10 PROPERTIES OF SOLUTIONS 



prepare a true saturated solution of any solute unless some of it is also present 

 in the solid state. Under certain conditions, and only when none of the un- 

 dissolved solid is present, a super-saturated solution can be prepared ; that is, a 

 solution in which the concentration of the solute is greater than that in the 

 saturated solution. If a fragment of the solid solute be added to such a solu- 

 tion the excess solute usually crystallizes out immediately, and the concen- 

 tration of the solution decreases to that usually present at saturation. 



Usually increase in temperature increases the solubility of a solid in a liquid 

 but there are some exceptions to this principle. The solubility of common 

 salt (NaCl) in water, for example, is only slightly influenced by tempera- 

 ture, while the solubility of many calcium compounds is decreased by an in- 

 crease in the temperature of water. 



Methods of Expressing the Composition of Solutions. — If a mol ^ of 

 any soluble compound be dissolved in just enough water to make exactly one 

 liter of solution the result is a volume molar solution. Since the volume of 

 water changes with temperature it is usual to specify that the solution is to 

 be made up to a liter volume at 20° C. Whenever the word molar is used 

 without qualification this type of solution is meant. Similarly, if half the 

 molar weight, or one-tenth the molar weight of a substance be dissolved in 

 enough water to make exactly one liter of solution the result is a half molar 

 (0.5 M) or tenth molar (o.i M) solution, respectively, etc. Gram molar 

 weights of all substances contain the same number of molecules. Estimates 

 made by various methods show this number to be about 6.06 X 10^3 molecules. 

 Hence one liter of any volume molar solution will contain this number of 

 solute molecules, one milliliter (o.ooi liter) will contain one thousandth of 

 this number, and so on. Equal volumes of all solutions of the same volume 

 molarity contain the same number of solute molecules but different numbers 

 of solvent molecules. If a given volume of a volume molar solution be diluted 

 with an equal volume of water, the result is a 0.5 M solution; if a given 

 volume be diluted with nine volumes of water the result is a O.i M solution, 

 etc. Therefore a volume molar solution of any strength may be diluted with 

 water, and the resulting more dilute solution will have a volume molarity in 

 proportion as it has been diluted. 



If a mol of any soluble substance be completely dissolved in lOOO g. of 

 water the result is a weight molar solution. Such solutions are often called 

 molal solutions. They are used principally in experimental work upon various 

 osmotic phenonema. The addition of a mol of most solids to a liter of water 

 will increase the volume of the resulting solution to more than one liter. This 

 increase in volume is called the solution volume of the solute. The solution 



^ A mol is the molecular weight of a compound in grams. 



