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

# Boiling Water in One Half of the Test Tube

## Objective

To demonstrate heat transfer by convection.

## Introduction

When heated, water above the source of heat gets heated while water below the source of heat remains as it is.

## Apparatus

test tube, candle, water

## Procedure

Fill a test tube three-fourth with water. Hold it at the bottom and start heating its middle part over a candle flame. After sometime water in the upper half start boiling.

## Discussion

In solids, heat is transferred from the hot region to the cold region. The heat gets transferred without movement of the particles. This mode of heat transfer is called conduction. But in water (fluid), the heat transfer takes place through movement of the particles. The hotter particles moves up and surrounding cooler particles take their place. They also get heated and move up. This mode of transfer of heat is called convection. As a result, only water above the source of heat gets heated up while water below the source of heat remains as it is.

## Note

Hazard: be careful while handling the flame. Do not touch the upper portion of the test tube as it becomes very hot.