Question

Which of the following is a chemical change? Explain your reasoning: (a) boiling canned soup; (b) toasting a slice of bread; (c) chopping a log: (d) burning a log.

Short answer

Burning a log is a chemical change because it forms new substances.

Step-by-step solution

Understanding Physical and Chemical Changes

Define and understand the difference between physical and chemical changes. Physical changes do not result in the formation of new substances (e.g., state changes like melting or boiling). Chemical changes result in new substances being formed (e.g., burning, rusting).

Evaluating Boiling Canned Soup

Boiling canned soup mainly changes its temperature and state (liquid to gas). This is a physical change because no new substances are formed.

Evaluating Toasting a Slice of Bread

Toasting bread results in browning and the formation of new compounds, such as when starches convert into different molecules under heat, causing a change in color, texture, and flavor. This is a chemical change.

Evaluating Chopping a Log

Chopping a log means changing its size and shape but the wood remains chemically the same. This is another example of a physical change.

Evaluating Burning a Log

Burning a log involves combustion, where the wood reacts with oxygen to form new substances such as carbon dioxide, water vapor, and ash. This is a chemical change.

Identifying the Chemical Change

Considering the new substances formed, toasting a slice of bread and burning a log are chemical changes. Burning a log is the most definitive example of a chemical change as it is easily identifiable through the creation of new substances.

Key concepts

Physical Change

To understand physical changes, we need to know that they do not involve creating new substances. Instead, these changes might affect the shape, size, or state of a material.
For example, melting ice, boiling water, or chopping a log are physical changes. In each case, the materials (ice, water, wood) remain chemically the same.
Here are some key points about physical changes:

• No new substances are formed.
• The change is usually reversible.
• The state of the substance might change (solid, liquid, gas).
• Typical examples include melting, freezing, condensation, and breaking objects.

Chemical Reaction

A chemical reaction happens when substances interact and form new substances with different properties.
During these reactions, the original materials undergo a chemical change.
Key signs of a chemical reaction could include:

• Color change
• Formation of a gas (bubbles)
• Formation of a precipitate (solid)
• Energy change (heat, light)

For instance, toasting bread is a chemical reaction. The heat transforms the starches in the bread into new molecules, which alters its flavor, color, and texture.
Burning a log is another chemical reaction. The wood reacts with oxygen, producing new substances like carbon dioxide, water vapor, and ash.

Combustion

Combustion, also known as burning, is a specific type of chemical reaction. It is a process in which a substance combines with oxygen, releasing energy in the form of light and heat.
This reaction typically results in new products, such as carbon dioxide and water.
Key features of combustion include:

• Combustible material (like wood or gasoline) reacts with oxygen.
• Energy is released, often as heat and light (flames).
• New substances are formed (for example, ash, carbon dioxide).

An everyday example of combustion is burning a log in a fireplace. The initial material, wood, combines with oxygen, releasing energy that produces heat and light. The combustion process leaves behind new substances such as ash and gases, clearly indicating a chemical change.