Why Do Candles Make Holes In The Middle?

Candles have been used for light and ambiance for centuries. The basic structure of a candle includes wax fuel, commonly paraffin wax, and a wick made from braided cotton threads that runs through the middle. When the candle is lit, the wick ignites and melts the surrounding wax which is then drawn up the wick via capillary action. This melted wax combusts in the flame providing continued fuel for burning. As the candle burns down, the flame melts the wax and travels down through the candle, leaving an empty space behind. This results in the unique hole in the middle of a burned candle.

The candle wick is the key to this process, acting as a continuous fuel source to keep the flame burning. The wick transports melted wax to the flame through capillary action so that the wax can vaporize and combust.

The Candle Wick

The wick is a crucial part of how a candle works. As the candle burns, the wick acts like a straw, drawing melted wax up to the flame through capillary action. This ensures that fresh fuel is continuously delivered to the flame. Without the wick, the melted wax would simply pool at the base of the candle and not reach the flame.

There are two main types of candle wicks – cotton and wood wicks. Cotton wicks are made from twisted strands of cotton fiber. They have a soft texture and tend to curl as they burn. Wooden wicks contain wood fibers and are stiffer than cotton wicks. Wood wicks make a crackling sound as they burn due to the wood fibers being consumed by the flame. Both materials are effective at drawing wax up to the flame through capillary action (Homesick). The type of wick impacts the size and brightness of the flame, as well as other characteristics like crackling sounds.

Melting of Wax

The wax used to make candles melts and liquefies when heated. Most candle waxes are made of hydrocarbons and melt between 120-200°F depending on the type of wax (https://nikura.com/blogs/discover/what-temperature-does-wax-melt). As the candle burns, the flame provides heat that melts the solid wax above it into liquid fuel. This liquefied wax is then drawn up the wick by capillary action to provide more fuel for the flame.

Paraffin wax, a common candlemaking material, melts between 115-145°F. Beeswax melts around 144-147°F. Soy wax, made from soybean oil, melts between 115-135°F. The variations in melting points depend on the different hydrocarbon molecules that make up each type of wax.

The melting wax is essential for candle burning. As the heat of the flame melts the solid wax, the liquid wax can travel up the wick to fuel the flame. This melting process sustains the candle’s burn until all the wax is used up.

Capillary Action

Capillary action refers to the ability of a liquid to flow against gravity in narrow spaces such as thin tubes, or in this case, the cotton fibers that make up a candle wick (Merriam-Webster, 2022). It occurs because of intermolecular attractive forces between the liquid and the walls of the tube or fibers. In a candle, the wax liquefies via the heat of the flame. The melted wax is then drawn up the wick through capillary action.

As the wax melts, it forms a concave meniscus around the wick. This happens because of the adhesive intermolecular forces between the liquid wax and the wick fibers. These forces pull the wax upward. The narrower the tube or fiber, the farther the liquid can travel upwards. This allows the melted wax to be transported through the wick and towards the flame where it can be vaporized and burned (Thoughtco, 2020).

Capillary action is essential for allowing the melted wax to travel up the wick. This brings fresh liquid fuel to the flame so that the candle can continue burning. Without capillary forces drawing wax up the wick, the flame would quickly extinguish once the immediately adjacent fuel was exhausted.

Combustion

Combustion is the process by which the wax vapor generated from the melted wax undergoes an exothermic chemical reaction with oxygen. At the top of the wick, where the temperature is the highest, the vaporized wax molecules combine with oxygen rapidly, releasing a significant amount of energy in the form of heat and light.

Specifically, the hydrocarbon chains that make up paraffin wax chemically react with oxygen to produce carbon dioxide, water vapor, and energy. This highly exothermic reaction reaches temperatures of around 1400°C at the top of the non-luminous zone of the flame.

The release of energy sustains the candle flame and keeps the wax hot enough to continue vaporizing. This combustion process is what powers the self-sustaining flame of a burning candle.

Heat Transfer

As the candle burns, the flame generates heat that melts the wax around the wick. This happens through the process of heat conduction. The heat from the flame is transferred to the adjacent solid wax, raising the temperature of the wax molecules until they reach their melting point of around 140-200°F. The liquid wax is then drawn up the wick via capillary action to fuel the flame. This creates a constant cycle of heat transfer from the flame melting solid wax into liquid fuel.

