Question

When a wax candle is burned, the fuel consists of gaseous hydrocarbons appearing at the end of the candle wick. Describe the phase changes and processes by which the solid wax is ultimately consumed.

Short answer

The burning of a candle involves various phase changes where the solid wax is melted into a liquid by flame's heat, drawn up the wick, and then evaporated into a gas. The gaseous wax reacts with oxygen in a combustion process, producing heat, light, water vapor, and carbon dioxide. The wax, which is the fuel for the flame, is ultimately consumed in this process.

Step-by-step solution

Melting of the Wax

When the candle is lit, the heat from the flame melts the solid wax near the wick. This turns the solid wax into a liquid.

Transportation of the Wax

The molten wax is then drawn up the wick by capillary action. This process is similar to how a sponge soaks up water or how plants draw up water from the ground.

Evaporation of the Wax

Near the flame, the heat causes the liquid wax to completely vaporize or turn into a gas. This gaseous wax can now mix with oxygen in the air.

Combustion of the Wax

The flame heats the gaseous wax molecules to high temperatures causing them to react with oxygen in a chemical process known as combustion. This releases heat, light, water vapor, and carbon dioxide.

Consumption of the Wax

The wax, which was the fuel for the flame, is consumed in this process. The heat produced by the combustion maintains the flame, which in turn, continues to melt more wax, resulting in a self-sustaining process until all the wax is consumed.

Key concepts

Phase Changes in Wax

Understanding phase changes in wax is fundamental to grasping how a candle burns.
The transition from solid to liquid, and eventually to gas, reveals the fascinating interplay between temperature and physical states.

Initially, when you light a candle, the heat from the flame melts the wax around the wick. This is the first phase change, where solid wax becomes liquid as it hits its melting point. The liquid phase doesn't last long, though, as the close proximity to the flame rapidly heats the liquid wax. This heat provides enough energy for the molecules to break free from their liquid state in a process called evaporation, transforming into a wax vapor, which is the gaseous state.

This transition is key to the candle's ability to burn and demonstrates the classic physics principle of phase changes, where matter changes states—from solid to liquid to gas—when energy is applied or removed.

Capillary Action

Capillary action is a principle of physics and fluid dynamics that might seem like magic at first glance.
It allows the wax to move against gravity, climbing up the wick as if being pulled by an invisible force. But there's no magic here, just science.

Due to the combined effects of cohesion and adhesion—molecular forces of attraction—the liquid wax is drawn up the wick's small fibers. This is comparable to how plants draw water from their roots up to their leaves. In wicks, the process is fine-tuned so that the liquid wax ascends at just the right pace to fuel the flame without drowning it out or causing it to smoke excessively.

This phenomenon ensures that the fuel (melted wax) reaches the site of combustion efficiently, maintaining the consistency of the flame.

Chemical Reactions

Once the phase changes have occurred and the wax has traveled the wick through capillary action, it's time for chemistry to take center stage.
Chemical reactions are at the heart of a candle's glow. When the wax vapor reaches the flame, it undergoes a reaction with oxygen from the air. This is an example of a combustion reaction, specifically one that consumes hydrocarbons in the wax to release energy.

The heat from the flame provides the energy needed to break molecular bonds, allowing carbon in the wax to combine with oxygen, forming carbon dioxide, and hydrogen to form water vapor. This exothermic reaction releases a substantial amount of energy in the form of heat and light—what we recognize as the candle's flame.

This aspect underscores the role of chemistry in converting potential chemical energy stored in the wax into the observable energy that is emitted every time you light a candle.

Combustion Process

The combustion process is what keeps the candle flame alight, releasing energy through the interaction of fuel and an oxidizer — in this case, wax vapor and oxygen.
Despite its seemingly simple appearance, the combustion of wax in a candle flame is a complex event involving numerous reactions and resulting in light, heat, water vapor, and carbon dioxide. It's a delicately balanced sequence, where continuous heat from the flame ensures ongoing melting and vaporization of wax, hence sustaining the flame.

A candle's flame burns brightest where the oxygen supply is optimal, showing an outer blue region and a yellow, brighter area closer to the wick. This difference in colors reflects varying levels of heat and oxygen throughout the flame and the extent of the combustion reaction.

Law of Conservation of Mass

The law of conservation of mass is a fundamental concept in chemistry and physics that applies to the burning of a candle just as it would in any chemical reaction.
It states that mass cannot be created or destroyed in a closed system through ordinary chemical reactions or physical transformations. During the combustion of wax, the mass of the wax and the oxygen consumed equals the mass of the products (water vapor and carbon dioxide) and the energy released.

In essence, when you light a candle and it burns down, the wax doesn't disappear; it changes form, transforming into new substances that are released into the air. This principle helps explain why we don't have an accumulation of wax after a candle burns; it's not lost but converted. This illustrates the elegance and consistency of physical laws even in actions as mundane as the burning of a candle.