Hydrocarbons are compounds composed of carbon and hydrogen atoms. Alkanes are hydrocarbons. As the number of carbons increases in straight-chain alkanes, there is a steady gradation of properties which can be easily compared and predicted. The properties of branched alkanes vary considerably and are hard to predict because of other intervening forces that come into play. The boiling points of a few alkanes are given below:
The melting point of butane is close to:
|A|| 37.5o C|
|B|| 55.1o C|
|C|| 24o C|
|D|| –138o C|
Ans. D To answer this question you have to do some intuitive thinking based on the information given in the passage. Butane is a four-carbon alkane with the formula C4H10. It is a bigger molecule than propane, but a smaller molecule than pentane. So the boiling point of butane should be between propane and pentane. We are talking in terms of the relative boiling points. But the question asks for the melting point. So let's translate this idea in terms of melting point. Obviously, the melting point of a substance should be below its boiling point. So the answer cannot be either 37.5oC or 55.1oC. This rules out Choices A and B. Choice C is not a sensible answer because butane is a gas at 25oC.