Heat Transfer & Ignition

Introduction

Solids and liquids do not burn. Gases burn. Solids and liquids are broken down into simpler molecular compounds (a chemical change) by the effects of heat. These gases are the products of decomposition and that is what burns in a fire.

In a candle, how do we go from solid wax to gaseous fuel? The wax isn’t burning until we add heat from the lighter or match, and then the heat is consistently applied by the burning wick. The solid wax is fuel, but does not burn until heat is applied.

Many things are fuel and can burn – things around us every day like clothes, hair, carpeting, furniture, etc. If heat comes into contact at a fast-enough rate, solids can turn to gaseous fuel through pyrolysis.

Knowing that when heat is applied, solids and liquids can turn to gaseous fuels, we need to know how heat is transferred. There are three main methods of heat transfer: 1) conduction, 2) convection, and 3) radiation.

Questions

  • How do fires spread?
  • How do fires start?

Problem to be Solved

  • When we know how heat is transferred, we can better understand the fire scene.
  • Knowing the ignition of different materials helps fire investigators understand/read a burn scene.

Module

Heat Transfer & Ignition

Age Group

Middle School

Category

Fire Forensics

Materials

  • Pillar or small candle in center of metal/non-flammable pie pan or tray (not! plastic, paper, or wax coated material)
  • Beaker of ice water
  • 2 6” (15cm) lengths of copper wire 5 popsicle sticks/also known as craft sticks (non-coated, not colored)
  • 4 index cards
  • Tongs to handle materials that are hot or briefly flaming
  • Long reach lighter
  • Metal/non-flammable pie pan or tray (not! plastic, paper, or wax coated material) with 1-2” (2.5 – 5cm) of water
  • Student Xplorlab pages
  • Xplorlabs video: Investigation 2

Summary

Students make observations about how thermal energy is transferred and how transferred heat starts fires.

Students compare the surface to mass ratio of a block of wood and a toothpick, then compare how easily they ignite. Students then make observations about how different materials ignite and burn.