Learning Outcomes Distinguish between physical and chemical changes. Give examples of physical and chemical changes. Physical Changes As an ice cube melts, its shape changes as it acquires the ability to flow. However, its composition does not change. Melting is an example of a physical change . A physical change is a change to a sample of matter in which some properties of the material change, but the identity of the matter does not. Physical changes can further be classified as reversible or irreversible. The melted ice cube may be refrozen, so melting is a reversible physical change. Physical changes that involve a change of state are all reversible. Other changes of state include vaporization (liquid to gas), freezing (liquid to solid), and condensation (gas to liquid). Dissolving is also a reversible physical change. When salt is dissolved into water, the salt is said to have entered the aqueous state. The salt may be regained by boiling off the water, leaving the salt behind. When a piece of wood is ground into sawdust, that change is irreversible since the sawdust could not be reconstituted into the same piece of wood that it was before. Cutting the grass or pulverizing a rock would be other irreversible physical changes. Firewood also represents an irreversible physical change since the pieces cannot be put back together to form the tree. Chemical Changes When exposed to air, an object made of iron will eventually begin to rust (see figure below). As the rust forms on the surface of the iron, it flakes off to expose more iron, which will continue to rust. Rust is clearly a substance that is different from iron. Rusting is an example of a chemical change. A chemical property describes the ability of a substance to undergo a specific chemical change . A chemical property of iron is that it is capable of combining with oxygen to form iron oxide, the chemical name of rust. A more general term for rusting and other similar processes is corrosion. Other terms that are commonly used in descriptions of chemical changes are burn, rot, explode, and ferment . Chemical properties are very useful as a way of identifying substances. However, unlike physical properties, chemical properties can only be observed as the substance is in the process of being changed into a different substance. A chemical change is also called a chemical reaction. A chemical reaction is a process that occurs when one or more substances are changed into one or more new substances . Zinc \(\left( \ce{Zn} \right)\) is a silver-gray element that can be ground into a powder. If zinc is mixed at room temperature with powdered sulfur \(\left( \ce{S} \right)\), a bright yellow element, the result will simply be a mixture of zinc and sulfur. No chemical reaction occurs. However, if energy is provided to the mixture in the form of heat, the zinc will chemically react with the sulfur to form the compound zinc sulfide \(\left( \ce{ZnS} \right)\). Pictured below are the substances involved in this reaction. The reaction between zinc and sulfur can be depicted in something called a chemical equation. In words, we could write the reaction as: \[\text{zinc} + \text{sulfur} \rightarrow \text{zinc sulfide}\] A more convenient way to express a chemical reaction is to use the symbols and formulas of the substances involved: \[\ce{Zn} + \ce{S} \rightarrow \ce{ZnS}\] The substance(s) to the left of the arrow in a chemical equation are called reactants. A reactant is a substance that is present at the start of a chemical reaction. The substance(s) to the right of the arrow are called products. A product is a substance that is present at the end of a chemical reaction . In the equation above, zinc and sulfur are the reactants that chemically combine to form zinc sulfide as a product. Recognizing Chemical Reactions How can you tell if a chemical reaction is taking place? Certain visual clues indicate that a chemical reaction is likely (but not necessarily) occurring, including the following examples: A change of color occurs during the reaction. A gas is produced during the reaction. A solid product, called a precipitate, is produced in the reaction. A visible transfer of energy occurs in the form of light as a result of the reaction. When zinc reacts with hydrochloric acid, the reaction bubbles vigorously as hydrogen gas is produced (see figure below). The production of a gas is also an indication that a chemical reaction may be occurring. When a colorless solution of lead (II) nitrate is added to a colorless solution of potassium iodide, a yellow solid called a precipitate is instantly produced (see figure below). A precipitate is a solid product that forms from a reaction and settles out of a liquid mixture . The formation of a precipitate may also indicate the occurrence of a chemical reaction. \[\ce{Pb(NO_3)_2} \left( aq \right) + 2 \ce{KI} \left( aq \right) \rightarrow \ce{PbI_2} \left( s \right) + 2 \ce{KNO_3} \left( aq \right)\]