Distillation and the Particle Theory
fu you saw in Conduct
an Investigation 5-A,
distillation is a method for
separating the parts of a liquid solution. In distillation, the solvent is heated to change it
to a gas, then condensed to a liquid again, as shown in Figure 5.6. Condensation is
the change hom a gas to a liquid. Because the solutes do not change state, they
remain behind. Distillation can remove other dissolved solids as well as salt by
leaving them behind as the pure water evaporates.
water out
steam
seawater
steam condenses
as it cools
condenser
water in
--_7
salt (solute) collects
on bottom of flask as
water evaporates
pure water
Figure 5.6 Distillation involves evaporating a solvent to separate it from the solute and then
condensing it to a liquid. Water circulating in the condenser helps cool the steam as it passes through
the tube.
be explained by the particle theory? According to the particle
theory solute particles and solvent particles must attract each other in order to
form a solution. What overcomes the attraction and allows them to separate? The
particle theory also states that particles are constandy moving. This idea can be
used to explain why water evaporates: occasionally some particles move away from
the others and go offinto the air. Now we can add one more idea to the theory:
IIow can distillation
.
You have just considered
the fifth (and last) idea in
the particle theory, as it
is discussed in this
textbook. ln your Science
Log, list the five ideas
with page references,
so you can refer to them
again.
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Particles at a higher temperature are moving faster than particles at a lower
temperature.
When \ryater is heated, for example, its particles move faster, so more of them
escape. Water changes to a gas at a lower temperature than salt. Does this piece of
the theory fit with what you already know? When you want to dry your hair quickly, does it help if you use a hair dryer set to "hot"? Of course it does. When the
particles in the liquid water are heated, they move fasteq so more of them escape
the attractions of the other particles, and they move into the air.
salt and water
In distillation, heating the solution causes all of the particles
alike
to move more quickly. The salt and water particles have different properties, however. Although both kinds of particles move more quickly, the water
particles escape and the salt particles do not. The water evaporates, and the salt
does not. In the tubing, as the hot gas cools, the water particles move more slowly
again. The gas changes back into a liquid, and water drips out the end of the tube.
The salt is left behind. The product is pure watet or distilled water.
Pure Substances and Mixtures