Chemical Reactions, Acids and Bases

Physical vs. Chemical Changes and Chemical Equations

In your everyday life and on the General Science subtest of the ASVAB, understanding how matter transforms is fundamental. A physical change alters the form or appearance of a substance without changing its chemical composition. Everyday actions such as tearing, crushing, cutting, and grinding are physical changes because they alter only the size and shape of the material. Similarly, phase changes, including melting, freezing, boiling, and condensing, are physical changes. Dissolving sugar or salt in water is also a physical change because the chemical identity of the substance remains unchanged.

In contrast, a chemical change results in the formation of one or more new substances with different chemical properties. Common real-world examples of chemical changes include rusting iron and burning wood. When a chemical reaction occurs, several possible signs may be observed: a color change, gas production, the formation of a precipitate, or a temperature change. A precipitate is an insoluble solid that forms in a solution, often when dissolved substances react. It is critical to remember that no single sign proves that a chemical reaction occurred.

All chemical changes follow the Law of Conservation of Mass, which states that matter cannot be created or destroyed in a chemical reaction. Because of this principle, the total mass of the reactants must equal the total mass of the products in any chemical reaction. Reactants are the starting substances in a chemical reaction and are written on the left side of the reaction arrow. Products are the new substances formed by the reaction and are written on the right side of the reaction arrow.

To represent this conservation on paper, scientists write chemical equations using subscripts and coefficients. Subscripts in a chemical formula show the numbers or ratios of atoms of each element. Because changing subscripts changes the chemical formula itself, you must never change them to balance an equation. Instead, coefficients, which are whole numbers placed before chemical formulas, are used to balance the number of each type of atom on both sides. A chemical equation is balanced when the number of atoms of each element is identical on both sides of the arrow.