Concepts of Physics

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

Burning a 100 Rs Note

Objective

To demonstrate that ignition temperature of the alcohol is low.

Introduction

Water is somewhat bad conductor of heat.

Apparatus

alcohol mixed water, a hundred rupee note.

Procedure

Mix alcohol with water in equal proportion. Dip the note in the liquid. When it is wet, take it out and bring a lighted match-stick near it. The note burns with flame. After a little time the flame extinguishes but note comes out intact.

Discussion

Alcohol is a highly volatile liquid. It evaporates at ordinary temperature i.e., at room temperature. The ignition temperature of alcohol vapour is low. Hence, the vapour catches fire easily. Most of the heat produced by combustion of the alcohol is lost to the surrounding air with some amount used up in the vaporization of alcohol. Moreover, the water in the note also protects it from heat. So the note does not burn though there is a flame.