When lemonade soapy water and plain water are soaked up by a paper towel why are the first two absorbed more than the water?

Answer 1

There is a natural attractive tension that exists among the molecules of liquids and, therefore, within the surfaces of liquids. This Surface Tension is what enables rainwater, for instance, to form into separate droplets on a window pane.

The Surface Tension of the liquid also enables each droplet to assume its symmetrical, uniform shape rather than spreading itself thinly and randomly across any hard surface.

If a open window is horizontally hinged and thus sloping, the droplets will take a more circular shape as the window is opened further and the stretching effect of gravity is lessened. In a smaller droplet the surface tension becomes larger in proportion to any other forces acting on the liquid and this allows it to become more nearly spherical in spite of the pull of gravity.

It is the same attractive forces between molecules that keep a droplet clinging to a surface, due to the attractive forces between the liquid molecules and the molecules that make up the harder surface.

The same attractive forces cause the liquid to be taken up into an absorbing medium such as tissue paper. This time there is no clean, even and solid surface for the liquid to adhere to; the tissue comprises a tight tangle of the minute surfaces of all the very fine fibres that make up the paper.

If the adhesive force between liquid and tissue fibre is stronger than the force of molecular attraction within the liquid, then the liquid will be drawn into the tissue. In other words, if the Surface Tension is not too great a liquid will be absorbed into a paper towel. And, if the Surface Tension can be reduced the absorption will happen more quickly.

The Surface Tension of a liquid can be reduced by dissolving another substance into it.

The surface tension of pure water, for example, would be higher than the surface tension of the same water with soap or lemonade concentrate mixed into it. One of the deliberately selected properties of a soap or detergent is its ability to greatly reduce the surface tension of tap water. Heating it up also reduces the surface tension of water.

That is why your lemonade and your warm soapy water are more quickly absorbed by the paper towel.