Gradation in a homologous series

The idea

In a homologous series, melting and boiling points rise with molecular mass and other physical properties such as solubility show a similar gradation, while chemical properties, determined solely by the functional group, remain similar.

Going down the list

In a homologous series each member has a greater molecular mass than the one before it.

What do you expect to happen to the melting and boiling points as the molecular mass increases?

• They increase
• They decrease
• They stay exactly the same

Changing or similar?

Sort each property.

Gradation and sameness

Physical properties such as melting point change gradually as the molecules get bigger. Chemical properties stay the same because the functional group is the same.

In a homologous series, melting and boiling points rise with molecular mass and other physical properties such as solubility show a similar gradation, while chemical properties, determined solely by the functional group, remain similar.

Notes

In a homologous series, melting and boiling points rise with molecular mass and other physical properties such as solubility show a similar gradation, while chemical properties, determined solely by the functional group, remain similar.

Check yourself

What happens to the melting and boiling points as the molecular mass increases in a homologous series?

What can be said about the chemical properties of the members of a homologous series?

What determines the chemical properties in a homologous series?

Two members of a series differ in melting point but react in a similar way. Why?

• They increase — correct. Yes!
• They decrease. The book says melting and boiling points increase with increasing molecular mass.
• They do not change. A gradation is seen: melting and boiling points increase with increasing molecular mass.
• They change at every step. Chemical properties are determined solely by the functional group, so they remain similar.
• They remain similar — correct. Yes!
• They get stronger as the chain grows. The chemical properties remain similar, since the functional group is the same.
• The molecular mass. Molecular mass changes the physical properties, such as melting and boiling points. Chemical properties are determined solely by the functional group.
• The length of the carbon chain. The functional group decides the properties regardless of the length of the carbon chain.
• The functional group — correct. Yes!
• Physical properties change with molecular mass, but the functional group is the same — correct. Yes!
• They contain different functional groups. Members of a series have the same functional group, and that is why their chemical properties are similar.
• Their molecular masses are equal. Different members have different molecular masses, which is why their melting points differ.