IIT JEE Physics (1978-2018: 41 Years) Topic-wise Complete Solutions

# Cohesive and adhesive forces of water

## Introduction

The force between two surfaces of the same material is called cohesive force and that between the surfaces of dissimilar materials is called adhesive force. There are several popular demonstrations of water surfaces attracting each other for cohesive force. There are relatively smaller number of popular demonstrations to show the adhesive force. This experiment gives one such simple demo.

## Apparatus

A plastic bottle with two holes one below the other, tape, water

## Procedure

1. Close the two holes by fingers. Fill water in the bottle. Do not put the cap.
2. Remove fingers so that water starts coming out from the holes. You see two streams.
3. Using your thumb and a finger, bring the two streams in contact with each other. - Occasionally you will see that the two streams have merged into one. This is due to cohesive force between the two water streams. When do you find the merger more frequent, when the bottle is nearly filled or when a good amount of water has gone out?
4. Put a tape on the upper hole. Fill the bottle and let the stream come out from the open hole. Carefully watch as the stream slows down. The stream starts going along the surface of the bottle. This is due to adhesive force between the plastic surface and the water stream.
5. Repeat step-4, but the bottle tilted as shown in the figure. Does water still goes along the surface of the bottle? Write your observations.