Physics Part 4

Brownian Movements.

Questions

1. What is the molecular (kinetic) theory of gases?

2. What three kinds of evidence help to confirm the theory?

3. What have you seen that seems to show that a gas consists of molecules in motion?

4. How many meters long is a 10-ft. pole?

5. A 50-kg. boy weighs how many pounds?

6. What are three advantages of the metric system?

7. What will 12 qts. of milk cost at 8 cents a liter?

8. A cube 1 meter each way will contain how many cubic centimeters? How many liters? What will a cubic meter of water weigh?

(2) MOLECULAR MOTION IN LIQUIDS

=18. Diffusion of Liquids.=--From the evidence given in Arts. 14-17, (a) of diffusion of odors, (b) of the continued _expansion_ of air in the air pump, and (c) of the pressure exerted by a gas in all directions, one may realize without difficulty that a _gas consists of small particles in rapid motion_. Let us now consider some of the evidence of molecular motion in liquids. If a little vinegar is placed in a pail of water, all of the water will soon taste sour. A lump of sugar in a cup of tea will sweeten the entire contents. This action is somewhat similar to the diffusion of gases but it takes place much more slowly. It is therefore believed that the motion of liquid molecules is much slower than that of gas molecules.

Again, if a dish of water is left standing in the open air in fine weather, within a few days the dish will become dry though no one has taken anything from it. We say the water has _evaporated_. What was liquid is now _vapor_. If we were to observe carefully any dish of water we would find that it continually loses weight on dry days. That is, there is a constant movement of the molecules of water into the air.

This movement of the molecules is explained as follows. There appear to be in the dish of water some molecules that by moving back and forth acquire a greater velocity than their neighbors; when these reach the surface of the liquid, some vibration or movement sends them flying into the air above. They are now vapor or gas molecules, flying, striking, and rebounding like the air molecules. Sometimes on rebounding, the water molecules get back into the water again. This is especially apt to happen when the air is damp, _i.e._, when it contains many water molecules. Sometimes the air over a dish becomes _saturated_, as in the upper part of a corked bottle containing water. Although molecules are continually leaving the surface of the water they cannot escape from the bottle, so in time as many molecules must return to the water from the s.p.a.ce above as leave the water in the same time. When this condition exists, the air above the water is said to be _saturated_. On very damp days the air is often saturated. The explanation above shows why wet clothes dry so slowly on such a day (See Arts. 166-7 on Saturation.)

=19. Cooling Effect of Evaporation.= We have seen that warming a gas increases its volume. This expansion is due to the increased motion of the warmed molecules. Now the molecules that escape from a liquid when it evaporates are naturally the fastest moving ones, _i.e._, the hottest ones. The molecules remaining are the slower moving ones or colder molecules. The liquid therefore becomes colder as it evaporates, unless it is heated. This explains why water evaporating on the surface of our bodies cools us. In evaporating, the water is continually losing its warm, fast moving molecules. The _cooling effect_ of evaporation is, therefore an evidence of molecular motion in liquids.

[Ill.u.s.tration: FIG. 8.--Osmosis Shown by carrot placed in water.]

=20. Osmosis.=--If two liquids are separated by a membrane or porous part.i.tion, they tend to pa.s.s through and mix. This action is called osmose, or _osmosis_.

Such a movement of liquid molecules in osmosis may be ill.u.s.trated by filling a beet or carrot that has had its interior cut out to form a circular opening (see Fig. 8) with a thick syrup. The opening is then closed at the top with a rubber stopper through which pa.s.ses a long gla.s.s tube.

If the carrot is immersed in water, as in Fig. 8, a movement of water through the porous wall to the interior begins at once. Here, as in the experiment of the hydrogen and air pa.s.sing through the porous cup, the lighter fluid moves faster. The water collecting in the carrot rises in the tube. This action of liquids pa.s.sing through porous part.i.tions and mingling is called _osmosis_.

Gases and liquids are alike in that each will _flow_. Each is therefore called a _fluid_. Sometimes there is much resistance to the flow of a liquid as in mola.s.ses. This resistance is called _viscosity_. Alcohol and gasoline have little viscosity. They are _limpid_ or _mobile_. Air also has some viscosity. For instance, a stream of air always drags some of the surrounding air along with it.

Important Topics

1. Liquids behave as if they were composed of small particles in motion.

2. This is shown by (1) Diffusion, (2) Solution, (3) Evaporation, (4) Expansion, (5) Osmosis.

Exercises

1. Give an example or ill.u.s.tration of each of the five evidences of molecular motion in liquids.

2. When is air saturated? What is the explanation?

3. Why does warming a liquid increase its rate of evaporation?

4. Air molecules are in rapid motion in all directions. Do they enter a liquid with a surface exposed to the air? Give reason.

