What is a chemical reaction, and how can you tell one has occurred?

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Accepted Answer

A process in which substances (reactants) are transformed into new substances (products) with different properties, indicated by signs such as color change, gas production, temperature change, or precipitate formation. A chemical reaction is a process in which the chemical bonds in reactant molecules are broken and new bonds are formed to create product molecules with different chemical properties. Signs that a chemical reaction has occurred include a color change, production of gas (bubbles), change in temperature (exothermic reactions release heat; endothermic reactions absorb heat), formation of a precipitate (an insoluble solid formed from two liquids), and the production of light or sound. Chemical reactions involve changes at the molecular level that create new substances, unlike physical changes (like melting or dissolving) that alter appearance without changing chemical identity.

Simply mixing two substances is not necessarily a chemical reaction; mixing salt and sand is a physical change with no new substances formed. Chemical reactions occur everywhere, not only in laboratories - cooking, rusting, digestion, photosynthesis, and combustion are all everyday chemical reactions. Physical mixing to form solutions (like dissolving sugar in water) is a physical change, not a chemical reaction, because no new substances are created.

Chemical reactions and their indicators are important topics on the DSST Principles of Physical Science I exam. Students should be able to distinguish chemical changes from physical changes, recognize the signs of chemical reactions, understand the types of chemical reactions (synthesis, decomposition, single replacement, double replacement, combustion), and balance simple chemical equations to satisfy the law of conservation of mass.

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