Physical vs. Chemical Changes

Learning Objectives

Define and differentiate between a physical property and a chemical property.

Define and differentiate between a physical change and a chemical change.

Identify signs that a chemical change has occurred.

Observing Matter

When we observe a substance, we can describe it in two main ways: by its physical properties and its chemical properties. These properties determine how the substance will change.

Physical Properties and Changes

A physical property is a characteristic of a substance that can be observed or measured without changing the substance's identity. Examples include:

Color

Odor

Density

Melting point and boiling point

Hardness

State of matter (solid, liquid, gas)

A physical change is a change in the form or appearance of a substance, but not its chemical composition. The substance is still the same substance. Examples include:

Changing state: melting ice (solid water to liquid water), boiling water (liquid water to steam)

Changing shape or size: crushing a can, tearing paper, dissolving sugar in water.

In all these cases, the molecules themselves are not altered. H₂O is still H₂O whether it's ice, water, or steam.

Chemical Properties and Changes

A chemical property describes a substance's ability to undergo a change that transforms it into a different substance. Examples include:

Flammability: The ability to burn in the presence of oxygen.

Reactivity: How readily a substance combines chemically with other substances (e.g., iron's ability to rust).

A chemical change, also known as a chemical reaction, is a process where one or more substances are converted into one or more new and different substances. The original substance is gone, and a new one with new properties has taken its place.

Signs of a Chemical Change

How can you tell a chemical change has happened? Look for these clues:

1.Production of a gas: Bubbles forming (like when vinegar and baking soda are mixed).

2.Formation of a precipitate: A solid forming from the mixing of two liquids.

3.Unexpected color change: A change in color that isn't just due to mixing (like a green banana turning yellow).

4.Production of light or heat: The release or absorption of energy (e.g., burning wood gives off heat and light).

5.Production of an odor: A new smell being created (like a rotten egg).

Example: Burning wood is a classic chemical change. The wood (cellulose) reacts with oxygen and is transformed into new substances: ash, carbon dioxide, and water vapor. You can't get the original wood back.

Key Terms

**Physical Property: A characteristic of a substance that can be observed without changing it into another substance (e.g., color, density).~|~Chemical Property: A characteristic of a substance that describes its ability to change into different substances (e.g., flammability, reactivity).~|~Physical Change: A change in a substance that does not involve a change in its identity or chemical composition.~|~Chemical Change (Reaction): A process that involves one or more substances changing into new substances with different properties.~|~Precipitate: A solid that forms from a solution during a chemical reaction.