THE CHEMISTRY OF LIFE 43 



tolerance — some do best in soil which is slightly acid, others do best 

 when it is on the alkaline side. Farmers who want to get the most 

 from their crops test the pH value of their soil and adjust it to the 

 optimum needs of the specific crop to be planted. 



Kinds of Mixtures 



Chemists recognize nine different kinds of mixtures — gas in gas, 

 gas in liquid, gas in solid, liquid in gas, liquid in liquid, liquid in solid, 

 solid in gas, solid in liquid, and solid in solid. Since water is universally 

 present in living matter and since water is a liquid, we shall learn more 

 about the kinds of mixtures which substances can form with a liquid, 

 using water as a specific example. 



Solution. Whenever a substance is mixed with water, several dif- 

 ferent kinds of mixtures may result depending upon the size of the 

 particles of the substance in the water. Some substances break down 

 to form individual molecules, ions, or even atoms. Such a substance 

 is said to be dissolved in the water, and the water being known as the 

 solvent and the dissolved substance as the solute. This type of mixture 

 is known as a solution, and the dissolved substance may be a solid, 

 liquid, or a gas. Sugar is an example of a solid which dissolves in 

 water to yield a clear solution. Alcohol is a liquid which dissolves in 

 water, and oxygen is a gas which dissolves in water. (Were it not for 

 dissolved oxygen, fish or other water animals could not live.) In every 

 case, the physical and possibly the chemical properties of the water 

 are altered by the dissolved substance. Its boiling and freezing point 

 may be altered, as well as its ability to transmit electric currents, etc. 

 Also, in the case of a solution, the dissolved particles do not settle out 

 or rise to the surface on standing. When water containing solids in 

 solution is evaporated, the solids will form in characteristic crystals in 

 most cases. We call such substances crystalloids. 



Suspension. If we place some powdered clay in water and shake it 

 thoroughly, the water becomes cloudy and the particles will be sus- 

 pended in it. The clay particles have not gone into solution, however, 

 for each particle is composed of many molecules and these will settle 

 to the bottom if this mixture is allowed to stand for a time. These 

 particles can be seen under the microscope and appear as rather large 

 bodies. This type of mixture is known as a suspension. Water in 

 such a suspension has the same boiling and freezing points, as well as 

 the other physical and chemical properties, that are found in pure water. 



Emulsion. If we put a little olive oil in water and shake it vigor- 

 ously, the oil will become broken into many tiny particles which be- 



