Problem 43 - Chemistry Matter and Change

Question

Classify each as a physical change or a chemical change. \begin{equation} \begin{array}{l}{\text { a. breaking a pencil in two }} \\ {\text { b. water freezing and forming ice }} \\ {\text { c. frying an egg }} \\ {\text { d. burning wood }} \\ {\text { e. leaves changing colors in the fall }}\end{array} \end{equation}

Step-by-Step Solution

Answer

a. Physical change; b. Physical change; c. Chemical change; d. Chemical change; e. Chemical change.

Step 1: Understand Physical vs Chemical Change

A physical change involves a change in physical properties without altering the substance's chemical identity. Examples include changes in state or shape. A chemical change involves forming a new substance with different properties, often indicated by color changes, energy release/absorption, or gas formation.

Step 2: Analyze 'Breaking a Pencil in Two'

Breaking a pencil changes its shape, but the material composition (wood, graphite) remains the same. No new substances are formed, so this is a physical change.

Step 3: Analyze 'Water Freezing and Forming Ice'

Water changing from a liquid to solid state is a change in state. The chemical composition (H₂O) does not change, indicating a physical change.

Step 4: Analyze 'Frying an Egg'

Frying an egg involves cooking, which changes the proteins in the egg physically and chemically. The formation of new substances occurs, which characterizes a chemical change.

Step 5: Analyze 'Burning Wood'

Burning wood results in ash, smoke, and other gases, with new substances being formed. This is an exothermic reaction, so it is a chemical change.

Step 6: Analyze 'Leaves Changing Colors in the Fall'

Leaves changing color involves chemical processes where chlorophyll breaks down, and new pigments form, indicating a chemical change.

Key Concepts

Physical Properties

Physical properties are characteristics of a substance that can be observed or measured without changing the substance's identity. Understanding these properties helps determine if a change is purely physical. Some common examples include:

• Color: Although color itself can indicate a physical or chemical change, the inherent color of a substance is a physical property.
• Density: This measures how much mass is contained in a given volume.
• Shape and Size: Changes in shape or size, like breaking a pencil in two, are physical as the composition remains unchanged.
• State of Matter: Whether a substance is a solid, liquid, or gas.

Physical properties are usually easy to observe and measure. They do not require altering the basic nature of the substance, making them a key factor when distinguishing between physical and chemical changes.

Chemical Properties

Chemical properties are traits that determine how a substance can change its chemical identity. These properties only become apparent during a chemical reactions. Here is what you should know:

• Reactivity: Describes how easily a substance will react with others. For example, frying an egg exploits the reactivity of its proteins.
• Flammability: The ability to burn in the presence of oxygen, a critical property in the burning of wood.
• pH Level: This indicates if a substance is acidic, basic, or neutral, affecting reactions.
• Chemical Bonds: Changes in chemical bonds lead to the creation of new substances, observed in chemical changes.

Understanding chemical properties helps predict how a substance will behave under various circumstances, crucial in recognizing if a change is chemical.

State Change

State change refers to the transition of matter from one state to another—solid, liquid, or gas. Notably, this transformation is a hallmark of physical changes:

• Ice Formation: When water freezes, it shifts from liquid to solid, conserving its molecular composition (H₂O).
• Melting and Boiling: Similar to freezing, melting of chocolate or boiling of water alters the state but not the substance.

These processes are reversible and do not impact the chemical identity of the substance. Recognizing state changes helps students accurately classify physical changes in their exercises.

Substance Formation

Substance formation involves the emergence of new molecules or compounds, indicating a chemical change. This process is central to many everyday events:

• Cooking: The alteration in taste and texture when frying an egg comes from new compounds creating distinct flavors and structures.
• Combustion: Burning wood converts it into ash, a myriad of gases, and heat energy, confirming new substance creation.
• Color Change in Leaves: The breakdown of chlorophyll and emergence of new pigments during fall are classic signs of chemical change.

Such transformations are often irreversible and involve complex rearrangements of atoms. Recognizing substance formation is key to differentiating chemical changes from physical ones.