Matter, the substance that makes up everything around us, constantly undergoes various transformations. These changes can alter its appearance, physical state, or fundamental composition. Understanding how matter changes helps clarify many everyday phenomena.
What Are Physical Changes?
A physical change alters a substance’s form or appearance without changing its chemical identity; its molecular structure remains the same. For instance, when an ice cube melts, it changes from a solid to a liquid, but it is still water (H₂O). Similarly, boiling water transforms liquid water into steam, yet its chemical makeup does not change.
Other common examples include cutting paper, crumpling a can, or dissolving sugar in water. In these cases, the material’s size, shape, or state may change, but no new chemical substances are created. Many physical changes are often reversible, meaning the substance can return to its original form, such as refreezing melted water.
What Are Chemical Changes?
A chemical change, in contrast, forms new substances with different chemical properties than the original materials. This type of change involves a chemical reaction where atoms rearrange to form new molecules. For example, when wood burns, it transforms into ash, smoke, and gases, which are entirely different substances from the original wood.
Indicators often signal that a chemical change has occurred. These signs can include a change in color, gas production (seen as bubbles), a temperature change (heat being released or absorbed), or solid precipitate formation. Rusting iron, baking a cake, and digesting food are additional examples of chemical changes.
The Candle’s Transformation: Melting and Burning
A burning candle demonstrates both physical and chemical changes simultaneously. When a candle is lit, the solid wax near the wick absorbs heat from the flame and turns into a liquid. This melting is a physical change because the wax simply transitions from a solid to a liquid state, and its chemical composition remains unchanged. If the flame were extinguished, this melted wax would solidify again, reverting to its original form.
The liquid wax is then drawn upward through the wick by capillary action. As this liquid wax reaches the hotter parts of the flame, it vaporizes, becoming a gas. This gaseous wax then reacts with oxygen in the air, undergoing a chemical change known as combustion.
During this combustion, the wax breaks down and combines with oxygen to produce new substances, mainly carbon dioxide and water vapor. This chemical reaction also releases energy in the form of heat and light, which sustains the flame. While the melting of the wax is a physical transformation, the actual burning of the wax and wick is a chemical change.