FREEZING MIXTURES 141 



air in water. All water exposed to air has air dissolved in 

 it. It is upon this air in solution that fishes depend for the 

 oxygen they need. But while air may hold moisture, and 

 water may hold air, Experiments 37 and 61 show an impor- 

 tant point of difference between the capacity of air for water 

 and of water for air. We learned that when air is heated 

 it is capable of holding more water vapor. But when 

 water is heated, it is capable of holding less air. 



Experiment 62. Stir salt, a little at a time, into a test tube of 

 water which is no warmer than the temperature of the room. 

 Gradually increase the salt until the water will absorb no more, 

 and a little of the salt settles at the bottom of the test tube. Now 

 heat the solution. What happens to the salt at the bottom of 

 the test tube? Set the test tube containing the solution aside 

 to cool. Does any of the salt reappear in solid form ? 



If we put as much of a solid substance into a liquid as the 

 liquid will dissolve, we have a saturated solution. If any 

 more of the solid is added, it will remain undissolved. 

 As the temperature of water increases, it can hold more 

 solid matter in solution. If a liquid at a certain tempera- 

 ture is saturated with a solid and then is reduced to a 

 lower temperature, it will, under ordinary circumstances, 

 deposit some of the solid. What similar thing happens in 

 the atmosphere ? 



Freezing Mixtures. Experiment 63. Place some chopped 

 ice in a beaker, and test the temperature. Add a generous amount 

 of salt and test the temperature again. Has there been a fall of 

 temperature ? 



Salt and some other substances tend to absorb water and 

 to form a solution whenever it is possible. On a damp day 

 salt sticks in the salt-shaker. This simply indicates that 

 salt has absorbed moisture from the atmosphere. 



