When a candle burns, both physical and chemical changes occur, each identifiable by specific characteristics. Here's a breakdown of the changes and reasons that indicate which are physical and which are chemical:
Physical Changes:
-
Melting of Wax:
- Observation: When the candle is lit, the solid wax melts and turns into liquid.
- Reason: This is a physical change because it involves a change of state from solid to liquid. The chemical composition of the wax remains unchanged; only its physical form changes.
-
Solidification of Wax:
- Observation: When the candle is extinguished, the melted wax cools and solidifies again.
- Reason: This is another physical change, as the wax changes back to solid form without altering its chemical structure.
-
Change in Size and Shape:
- Observation: As the candle burns, it gets shorter and may change in shape due to melting.
- Reason: The reduction in size is a physical change since the substance (wax) is still present; it is just in a different physical state and quantity.
Chemical Changes:
-
Combustion of Wax:
- Observation: The wick ignites, and the wax vaporizes and burns, producing light, heat, carbon dioxide, and water vapor.
- Reason: This is a chemical change because new substances are formed (carbon dioxide and water), and the original substances (wax and oxygen) cannot be easily reverted back to their original form. This process involves a chemical reaction (combustion) where energy is released.
-
Formation of Soot:
- Observation: Incomplete combustion can produce soot (carbon particles).
- Reason: The formation of soot indicates a chemical change because it results from the breakdown of wax molecules during combustion. This again produces different substances, which cannot revert to the original wax.
Key Characteristics:
-
Physical Changes:
- Do not change the chemical identity of the substance.
- Often reversible (e.g., melting and freezing).
- Typically involve changes in state or appearance (e.g., size, shape).
-
Chemical Changes:
- Change the chemical identity of the substance, producing new substances.
- Usually not reversible or very difficult to revert (e.g., combustion).
- Often associated with energy changes (e.g., heat, light produced during burning).
In summary, the burning of a candle primarily illustrates chemical changes through the combustion process, while the melting and solidification of the wax are physical changes, highlighting the distinct characteristics that differentiate the two types of changes.