This is a perfect experiment that teaches kids about water density.
- food colorings (rainbow colors are red, orange, yellow, green, blue, purple)
- a clear straw
- 6 cups (at least 4 inch tall)
- adult supervision
- Fill each of the six cups with the same amount of water.
- Adding food colorings into the cups, one color in each cup.
- Line up the cups from left to right.
- In the first cup, do not add any sugar. In the second cup, add one tablespoon of sugar. In the third cup, add two tablespoons of sugar. In the fourth cup, add three tablepoons of sugar, and so on.
- Stir until all the sugar is dissolved.
- To make a sugar rainbow, grab the straw and place the end into the first cup, the one with no sugar. Only insert about half an inch into it. Cap the other end of the straw with your thumb before lifting it out of the water to prevent the water from falling out of the straw.
- Now dip the straw into the second cup (1 tablespoon sugar). This time, insert it deeper so that the end is one inch below the water level.
- In one quick move, release the thumb and recap again. Now you should have two layers of color.
- Keep dipping the straw into each solution from the one with the least sugar to the one with five tablespoons of sugar. Each time, the straw is inserted half an inch deeper.
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Density is the amount of substance (mass) within the volume occupied by the object.
If two cups have the same amount of water (i.e. same volume), the one with more sugar is denser than the one with less sugar.
Buoyancy is determined by relative density.
The solution with less density floats above the solution with higher density.
That is why the color don’t mix.
Sugar water has higher density than plain water.
The solution with more sugar has higher density than the one with less sugar.
If you have inserted the straw in the solutions from the least sugar to the most sugar, then the color don’t mix and you have a sugar water rainbow.
Another principle illustrated in this experiment is air pressure.
In this experiment, a straw is used to create the rainbow tower.
Alternatively, you can use a dropper to carefully layer the colored solution in a narrow test tube.
Why is it possible to keep the colored solution in the straw when it is lifted from the water?
When the straw is lifted out of the solution, the rainbow tower doesn’t fall out because water’s surface tension makes sure the colored water and your thumb together create a sealed air chamber inside the straw.
Gravity pulling the water downward makes the air volume slightly larger.
When the amount of air is constant, larger volume leads to lower air pressure.
The air pressure outside of the straw is therefore higher than that inside, pushing the water inward and canceling the gravitational force.