CHAPTER 5. THERMAL PROPERTIES AND IDEAL GASES 5.2
There are a number oflaws that describe howgases behave. It will beeasy to make sense of these
laws if you understand the kinetic theory of gases that was discussed above.5.2 Boyle’s Law: Pressure and volume of an enclosed gas
ESBAZ
If you have ever tried toforce in the plunger of asyringe or a bicycle pump while sealing the opening
with your finger, you will have seen Boyle’s Lawin action! This will nowbe demonstrated using a 10
ml syringe.General experiment: Boyle’s Law
Aim: To demonstrate Boyle’s law.
Apparatus: A syringe.m� (^105)
Method:
- Hold the syringe inone hand, and with theother pull the plunger out towards you so
that the syringe is now full of air. - Seal the opening of the syringe with your finger so that no air can escape the syringe.
- Slowly push the plunger in, and notice whether it becomes more or less difficult to push
the plunger in.
Results: What did you notice when you pushed the plunger in? What happensto the volume
of air inside the syringe? Did it become more or less difficult to push the plunger in as the
volume of the air in thesyringe decreased? In other words, did you haveto apply more or less
force to the plunger as the volume of air in the syringe decreased?As the volume of air inthe syringe decreases, you have to apply more force to the plunger to
keep pressing it down. The pressure of the gas inside the syringe pushingback on the plunger
is greater. Another wayof saying this is that as the volume of the gas inthe syringe decreases,
the pressure of that gas increases.Conclusion: If the volume of thegas decreases, the pressure of the gas increases. If the
volume of the gas increases, the pressure decreases. These results support Boyle’s law.In the previous experiment, the volume of thegas decreased when thepressure increased, andthe
volume increased whenthe pressure decreased.This is called an inverse relationship (or more-less
relationship). The inverse relationship between pressure and volume is shown in Figure 5.1.
Can you use the kinetictheory of gases to explain this inverse relationship between the pressureand
volume of a gas? Let’s think about it. If you decrease the volume of agas, this means that thesame
number of gas particlesare now going to comeinto contact with each other and with the sides of the
container much more often. You may rememberfrom earlier that we saidthat pressure is a measure of
the frequency of collisions of gas particles with each other and with the sides of the container theyare