Chapter 3 Resource: Matter and its Changes

Name Date Class
Meeting Individual Needs
2
Enrichment
One chemical property is the sensitivity of a
substance to heat. Some chemicals might
change form when their temperature changes;
others might undergo chemical reactions with
other substances. When these changes result in
a change in color, the chemicals involved are
called thermochromic. (Thermo means “heat,”
and chrom means “color.”)
Thermochromic Labels
Some thermochromic chemicals undergo
irreversible changes when they are heated. In
other words, once the color change occurs, the
chemical does not have the ability to return to
its original color even after it is cooled. These
types of thermochromic chemicals are being
used on a new type of labels for fresh meats.
All fresh meat contains a certain amount of
bacteria. Because the bacteria grow very slowly
at cool temperatures, the meat is safe to eat for a
certain amount of time as long as it is kept
refrigerated. However, if the meat is exposed to
high temperatures anywhere on its way from a
packing plant to a grocery store to your kitchen,
the bacteria in the meat will grow much more
quickly. They might even build up to levels that
would make the meat unsafe to consume.
Color Change
A thermochromic label is designed to help
people tell if meat has been exposed to high
temperatures. When the meat is packaged, a
thermochromic label is placed on the outside.
Using Chemical Reactions
on Food Labels
The label is then activated, which allows two or
more chemicals inside the label to mix. If the
temperature increases to a certain level, a reaction
will occur between these chemicals that
causes the label to change color. (For example,
some labels change from white to black or
from green to yellow.) This color change tells
customers that the meat is not safe to eat.
Time-Temperature Indicators
The chemicals in thermochromic labels
used on meat are not only sensitive to temperature.
They are also sensitive to time. For this
reason, these labels are sometimes called TTI
(Time-Temperature Indicator) labels. Regular
labels on meat usually have an expiration or
“sell-by” date. After this date, the meat is no
longer safe to be sold. A TTI label is not
marked with a date, however. Instead, the label
changes color after a certain amount of time
has passed (regardless of whether the meat has
been exposed to high temperatures).
Because different combinations of chemicals
have different sensitivities to temperature
and time, scientists can design TTI labels that
will change color at almost any temperature
or after almost any period of time. This
allows TTI labels to be used on a variety of
different foods, including milk, eggs, and even
fresh fruit and vegetables. You will probably
be seeing more of these labels in the near
future.
1. Why is it important that the color change in a thermochromic meat label is irreversible
2. How is a thermochromic chemical similar to an acid-base indicator How are they different
3. Is sensitivity to time a physical property or a chemical property Explain.
4. Think of another application for thermochromic chemicals besides food labels. On a separate
sheet of paper write a short paragraph describing this use. Will this application use chemicals
that undergo a reversible or irreversible color change Who would be likely to use or benefit
from the application
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30 Matter and Its Changes