Chemical Reactions - Physical and Chemical Changes

Objective

Identify physical and chemical changes, and describe the differences between them.

Physical Changes

Definition

A physical change alters the form or appearance of a substance, but does not change its chemical composition. The molecules of the substance remain the same.

Examples

• Melting ice (solid to liquid)
• Boiling water (liquid to gas)
• Dissolving sugar in water
• Cutting paper
• Crushing a can

Key Characteristics

• No new substances are formed.
• The change is often reversible.
• The chemical properties of the substance remain the same.

Indicators of a Physical Change

• Change in state (solid, liquid, gas)
• Change in shape or size
• Change in texture

Chemical Changes

Definition

A chemical change results in the formation of one or more new substances with different chemical properties. The chemical composition of the original substance is altered.

Examples

• Burning wood
• Rusting iron
• Cooking an egg
• Reaction of acids with bases
• Decomposition of hydrogen peroxide

Key Characteristics

• New substances are formed.
• The change is often irreversible.
• The chemical properties of the substance change.

Indicators of a Chemical Change

• Change in colour
• Formation of a gas (bubbles)
• Formation of a precipitate (a solid forming in a solution)
• Release or absorption of heat (temperature change)
• Production of light

Table: Comparing Physical and Chemical Changes

Feature	Physical Change	Chemical Change
New Substances Formed?	No	Yes
Reversible?	Often Yes	Often No
Chemical Properties Change?	No	Yes
Examples	Melting, Boiling, Dissolving	Burning, Rusting, Cooking