Few things feel as cosy as the soft flicker of candlelight on a winter evening. The gentle glow seems simple, but behind every flame lies a fascinating fusion of chemistry and physics.
Ingredients in a candle
At first glance, a candle seems uncomplicated. Most are made of wax, a cotton wick and maybe some fragrance oils to give it a pleasant smell.
The wax is typically made up of hydrocarbons, which are long chains of carbon and hydrogen atoms. The most popular type of wax used in candles is paraffin wax, which is made of hydrocarbons containing 20—40 carbon atoms and 42—82 hydrogen atoms.
Igniting the reaction
When you burn a candle wick, the heat melts some of the wax near the flame. This liquid wax then travels upward through the wick by capillary action.
Capillary action is the ability of a liquid to flow through a narrow space, against gravity. This is the same process that allows trees to pull water from their roots to their leaves.
On arrival to the flame, the heat then vaporises the wax, turning it into a gas. It’s this wax vapour, not the solid or liquid wax, that actually burns.
Combustion in action
Combustion is an oxidation reaction, which is a type of reaction where a fuel reacts with oxygen to release energy. For a candle, that reaction can be summarised as:
CnH2n+2+ O2 → CO2 + H2O + energy
The hydrocarbon wax vapour (CnH2n+2) is the fuel that reacts with oxygen (O2) to produce carbon dioxide (CO2), water vapor (H2O), and energy. The energy is what we see as light and feel as heat.
However, the process of a candle burning isn’t so perfectly clean or simple. The flame has distinct zones, where different stages of combustion occur.
The blue zone
At the very base of the flame, closest to the wick, is the blue zone. Here, wax vaporises from the wick and meets a plentiful supply of oxygen. As oxygen is abundant, complete combustion occurs, mostly producing carbon dioxide and water.
The yellow (or luminous) zone
Above the blue zone is the bright yellow part of the flame. In this middle region, oxygen is less abundant and incomplete combustion occurs. This produces tiny carbon particles (soot), which are then heated to incandescence, producing the warm golden light we associate with candle flames.
The dark inner zone
Closest to the wick is the dark inner zone, often appearing nearly invisible. Here, wax vaporises but hasn’t yet ignited. This region is rich in unburned hydrocarbons and contains relatively little oxygen. It essentially acts as a fuel reservoir for the flame above it.
So, the next time you light a candle, you’ll know that behind its calm glow is a surprisingly complex interplay of physics and chemistry—an elegant little engine turning wax into warmth and light.
Festive quiz
Last question solution
b) Peru
Your next seasonal question
Which SI unit is based on a “standard candle”, which was a candle used in older measurement systems?
a) Candela (cd)
b) Kelvin (K)
c) Mole (Mol)
Read the next article to find out!
Subscribe to updates
Like what you read? Sign up for news and updates like this.