248 PHYSICAL LIMITATIONS HAVE BEEN OVERCOME valuable mineral substances by selective dialysis. One method of eliminating this objection is the practice that many people have of drinking a glass or two of lukewarm normal saline solution about a half-hour before breakfast. The term "normal" here refers to the fact that the saline solution, which is about 0.9-1 per cent and contains two level teaspoons of common salt to a quart of water, is isotonic with the blood. The body receives its food by dialysis through the membranes in the intestinal tract, so that both dialysis and osmosis take place at the same time. A solution having the same osmotic pressure as the blood would be called isosmotic with the blood, but such a solution might still change in concentration as the result of dialysis either from it to the blood or vice versa. A solution which does not change either by osmosis or dialysis when placed on the opposite side of the membrane to that of the blood is said to be isotonic with the blood. The chemical garden is an excellent illustration of osmosis. It is prepared by dropping little crystals of very soluble colored salts such as ferric chloride, nickel chloride, cobalt nitrate, manganese nitrate, uranium nitrate, copper sulfate, etc., into a 10 per cent sodium silicate solution. These salts dissolve in the film of water surrounding the crystal, and the resulting solution reacts with the sodium silicate to form membranes. Water passes by osmosis into the crystal side of the membrane and finally bursts the membrane. The solutions thus form new membranes, and the growth continues. Your instructor will undoubtedly demonstrate the chemical garden for you. The lowering of the vapor pressure and the freezing-point, the elevation of the boiling-point, and the osmotic pressure of solutions are all due to the number of molecules present rather than to their chemical composition. It is possible, therefore, to estimate the relative number of molecules of any solute present in a given weight of solvent by measuring the above properties of the solutions in question. It should be added here that there is a group of substances for which the above properties are abnormally high. The simplest way to explain such abnormalities is by saying that the molecules must have subdivided to form a larger number of smaller particles, each of which produced the same effect as the original molecule. Such abnormalities, characteristic of solutions of electrolytes, will be studied in a later unit. STUDY QUESTIONS 1. Define: solvent, solute, and solution. 2. How does water behave when it is cooled from 20° C. to —10° C? 3. Give an example of a supersaturated solution.
REGULARITIES OF SOLUTIONS 249 4. What is deliquescence? Give an application. 5. What is the effect of temperature on the solubility of a gas in a liquid? 6. What is the effect of pressure on the solubility of a gas in a liquid? 7. What is the effect of dissolved substances on the boiling-point, freezing-point, and vapor pressure of liquids? 8. What is osmosis? Describe an experiment illustrating osmosis. 9. What is the relation of osmotic pressure to the concentration of a solution? 10. Explain the use of calcium chloride on dusty roads. 11. How would 3^ou prepare a saturated solution? 12. How would you prepare a supersaturated solution? 13. Explain the use of vinegar or cream of tartar in making candy. 14. Why do red blood cells burst when they are placed in distilled water? 15. Explain the physiological effect of Epsom salts. 16. How would you prepare a normal saline solution? Is it isotonic or isosmotic with the blood?