Question

If you place a glass rod over a burning candle, the glass appears to turn black. What is happening to each of the following (physical change, chemical change, both, or neither) as the candle burns? Explain each answer. a. the wax b. the wick c. the glass rod

Short answer

The wax undergoes both physical and chemical changes: physically, the solid wax melts into a liquid state; chemically, the wax's molecules are broken into smaller molecules, such as carbon dioxide and water vapor, during combustion. The wick also undergoes both types of changes: physically, the wick gets shorter; chemically, its cellulose fibers undergo pyrolysis and react with oxygen to produce byproducts. The glass rod experiences a physical change, as it appears black due to the deposition of soot but can be returned to its original appearance by wiping it off.

Step-by-step solution

a. The wax

When a candle burns, the wax undergoes both physical and chemical changes. The physical change occurs as the solid wax melts into a liquid state due to the heat from the flame. The liquid wax is then drawn up the wick via capillary action, where it undergoes a chemical change. The heat from the flame breaks the wax's hydrocarbon molecules into smaller molecules, such as carbon dioxide and water vapor, which are released as gases. This process, called combustion, is a chemical change.

b. The wick

The wick also undergoes both physical and chemical changes as the candle burns. Physically, the wick gets shorter as it is consumed by the flame. Chemically, the wick serves as a medium for the liquid wax, drawing it up to the flame where combustion occurs. The wick itself also undergoes a chemical change, as the heat from the flame causes the pyrolysis (thermal decomposition) of the wick's cellulose fibers into smaller molecules. These decomposed molecules react with oxygen and produce carbon dioxide, water vapor, and other byproducts, which is a chemical change.

c. The glass rod

When a glass rod is placed over a burning candle, it appears to turn black due to the deposition of soot (unburned carbon particles) formed during the incomplete combustion of the wax and the wick. This darkening of the glass rod is a physical change, as the glass itself does not undergo a chemical change. The process is reversible – the soot can be wiped away, and the glass rod would return to its original appearance.

Key concepts

Combustion

Combustion is a chemical process that occurs when a substance reacts with oxygen, releasing energy in the form of heat and light. This process is a fundamental concept in understanding how candles burn. In the candle scenario, combustion involves the wax and the wick, both composed of hydrocarbon molecules. These molecules decompose into smaller substances like carbon dioxide and water vapor during combustion. The candle flame provides the necessary heat energy to sustain these reactions.

Key points regarding combustion:

• It requires oxygen to occur.
• It releases energy, usually seen as a flame.
• Both the wax and wick undergo combustion in a burning candle.

Recognizing combustion as a chemical reaction helps us understand why the products, like water vapor and carbon dioxide, are different from the initial materials.

Physical Change

When we talk about physical changes, we refer to changes that alter the form or appearance of a material without changing its chemical identity. In the case of a candle burning, the most evident physical change occurs when the solid wax turns into a liquid due to the heat from the flame. This melting of wax is purely a physical change, as it does not change the chemical structure of the wax itself.

Key aspects of physical changes:

• No new substances are formed.
• The change can often be reversed (e.g., liquid wax solidifies again when cooled).
• Only physical states change, such as from solid to liquid.

Understanding physical changes helps distinguish between processes that are reversible and those that involve chemical transformations.

Chemical Change

Chemical changes result in the formation of one or more new substances and involve the breaking and forming of chemical bonds. In burning a candle, both the wax and the wick undergo chemical changes. The heat from the flame initiates these reactions by breaking down hydrocarbon molecules into smaller compounds like carbon dioxide and water vapor. Such changes are a defining characteristic of combustion.

Important features of chemical changes:

• Energy is either absorbed or released, usually in the form of heat or light.
• New chemical substances are formed with different properties.
• These changes are often irreversible without another chemical reaction.

Comprehending chemical changes is crucial to understanding how matter transforms in nature and various chemical processes.

Incomplete Combustion

Incomplete combustion occurs when there is not enough oxygen to allow the fuel to react completely to form carbon dioxide and water. Instead, this process results in the formation of soot, carbon monoxide, or other carbon-rich compounds. In the case of a candle, incomplete combustion happens when there isn't sufficient oxygen reaching the burning wick.

Key features of incomplete combustion:

• Less energy is released compared to complete combustion.
• Produces soot and other potentially harmful byproducts like carbon monoxide.
• Occurs due to limited oxygen availability.

Recognizing the signs of incomplete combustion, like soot deposition, helps understand combustion efficiency and safety aspects.

Soot Deposition

Soot deposition occurs when tiny black carbon particles settle on surfaces. It is often a product of incomplete combustion. When a glass rod is placed over a burning candle, it picks up soot produced from the candle flame. This deposition is purely a physical change because the soot can be easily wiped off, restoring the glass to its original state.

Key characteristics of soot deposition:

• It is a physical deposition of unburnt carbon particles.
• The process is reversible (e.g., soot can be cleaned away).
• Soot is typically produced when combustion isn't complete.

Understanding soot deposition helps identify the efficiency of burning processes and manage potential health and cleaning concerns.