Problem 22 - Chemistry: The Central Science in SI Units, Global Edition

Question

A match is lit to light a candle. The following observations are made: (a) The candle burns. (b) Some wax melts. (c) Melted wax solidifies on the candleholder. (d) Soot (carbon) is produced by the burning of the match and the candle. Which of these occurrences are due to physical changes, and which are due to chemical changes?

Step-by-Step Solution

Answer

Physical changes: (b) Wax melting and (c) Melted wax solidifying on the candleholder. Chemical changes: (a) Candle burning and (d) Soot (carbon) production.

Step 1: Observation (a): Candle Burns

When the candle burns, it is a chemical reaction taking place. The wax in the candle combines with the oxygen in the air to produce carbon dioxide, water, and heat. This is a process known as combustion. Since this process involves a change in the substance's chemical composition, it is a chemical change.

Step 2: Observation (b): Wax Melts

When wax melts, its state changes from solid to liquid, but its underlying chemical composition remains the same. In this case, the molecules of wax are just moving more freely, rather than changing their nature. This is a physical change, as there is no change in the chemical composition of the substance.

Step 3: Observation (c): Melted Wax Solidifies on the Candleholder

When the melted wax cools down and solidifies, it changes from its liquid state back to a solid state. Again, this does not involve any chemical reaction, as the composition of the wax remains the same. Therefore, this observation represents a physical change.

Step 4: Observation (d): Soot (Carbon) is Produced

As the match and the candle burn, soot (carbon) is produced, which is a result of incomplete combustion. In this process, the chemical composition of the original substances (match, candle, and oxygen) is changed, and new substances (soot, carbon dioxide, and water) are formed. Hence, this observation represents a chemical change. In conclusion: Physical changes: (b) Some wax melts, and (c) Melted wax solidifies on the candleholder. Chemical changes: (a) The candle burns, and (d) Soot (carbon) is produced.

Key Concepts

Combustion Reaction

When you light a candle, the process you're observing is a combustion reaction. Combustion is a type of chemical reaction where a substance combines with oxygen to release energy. This can be in the form of light, heat, or both. In the case of a candle, the wax (often made of hydrocarbons) reacts with oxygen in the air. This reaction produces carbon dioxide and water as by-products.
However, not only are new substances produced, but energy is also released, which is what allows the candle to emit light and warmth. Because the molecules in the wax are changing into entirely new products, this is classified as a chemical change.
Combustion reactions are typically characterized by the presence of a flame, indicating the release of energy.

State Change

State changes involve changing the physical form of a substance without altering its chemical composition. When wax melts, it shifts from a solid to a liquid state. This happens because heat causes the wax molecules to gain energy and move more freely.
Similarly, when melted wax cools and solidifies, the reverse occurs. The wax loses energy, and the molecules arrange themselves back into a solid form. These transformations are examples of physical changes.

• No new substances are formed during a state change.
• The process is reversible, like how melted wax can solidify again when cooled.

Chemical Composition

The chemical composition of a substance refers to the types and arrangements of atoms within the molecule. In the context of the candle burn, when wax undergoes a combustion reaction, it interacts with oxygen in the air.
As a result, the chemical composition of wax changes because the atoms rearrange to form new substances such as carbon dioxide and water. This type of change reflects a chemical change, indicating the breaking and forming of chemical bonds.
Understanding chemical composition is crucial in identifying whether a change is physical or chemical. Here, a change in composition signals a chemical process.

Incomplete Combustion

Incomplete combustion happens when there isn't enough oxygen available for a fuel, like wax or wood, to completely convert to carbon dioxide and water.
Instead of turning the wax fully into its combustion products, you might get carbon in the form of soot or other by-products. The formation of soot when burning a candle is an example of incomplete combustion.

• This process can result in less energy being released compared to complete combustion.
• It leads to the production of pollutants like carbon monoxide and soot.
• Incomplete combustion is a chemical change because the chemical composition of the burning material changes.