Flash point of ignitable Liquids

Demonstration Experiment on Video

Flash Point of ignitable Liquids

Objectives: Flash Point, Fire Point

Peter Keusch

German version

Supermarket products:
cleaning petrol
lampoil
fireplace matches

Glass wares:
2 gas collecting cylinder h = 9.5 cm, d = 5 cm
2 watch glasses d = 8 cm

Hazards and safety precautions:

cleaning petrol und lampoil ignite easily. They are harmful to the environment.

Safety glasses and protective gloves are required.

Experimental procedure:

A watch glass is placed on each of two gas collecting cylinders. One watch glass contains lampoil. The second watch glass is covered with cleaning petrol. A lighted fireplace match is held over the watch glasses without contacting the liquid surface.

Result:

While lampoil does not ignite under the given conditions, cleaning petrol inflames immediately.

Video clip (Download RealPlayer .rm file)

Cleaning petrol has a low flash point. Already at low temperatures sufficient volatile fumes are produced which ignite in the presence of flame and support a continous combustion of the fuel. Short chain aliphatic hydrocarbons can be identified by their low flash point.

Discussion:

Lampoil and mineral oil consist of long-chain hydrocarbons. The increased flash point is caused by the strong intermolecular forces (e.g. van der Waals forces).

The flame point is usually a few degree higher than the flashpoint for a given ignitable liquid. The 'flame or fire point' of a fuel is the temperature at which it will continue to burn when the ignition source is removed.

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