CHAPTER IV LIQUIDS I. MOLECULAR FORCES IN LIQUIDS Since water is 125. Cohesion, in liquids, is the mutual molecular attraction of the particles of a liquid for one another. the most common liquid, the demonstrations that follow will be made with water unless there is a special reason for using some other liquid. If a glass rod is dipped in water and then removed, a drop will form on the end of the rod and will grow larger and larger as the water runs down the side until the weight of the drop becomes great enough to break it away from the rod, when, as it falls, it takes the form of a sphere. In this experiment cohesion does two things: it keeps the water from falling as soon as it runs down the side of the rod; and it gives the drop the form of a sphere. The spherical form of liquids can be studied by making a mixture of alcohol and water, using 4 such proportions that the mixture will have the same density as olive oil. Introduce a small quantity of the oil below the surface of the mixture, by the use of a glass tube, and the oil will assume the globular form, as in Fig. 99. FIG. 99 Shot are formed by pouring molten lead through sieves at the top of a high tower; the lead is thus separated into small masses, each of which assumes the form of a sphere. Demonstration. - Cover a smooth board with lycopodium powder or powdered lampblack. Drop a small quantity of water upon it from a height of 2 or 3 ft., and the water will scatter and take the form of spheres. NOTE. Lycopodium powder - which is made up of the spores from certain plants can be obtained from any drug store. few cents' worth will be found very useful for many experiments. A Demonstrations. Make one end of a small brass wire very sharp, and bend it into the form of a hook. Put the hook into a glass of clean water so that the point shall be below the surface. Bring the point of the hook up to the surface, and observe that the point, before breaking through the surface, lifts it as FIG. 100 if it were a thin flexible blanket stretched over the water. Observe that the reflection seen from the surface of the water is distorted at the point where the hook lifts the surface. Bend a wire into the shape shown at A (Fig. 101). Place a sewing needle in the hook and lay it carefully upon the surface of clean water, and the needle will float in a little depression upon the surface, as shown in the lower part of the figure. If the needle is placed below the surface, it will sink at once. FIG. 101 NOTE. In all experiments on liquid surfaces great care must be taken to keep the water, and everything that comes in contact with it, clean. The touch of a greasy finger is enough to change the surface tension of the water. Certain insects make use of the above phenomena and are able to run over the surface of water, their feet resting in depressions in its surface just as the needle does. 127. Surface Tension. acting upon a molecule at Let us study the attractions different distances from the sur face, as shown in Fig. 102. At A the molecule is attracted B FIG. 102 equally in all directions by the molecules that are within the distance of molecular attraction (cohesion); hence it can move readily in any direction. At B, very near the surface, the horizontal attractions are equal in all directions, but the inward (downward) attraction is greater than the outward (upward). At the surface the molecule C has no outward attraction, and hence it is held in place by the inward force. As this is true of every molecule on the surface, the result is a tension upon the surface layer much greater than upon any other layer. The inward attraction tends to pull each molecule in from the surface layer, but if a molecule were drawn into the interior of the liquid, it would displace some other molecule and force it to the surface against a similar attraction; so there is no change unless the shape of the liquid body can be changed so as to decrease the area of the surface. The surface tension causes a tendency of the surface to contract to the smallest area possible. This is the reason why liquids take the spherical form (§ 125); a sphere has a smaller surface than any other form of solid of the same volume. Surface tension varies with the liquid and with the temperature of the liquid. The surface tension of pure water, which is very great, compared with that of most other liquids, is illustrated by the following: Demonstrations. Pour some hot water into a shallow dish, like a soup plate. Cover the surface with pepper. Hold a small piece of butter in the surface of the water at the middle, and observe how the pepper goes away from the melting butter to the sides of the plate. Spread a thin layer of clean water upon a clean glass plate, and then let a drop of alcohol fall upon the middle of it. The water will at once retreat, leaving a space around the drop of alcohol. Why? Viscous liquids are stronger than water though their surface tension is less, and for this reason oil is sometimes thrown upon the water around a ship during a storm. The effect of this is to smooth out the surface as though a strong elastic blanket were stretched over the water; and the waves are then kept from breaking over. A drop of kerosene placed upon water has less surface tension than the water and hence is pulled out by the tension of the water into a thin circular film. 128. Films. If we make the thickness of the liquid mass very little, and give to it two free surfaces, surface tension may be studied to better advantage. Demonstrations. Make a strong solution of soap by dissolving castile soap in water until a large bubble can be blown. Bend a A L FIG. 103 B piece of iron wire so as to form an open frame with a handle, and from one side of the frame hang a loop L made of one strand of a silk thread. Dip this frame in the soap solution, and the loop will hang as at A in Fig. 103. Remove the film within the loop by touching it with the point of a piece of blotting paper, and the loop will at once spring out into the form of a circle, as at B. Why? Using a clay pipe or a glass tube, blow a small-sized bubble. Remove the tube from the mouth, and hold the end of it toward the flame of a lighted candle. The pressure exerted by the surface tension of both sides of the film will force out a current of air strong enough to blow the flame to one side. What change takes place in the size of the bubble? Let us con 129. Adhesion between Liquids and Solids. sider what may happen when a solid is brought into contact with a liquid. If a lump of sugar is dipped into water, the adhesion between the two is greater than the cohesion of the sugar, and the sugar is dissolved. If a clean glass rod is dipped into pure water, the adhesion is greater than the cohesion of the water, and the rod will be found wet when it is removed. If the glass rod is dipped into mercury, the adhesion is less than the cohesion, and none of the mercury will cling to the rod. B FIG. 104 When the glass rod is dipped into mercury, the surface of the liquid is not broken, but extends down beside and below the rod. The surface tension, tending to decrease the area of this surface, rounds off the corners at A and B, Fig. 104, convex upward. When the rod is dipped into water (Fig. 105), the adhesion causes the water next the glass to rise above the general level, so that the surface FIG. 105 B would be as at A if it were not for the surface tension; but the surface tension decreases the area of the surface by rounding off the corners as at B, concave upward. If two plates of glass are thrust into water with their faces parallel to each other, the liquid will rise between them, the height being greater, the nearer the plates are to each other. If the plates are held tightly together at one edge and slightly |