The intensity and temperature of the flame determines how much heat is conducted downwards to melt more wax. A hotter flame will transfer more thermal energy down the wick and melt wax faster. This efficient heat transfer results in a larger pool of liquid wax forming around the wick as more solid wax gets melted.

Additionally, convection currents in the air also transfer heat outwards from the flame to the solid wax surface. This supplemental heat helps melt wax not directly in contact with the wick itself. The combined heat conduction and convection is what allows candles to continuously melt wax to sustain the flame.[1]

Oxygen Access

For a candle flame to burn, it needs access to oxygen. As the candle wax melts and travels up the wick, it vaporizes into a gaseous fuel. This fuel then mixes with oxygen in the air surrounding the flame and undergoes combustion, releasing energy in the form of light and heat. The flame needs a constant supply of oxygen to sustain this combustion process.

This explains why the flame flickers when there is an insufficient supply of oxygen. Air currents in the surrounding environment can disrupt access to oxygen momentarily. The fluctuation in oxygen causes the flame’s intensity to weaken and strengthen, seen as a flicker. Enclosing the flame in a glass container limits air flow, which also reduces oxygen supply and makes the flame flicker and potentially go out.

The melted wax pool creates a cavity around the wick. This allows air to circulate and bring fresh oxygen to the base of the flame. Without this oxygen access to the wick, the combustion reaction could not occur and the flame would cease.

Sources:
https://brainly.com/question/24213015

Air Currents

Air currents play an important role in influencing the shape and size of a candle flame. As hot air from the flame rises, it pulls in cooler surrounding air towards the base of the flame. This influx of air provides the oxygen needed to sustain combustion. However, air drafts in the environment can disrupt this regular air flow and cause the flame to flutter or dance around [1].

A steady breeze or draft will bend the flame to one side as it draws in more air from that direction. This can make the flame appear tilted or angled. A stronger draft may cause the flame to wobble more violently as it struggles to draw in sufficient oxygen. In some cases, a draft may even blow out the flame by disrupting the air supply too much.

Candles placed near vents, windows, doors or fans are especially prone to air currents that affect the flame. But even small convection currents in a room can cause subtle movements in the flame. The flame shape and size essentially mirrors the patterns of air flow in its immediate vicinity.

Soot Deposits

As a candle burns, it produces soot, which is composed of very fine carbon particles resulting from the incomplete combustion of the candle wax. This soot floats up into the air and some of it gets deposited on surfaces above and around the candle flame. The hottest part of the candle flame is in the middle, which is why the melted wax gets drawn up here by capillary action through the wick. This central area burns the wax vapor most efficiently, resulting in the least soot production.

The coolest parts of the flame are on the outer edges and at the tip. More soot gets created in these cooler regions because the hydrocarbon fuel does not burn as completely here. As the soot rises, some of it gets captured in the stream of hot air above the central candle flame. This concentrated stream of hot air keeps the soot suspended above the wick, forming the hollow cavity in the middle of the flame. Some amount of soot inevitably escapes the hot air stream and floats to the edges, resulting in black soot deposits around and above the candle over time.

According to research by the Lead Education and Abatement Design Group, candle soot contains very fine particles that can deposit on surfaces like walls, carpets, and electronics. Candle soot is difficult to remove once it has settled onto household items and furniture.

Conclusion

So in summary, there are a few key scientific reasons why candles tend to burn and create a central hollow as they are used:

The candle wick provides a continuous source of fuel via capillary action, drawing melted wax up through the wick’s fibers. This melted wax is then vaporized by the heat of the flame.

The flame requires oxygen to burn, and draws in air from around the sides. This inward air flow helps shape the melted wax into a hollow cavity.

Soot and other byproducts of the combustion process also collect on the wick and edges of the melted wax pool. This carbon deposit eats away at the solid wax and contributes to the hollow.

Thus the need for oxygen, the capillary motion of the melted wax, and the accumulation of carbon deposits all work together to produce the characteristic central hollow in a burning candle.