Page 20 - Curriculum Visions Dynamic Book

Lead nitrate and sulphide
Lead is not a very reactive metal, and the compounds it forms are therefore not very stable. As a result it is relatively easy to get lead compounds to break down, or decompose.
Some lead compounds also have photoelectric properties, that is, they will change their conductivity when infra-red light reaches them.
Both of these properties are demonstrated on these pages.
Lead sulphide as a photoconductor
When some lead compounds absorb light, electrons are freed, making them more conductive. Lead sulphide (PbS), lead selenide (PbSe) and lead telluride (PbTe) are all sensitive to infra-red radiation (although they are not sensitive to visible light).
Each of these photo-conducting substances finds a role in detecting infra-red radiation. For example, photoconductors can detect the infra-red radiation given out by warm bodies. It is for this reason
that they are used to
make PIR (Passive infra-
red) detectors, now
commonplace as part of
home security. They are
used to trigger alarms
or switch on lights when
a warm body (such as
a person or a vehicle)
comes within their
detection range.
 A PIR detector containing a lead sulphide photoconductor.
Also:
Cadmium sulphide is sensitive to changes in visible light, and it is the semi-conducting material used in dusk-to-dawn light switches and camera photocells.
Decomposition of lead nitrate
Lead nitrate decompose to lead oxide when it is heated. As the lead nitrate decomposes, energy is released, and the crystals
of lead nitrate break up. The process of breaking down is called decrepitation; it produces a loud crackling sound.
Decrepitation releases both nitrogen dioxide and oxygen, the gases collected at the left hand end of the apparatus.
 The apparatus set up before the experiment
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