How many drops of water they think a coin can hold? Were your predictions correct? Does it matter if the coin is heads or tails? How is it possible to get so many water drops on a coin? Would a 10 cent coin hold more or less drops?
Teacher Tip: Younger students may believe a 10 cent coin holds more because it is worth more despite being smaller. Teacher Tip: To make sure your count is accurate, hold the eyedropper far enough above the coin so that the drop has to fall a short distance before fusing with the droplet on the coin. Details Activity Length mins. Objectives Describe the cohesive and adhesive properties of water.
What To Do Place a coin on a table or desk. Holding the eyedropper close to the surface of the coin, carefully squeeze water droplets onto the coin, one at a time. The droplets should pool up on the coin, creating a big droplet of water. Get the students to count the drops. Stop squeezing when the droplet on the coin breaks up and overflows. The count is the number of drops that the coin could hold before the one that caused the coin to overflow.
They increase with temperature and are independent of the presence of other gases, such as air. They depend only on the vapor pressure of water. The fact that hydrogen bonds need to be broken for water to evaporate means that a substantial amount of energy is used in the process. As the water evaporates, energy is taken up by the process, cooling the environment where the evaporation is taking place.
In many living organisms, including humans, the evaporation of sweat, which is 90 percent water, allows the organism to cool so that homeostasis of body temperature can be maintained. Water, which not only dissolves many compounds but also dissolves more substances than any other liquid, is considered the universal solvent.
A polar molecule with partially-positive and negative charges, it readily dissolves ions and polar molecules. Water is therefore referred to as a solvent: a substance capable of dissolving other polar molecules and ionic compounds.
The charges associated with these molecules form hydrogen bonds with water, surrounding the particle with water molecules. This is referred to as a sphere of hydration, or a hydration shell, and serves to keep the particles separated or dispersed in the water. When ionic compounds are added to water, individual ions interact with the polar regions of the water molecules during the dissociation process, disrupting their ionic bonds. Dissociation occurs when atoms or groups of atoms break off from molecules and form ions.
Dissociation of NaCl in water : When table salt NaCl is mixed in water, spheres of hydration form around the ions. Since many biomolecules are either polar or charged, water readily dissolves these hydrophilic compounds. Water is a poor solvent, however, for hydrophobic molecules such as lipids. Nonpolar molecules experience hydrophobic interactions in water: the water changes its hydrogen bonding patterns around the hydrophobic molecules to produce a cage-like structure called a clathrate.
Thermodynamically, such a large decrease in entropy is not spontaneous, and the hydrophobic molecule will not dissolve. Cohesion allows substances to withstand rupture when placed under stress while adhesion is the attraction between water and other molecules.
Have you ever filled a glass of water to the very top and then slowly added a few more drops? Before it overflows, the water forms a dome-like shape above the rim of the glass. This water can stay above the glass because of the property of cohesion. In cohesion, water molecules are attracted to each other because of hydrogen bonding , keeping the molecules together at the liquid-gas water-air interface, although there is no more room in the glass.
Cohesion allows for the development of surface tension, the capacity of a substance to withstand being ruptured when placed under tension or stress. This is also why water forms droplets when placed on a dry surface rather than being flattened out by gravity. When a small scrap of paper is placed onto the droplet of water, the paper floats on top of the water droplet even though paper is denser the mass per unit volume than the water.
Cohesion and surface tension keep the hydrogen bonds of water molecules intact and support the item floating on the top. Surface Tension : The weight of the needle is pulling the surface downward; at the same time, the surface tension is pulling it up, suspending it on the surface of the water and keeping it from sinking. Notice the indentation in the water around the needle.
This is because the water molecules are attracted to the charged glass walls of the capillary more than they are to each other and therefore adhere to it. This type of adhesion is called capillary action. Adhesion : Capillary action in a glass tube is caused by the adhesive forces exerted by the internal surface of the glass exceeding the cohesive forces between the water molecules themselves.
Why are cohesive and adhesive forces important for life? Cohesive and adhesive forces are important for the transport of water from the roots to the leaves in plants. This pull results from the tendency of water molecules being evaporated on the surface of the plant to stay connected to water molecules below them, and so they are pulled along. Plants use this natural phenomenon to help transport water from their roots to their leaves.
Without these properties of water, plants would be unable to receive the water and the dissolved minerals they require. In another example, insects such as the water strider use the surface tension of water to stay afloat on the surface layer of water and even mate there. By convention, scientists refer to hydrogen ions and their concentration as if they were free in this state in liquid water.
