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) isdeposited on the metal. Which of these occurrences are due to physical changes, and which are due to chemical changes?

Short answer

Observations (a) and (d) are due to chemical changes, as burning and formation of soot involve the formation of new substances. Observations (b) and (c) are due to physical changes, as the metal getting warmer and water condensing are changes in state, appearance, or location without altering the chemical composition.

Step-by-step solution

Observation (a): The match burns

Burning is a process in which a substance reacts with oxygen to give out heat and light. This is known as combustion, which is a chemical change. During the burning of the match, the chemicals in the match react with oxygen to produce new substances like carbon dioxide, water, and heat. So, observation (a) is due to a chemical change.

Observation (b): The metal gets warmer

When the match burns, it releases heat. This heat is transferred to the cold piece of metal, causing it to become warmer. The metal's increase in temperature is a result of heat transfer and not a change in its chemical composition. Therefore, observation (b) is due to a physical change.

Observation (c): Water condenses on the metal

The combustion of the match produces water vapor as one of its products. As the hot water vapor comes into contact with the cold metal surface, it cools down and condenses back into liquid water. Condensation is a change in the state of matter, from gas to liquid, and does not involve any changes in the chemical composition. Hence, observation (c) is due to a physical change.

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

During the burning or combustion process of the match, carbon is produced as one of the by-products. The carbon in the form of soot then deposits on the metal surface. As carbon is a new substance produced as a result of a chemical reaction, observation (d) is due to a chemical change. In conclusion, observations (a) and (d) are due to chemical changes, while observations (b) and (c) are due to physical changes.

Key concepts

Combustion Reaction

When you light a match, you're witnessing a combustion reaction in action. Combustion is a chemical process where a substance, like the chemicals in a match, reacts with oxygen from the air. This reaction releases heat and light, and it produces new substances. In the case of a match burning, the main products are carbon dioxide and water. It's important to note that combustion is always a chemical change because new substances are created during the process. The original materials, such as the chemicals in the match, are transformed into different compounds entirely. Therefore, the burning of a match is a classic example of a combustion reaction.

Heat Transfer

In the process of lighting a match and holding it under a piece of metal, you can observe heat transfer in action. When the match burns, it releases heat. This heat is transferred from the flame to the cooler metal, causing the metal to become warmer. The increase in the metal's temperature is not due to any change in its chemical makeup. Instead, when heat flows from one object to another, it's simply moving energy around. This is known as heat transfer, a physical change. It doesn't alter the identity of the metal; it just changes its temperature.

Condensation

When water vapor from the burning match comes into contact with the cold metal, you might notice water droplets forming. This process is called condensation. What's happening here is a change in the state of matter from gas to liquid. The hot water vapor cools down upon touching the metal and turns back into liquid water. This is a physical change because the chemical structure of water vapor doesn't change during this process. It's simply transitioning between two physical states: gas and liquid.

Soot Formation

During the combustion of the match, not all carbon combusts completely. Some carbon forms tiny black particles known as soot. These soot particles can be seen as they get deposited onto the metal surface. Since soot is formed as a by-product of the combustion process, it involves a chemical change. The creation of soot represents a new substance resulting from the incomplete combustion of carbon in the match. While it may seem like just a minor detail, soot formation highlights the complexity of chemical reactions, illustrating that not all reactions proceed perfectly or cleanly.