Page 53 - Physical chemistry understanding our chemical world

20 INTRODUCTION TO PHYSICAL CHEMISTRY

SAQ 1.2 Identify the variable, number, factor and unit in the phrase,
‘length = 3.2km’.

1.3 Properties of gases and the gas laws

Why do we see eddy patterns above a radiator?

The effects of temperature on density

The air around a hot radiator soon acquires heat. We explain this observation from
the ‘minus oneth law of thermodynamics’ (see Section 1.1), since heat travels from
hot to cold.
The density of a gas depends quite strongly on its temperature, so hot air has
a smaller density than does cold air; colder air is more dense than hot air. From
everyday experience, we know that something is dense if it tries to drop, which is
why a stone drops to the bottom of a pond and a coin sinks to the bottom of a pan of
water. This relative motion occurs because both the stone and the coin have higher
densities than does water, so they drop. Similarly, we are more dense than air and
will drop if we fall off a roof.
Just like the coin in water, cold air sinks because it is denser than warmer air.
We sometimes see this situation stated as warm air ‘displaces’ the cold air, which
subsequently takes its place. Alternatively, we say ‘warm air rises’, which explains
why we place our clothes above a radiator to dry them, rather than below it.
Light entering the room above the radiator passes through these pockets of warm
air as they rise through colder air, and therefore passes through regions of different
density. The rays of light bend in transit as they pass from region to region, much in
the same way as light twists when it passes through a glass of water. We say the light
is refracted. The eye responds to light, and interprets these refractions and twists as
different intensities.
So we see swirling eddy (or ‘convective’) patterns above a radiator because the
density of air is a function of temperature. If all the air had the same temperature, then
no such difference in density would exist, and hence we would see no refraction and
no eddy currents – which is the case in the summer when the radiator is switched off.
Then again, we can sometimes see a ‘heat haze’ above a hot road, which is caused
by exactly the same phenomenon.

Why does a hot-air balloon float?
The effect of temperature on gas volume

A hot-air balloon is one of the more graceful sights of summer. A vast floating
ball, powered only by a small propane burner, seems to defy gravity as it floats
effortlessly above the ground. But what is it causing the balloon to fly, despite its
considerable weight?