Heating water to steam is a physical change, not a chemical one. This transformation is used to illustrate the difference between changes that alter a substance’s identity and those that only change its appearance or state. Understanding this distinction requires focusing on the molecular level, specifically whether chemical bonds within the molecules are broken or formed. The following sections will detail the differences between physical and chemical processes to clarify why steam remains the same substance as liquid water.
What Defines a Physical Change
A physical change occurs when a substance changes its form, appearance, or state of matter, but its underlying molecular structure remains unchanged. No new substance is created. For example, liquid water, ice, and steam all retain the molecular formula $\text{H}_2\text{O}$. These changes primarily affect the physical properties of the substance, such as its size, shape, volume, or density. Physical changes are often easily reversible; a change in temperature or pressure can return the substance to its original state. Examples include crushing a metal can, shredding paper, or separating a mixture like sand and water.
What Defines a Chemical Change
A chemical change, also known as a chemical reaction, results in the formation of one or more entirely new substances with different properties from the starting materials. This transformation involves the breaking of existing chemical bonds and the formation of new ones. The atoms of the original reactants are simply reorganized into new products. Chemical changes often release or absorb energy, which can be observed through signs like light emission, a noticeable temperature change, or the production of a gas. Because the identity of the material is fundamentally altered, these changes are generally difficult to reverse. A classic example is the reaction between baking soda and vinegar.
The Case of Water Changing to Steam
The transition from liquid water to steam is definitively a physical change because the chemical identity of the water molecules is not altered; both liquid water and steam are $\text{H}_2\text{O}$. The energy supplied for boiling is used only to overcome the forces of attraction between the water molecules, not the bonds within them. These weak attractive forces are known as intermolecular forces, specifically hydrogen bonds. Heating the water gives the molecules enough kinetic energy to break these intermolecular bonds, allowing the $\text{H}_2\text{O}$ molecules to escape the liquid phase and move far apart as a gas. Crucially, the strong covalent bonds defining the $\text{H}_2\text{O}$ molecule remain completely intact.
How to Identify Other Types of Changes
To classify other common processes, the most reliable test is determining whether a new substance is generated. Dissolving table salt in water is a physical change because the salt molecules can be recovered simply by evaporating the water. Shattering a pane of glass is also a physical change, as the material is still glass, only in smaller pieces. In contrast, burning wood is a chemical change because the wood reacts with oxygen to form entirely new substances, namely ash, carbon dioxide, and water vapor. Rusting metal is also a chemical change where iron reacts with oxygen and water to form a new compound, iron oxide.