Igniting a Sugar Cube

Sugar burns in air. But igniting a sugar cube is much more difficult than expected. Put a sugar cube on the end of a toothpick and bring a lighted match flame under a remote corner. The sugar melts instead of burning, and the brown, gooey stuff is caramel.

However, we wish to burn the sugar, not melt it! We want to see it on fire with a flame of its own. Why is this process so difficult to achieve? How can we succeed in lighting the sugar cube with the burning match?

Answer

Very small particles tend to ignite more easily. The large surface-area-to volume ratio for a collection of small particles aids ignition, providing a large combustion area for the chemical interaction of their surface molecules with oxygen and also providing a nearby heat source for sustenance. Therefore, rub the far corner of the sugar cube in some cigarette ash or tiny ash particles from burned paper, then light the ashen cube with the burning match. Ignition is now easy. Oxygen molecules react with molecules in the ash to produce thermal energy and product molecules, including water.

Historically, there have been many examples of the spontaneous ignition of dust particles in the air, such as explosions in granaries where grains crops are stored and in mills that grind grain into smaller particles. A small warm spot in the dusty air, perhaps produced by sunlight, by a match, or by friction, can rapidly spread into a full-scale explosion.

On a less violent scale, simply lighting a campfire outdoors begins with kindling, very small sticks and shavings of wood, which have a very large surface-area-to-volume ratio. The slightly larger sticks can be added once the flame sustains itself. Finally, a whole faggot of sticks can be placed in the firepit to generate a lasting fire.

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