WATER 69 



calcium hydroxide. Other oxides, for example sulphur dioxide or 

 phosphorus pentoxide, give acids (sulphurous acid or phosphoric 

 acid). Cases in which water acts chemically upon substances 

 dissolved in it will be noted later (p. 145). 



Reversible Chemical Actions. The contrary effect upon 

 an unstable hydrate of leaving the bottle open or closed, referred 

 to on p. 68, deserves a moment's notice. When understood, it 

 explains many things in chemistry. The hydration and dehy- 

 dration are opposite directions of the same chemical change, and 

 the condensed statements of the actions were written with the 

 double arrow to indicate this (p. 68). When the bottle is closed, 

 the water vapor is unable to escape and recombines with the 

 anhydrous particles as fast .as other particles of the hydrate de- 

 compose. A reversible action therefore can not complete itself, if 

 the products of the action are kept together and not allowed to 

 separate. On the other hand, a reversible action will go to com- 

 pletion, if one of the products escapes, as the water vapor does 

 when the bottle is left open. This idea enables us to answer 

 several interesting questions. 



For example, why does steam decompose to the extent of 1.8 

 per cent at 2000 degrees, but not any further? All its parts are 

 alike and are therefore equally capable of decomposing. The 

 answer is, because neither the oxygen nor the hydrogen is removed, 

 or can easily be removed, from the steam, and so the completion 

 of the decomposition is prevented by continual recombination of 

 these gases. 



When a reversible action has come to a standstill, we say that 

 equilibrium has been reached. This means that two opposing 

 tendencies are neutralizing one another's effects. 



When will reversible actions go to completion? The products 

 must be of such a nature that they separate easily. In practice 

 this happens when one is a gas or vapor, like the water vapor 



