Using glass columns with brightly colored liquids layered by density makes a great demonstration of liquid density. However, you can encourage students to more fully grasp the concept of liquid density by inviting them to experiment with familiar materials, such as ice cubes, rubbing alcohol, eggs and saltwater.
Floating Ice Cube
Pour rubbing alcohol into the glass so it is about a third of the way full.
Tilt the glass slightly and pour water so it slides down the inside of the glass. This will prevent the liquids from mixing. The water should sink to the bottom because it is more dense than the alcohol.
Place the ice cube into the cup. It should sink in the alcohol but float in the water. This looks especially cool because all three materials are clear and colorless, so students will have to look closely to see why the ice cube stopped in the middle of the cup. Tilt the cup and watch the ice cube bob but stay right between the liquids. Observe what happens as the ice cube melts.
Boil enough water to fill six large beakers three-quarters of the way full.
Add salt to the first beaker and stir. Keep adding until no more salt will dissolve. Label this beaker "Saturated = ___ tsp salt."
Add no salt to the next beaker. Label this beaker "0 tsp salt."
Create different concentrations of saltwater in the other four beakers by adding different amounts of salt. Label each beaker with the amount salt dissolved in it: "____ tsp salt."
Drop a fresh egg gently into each beaker. Record whether it floats, sinks or hovers. Water has a density of 1 gm/ml. Adding salt to the water increases its density, so at some point the saltwater will have a greater density than the egg, and the egg will float.
Getting Into Hot Water
Fill a large beaker three-quarters of the way full with room-temperature water.
Fill a small beaker with ice water and add food coloring until it is very dark. Make sure all ice cubes are removed and the colored water is filled to the top.
Use tongs to gently lower the small beaker of cold, colored water into the larger beaker that is filled three-quarters full of hot water. Allow the small beaker to rest on the bottom, totally submerged. Observe. The ice water, being more dense than the hot water, will stay in the small beaker and not mix with the hot water.
Fill another large beaker three-quarters of the way full with cold water.
Fill a small beaker with hot, colored water.
Use tongs to gently lower the small beaker into the large beaker of cold water until it rests on the bottom. Because the hot, colored water is less dense, this time it will quickly rise out of the small beaker.
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In Experiment 3, lower the small beaker gently. If you bump or tilt it, your results will not be as clear.
Students at this age like to make their own decisions. After they have completed the experiment, allow them to choose their own variables to extend the experiments when possible, as this is the fundamental basis for scientific inquiry.
Experiment with different containers if you do not have lab-grade glassware.
Use caution with glassware. Use tongs when handling hot materials. Do not drink any of the experimental liquids. Only place hot liquids into heat-safe containers.
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