Read the section "What Is A Chemical Change?" Select the paragraph that BEST explains several ways to identify a chemical change through observation alone.
(A) A chemical change occurs when the composition of a substance is changed. Chemical changes require
. the breaking and forming of chemical bonds during a chemical reaction. This results in the rearranging
of atoms in substances to form the products of a chemical reaction, which become brand new
molecules. These new molecules cannot be easily reverted back to their original state,
(B) Sometimes it is difficult to tell if a chemical reaction has taken place. To help determine whether there
has been a reaction, chemists consider a few basic indicators. These include a change in temperature,
a change in color, or the development of an odor. The formation of a precipitate or the formation of a
gas may also indicate a chemical reaction.
(C)
In a chemical alteration, a temperature change occurs as a result of the breaking or formation of
chemical bonds. When the chemical bonds of the reactants are broken, sometimes excess energy is
released. This can cause heat to be discharged, leading to an increase in temperature. Burning wood is
an example of a reaction that releases excess energy as heat. A chemical cold pack in a first aid kit is
an example of a chemical reaction that absorbs heat energy, rather than releasing it. This results in
cooling
A chemical reaction may also release a gas. For example, a mix of vinegar and baking soda will
immediately start bubbling and foaming. The bubbles are a release of carbon dioxide gas, a product of
the chemical reaction between these two household substances.
(D)