Question

Methane, ethane and propane are said to form a homologous series because all are:
a.) Hydrocarbon
b.) Saturated compounds
c.) Aliphatic compounds
d.) Differ from each other by a $C{H}_2$ group

Answer 1

Hint: In order to deal with this question first we will define the term homologous series further we will discuss its function and with the help of example as alkanes we will get our required answer. Homologous series: A sequence of compounds having the same functional group and also having similar chemical properties in which the number of the series can be branched or unbranched is called a homologous series.
For example, the main straight-chained alcohol sequence has hydroxyl at the end of the carbon chain. These properties usually change slowly over the sequence, and the variations can also be clarified by simple differences in molecular size and density.

Complete step by step answer:
The homologous series of straight-chained alkanes begins from methane $(C{H}_4)$, ethane$(C_2{H}_6)$ , propane $(C_3{H}_8)$, butane $(C_4{H}_8)$, and pentane $(C_5{H}_{12})$. In that series, successive members differ in mass by an extra methylene bridge $(-C{H}_2-)$unit inserted in the chain. Thus, the molecular mass of each member differs by 14 atomic mass units. Adjacent members in such a series, such as methane and ethane, are known as "adjacent homologues".
Therefore, Methane, ethane and propane are all alkanes with the general formula $(C_n{H}_{2n+2})$. They only differ by a $(C{H}_2)$ group.
So, the correct answer is “Option D”.

Note: Hence, there are some characteristics of the homologous series as follows-
The members of the homologous series are having a common general method of preparation.
The functional group for each member of the homologous series are the same.
There is a same general formula for each member of the series.

Due to the same functional group the chemical properties of the members are almost the same.
Also sometimes used is the term "homologous sequence" for any set of compounds that have identical structures that have the same functional group, such as general alkanes (straight and branched), alkenes (olefins), carbohydrates, etc. If the members cannot be ordered by a single parameter in a linear order, though, the set may best be called a "chemical family" or "class of homologous compounds" rather than a "list".