Edexcel GCSE Chemistry (1CH0) — Qualitative Analysis, Hydrocarbons and Polymers
Think about the fuel inside a camping gas stove—it burns incredibly cleanly because of its simple, two-element chemical makeup. An alkane is a specific type of organic compound classified as a hydrocarbon. This means it is a molecule made up of carbon and hydrogen atoms only. There are absolutely no other elements, such as oxygen or nitrogen, present in the chemical structure.
The word saturated describes the specific type of bonding within the molecule. In an alkane, every carbon atom forms exactly four single covalent bonds. They are described as saturated because they contain only single carbon-carbon bonds (), and feature absolutely no carbon-carbon double bonds ().
Because there are only single bonds present, there are no reactive double bonds to "open up". As a result, the carbon skeleton is bonded to the maximum possible number of hydrogen atoms. This means no more hydrogen atoms can be added to the molecule without breaking the existing carbon chain. Because of this stable, saturated nature, alkanes do not react with bromine water, leaving the testing solution its original orange/brown colour.
Alkanes belong to a homologous series. This is a family of organic molecules that share the same general formula and exhibit very similar chemical properties. They also show a predictable, gradual trend in their physical properties, such as their boiling points increasing steadily as the carbon chains become longer.
The general formula for the alkane family allows you to calculate the exact number of hydrogen atoms for any given number of carbon atoms ():
How many hydrogen atoms are in an alkane molecule that contains 8 carbon atoms?
Step 1: State the general formula for an alkane.
Step 2: Substitute the number of carbon atoms () into the formula.
Step 3: Calculate the final number.
Students often incorrectly state that a hydrocarbon contains carbon and hydrogen 'molecules' — you must use the word 'atoms' to describe its components accurately.
In Edexcel mark schemes, failing to include the word 'only' (or 'solely') when defining a hydrocarbon will almost always guarantee zero marks for the definition.
For full marks when explaining why an alkane is saturated, you must provide two specific points: state that it contains only single carbon-carbon bonds AND that no more hydrogen atoms can be added.
If asked to distinguish alkanes from alkenes using a chemical test, remember that saturated alkanes cannot undergo addition reactions, so they will leave bromine water orange/brown.
Alkane
A homologous series of saturated hydrocarbons with the general formula CₙH₂ₙ₊₂.
Hydrocarbon
A chemical compound consisting of hydrogen and carbon atoms only.
Atoms
The smallest individual particle of a chemical element, such as carbon or hydrogen, that makes up a molecule.
Saturated
An organic molecule containing only single carbon-carbon covalent bonds, meaning it holds the maximum possible number of hydrogen atoms.
Single covalent bonds
A strong chemical bond formed when two non-metal atoms share exactly one pair of electrons.
Homologous series
A family of organic compounds that share the same general formula, similar chemical properties, and a clear trend in physical properties.
Put your knowledge into practice — try past paper questions for Chemistry
Alkane
A homologous series of saturated hydrocarbons with the general formula CₙH₂ₙ₊₂.
Hydrocarbon
A chemical compound consisting of hydrogen and carbon atoms only.
Atoms
The smallest individual particle of a chemical element, such as carbon or hydrogen, that makes up a molecule.
Saturated
An organic molecule containing only single carbon-carbon covalent bonds, meaning it holds the maximum possible number of hydrogen atoms.
Single covalent bonds
A strong chemical bond formed when two non-metal atoms share exactly one pair of electrons.
Homologous series
A family of organic compounds that share the same general formula, similar chemical properties, and a clear trend in physical properties.
