Problem 20 - 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

In summary, observations (a) and (d) are chemical changes because they involve the formation of new substances, while observations (b) and (c) are physical changes since they only involve changes in state or temperature without forming new substances.

Step 1: Observation (a): The match burns

Burning is a process that involves breaking and forming new chemical bonds. When the match burns, the wooden material reacts with oxygen in the air to produce flames and release heat. This is a chemical change because a new substance is formed, and energy is being released during the process.

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

As the match burns, it releases heat energy which transfers to the metal. There is no change in the composition of the metal. Thus, this is a physical change because it involves a change in the metal's temperature, but no new substance is formed.

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

The cold metal causes the water vapor present in the air to lose heat and condense into water droplets. This is a change in the state of water from vapor to liquid but doesn't involve any conversion into a new substance. Therefore, this is a physical change.

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

Soot, or carbon, is produced as a result of the incomplete combustion of the wooden material of the match. The deposit of soot on the metal indicates that a new substance was formed due to the burning of the match. This is a chemical change because of the formation of a new substance. As a result, observations (a) and (d) involve chemical changes, and observations (b) and (c) involve physical changes.

Key Concepts

Burning Process

The burning of a match is an interesting and complex process, but it can be understood in simple terms. When you strike a match, the heat generated from the friction increases the temperature enough for the chemicals in the match head to react with oxygen in the air. This reaction is known as combustion. It ignites the flammable materials in the match, primarily made of wood and coated with chemicals like phosphorus.

During the burning process:

• Chemical bonds in the match head break and new bonds form, creating new substances like ash and smoke.
• This reaction releases energy in the form of heat and light, which is why we see a flame.
• The main components produced include carbon dioxide, water vapor, and sometimes soot (tiny carbon particles).

The burning of the match is a clear example of a chemical change, as the original substances are transformed into different materials.

Heat Transfer

Heat transfer occurs when thermal energy moves from one place to another due to a temperature difference. In the case of our burning match, the heat generated from the flame naturally transfers to the surrounding air and objects, such as the cold piece of metal.

There are three methods through which heat can be transferred:

• Conduction: Direct contact allows heat to transfer from the hot object (the match) to the cooler one (the metal).
• Convection: Occurs when heat circulates through fluids—usually air or liquid—making the surrounding air warmer.
• Radiation: Heat is emitted in the form of rays or waves that travel through space, not requiring a medium.

For our burning match, conduction is primarily responsible for making the metal warm. This physical change doesn’t alter the metal itself; it only increases its temperature.

Condensation

Condensation is a physical process where gas turns into a liquid. This happens when water vapor in the air cools down and loses energy, condensing into liquid water. In the context of our exercise, when the warm water vapor from the air comes into contact with the cold metal surface, it loses heat and turns into droplets of water.

This transformation can be described as:

• Temperature decrease causes the water molecules to slow down.
• Molecules come closer due to loss of energy, changing from the gaseous to the liquid phase.
• No new substances are formed during this process.

Therefore, condensation is purely a physical change as it involves state change without altering chemical composition.

Combustion Reactions

Combustion reactions are chemical reactions where a substance combines with oxygen and releases energy in the form of light or heat. In our scenario, combustion occurs when the wooden material of the match reacts with oxygen, resulting in a burning process.

Key aspects of combustion reactions include:

• They require oxygen and a fuel source—like the wood in a match.
• They produce heat and light, evidenced by the match flame.
• Byproducts like carbon dioxide, water vapor, and sometimes soot are formed.

The incomplete combustion of wood can lead to the production of soot, a form of carbon, which gets deposited as tiny particles. This formation of new substances characterizes the combustion reaction as a chemical change.