Put your favorite food coloring in a ice cube tray with water and freeze.
Fill one transparent cups up with tap water, and the other with tap water and add salt.
Observe the rate of melting of each ice cube.
I love the results of this experiment as it really surprise me. When I put salt on a ice cube it melts the ice very fast and when I saw this I could not believe it. The ice cube in the salt water melts much slower then the one in the freshwater.
In the salt water the colored water from the melted ice cube forms a distinct layer that floats on top of the salt water. In the fresh water, the melted water sinks to the bottom and is evenly spread out.
These observations can be interpreted by comparing the density of fresh water and salt water. When the ice melts, the water that results in very cold. It is more dense than the warmer tap water in the cup, and will thus sink. As we see on the left side.
As the colder water sinks, it displaces and pushes up the warmer tap water from the bottom. As this warmer water comes into contact with the ice cube, it hastens its rate of melting. A convection current has been set up, where the colder water sinks and the warmer water is pushed up. Convection is the transfer of heat by the movement of fluids.
In the cup of salt water, the cold water that results from the melting of the ice cube is less dense than the salt water, so it floats on top. There are no convection currents occurring here. Since the ice cube is constantly in contact with the very cold water, which prevents it from contacting the warmer water below, it takes a longer time to melt.
This happens in the oceans where Icebergs last much longer in salt water then they do in freshwater. The melted ice from an iceberg will float on top of the salt water in the ocean, but will sink in fresh water. Ice cubes will cool a diet drink much faster then they will a sugary drink for this same reason.