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224 PHASE EQUILIBRIA

How do carbon monoxide sensors work?

Henry’s law and solid-state systems

Small, portable sensors are now available to monitor the air we breathe for such
toxins as carbon monoxide, CO. As soon as the air contains more than a critical
concentration of CO, the sensor alerts the householder, who then opens a window or
identifies the source of the gas.
At the ‘heart’ of the sensor is a slab of doped transition-metal oxide. Its mode
of operation is to detect the concentration of CO within the oxide slab, which is in
direct proportion to the concentration of CO gas in the air surrounding it, according
to Henry’s law.
A small voltage is applied across the metal oxide. When it contains no CO, the
electrical conductivity of the oxide is quite poor, so the current through the sensor is
minute (we argue this corollary from Ohm’s law). But increasing the concentration of
CO in the air causes a proportionate increase in the amount of CO incorporating into
the solid oxide, which has a profound influence on electrical conductivity through the
slab, causing the current through the slab to increase dramatically. A microchip within
the sensor continually monitors the current. As soon as the current
In general, Henry’s increases above its minimum permissible level, the alarm sounds.
law only applies over So, in summary, CO gas partitions between the air and carefully
relatively small ranges formulated solid oxides. Henry’s law dictates the amount of CO in
of gas pressure. the oxide.

Why does green petrol smell different from
leaded petrol?

Effects of amount of material on vapour pressure

A car engine requires petrol as its source of fuel. Such petrol has
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Petrol is only useful in a low boiling temperature of about 60 C. Being so volatile, the
a car engine because it liquid petrol is always surrounded with petrol vapour. We say it has
is volatile. ahigh vapour pressure (also called ‘saturated vapour pressure’),
which explains why we smell it so readily.
Once started, the engine carburettor squirts a mixture of air and volatile petrol
into a hot engine cylinder, where the mixture is ignited with a spark. The resultant
explosion (we call it ‘firing’) provides the ultimate source of kinetic energy to propel
the car.
A car engine typically requires four cylinders, which fire in a carefully synchro-
nized manner. Unfortunately, these explosions sometimes occur prematurely, before