How Does It Work
Density is the measurement of how much “stuff” is packed into a measured space. Nearly every substance and material imaginable has a different density. This is especially true for the six solutions you made using salt and water. By increasing the amount of salt in the solution but keeping the amount of water constant, you create solutions that have increasing densities. The more salt that is mixed into a measured amount of water, the higher the density of the solution. As the Density Straw shows, a solution with a low density stacks on top of a solution with a higher density.
So, density explains why the solutions stack on top of each other inside the straw, but what keeps the solutions in the straw? You might expect the solutions to just fall out of the straw as you lift the straw from a solution. However, because of cohesion (similar molecules attracting each other) and adhesion (different molecules attracting each other), there is surface tension sealing the water at the bottom of the straw. The surface tension is strong enough to help hold the solutions in the straw as long as air pressure inside the straw is lower than all the air pressure outside the straw. Gravity tugs the solutions downward which creates a slight vacuum in the empty part of the straw. That lowers the air pressure inside the straw which is why you need your thumb to cap the straw. This prevents air pressure from equalizing in the straw. If you remove your thumb, the air pressure equalizes, and gravity simply moves the colored solutions out.
