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Learning Objectives
- Define physical property.
- Define physical change.
- Define chemical change.
- Define precipitate.
- Compare and contrast physical and chemical changes.
- Classify the alteration of a substance as either a physical or a chemical change.
Recall that a pure substance, which is a form of matter that has a definite composition, can be categorized as either an element, which consists of only one type of atom, or a compound, which must contain two or more elements that are combined in a specific ratio. The types and relative ratio of atoms that are present in a substance, as represented by an elemental symbol or in a chemical formula, are both unique and intrinsic to the identity of a substance. For example, carbon is carbon because it is comprised solely of carbon atoms. Water is the only compound that forms through a two-to-one ratio of covalently-bonded hydrogen and oxygen atoms.
In contrast, the physical properties of a substance are its observable, descriptive characteristics, such as shape, color, state of matter, melting point, boiling point, and density. Furthermore, unlike chemical formulas, the physical properties of a substance are not uniquely-associated with a single material. For example, both hexane, an organic solvent, and water can be described as clear and colorless liquids.
At its core, chemistry is the branch of science that studies the effects of allowing different substances to interact with one another. Any resultant modification of those substances can be classified as either physical or chemical in nature. As will be described in greater detail in the following paragraphs, while both physical and chemical changes are characterized by alterations to the appearance of a substance, only the latter involves a transformation of the underlying identity of the chemical that is being studied.
Physical Changes
A physical change is defined as a transformation of the appearance of a substance that is not brought about by a corresponding change in the identity of that substance. In other words, during a physical change, the physical properties of a substance are transformed, but its chemical formula is not. Physical changes, which are often reversible, do not result in the creation of a new substance. Instead, the physical form of the material that is present changes, through manual manipulations, such as cutting or denting, undergoing a phase change, or dissolving in another chemical.
Water can exist as a solid, a liquid, or a gas. Applying heat to solid water, which is more commonly-known as "ice," causes it to melt. The result of this transformation, which is an example of a phase change, is liquid water. As shown below in Figure \(\PageIndex{1}\), the appearance of ice is distinctive from that of liquid water, but both substances share a common chemical formula, H2O. Therefore, melting an ice cube is a physical change, as only the physical properties, not the chemical identity, of the water is changed during this process. Finally, as is typical for most physical changes, this transformation can be easily reversed by cooling the liquid water until ice reforms.
Chemical Changes
In contrast, both the physical properties and the chemical formula of a substance must be altered during a chemical change. A chemical formula is the symbolic representation of a substance's chemical composition, and altering either the elements that are present or their relative amounts changes the identity of the substance that is being considered. Therefore, because a chemical change requires that the chemical formula of the resultant substance be different from that of the initial material, a chemical change must, by definition, create a chemical or chemical mixture whose identity is unique, relative to the substances that were originally present. While a chemical change can be described verbally, the overall transformation, which is known as a chemical reaction, is more often represented symbolically, in the form of a chemical equation, as will be discussed in Section 4.12. Finally, the physical properties of a chemical undergoing a chemical change are also altered, often as a direct result of the aforementioned changes in that substance's chemical composition.
Chemical changes are often indicated by
- the appearance of an unexpected color in a chemical mixture,
- the emission of light,
- the evolution of a gas, which results in the generation of bubbles within a chemical mixture, or
- the formation of an insoluble solid precipitate that had not previously existed.
Because neither gases nor precipitates are soluble, they do not dissolve readily and, therefore, are unable to interact with any other chemicals that have been generated. As a result, most chemical changes are irreversible, because these newly-created substances cannot be utilized to reform the chemicals that were initially present.
Figure \(\PageIndex{2}\) shows three distinctive, but related, chemicals. The left-most and central vials contain elemental sodium, Na, and molecular chlorine, Cl2, respectively. When combined with one another, these substances produce sodium chloride, NaCl, which is more commonly-known as "table salt." The appearances of both of sodium and molecular chlorine are drastically different from that of sodium chloride, which is a crystalline solid, as shown in the final image in Figure \(\PageIndex{2}\). Furthermore, elemental sodium is an extremely soft, reactive metal that cannot be exposed to air or water, and molecular chlorine is a deadly gas. In contrast, sodium chloride is a highly-stable, water-soluble solid that is an essential mineral component of the human diet. This transformation in physical properties could be indicative of either a physical or a chemical change. However, the generation of a chemical with a unique formula, NaCl, that occurs upon the combination of sodium, Na, and molecular chlorine, Cl2, requires that the interaction of these chemicals be classified as a chemical change.
Example \(\PageIndex{1}\)
State whether each of the following transformations corresponds to a physical or a chemical change.
- Melting vanilla ice cream
- Burning a piece of wood
- Breaking a piece of glass
- Cooking sugar to create caramel
- Cutting a piece of paper into triangles
Solutions
A physical change is defined as a transformation of the appearance of a substance that is not brought about by a corresponding change in the identity of that substance. In contrast, both the physical properties and the chemical formula of a substance must be altered during a chemical change. Therefore, a physical change can be distinguished from a chemical change based on an analysis of the identities of the chemicals that are present before and after the transformation that is described.
- The same substance, ice cream, is present before and after the melting process. Therefore, this is a physical change. Melting is also both reversible and a type of phase change, which also indicates that this is a physical change.
- Burning is an irreversible process. Therefore, this is a chemical change.
- The same substance, glass, is present before and after the breaking process. Therefore, this is a physical change.
- Different chemicals are present before (sugar) and after (caramel) the cooking process. Therefore, this is a chemical change. The irreversible nature of caramelization is also an indicator that this transformation is a chemical change.
- The same substance, paper, is present before and after the cutting process. Therefore, this is a physical change.
Exercise \(\PageIndex{1}\)
State whether each of the following transformations corresponds to a physical or a chemical change.
- The formation of bubbles (carbon dioxide, CO2) upon mixing baking soda (NaHCO3) and vinegar (HC2H3O2)
- The formation of a new solid upon mixing two clear and colorless liquids.
- The formation of iron (III) oxide (Fe2O3), which is more commonly-known as "rust," on a piece of iron (Fe)
- Molding gold into a piece of jewelry
- Adding salt to water
- Adding an Alka-Seltzer® tablet to water
- Answer a
- Different chemicals are present before (baking soda, NaHCO3, and vinegar, HC2H3O2) and after (carbon dioxide, CO2) the mixing process. Therefore, this is a chemical change. The formation of bubbles is caused by the evolution of a gas, which is also an indicator that this transformation is a chemical change.
- Answer b
- The formation of a new solid corresponds to the generation of precipitate, which is an indicator that this transformation is a chemical change.
- Answer c
- Different chemicals are present before (iron, Fe) and after (iron (III) oxide, Fe2O3) the rusting process. Therefore, this is a chemical change. The irreversible nature of rusting is also an indicator that this transformation is a chemical change.
- Answer d
- The same substance, gold, is present before and after the molding process. Therefore, this is a physical change.
- Answer e
- The same substances, salt (NaCl) and water (H2O), are present before and after the addition has occurred. Therefore, this is a physical change. This process can be reversed by boiling the water in which the salt has dissolved, which also indicates that this is a physical change.
- Answer f
- Adding an Alka-Seltzer® tablet to water results in the formation of bubbles, which is indicative of the evolution of a gas. Therefore, this transformation is a chemical change.
