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The Street Chemist - Part 19

In the previous “Street Chemist” we discussed the first of several combustion process physical characteristics boiling point. Factors affecting boiling point include molecular weight, polarity, and branching.In the previous "Street Chemist" we discussed the first of several combustion process physical characteristics boiling point. Factors affecting boiling point include molecular weight, polarity, and branching. Molecular weight was discussed in Parameters of Combustion Part I. This edition of "Street Chemist" will discuss the remaining two effects on boiling point, polarity and branching.



Hydrocarbon derivatives are compounds with other elements in addition to hydrogen and carbon. Weight will still determine boiling points when comparing hydrocarbon derivatives within the same family. However, when comparing different families, the concept of polarity has to be considered with some of the compounds.

Polarity is the second factor that affects boiling point. There is a rule in chemistry that says, "like materials dissolve like materials." Materials that are polar will mix together. Another term used when materials mix together is miscibility. If a material is miscible, it will mix with another. If a material is immiscible, it will not mix with another. All polar compounds are alike in terms of being polar. Therefore, polar compounds are soluble in polar compounds.

One of the main reasons polarity is discussed in terms of emergency response is because of foam used for fighting flammable liquid fires. Two general types of foam are used to extinguish flammable liquid fires: hydrocarbon foam and polar-solvent foam, which is sometimes referred to as alcohol-type foam. The reason that different types of foam are necessary for flammable liquid fires is polarity. Water is a polar compound. Because "like dissolves like", water is miscible with most polar solvents. The main ingredient of firefighting foam is water. Foam is a mixture of foam concentrate, water, and air. If regular foam is put on a polar-solvent liquid such as alcohol, the water will be removed from the foam by the polar solvent and the foam blanket will break down. To effectively extinguish fires with polar solvents, it is necessary to use polar-solvent or alcohol type foam.

Polarity is presented here to discuss the types of flammable liquids that are polar solvents and the effect that polarity has on them. Polar solvents, such as alcohols, aldehydes, organic acids, and ketones, require special foam to extinguish fires. Choosing the right type of foam is part of the process of effectively managing a flammable-liquid incident. Polar liquids tend to have higher boiling points than non-polar liquids. Polarity is said to have the effect of raising the boiling point of a liquid. There are two types of structure that represent polarity in hydrocarbon derivative compounds: the carbonyl structure and hydrogen bonding.

A carbonyl contains a carbon-to-oxygen double bond. Most double bonds are reactive; however, with the carbonyl family, except for aldehyde, the double bond is protected by hydrocarbon radicals on either side of the carbonyl. This prevents oxygen from getting to the bond and breaking it. Double bonds are stable within the carbonyl families, except for aldehydes. The carbonyl structure is shown in the following illustration, first by itself, and then in the hydrocarbon functional groups: ketone, aldehyde, organic acid, and ester. All four compounds are polar because of the carbonyl, and the organic acid also has a hydrogen bond. The amount of polarity is generally the same between the ketone, ester, and aldehyde. The polarity of an acid is higher because of the hydrogen bond and the carbonyl in the same compound, like a double dose of polarity.

When oxygen and hydrogen covalently bond, the bond is very polar. Water has a hydrogen-oxygen bond that gives water polarity. It is well known that water is a liquid between 32