Physical Change Explained: Definition and Examples

A physical change affects the form of a substance but not its chemical identity. The substance might change its state (solid, liquid, gas), shape, or size, but the molecules themselves remain the same. Examples include melting ice, cutting paper, or dissolving sugar in water.

A chemical change, also known as a chemical reaction, results in the formation of new substances with different chemical properties. Signs of a chemical change can include a change in color, production of gas, formation of a precipitate, or release of energy. Examples include rusting, burning, and cooking.

Physical Change Examples: Analyzing Options

Let's examine each option provided to determine which is a physical change:

• Metal rusting: This is a chemical change. For instance, iron rusts when it reacts with oxygen and moisture in the air to form iron oxide. The chemical composition changes from elemental iron ($Fe$) to iron(III) oxide ($Fe_2O_3$). This is a new substance.
• Silver tarnishing: This is also a chemical change. Silver reacts with sulfur compounds (like hydrogen sulfide, $H_2S$) in the air to form silver sulfide ($Ag_2S$), which appears as a black coating. The original silver ($Ag$) is chemically transformed.
• Water boiling: This is a classic physical change. When water boils, it transitions from liquid water ($H_2O(l)$) to steam, which is gaseous water ($H_2O(g)$). Importantly, the chemical substance remains water ($H_2O$) throughout the process; only its physical state changes.
• Paper burning: This is a chemical change. Burning, or combustion, is a reaction between the paper (cellulose) and oxygen ($O_2$). It creates entirely new substances such as ash, carbon dioxide ($CO_2$), and water vapor ($H_2O$). This reaction releases energy and is generally irreversible.

Physical Change Identified: Water Boiling

Therefore, out of the given options, water boiling is the correct example of a physical change. It demonstrates a change in state (liquid to gas) without altering the fundamental chemical nature of water ($H_2O$).