Physics Part 5

=23. Explanation of the Surface Film.=--Beneath the surface of a liquid each molecule is attracted by all the other molecules around it. It is attracted equally in all directions. Consequently the interior molecules move very easily over each other in any direction. A molecule at the surface, as at _A_, Fig. 14, is not attracted _upward_ by other liquid molecules. Its freedom of motion is thereby hindered with the result that a molecule at the surface behaves differently from one beneath the surface. The surface molecules act as if they form an elastic skin or membrane upon the liquid surface.

[Ill.u.s.tration: FIG. 15.--Capillary attraction in tubes.]

=24. Capillarity.=--A striking action of the surface film of a liquid is seen in the rise of liquids in tubes of small bore when the liquid _wets_ them. If the liquid _does not wet_ the tube, as when mercury is placed in gla.s.s, the liquid is depressed. It is found in general that: _Liquids rise in capillary tubes when they wet them and are depressed in tubes which they do not wet; the smaller the diameter of the tube the greater the change of level._ (See Fig. 15.) This action is explained as follows: The molecules of a liquid have an attraction for each other and also for the sides of a tube. The former is called "cohesion for itself," the latter is called "adhesion for the sides of the containing vessel." If the cohesion for itself is greater than the adhesion for the side of the containing vessel, the liquid is pulled away from the side and is depressed. If the adhesion is greater, the liquid is elevated.

This action is called "capillary action" from the Latin word (_capillus_) signifying hair, since it shows best in fine hairlike tubes.

There are many common ill.u.s.trations of capillary action: oil rising in a wick; water rising in a towel or through clothes; ink in a blotter, etc.

The minute s.p.a.ces between the fibers composing these objects act as fine tubes. If cloth is treated with a preparation which prevents water from adhering to its fibers, the material will not be wet when water is poured upon it, because the water will not run in between the fibers; a surface film spreads over the cloth so that no water enters it.

_Cravenette cloth_ has been treated in this way and hence is waterproof.

The action of this film may be shown by the following experiment.

Dip a sieve of fine copper gauze in melted paraffin, thus coating each wire so that water will not adhere to it. Water may now be poured into the sieve, if a piece of paper is first laid in it to break the force of the water. On carefully removing the paper the surface film of the water will prevent the pa.s.sage of the water through the sieve.

=25. Capillary Action in Soils.=--The distribution of moisture in the soil depends largely upon capillary action. When the soil is compact the minute s.p.a.ces between the soil particles act as capillary tubes, thus aiding the water to rise to the surface. As the water evaporates from the surface more of it rises by capillary action from the damper soil below. Keeping the soil loose by cultivation, makes the s.p.a.ces between the particles too large for much capillary action, thus the moisture is largely prevented from rising to the surface.

In the semi-arid regions of the West "_dry farming_" is successfully practised. This consists in keeping the surface covered with a "dust mulch" produced by frequent cultivation. In this way the moisture is kept below the surface, where it can be utilized during the hot dry summer by the roots of growing plants.

Important Topics

1. Attractive forces between liquid molecules.

2. Cohesion (like molecules); adhesion (unlike molecules).

3. Special effects of this force are cla.s.sified as (a) capillary action, and (b) surface tension.

Exercises

1. What evidence of capillary action have you seen outside of the laboratory?

2. What is the explanation for capillary action?

3. Where are surface films found?

4. What are three common effects of surface films?

5. Explain why cravenette cloth sheds water.

6. If a circular gla.s.s disc 10 cm. in diameter requires 50 grams of force to draw it from the water, what is the cohesion of water per square centimeter?

7. What is the weight in grams of 1 ccm. of water? of a liter of water?

8. Name five examples of adhesion to be found in your home.

9. Under what conditions will a liquid wet a solid and spread over it?

10. When will it form in drops on the surface?

11. Explain the proper procedure for removing a grease spot with benzine.

12. What difference is there between a liquid and a fluid?

13. Why cannot a "soap bubble" be blown from pure water?

14. Which are larger, the molecules of steam or those of water? Why?

15. Why is the ground likely to be damp under a stone or board when it is dry all around?

16. Why does any liquid in falling through the air a.s.sume the globule form?

17. Give three examples of capillary attraction found in the home. Three out of doors.

18. Why does cultivation of the soil prevent rapid evaporation of water from the ground?

(4) EVIDENCES OF MOLECULAR FORCES IN LIQUIDS AND SOLIDS

=26. Solutions.=--A crystal of pota.s.sium 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 ill.u.s.tration 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 sh.e.l.lac, 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 s.p.a.ce 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 acc.u.mulating 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 b.u.t.ter 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 pa.s.s 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.

Important Topics

1. The solution of solids is increased by heating.

2. The solution of gases is decreased by heating.