Question

Identify each of the following as a physical or chemical change: a. A plant grows a new leaf. b. Chocolate is melted for a dessert. c. Wood is chopped for the fireplace. d. Wood burns in a fireplace.

Short answer

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

Step-by-step solution

Understanding Physical and Chemical Changes

Physical changes are changes in the appearance or physical state of a substance without changing its chemical composition. Chemical changes, on the other hand, result in the formation of new substances with different chemical properties.

Analyzing a. A plant grows a new leaf.

When a plant grows a new leaf, it involves complex biological processes that change the plant’s chemical composition. This qualifies as a chemical change.

Analyzing b. Chocolate is melted for a dessert.

When chocolate is melted, it changes from a solid to a liquid state without altering its chemical composition. This is a physical change.

Analyzing c. Wood is chopped for the fireplace.

Chopping wood involves breaking it into smaller pieces without changing its chemical composition. This makes it a physical change.

Analyzing d. Wood burns in a fireplace.

Burning wood involves a chemical reaction with oxygen in the air to form new substances such as ash, carbon dioxide, and water vapor. This is a chemical change.

Key concepts

Physical Change

A physical change involves a change in the form or physical properties of a substance, without any change in its chemical composition. For example, ice melting into water is a physical change, because even though ice changes from a solid to a liquid, its molecular structure, H2O, remains unchanged. In a physical change:

• The substance's state or appearance changes.
• Its chemical identity remains the same, meaning no new substances are formed.
• Physical changes are generally reversible.

Examples include breaking a glass, dissolving sugar in water, and stretching a rubber band. These actions alter the substance's shape or state, but not its chemical properties.

Chemical Change

Chemical changes occur when a substance transforms into one or more different substances with new chemical properties. This happens through chemical reactions. For example, when wood burns in a fireplace, it reacts with oxygen to form ash, carbon dioxide, and water vapor. In a chemical change:

• New substances with different chemical properties are formed.
• The original substance's chemical composition changes.
• Chemical changes are often irreversible under normal conditions.

Indicators of a chemical change can include a change in color, production of gas, formation of a precipitate, or a change in temperature. Rusting of iron, baking a cake, and digesting food are all examples of chemical changes.

Examples of Physical Changes

To further understand physical changes, here are a few more examples:

• Melting Ice: When ice melts into water, it changes its state from solid to liquid. The chemical structure of H2O remains unchanged.
• Boiling Water: Boiling turns liquid water into water vapor (gas). This is a physical transition without altering the chemical nature of the water.
• Shredding Paper: Shredding paper changes its size and shape, but the paper still remains chemically identical.
• Mixing Sand and Salt: When mixed together, sand and salt retain their individual properties and can be separated again through physical means.

These examples highlight how physical changes are purely alterations in physical appearance or state without any chemical transformation.

Examples of Chemical Changes

Chemical changes, on the other hand, result in the formation of new substances. Here are some classic examples:

• Burning Wood: As mentioned, burning wood in a fireplace reacts with oxygen to create ash, water vapor, and carbon dioxide, transforming the wood into entirely new substances.
• Baking a Cake: When baking, ingredients like flour, sugar, and eggs undergo chemical reactions to form a new product with different properties, the cake.
• Rusting of Iron: Iron reacts with oxygen and moisture in the air to form rust (iron oxide), a new chemical compound.
• Food Digestion: In our bodies, enzymes break down food into smaller molecules through chemical reactions, which can be utilized for energy.

These examples illustrate how chemical changes alter the chemical composition of substances, resulting in new products with different properties.