Figure 1. A top view of a floating ice cube.
Here is a greener and better way to do it--using an infrared (IR) camera. An IR camera shows hot and cold (typically) in red and blue colors, which can be considered as "IR ink" that can be seen only through an IR camera. With the tool, all you can do is to add some ice cubes or hot water to a container of water every time you need to do a demo. There is no need to change the water.
|Figure 2. A side view of a floating ice cube showing "cold fingers."|
One thing to notice is that you should not use a glass container--because it reflects off IR rays that will get into the image. A clear plastic one is the best as it does not reflect much and it allows you to observe what happens inside (if anything visible) with naked eyes.
|Figure 3. A view from another side|
showing the the cooling at the
|Figure 4. An IR image after hot water was added to room temperature water in a container showing hot water tended to float atop.|
|Figure 5. An IR image of a fish tank showing a clear pattern of temperature stratification.|