Terminology
Section: Chemistry | Syllabus: Cambridge IGCSE Combined Science (0653)
Saturated and Unsaturated Compounds
- A saturated compound has molecules in which all the carbon-carbon bonds are single bonds
- Alkanes are the saturated hydrocarbons - only C-C and C-H single bonds are present
- An unsaturated compound has molecules in which one or more carbon-carbon bonds are not single bonds (i.e. they contain a C=C double bond)
- Alkenes are the unsaturated hydrocarbons - they contain at least one C=C double bond
Common Mistakes
- ❌ Describing a compound with a C=C double bond as "saturated"
- ✓ the presence of a C=C double bond is exactly what makes a compound unsaturated - "saturated" means single carbon-carbon bonds only
Homologous Series
A homologous series is a family of similar compounds with similar chemical properties.
The general characteristics of a homologous series are that its members:
- have the same general formula (you do not need to recall the specific general formula of any particular series)
- display a trend in physical properties (e.g. boiling point increases as chain length increases)
For example, alkanes and alkenes (covered later in this section) are each a homologous series in their own right - every alkane shares the same type of bonding and similar chemical reactions as every other alkane, and the same is true for alkenes.
Worked Example
- Question: a family of organic compounds all show similar chemical properties to one another, and their boiling points increase steadily as the molecules get larger. What term describes this kind of family? [1]
- Answer: a homologous series
Common Mistakes
- ❌ Thinking every member of a homologous series has the identical boiling point
- ✓ members show a gradual trend in physical properties (e.g. boiling point increasing with chain length) - they do not all have identical values
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