Problem 22 - Chemistry: The Central Science

Question

A match is lit and held under a cold piece of metal. The following observations are made: (a) The match burns. (b) The metal gets warmer. (c) Water condenses on the metal. (d) Soot (carbon) is deposited on the metal. Which of these occurrences are due to physical changes, and which are due to chemical changes?

Step-by-Step Solution

Answer

Observations (a) and (d) are chemical changes as they involve the formation of new substances. In (a), the match burns, forming water vapor and carbon dioxide, and in (d), soot (carbon) is deposited on the metal. Observations (b) and (c) are physical changes, as the metal getting warmer and water condensing on the metal do not involve the formation of new substances.

Step 1: Observation (a): The match burns

Burning is a chemical change as it involves the formation of new substances. In this case, when the match burns, it reacts with oxygen in the air and forms water vapor and carbon dioxide, both of which are new substances with different properties from the original match.

Step 2: Observation (b): The metal gets warmer

The metal getting warmer is a physical change. This is because no new substances are formed, and the changes in temperature are only temporary. The identity of the metal remains the same, it just has more energy in the form of heat.

Step 3: Observation (c): Water condenses on the metal

Water condensation on the metal is a physical change. This is because the water is simply changing states (from gas to liquid) due to the temperature difference between the metal and the surrounding air. The identity of the water does not change in this process.

Step 4: Observation (d): Soot (carbon) is deposited on the metal

Soot deposition is a chemical change. During the combustion of the match, carbon is produced as a byproduct, and this is deposited onto the metal. Carbon is a new substance with different properties from the original match and oxygen in the air, thus, it is a chemical change. In summary, Observations (a) and (d) are chemical changes, while Observations (b) and (c) are physical changes.