tin foil
• Duct tape
• 2 x food coloring (one red, one blue)
• Salt
• Kettle
• Water
• Mixing utensil
• Pointy object to poke holes in tinfoil
• Ice (optional)
• Boxes to raise to eye level

Preparation:

• First, start heating up water in a kettle
• Build a tall, watertight tin-foil wall in the middle of each bin (dividing the bin in to two compartments)
• Fill Bin 1 with tap water on both sides of wall
• Dissolve salt in water on one side of Bin 1 (~2.5 tbps/1 litres)

*NOTE* ocean water contains ~ 35g salt to 1L of water

• In Bin 2, fill one side with cold tap water (can include 2-3 ice cubes to ensure water is extra cold), fill other side with hot water from the kettle (doesn’t need to be boiling, simply hot water will work)

Dye:

• Bin 1:
  • Freshwater : dye blue
  • Saltwater on opposite side : dye red
• Bin 2:
  • Cold water : dye blue
  • Hot water on opposite side : dye red

Safety Considerations

• Boiling water : teacher handles this material to reduce chance of burning students
• The stick used to poke holes in the aluminum foil is quite sharp
• Spilling water (have a towel at the ready)

Description of Demonstration:

1. Explain to students that we will be continuing our lesson about mixtures, both heterogeneous and homogeneous, and our demonstration will be relating specifically to the ocean today.
   • Questions:
     1. What do we know about the water in our oceans? (salty) Where do we find freshwater? (rain, rivers, lakes, ice…)
     2. Do you think some places in our ocean are saltier or more fresh than others? (as you go deeper in the ocean, it should get saltier. Also, closer to tropics – increased evaporation makes for higher salinity)
2. Point out Bin 1 and explain that we have salt water on one side and freshwater on the other.
   • Question: Do we all remember what hetero/homo-geneous mixtures are? *point out that the mixture of salt and water was heterogeneous, then it dissolved in the water, distributing evenly, making it homogeneous (a good quick review)
3. Introduce the graphic organizer and have students discuss with a partner and record predictions on what might happen if we were to poke holes through the aluminum foil wall between the salt and freshwater in Bin 1.
4. Invite students up around the table, tell them to bring their journals, graphic organizers, and a pencil.
5. Invite a volunteer to poke 6 holes (low and high) throughout the aluminum foil wall in Bin 1 (avoid shaking the bin).
6. Transfer attention to Bin 2 and explain that we will keep an eye on Bin 1 and come back to it in a few minutes.
7. Explain that Bin 2 contains hot freshwater on one side and cold freshwater on the other side. This is another property of water that occurs in our oceans.
   • Questions:
     1. What do you expect might happen in this case when we poke holes in the wall?
     2. Where in the ocean would we find cold water? Hot water?
     3. Where in the water column would it be hotter or colder?
     4. Where in the world would it be hotter or colder..?
     5. What are some of the ways ocean water is heated? (sun, geothermal)
8. Prompt the students to make predictions and repeat the hole-poking process with Bin 2.
9. Return attention to Bin 1 and discuss what we are observing (red/salt-water should be sitting on top of the blue/fresh-water)
   • Questions:
     1. What are you observing? Is this what you expected?
     2. Do you think we could observe this layering effect in our natural oceans?
     3. What locations might we see this layered separation in an exaggerated way? (where it rains, where rivers meet ocean, when ice melts…etc.)
     4. When might we not see this layered separation as much in the ocean? (places where the ocean is mixing substantially vertically…places where a lot of the freshwater is evaporating rapidly off the surface)
     5. Why are we seeing this layering? (check to see if students mention density or can explain it before bringing it up)
   • Explain density
     1. Density is a word we use to describe how much space a substance takes up (its volume) in relation to the amount of matter in that substance (its mass). In other words “weight per volume.” If an object is heavy and compact, it has a high density.
        1. Salt water density = salt molecules take up space between water molecules, making it have more mass in roughly the same amount of volume as the fresh water.
   • Point out that we created a heterogeneous mixture by allowing the substances to mix slowly – the ocean can be both a homogeneous and heterogeneous mixture depending on what components and zones you are looking at!
10. Continue the discussion, returning attention to Bin 2, as students record their observations.
    • Questions:
      1. What are you observing? Is this what you expected?
      2. Why does cold water sink?
         1. The cold water has a higher density because molecules move more slowly and are, therefore, more compact and occupy a smaller volume.
         2. The hot water molecules have more energy and are jostling around, occupying a larger space. This means less molecules fit in that same volume, therefore less dense and lighter weight than cold water
         3. *more cold molecules can fit in a certain volume than hot molecules because of molecular movement
      3.  Future research question – what about warm, salty water? Or cold, fresh water? Which attribute has the greater effect in these cases?

