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Chemical vs. Physical Changes
The following was excerpted from Chemistry, Addison-Wesley by Wilbraham, Staley, Simpson, &
Physical Changes
Matter can be changed in many ways. A physical change will alter a substance without changing its
composition. Cutting, grinding, or bending a material will cause a physical change. The substance
remains the same. A change in temperature may also bring about a physical change. The melting of
ice, the freezing of water, the conversion of water to steam, and the condensation of steam to
water are all examples of physical changes. We know that these physical properties of water are
the same for water that has been frozen and melted as they are for water that has been converted
to steam and then condensed.
Words like boil, freeze, melt, condense, break, split, crack, grind, cut, crush, and bend usually
signify a physical change. Table salt (sodium chloride) is a white solid at room temperature. It can
also exist in each of the three physical states. However, the temperatures at which the changes of
state occur in sodium chloride are much higher than for the corresponding changes in water.
Sodium chloride melts at 801 C and boils at 1413 C.
Chemical Reactions
Energy changes occur whenever a chemical reaction takes place. In a chemical reaction one or
more substances are changed into new substances. For example, when refined cane sugar is
heated it undergoes a process called caramelization. The sugar melts to a colorless liquid that soon
develops a brown color and a caramel odor as the temperature is raised. At this point the sugar is
breaking down, and a chemical change is taking place. The important point is that when the hot
liquid is allowed to cool, the sugar has lost its identity. It does not turn back into sugar, because
the sugar has been changed to other substances. The change in composition of the sugar is the
result of a chemical reaction. In a chemical reaction, the starting substances are called reactants,
and the new substances are the products. Chemists use a n arrow as a shorthand form of the
phrase “are changed into.”
… Several occurrences are common indicators of a chemical change. Energy is usually absorbed or
evolved in chemical reactions. Burning coal evolves heat; cooking food absorbs heat. Energy is also
absorbed or evolved, however, in physical changes of state. A color change, as in leaves turning in
the fall, or an odor change, as in meat rotting, often accompanies a chemical change. The
production of a gas or a solid can be the result of a chemical change. A solid that separates from a s
solution is called a precipitate. Gas bubbles rising from the bottom of a stagnant lake are formed
by the chemical decay of plant matter on the lake floor. Gas or vapor formation can also be a the
result of a change of state. Boiling water is and example. Finally, it is sometimes most helpful to
consider whether the change can be easily reversed. Physical changes, especially those involving a
change of state, are usually reversible. Ice can be melted and the water refrozen. In contrast, most
chemical changes are not easily reversed. Once iron reacted with oxygen to form rust, as it often
does on a car, you cannot easily reverse the process.