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26. Solutions.—A crystal of potassium permanganate is placed in a liter of water. It soon dissolves and on shaking the flask each portion of the liquid is seen to be colored red. The dissolving of the permanganate is an illustration of the attraction of the molecules of water for the molecules of the permanganate. We are familiar with this action in the seasoning of food with salt and sweetening with sugar.

Water will dissolve many substances, but in varying degrees, i.e., of some it will dissolve much, of others, little, and some not at all. Further, different liquids have different solvent powers. Alcohol will dissolve resin and shellac, but it will not dissolve gum arabic, which is soluble in water. Benzine dissolves grease. Beeswax is not dissolved by water, alcohol or benzine, but is soluble in turpentine.

It is found that the temperature of the liquid has a marked effect upon the amount of substance that will dissolve. This is an indication that the motions of the molecules are effective in solution. It appears that dissolving a solid is in some respects similar to evaporation, and just as at higher temperatures more of the liquid evaporates, because more of the molecules will escape from the liquid into the air above, so at higher temperatures, more molecules of a solid will detach themselves through greater vibration and will move into the liquid.

Further, just as an evaporating liquid may saturate the space above it so that any escape of molecules is balanced by those returning, so with a dissolving solid, the liquid may become saturated so that the solution of more of the solid is balanced by the return of the molecules from the liquid to the solid condition.

27. Crystals and Crystallization.—This return from the liquid to the solid state, of molecules that are in solution, is especially noticeable when the solution is cooling or evaporating and hence is losing its capacity to hold so much of the solid. On returning to the solid, the molecules attach themselves in a definite manner to the solid portion, building up regular solid forms. These regular forms are crystals. The action that forms them is called crystallization.

Each substance seems to have its own peculiar form of crystal due to the manner in which the molecules attach themselves to those previously in place. The largest and most symmetrical crystals are those in which the molecules are deposited slowly with no disturbance of the liquid. Beautiful crystals of alum may be obtained by dissolving 25 g. of alum in 50 ccm. of hot water, hanging two or three threads in the solution and letting it stand over night. The thread fibers provide a foundation upon which crystals grow.

When a solution of a solid evaporates, the molecules of the liquid escape as a gas, the molecules of the solid remain accumulating as crystals. This principle has many uses: (a) sea water is purified by evaporating the water and condensing the vapor, which of course forms pure water. (b) water is forced down to salt beds where it dissolves the salt. The brine is then raised and evaporated, leaving the salt in the evaporating pans.

28. Absorption of Gases by Solids and Liquids.—If a piece of heated charcoal is placed in a test-tube containing ammonia gas, inverted in mercury, the ammonia is seen to disappear, the mercury rising to take its place. The ammonia has been absorbed by the charcoal, the gas molecules clinging closely to the solid. The charcoal being very porous presents a large surface to the action of the gas.

This experiment indicates that attraction exists between gas molecules and other molecules. Many porous substances have this power of absorbing gases. We have all noticed that butter has its flavor affected by substances placed near it.

That liquids absorb gases is shown by slowly heating cold water in a beaker. Small bubbles of air form on the sides and rise before the boiling point is reached. Ammonia gas is readily absorbed in water, the bubbles disappearing almost as soon as they escape into the water from the end of the delivery tube. Household ammonia is simply a solution of ammonia gas in water. On warming the solution of ammonia the gas begins to pass off; thus, warming a liquid tends to drive off any gas dissolved in it.

Soda water is made by forcing carbon dioxide gas into water under strong pressure. When placed in a vessel open to the air the pressure is lessened and part of the gas escapes. The dissolved gas gives the characteristic taste to the beverage.