Page 46 - Curriculum Visions Dynamic Book

not dissolve. This is, of course, important in our bodies too, because otherwise everything we are made of would dissolve! Washing-up detergent and soap,
on the other hand, have properties that
are similar to water and some that are similar to fat. So, they are able to act as a chemical go-between and allow water to be used to remove fat.
Water at high temperature
and pressure
Interestingly, water only works on the
“like dissolves like” principle when it is
at normal temperatures. If water is made
very hot (and kept liquid by keeping
it under pressure), it will dissolve even
substances it is not similar to. This is an extremely
odd but useful property. It is used in industry, for example, for destroying organic wastes. In this process oxygen is mixed with the organic material under very hot high pressure. The organic material combusts (catches fire) with a flame burning under water, the very opposite to the way water normally behaves when it is cold and not under pressure!
Conducting electricity
Pure water cannot conduct electricity. That is because water molecules carry no electrical charge. But water is rarely pure and usually contains substantial quantities of materials.
Dissolved solids such as salt are called ionic substances. That is, they can split up into negatively charged and positively charged particles, which are called ions. Although ionic solids such as salt do not conduct electricity, and pure water is also a poor conductor, when salt dissolves in water, the ions are free to move around. When it dissociates in this way, the salt is said to have been ionized. As a result, the aqueous solution of salt in water is a very good conductor of electricity. Solutions like this are called electrolytes.
Not all solids dissolve and form ions, however. When sugar dissolves, it forms molecules with no charge. Sugary water is therefore as good an insulator as pure water.
(Above) A storage battery, such as is found in a car, uses an acid dissolved in water as an electrolyte. Sulfuric acid is usually used between lead plates.
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