This lab explores the effects of temperature and saltiness on density.
- lab handout
- clear, plastic cups and foam cups
- table salt
- red food coloring
- table salt
- hot and cold water
Creating an Underwater Waterfall
1. Fill a plastic cup nearly full with cool water.
2. Fill a foam cup half-full with hot water. Add a pinch of salt. Add 6 drops of red food coloring. Stir until the salt dissolves.
3. Place the eyedropper into the hot red water to warm it up. After a minute, fill the dropper with the water.
4. Hold the dropper so that it lies at a flat angle at the surface of the clear water with the tip just below the surface. Gently squeeze out a layer of hot red water onto the surface of the clear water.
5. After a short cooling time, the red layer will form little waterfalls that sink through the clear water.
1. Explain why the red water floats at first.
(The hot, red water floats because it is less dense than the cold water.)
2. Explain why the red water eventually sinks.
(The red water eventually sinks because heat flows out of the hot water to the cold water until the temperatures are the same. Then, the higher concentration of salt in the red water becomes the controlling factor. The high slainity of the red water makes it more dense than the cold water. As a result, it sinks.)
This activity was based upon material outlined in CPO Earth Science.