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Chapter 1: Problem 6

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

Expert verified
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

01

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.
02

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.
03

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.

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