Chapter 12.3, Problem 26SSC

Interpretation Introduction

Interpretation: An experiment needs to be designed to compare the relative abilities of water and isopropyl alcohol to support skipping stones. The better liquid needs to be predicted with explanation.

Concept Introduction: The skipping of stone is an activity in which a stone is thrown across the liquid to observe the number of times it can bounce before it sinks into the liquid.

Explanation of Solution

The ability of skipping of stone by a liquid is based on the surface tension of the liquid. There are two types of forces; adhesive forces which are act between two different types of molecules and cohesive forces which act between same type of molecules.

In any liquid, the molecules present in the middle can be attracted to particles above, below or at either side of them. The molecules present at the surface do not have balanced force of attraction due to the absence of molecules above them. Thus, the net attractive forces pull down the molecules from below. This causes surface to have minimum possible area and it acts like a stretched film.

This phenomenon is known as surface tension. It is the measure of inward pull by particles in the interior.

If the attraction forces between the particles are strong, the surface tension will be high. Due to presence of strong hydrogen bonding in water molecules, water will have greater surface tension than isopropyl alcohol.

The hydrogen bonding is also there in isopropyl alcohol, but due to presence of 2 −OH groups in 1 molecule of water, the hydrogen bonding in water is stronger than isopropyl alcohol with 1 −OH group per molecule.

Now, due to greater surface tension, the surface of water acts like a stretched film. The water surface is more stretched as compared to the surface of isopropyl alcohol. This stretched surface causes stone to bounce more number of times as compared to isopropyl alcohol, when thrown at certain angle.

The experiment can be set up to check the relative surface tension of two liquids.

Take equal volumes of water and isopropyl alcohol in two separate containers. Place a paper clip on the surface of both the liquids carefully. Due to high surface tension of water, the paper clip (with more density than water) can be suspended on the water surface.

As the surface tension of isopropyl alcohol is less than water, the paper clip cannot be suspended on it and paper clip immediately sinks in it.

Thus, it can be said that water is better liquid in this case.

Conclusion

Therefore, skipping of stone is better in water than isopropyl alcohol due to greater surface tension in water.