# Man's physical universe

242 PHYSICAL LIMITATIONS HAVE BEEN OVERCOME

is the ratio of the quantity of solute to the quantity of solvent in a

saturated solution.

A saturated solution is one which is in equilibrium with the undissolved

solute.

It represents a dynamic condition in which molecules of solute

are dissolving just as rapidly as they are coming out of solution.

saturated solution of sugar in water can be prepared by stirring sugar

in water until no more sugar dissolves. A quicker way to prepare it is

to heat the water and stir in sugar hastily, making sure that there is an

excess of undissolved sugar, and then cool to the desired temperature.

Inasmuch as the amount of sugar which dissolves in water increases

with a rise in temperature, a nearly saturated solution at a high temperature

contains more sugar than a saturated solution at a lower

temperature. Therefore, the excess sugar crystallizes out when the

solution is cooled, leaving a saturated solution.

Supersaturated Solutions Are Well Known in the Home.

If the nearly saturated solution should be allowed to cool after

removing the undissolved solute, the excess sugar might not crystallize

out, and the solution would be said to be supersaturated.

A

If a crystal

of undissolved solute should be added to a supersaturated solution, the

excess solute would begin to crystallize at once. A supersaturated

solution may readily be prepared for demonstration by melting crystalline

"photographer's hypo" and allowing it to cool. The cold supersaturated

solution quickly crystallizes upon dropping in a small crystal

of "hypo." The solution expands as it crystallizes and often breaks

beakers or other containers in which the crystallization is allowed to

complete itself.

Supersaturated solutions are frequently met with in the home.

Honey, for example, is a supersaturated solution. Sometimes crystals

of sugar form in the honey. Addition of sugar would bring this about,

but the usual cause of the formation of sugar crystals in honey is undue

cooling. If a supersaturated solution is cooled sufficiently, the supersaturation

becomes so great that crystals form even without seeding

with sugar or some other small particles around which sugar might

form. Honey is likely to "sugar " in the winter, and it is obvious that it

should not be kept in a refrigerator.

In making some types of candy, it is desirable to obtain a supersaturated

solution. Inasmuch as glucose crystallizes less readily than

cane sugar, glucose is used in making such candies; or some acid substance

like vinegar, lemon juice, or cream of tartar is added to cane

sugar to cause it to change to glucose and fructose. Such candies are

stirred as little as possible.

Taffy and butterscotch candy are typical

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