Wrap-up/Connections:

Connection to ocean currents and ocean mixing:

• Temperature and salinity differences can cause ocean mixing vertically in the water column.
  • There are separate thermal zones and salinity zones in the ocean, just like what was seen in our demonstration today. Oceanographers actually have a name for that distinct line you see between the layers (thermocline for temperature change, halocline for salinity change) – this really exists in nature! However, it is not as visible without the food dye.
  • As the warm surface water cools, it increases in density and sinks. Sometimes, cold deep water warms, so it rises. This causes these layers to shift around and mix = Vertical Mixing!
• Temperature and salinity, therefore, also affect ocean currents – how the ocean circulates around the world, from the equator/tropics to the north or south pole!
  • example: Water is cooled at polar latitudes → gets denser and sinks → vertical sinking motion causes horizontal water motion as surface water replaces the sinking water (thermohaline flow)
  • Salt example: Mediterannean sea – water gets heavier/more dense in the summer (why?) because of strong evaporation and increased salinity → this causes water to sink and go through the same process as above
  • FUN FACT: the salty water from the Mediterranean Sea can be found in the Atlantic ocean at depths of 1000 to 1500 m!
• Questions:
  • Why might ocean currents and mixing be important and what effects do they have on the world?
    • Weather!
    • Transport of nutrients and dissolved gases
    • Transport of plankton – Dispersal! Food source!
    • The movement of nutrients and dissolved gases from high to low in the ocean, photosynthesis can only occur at the ocean surface, way less life down below
  • What else might affect currents and mixing that we didn’t demonstrate today? (wind – major driver of currents, Coriolis effect, moon/tides)

Connection to the water cycle (they should have learned about this around grade 2):

• Questions:
  • What happens to the freshwater on the surface in warm areas like the tropics? (evaporates)
  • Where does it end up? (In the atmosphere/clouds .. travels north or south and rains into rivers/lakes/snows on mountains, etc.. → what if it snowed on mountains? It could then melt and travel through rivers and streams and end up back in the ocean!)
  • Why do we get storms and rain?
    • As ocean currents distribute this heat around the world, it plays into the hydrological cycle as it moves and interacts with the atmosphere – ocean evaporation can increase temperature and humidity in the surrounding area – forming rain and storms (this is why tropics are particularly rainy and stormy!)

Connection to Climate Change:

• Scientists have studied the Atlantic ocean and found that it has gotten slightly saltier over the years. They have also found ocean temperatures to be rising. What could some impacts of this be?
• Question: Can you think of the effects this may have on ocean currents and therefore what else? (weather, distribution of nutrients/gases/plankton, coral reefs (fragile biomes), etc.)
  • The majority of radiation from the sun is absorbed by the ocean (particularly in the tropics). The world’s oceans are huge heat sinks and keep our world warm and livable!
  • Ocean currents act like conveyer belt – transporting warm water and precipitation from equator toward the poles, and cold water from the poles back to the tropics → THUS regulating the global climate (helping to counteract the uneven distribution of solar radiation reaching Earth’s surface) — without this, regional temperatures would be more extreme and our climates would be very different! Much less of Earth’s land would be habitable.

Graphic Organizer

• Can also instruct students to use their journals and draw an example on the board

Ocean Demo – Graphic Organizer

Featured Image by Ivan Bandura on Unsplash