68 FIRST YEAR COURSE IN GENERAL SCIENCE 



at a much higher temperature than lead, but at a much 

 lower temperature than platinum. 



If a liquid is sufficiently cooled, it returns to the solid 

 state. The highest temperature at which it does this is 

 called the temperature of solidification or the freezing point. 

 The freezing point of water is 32 F.; of sea water, 28 F.; 

 of sulphur, 240 F.; of copper, 2000 F. 



The temperature of solidification and the melting point 

 are the same for a given substance. If a thermometer is 

 placed in cracked ice in a room of moderate temperature, 

 it will show the temperature of the melting mass to be 

 32 F. If the thermometer were placed in water out of 

 doors on a cold winter day, the temperature of the freezing 

 water would be 32 F. 



The temperature at which a substance changes from the 

 liquid to the gaseous form, when heated in an open vessel, 

 is called its boiling point. For water, this temperature is 

 212 F. On the other hand, if a gas is cooled sufficiently, 

 it will condense or change into the liquid form. The high- 

 est temperature at which the gas condenses is the same as 

 the boiling point of the liquid which it forms. 



71. Distillation. When water and other liquids are 

 changed into the gaseous form by boiling, many of the im- 

 purities which were dissolved in the liquid do not change to 

 a vapor. Hence, if the vapor is collected in a separate 

 vessel and cooled until it condenses, a product purer than 

 the original liquid may be obtained. This process is called 

 distillation, and the product formed by the condensed vapor 

 is called the distillate. 



When a mixture of two or more liquids is heated, the in- 

 gredients become gases at different temperatures, because 

 each substance has its own boiling point. The ingredient 

 with the lowest boiling point escapes as a vapor first. If 

 a mixture contains three liquids whose boiling points are 

 150, 180, and 212 respectively, the liquids can be sepa- 



