Bath time is generally a fun time around here, and given that much of my current education is focused on the physics of water and that Johann always wants to know what I have learned…well, bath time often ends up as science time as well. Our poor bathtub is now the scene of many experiments.
The latest round of experiments are a continuation of previous experiments with density. Early last spring we examined in some detail the difference between density and weight. We carefully weighed cans and bottles of cold, room temperature, and warm soda, both diet and regular. After finding they were the same volume and slightly different weights (the regular coke weighed more), we went into the bathroom and put the cans into the tub filled with warm water and recorded the results. Johann was thrilled to find the warm diet soda bobbing high in the water, while the cold regular coke sank much lower.
A week or so later we duplicated the same experiment using coke bottles filled with tap water, sugar water, and salt water — ~35 parts per thousand (ppt) or seawater salinity — to the same weight. While the coke measurements were interesting to Johann, the lesson really came home with the salt water. He helped prepare the solution, saw that the bottles all were equal in weight, and could see a slight, but definite difference in the volume of the liquids through the plastic bottles.
We did this second experiment with 9 bottles: one of each solution at ~10°C (50°F), ~30°C (86°F), and 50°C (120°F). Using all 9 bottles, we were able to examine the effects of both mass and temperature on water density, by observing how high or low the bottle floated. Because we filled the bottles very full, the cold high salinity water sank straight to the bottom of the tub, matching Johann’s predictions. Of course, taking a bath in a tub with nine floating half liter coke bottles was quite…interesting.
Now we have taken the experiment up one more level, looking at two masses of water in the tub, much like the processes happening in the estuarine river right nearby. We mixed salt into two liters of cold water to approximate Long Island Sound seawater (~30 ppt), then added some of Johann’s non-staining bathtub water coloring tablets, enough to make a strong color. After filling the tub most of the way up with warm water, I slowly added the “seawater”, pouring it down the slanted side of the tub so it would sink down with a minimum disturbance of the water in the tub. Johann then watched the colored wave of cold salty water move across the bottom of the tub as a wave with a clear vertical boundary that we could watch on the sides of the tub. When it hit the far side, it piled up the side, then collapsed under its own weight and returned to the starting end of the tub. The wave continued to reflect back and forth across the tub as Johann pondered the cause and the implications.
I was extremely proud when he understood that it was the density of the water masses that caused the warm water and the colored salt water to remain separate, and it was the higher density which caused the salt water to sink. I explained that the water in this system would eventually become uniform, since it is a closed system. The temperature would homogenize soon and the salt would diffuse throughout the water. (Especially since we would be introducing “fall winds and storms” when we got into the bath!)
I’m beginning to think he’s almost ready for a look at the charts of salinity and temperature measurements we have been taking and could understand where the layering would or would not occur. Not yet, but soon enough.
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