What Is a Chemical Change and How to Recognize It?


    What Is a Chemical Change and How to Recognize It?

    A chemical change, also known as a chemical reaction, is a process in which one or more substances are altered into one or more new and different substances. In other words, a chemical change is a chemical reaction involving the rearrangement of atoms. While a physical change can often be reversed, a chemical change typically cannot be, except through more chemical reactions. When a chemical change occurs, there is also a change in the energy of the system. A chemical change that gives off heat is called an exothermic reaction. One that absorbs heat is called an endothermic reaction.

    Examples of Chemical Changes

    Any chemical reaction is an example of a chemical change. Examples include:

    • Combining baking soda and vinegar (which bubbles off carbon dioxide gas)
    • Combining any acid with any base
    • Cooking an egg
    • Burning a candle
    • Rusting iron
    • Adding heat to hydrogen and oxygen (produces water)
    • Digesting food
    • Pouring peroxide on a wound

    How to Recognize a Chemical Change


    Examples of Chemical Changes

    Chemical changes may be identified by:

    • Temperature Change: Because there is an energy change in a chemical reaction, there is often a measurable temperature change.
    • Light: Some chemical reactions produce light.
    • Bubbles: Some chemical changes produce gases, which can be seen as bubbles in a liquid solution.
    • Precipitate Formation: Some chemical reactions produce solid particles that may remain suspended in a solution or fall out as a precipitate.
    • Color Change: A color change is a good indicator that a chemical reaction has occurred. Reactions involving transition metals are particularly likely to produce colors.
    • Odor Change: A reaction may release a volatile chemical that produces a characteristic scent.
    • Irreversible: Chemical changes are often difficult or impossible to reverse.
    • Change in Composition: When combustion occurs, for example, ash may be produced. When food rots, its appearance visibly changes.

    It’s important to know that chemical change may occur without any of these indicators being obvious to the casual observer. For example, the rusting of iron produces heat and a color change, but it takes a long time for the change to be evident, even though the process is ongoing.

    Types of Chemical Changes


    How to Recognize a Chemical Change

    Chemists recognize three categories of chemical changes: inorganic chemical changes, organic chemical changes, and biochemical change. Inorganic chemical changes are chemical reactions that don’t generally involve the element carbon. They include reactions like neutralization, oxidation, and redox. Organic chemical changes are chemical reactions that involve carbon and usually other elements like hydrogen, oxygen, and nitrogen. They include reactions like halogenation, methylation, and polymerization. Biochemical changes are chemical reactions that happen within living organisms and are driven by hormones and enzymes. They include reactions like photosynthesis, respiration, digestion, and fermentation.

    What Does a Chemical Change Produce?


    Types of Chemical Changes

    A chemical change produces one or more new substances that have different chemical properties from the original substance. These new substances may be elements or compounds. For example, when iron rusts, it reacts with oxygen to form iron oxide, a new compound. When hydrogen and oxygen combine with heat, they form water, a new compound. When sodium metal reacts with chlorine gas, they form sodium chloride, a new compound.

    Physical vs. Chemical Changes


    What Does a Chemical Change Produce?

    Physical changes are changes that do not alter the chemical composition of a substance. They only affect the physical properties of a substance, such as its shape, size, state, or color. Physical changes can often be reversed by changing the conditions. For example, when water freezes into ice, it undergoes a physical change from liquid to solid. This change can be reversed by heating the ice. When sugar dissolves in water, it undergoes a physical change from solid to solution. This change can be reversed by evaporating the water.

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