2.3. Changes in Matter http://www.ck12.org
mixture that contains naturally occurring biological organisms like bacteria, viruses, and protozoans. Some water
filters can filter out bacteria, the length of which is on the order of 1 micrometer. Other mixtures, like wet soil,
contain relatively large particles that can be filtered out using something like a coffee filter.
Chemical Change
Much of the field of chemistry is devoted to the study of chemical changes. Achemical change, also referred to as
a chemical reaction, is one in which the chemical identity of a substance is altered. We witness chemical changes
every day. For example, the burning of wood or the rusting of iron are chemical changes. The burning of wood is a
process in which cellulose molecules break down into water and carbon dioxide. The rusting of iron is a process in
which elemental iron combines with oxygen (from air or water) to produce iron oxide (Figure2.11).
FIGURE 2.11
Rust (iron oxide) forms on an unprotected
iron surface.As the rust forms on the surface of the iron, it flakes off to expose more iron, which will continue to rust. Rust
is clearly a substance that is different from iron. Rusting is an example of a chemical change. Some chemical
changes are not as obvious but are still hugely important. For example, photosynthesis and cellular respiration are
chemical changes that we could not live without. Chemical changes involve the combination, decomposition, or
rearrangement of elements and compounds to form new substances.
Achemical propertydescribes the ability of a substance to undergo a specific chemical change. A chemical
property of iron is that it is capable of combining with oxygen to form iron oxide, the chemical name of rust. A more
general term for rusting and other similar processes is corrosion. Other terms that are commonly used in descriptions
of chemical changes are burn, rot, explode, decompose, and ferment. Chemical properties are very useful as a way
of identifying substances. However, unlike physical properties, chemical properties can only be observed as the
substance is in the process of being changed into a different substance.
Recognizing Chemical Changes
How can you tell if a chemical change is taking place? Certain visual clues indicate that a chemical change is likely
(but not necessarily) occurring, including the following examples:
- A change of color occurs.