5. What are some of the inconveniences of living in a saturated atmosphere?

6. Fish require oxygen. How is it obtained?

(3) MOLECULAR FORCES IN LIQUIDS

=21. Cohesion and Adhesion.=--In liquids "the molecules move about freely yet tend to cling together." This tendency of molecules to cling together which is not noticeable in gases is characteristic of =liquids= and especially of =solids=. It is the cause of the viscosity mentioned in the previous section and is readily detected in a variety of ways.

For instance, not only do liquid molecules cling together to form drops and streams, but they cling to the molecules of solids as well, as is shown by the wet surface of an object that has been dipped in water. The attraction of like molecules for one another is called _cohesion_, while the attraction of =unlike molecules= is called _adhesion_, although the force is the same whether the molecules are alike or unlike. It is the former that causes drops of water to form and that holds iron, copper, and other solids so rigidly together. The adhesion of glue to other objects is well known. Paint also "sticks" well. Sometimes the "joint"

where two boards are glued together is stronger than the board itself.

The force of attraction between molecules has been studied carefully.

The attraction acts only through very short distances. The attraction even in liquids is considerable and may be measured. The cohesion of water may be shown by an experiment where the force required to pull a gla.s.s plate from the surface of water is measured.

[Ill.u.s.tration: FIG. 9.--The water is pulled apart.]

Take a beam balance and suspend from one arm a circular gla.s.s plate, Fig. 9. Weigh the plate and its support. Adjust the gla.s.s plate so that it hangs horizontally and just touches the surface of clean water, the under side being completely wet. Now find what additional weight is required to raise the gla.s.s plate from the water.

Just as the plate comes from the water its under side is found to be wet. That is, _the water was pulled apart_, and the plate was not pulled from the water. The cohesion of the water to itself is not so strong as its adhesion to the gla.s.s.

The cohesion of liquids is further shown by the form a drop of liquid tends to take when left to itself. This is readily seen in small drops of liquids. The spherical shape of drops of water or mercury is an example. A mixture of alcohol and water in proper proportions will just support olive oil within it. By carefully dropping olive oil from a pipette into such a mixture, a drop of the oil, an inch or more in diameter suspended in the liquid, may be formed. It is best to use a bottle with plane or flat sides, for if a round bottle is used, the sphere of oil will appear flattened.

[Ill.u.s.tration: FIG. 10 _a_.

FIG. 10 _b_.

FIGS. 10 _a_ AND _b_.--Surface tension of a liquid film.]

=22. Surface Tension.=--The cohesion of liquids is also indicated by the tendency of films to a.s.sume the smallest possible surface. Soap bubble films show this readily. Fig. 10 _a_ represents a circular wire form holding a film in which floats a loop of thread. The tension of the film is shown in Fig. 10 _b_ by the circular form of the loop after the film within it has been pierced by a hot wire, Fig. 11 shows a rectangular wire form with a "rider." The tension in the film draws the rider forward.

[Ill.u.s.tration: FIG. 11.--The rider is drawn forward.]

[Ill.u.s.tration: FIG. 12.--Surface tension causes the pointed shape.]

A soap bubble takes its spherical shape because this form holds the confined air within the smallest possible surface. A drop of liquid is spherical for the same reason. Many ill.u.s.trations of the tension in films may be given. Users of water colors notice that a dry camel's-hair brush is bushy. (Fig. 12 _A_). When in water it is still bushy. (Fig. 12 _B_.) But when it is taken from the water and the excess is shaken from it, it is pointed as in Fig. 12 _C_. It is held to the pointed shape by the tension of the liquid film about the brush.

[Ill.u.s.tration: FIG. 13.--A needle depresses the surface when floating.]

The surface of water acts as if covered by a film which coheres more strongly than the water beneath it. This is shown by the fact that a steel needle or a thin strip of metal may be floated upon the surface of water. It is supported by the surface film. (See Fig. 13.) If the film breaks the needle sinks. This film also supports the little water bugs seen running over the surface of a quiet pond in summer. The surface film is stronger in some liquids than in others. This may be shown by taking water, colored so that it can be seen, placing a thin layer of it on a white surface and dropping alcohol upon it. Wherever the alcohol drops, the water is seen to pull away from it, leaving a bare s.p.a.ce over which the alcohol has been spread. This indicates that the alcohol has the weaker film. The _film of greasy benzine is stronger_ than the film of the pure material. If one wishes to remove a grease spot and places pure benzine at the center of the spot, the stronger film of the greasy liquid will pull away from the pure benzine, and spread out, making a larger spot than before, while if pure benzine is placed _around the grease spot_, the greasy liquid at the center pulls away from the pure benzine, drawing more and more to the center, where it may be wiped up and the grease entirely removed.

[Ill.u.s.tration: FIG. 14.--The molecule at _A_ is held differently from one within the liquid.]