Chemistry for Liberal Studies - Forensic Academy / Dr. Stephanie R. Dillon

Flammable Liquids

Flammable and Combustible Liquids

We keep talking about accelerants and the chemicals they are composed of and how to detect them at crime scenes but there is one aspect of the accelerant liquid we have yet to cover: Why are they flammable?

The chemist always wants to know why when you put a match to water the flame is extinguished but the same match placed in alcohol bursts into flame. Why is alcohol flammable but water is not?

The reason comes from the very structure of the molecules. Earlier we talked about the unique properties of water stemming from its ability to make hydrogen bonds. The hydrogen bonds in water limit its vapor pressure and therefore make it very hard to transition into the gas phase. Molecules that do not have hydrogen bonds or that have very weak forces holding them together are much easier to get into the gas phase. The degree to which a liquid transitions into the gas phase naturally (the volatility) can be described by its Vapor Pressure.

Vapor Pressure

The pressure exerted by the vapor of the liquid at any given temperature is called the Vapor Pressure.

The vapor of the liquid exerts pressure on its container this is its vapor pressure.

7 Vapor Pressure vs Boiling Point
ChemistryConnected (YouTube)

As we said previously, in order to start a fire you must have enough energy to get the reaction over the activation energy barrier. Liquids with high vapor pressures have lower boiling points and therefore lower activation energies. The boiling point is the temperature at which the vapor pressure equals atmospheric pressure, such that the pressure of the atmosphere can no longer hold the liquid in a liquid state and bubbles begin to form. This means that liquids with lower boiling points and higher vapor pressures are easier to burn.

It is the liquid's vapor rather than the liquid itself that ignites when mixed in certain proportions with air in the presence of an ignition source. Flammable and combustible liquids vaporize and form flammable mixtures with air when in open containers, when leaks occur, or when heated.

Flash Point

The Flash Point of a liquid is the minimum temperature at which a liquid gives off vapor within a test vessel in sufficient concentration to form an ignitable mixture with air near the surface of the liquid. The flash point is therefore dependent on the boiling point and consequent vapor pressure of the liquid.

By definition, any liquid with a flashpoint less than 100°F is considered to be a flammable liquid. Any liquid with a flashpoint between 100°F - 200°F is considered combustible. In general, the relative hazard of a flammable liquid increases as the flashpoint decreases.

Flammable Liquids Boiling Point, °C (1 atm) Flash Point, °C
acetone 56 -18
hexane 69 -7
pentane 36 -40
heptane 98.4 -4
water 100 N/A
ethyl ether 35 -45
ethanol (absolute) 78 12
ethanol (95%) 17
methanol 65 11
isopropyl alcohol 82 12