The pH is calculated as the negative of the base 10 logarithm of this concentration:. Human cells and blood each maintain near-neutral pH. The pH of a solution indicates its acidity or basicity alkalinity.
The pH scale is an inverse logarithm that ranges from 0 to anything below 7. Extremes in pH in either direction from 7. The pH in cells 6. Non-neutral pH readings result from dissolving acids or bases in water. Using the negative logarithm to generate positive integers, high concentrations of hydrogen ions yield a low pH, and low concentrations a high pH. Emphysema produces the opposite problem with alveoli. Alveolar walls of emphysema victims deteriorate, and the sacs combine to form larger sacs.
Because pressure produced by surface tension decreases with increasing radius, these larger sacs produce smaller pressure, reducing the ability of emphysema victims to exhale. A common test for emphysema is to measure the pressure and volume of air that can be exhaled. Why is it that water beads up on a waxed car but does not on bare paint?
The answer is that the adhesive forces between water and wax are much smaller than those between water and paint. Competition between the forces of adhesion and cohesion are important in the macroscopic behavior of liquids. See Figure 7. Table 2 lists contact angles for several combinations of liquids and solids. Figure 7. In the photograph, water beads on the waxed car paint and flattens on the unwaxed paint.
One important phenomenon related to the relative strength of cohesive and adhesive forces is capillary action —the tendency of a fluid to be raised or suppressed in a narrow tube, or capillary tube. This action causes blood to be drawn into a small-diameter tube when the tube touches a drop. If a capillary tube is placed vertically into a liquid, as shown in Figure 8, capillary action will raise or suppress the liquid inside the tube depending on the combination of substances.
Mercury, for example, has a very large surface tension and a large contact angle with glass. When placed in a tube, the surface of a column of mercury curves downward, somewhat like a drop.
The curved surface of a fluid in a tube is called a meniscus. The tendency of surface tension is always to reduce the surface area. Surface tension thus flattens the curved liquid surface in a capillary tube. This results in a downward force in mercury and an upward force in water, as seen in Figure 8.
Figure 8. Surface tension exerts a downward force as it flattens the mercury, suppressing it in the tube. The dashed line shows the shape the mercury surface would have without the flattening effect of surface tension. Surface tension therefore exerts an upward force when it flattens the surface to reduce its area. Capillary action can move liquids horizontally over very large distances, but the height to which it can raise or suppress a liquid in a tube is limited by its weight.
It can be shown that this height h is given by. If we look at the different factors in this expression, we might see how it makes good sense. Furthermore, the height is inversely proportional to tube radius—the smaller the radius r , the higher the fluid can be raised, since a smaller tube holds less mass. See Figure 9. Figure 9. The smaller the tube, the greater the height reached. The height is negligible for large-radius tubes.
Can capillary action be solely responsible for sap rising in trees? This result is unreasonable. Sap in trees moves through the xylem , which forms tubes with radii as small as 2. This value is about times as large as the radius found necessary here to raise sap m. This means that capillary action alone cannot be solely responsible for sap getting to the tops of trees. How does sap get to the tops of tall trees?
Recall that a column of water can only rise to a height of 10 m when there is a vacuum at the top—see Example 3 from Variation of Pressure with Depth in a Fluid. The question has not been completely resolved, but it appears that it is pulled up like a chain held together by cohesive forces. As each molecule of sap enters a leaf and evaporates a process called transpiration , the entire chain is pulled up a notch. So a negative pressure created by water evaporation must be present to pull the sap up through the xylem vessels.
In most situations, fluids can push but can exert only negligible pull , because the cohesive forces seem to be too small to hold the molecules tightly together. But in this case, the cohesive force of water molecules provides a very strong pull. Figure 10 shows one device for studying negative pressure.
Some experiments have demonstrated that negative pressures sufficient to pull sap to the tops of the tallest trees can be achieved. Figure The density of oil is less than that of water, yet a loaded oil tanker sits lower in the water than an empty one. Birds such as ducks, geese, and swans have greater densities than water, yet they are able to sit on its surface. Explain this ability, noting that water does not wet their feathers and that they cannot sit on soapy water.
Water beads up on an oily sunbather, but not on her neighbor, whose skin is not oiled. Explain in terms of cohesive and adhesive forces. What effect does capillary action have on the reading of a manometer with uniform diameter? Explain your answer. Pressure between the inside chest wall and the outside of the lungs normally remains negative. Explain how pressure inside the lungs can become positive to cause exhalation without muscle action.
What is the pressure inside an alveolus having a radius of 2. You may assume the pressure is the same as that created by a spherical bubble. Assuming the alveolus acts like a spherical bubble, what is the surface tension of the fluid?
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