Matter around us constantly undergoes transformations, changing its appearance or composition. Understanding these changes helps us categorize and predict how substances will behave. A common process observed daily is dissolving, which raises a fundamental question about the nature of these transformations. The distinction between a physical and a chemical change helps clarify whether a new substance is formed during such processes.
What Defines a Physical Change?
A physical change involves an alteration in a substance’s physical properties without changing its chemical identity. The chemical composition of the material remains the same throughout the process. Examples include changes in state, such as ice melting into liquid water, where H2O molecules remain H2O molecules. Boiling water into steam or tearing a piece of paper are physical changes; the water molecules or paper fibers retain their original chemical structure. These changes are often reversible.
What Defines a Chemical Change?
A chemical change results in the formation of new substances with different chemical properties. During a chemical change, the atoms rearrange, and new chemical bonds are formed or broken. This process leads to a product that cannot be easily converted back into the original reactants by physical methods. Common examples include burning wood, which transforms cellulose into ash, carbon dioxide, and water, or iron rusting, where iron reacts with oxygen to form iron oxide.
Dissolving: Primarily a Physical Transformation
Most instances of dissolving are considered physical changes because the solute’s chemical identity remains intact. When a substance like salt (sodium chloride, NaCl) dissolves in water, the individual sodium ions (Na+) and chloride ions (Cl-) separate from the solid crystal lattice. These ions then become surrounded by water molecules, but they do not form new chemical compounds.
Similarly, when sugar (sucrose, C12H22O11) dissolves in water, the individual sucrose molecules disperse throughout the water. They remain sucrose molecules, simply separating from each other. No new chemical bonds are formed or broken within the sucrose molecule itself, nor does it react to form a new substance. Evaporating the water from a saltwater or sugar-water solution will recover the original salt or sugar, demonstrating that their chemical compositions were not altered.
When Dissolving Crosses into Chemical Territory
While most dissolving is physical, some instances involve chemical reactions, leading to the formation of new substances. When highly reactive metals, such as sodium, are placed in water, a chemical reaction occurs. The sodium reacts with water to produce sodium hydroxide and hydrogen gas, forming new chemical compounds. This process differs from simple physical dissolution because the original sodium metal is consumed, and new products with different chemical properties are generated.
Another example involves the dissolution of gases, such as hydrogen chloride (HCl) gas, in water. While the initial dissolving of HCl gas is a physical process, the subsequent reaction of HCl with water to form hydronium ions (H3O+) and chloride ions (Cl-) is a chemical change. Here, the strong acid donates a proton to a water molecule, altering the chemical species present. These scenarios highlight that dissolving can sometimes initiate or be accompanied by chemical reactions.