Page 18 - Curriculum Visions Dynamic Book

the water is superheated and only then is corrosion important. Observation windows used in boilers can be vulnerable to this kind of attack. Very often mica (which in thin sheets is also reasonably transparent) is used instead of glass or as an inner protective sheet to keep the water from the glass.
Finally, a few parts of alumina (aluminium oxide) in the glass composition greatly improves the way glass can stand up to attack by acids and alkalis.
Electrical properties
Glass is a nonmetal. Like the majority of nonmetals, it is an insulator that has very high electrical resistance. Copper, for example, conducts electricity 1018 (ten million, million, million) times as well as glass.
In general, the electrical conductivity of glass increases with the proportion of soda, lime, and other alkalis in it.
On the other hand, some glasses are very good at storing electricity on their surfaces. This is an extremely important property because electrical charge storage devices called capacitors are used in many electronic circuits.
Such glasses separate the metal plates of a capacitor by their good insulating properties.
Any glass that is developed for its electrical storage properties is known as a high-k glass.
Thermal properties
Glasses can be very good at holding heat compared with many substances, but still only a fifth to a third as good as water.
Glass is also a reasonable insulator of heat, but not as good as air. That is one reason why in double glazing air is used as the insulator trapped between two layers of glass.
Single-pane glass has no such insulating layer and therefore allows lots of heat to escape.
(Above) The sodium in glass can be identified by a flame test. The flame changes colour as the glass melts, showing the typical orange colour of the element sodium and the red colour of the element calcium.
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