2.3. Changes in Matter http://www.ck12.org
Physical Change
Any time the physical properties of a substance are changed, we can say the substance has undergone aphysical
change. All substances undergo physical changes where there is a change in the form of the substance but not in
its chemical composition. For instance, the grinding of granular sugar into powdered sugar is a physical change.
Similarly, dissolving sugar in water is a physical change. We can also use physical changes to separate mixtures into
their components. There are a variety of methods used, and the best procedure depends largely on the nature of the
mixture. Depending on the states of matter involved, the relative sizes of the mixtures components, and whether the
mixture is homogeneous or heterogeneous will determine the necessary separation technique.
Methods for Separating Mixtures
Chromatography
Chromatographyis the separation of a mixture by passing it through a medium in which the components move
at different rates. Mixtures that are solutions (such as salt water), suspensions (such as sand mixed with water),
or even vapors can be separated in this way. Paper chromatography is a type of chromatography that can be used
for separating and identifying mixtures in which one or more components are colored, especially pigments. The
following video shows paper chromatography being used to separate out the dyes present in a variety of water-
soluble inks: http://www.youtube.com/watch?v=ac9vALSoxbY (0:43).
MEDIA
Click image to the left for more content.In this video, we see several different dyes that have been placed on paper through which water was absorbed.
Dyes, such as the ink in colored markers, are usually a mixture of several different colored compounds. The dyes in
water-soluble inks dissolve easily in water, while permanent inks dissolve more readily in organic solvents such as
ethanol.
Distillation
Distillationis an effective method to separate mixtures comprised of two or more pure liquids. Distillation is a
purification process in which the components of a liquid mixture are vaporized (transformed from liquid to gas) and
then condensed (transformed from gas back to liquid) and isolated. In a simple distillation, a mixture is gradually
heated. The solution with the lowest boiling point will change into a gas first. This gas, or vapor, then passes through
a cooled tube (a condenser) where it condenses back into its liquid state. This condensed liquid is called the distillate.
Figure2.9 illustrates this.
When a mixture contains several components with similar boiling points, the one-step distillation may not give
a pure substance in the receiving vessel. Therefore, more elaborate methods are used to completely separate a
mixtures components. Distillation is an especially effective physical technique in separating out a homogeneous
mixture comprised of two or more pure liquids, such as alcohol and water.