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5.3.1: Surface Tension, Viscosity, and Capillary Action (Problems)

  • Page ID
    235778
  • PROBLEM \(\PageIndex{1}\)

    The test tubes shown here contain equal amounts of the specified motor oils. Identical metal spheres were dropped at the same time into each of the tubes, and a brief moment later, the spheres had fallen to the heights indicated in the illustration. Rank the motor oils in order of increasing viscosity, and explain your reasoning:

    An image of four graduated cylinders sitting on a table labeled “Oil viscosity ( S A E )” is shown. The left-hand cylinder, labeled “20,” is mostly filled with light tan liquid and a metal ball is drawn in the lower fifth of the cylinder, but not on the bottom. The second cylinder, labeled “30,” is mostly filled with light brown liquid and a metal ball is drawn about three-fourths of the way down cylinder. The third cylinder, labeled “40,” is mostly filled with medium brown liquid and a metal ball is drawn halfway down the cylinder. The right-hand cylinder, labeled “50,” is mostly filled with brown liquid and a metal ball is drawn near the top of the liquid in the cylinder.

    Answer

    20 < 30 < 40 < 50

    PROBLEM \(\PageIndex{2}\)

    Although steel is denser than water, a steel needle or paper clip placed carefully lengthwise on the surface of still water can be made to float. Explain at a molecular level how this is possible:

    A photo shows a close-up, above-view, of a needle lying on the surface of a sample of water.

    (credit: Cory Zanker)

    Answer

    The water molecules have strong intermolecular forces of hydrogen bonding. The water molecules are thus attracted strongly to one another and exhibit a relatively large surface tension, forming a type of “skin” at its surface. This skin can support a bug or paper clip if gently placed on the water.

    PROBLEM \(\PageIndex{3}\)

    The surface tension and viscosity values for diethyl ether, acetone, ethanol, and ethylene glycol are shown here.

    This table has four columns and five rows. The first row is a header row, and it labels each column: “Compound,” “Molecule,” “Surface Tension ( m N / m ),” and “Viscosity ( m P a dot s ).” Under the “compound” column are the following: diethyl ether C subscript 2 H subscript 5 O C subscript 2 H subscript 5; acetone C subscript 2 H subscript 5 O C subscript 2 H subscript 5; ethanol C subscript 2 H subscript 5 O H; ethylene glycol C H subscript 2 ( O H ) C H subscript 2 ( O H ). Under the “Molecule” column are ball-and-stick representations of each compound. The first shows two grey spheres bonded together. The first grey sphere is also bonded to three white spheres. The second grey sphere is bonded to two white spheres and a red sphere. The red sphere is bonded to another grey sphere. The grey sphere is bonded to two white spheres and another grey sphere. The last grey sphere is bonded to three white spheres. The second shows three grey spheres bonded tighter. The two grey spheres on the end are each bonded to three white spheres. The grey sphere in the middle is bonded to one red sphere. The third shows two grey spheres bonded together. The first grey sphere is bonded to three white spheres and the second grey sphere is bonded to two white spheres and a red sphere. The red sphere is bonded to a white sphere. The fourth shows two grey spheres bonded together. Each grey sphere is bonded to two white spheres and a red sphere. Each red sphere is also bonded to one white sphere. Under the “Surface Tension ( m N / m )” column are the following: 17, 23, 22 and 48. Under the “Viscosity ( m P a dot s )” column are the following: 0.22, 0.31, 1.07, and 16.1.

    1. Explain their differences in viscosity in terms of the size and shape of their molecules and their IMFs.
    2. Explain their differences in surface tension in terms of the size and shape of their molecules and their IMFs.
    Answer a

    The viscosity increases as the molecular weight (size) of the molecules increases. Additionally, the more polar the molecule, the more viscous.

    Answer b

    The surface tension increases as the molecular weight of the molecule increases. Additionally, the more polar the molecule, the higher the surface tension.

    PROBLEM \(\PageIndex{4}\)

    You may have heard someone use the figure of speech “slower than molasses in winter” to describe a process that occurs slowly. Explain why this is an apt idiom, using concepts of molecular size and shape, molecular interactions, and the effect of changing temperature.

    Answer

    Temperature has an effect on intermolecular forces: the higher the temperature, the greater the kinetic energies of the molecules and the greater the extent to which their intermolecular forces are overcome, and so the more fluid (less viscous) the liquid; the lower the temperature, the lesser the intermolecular forces are overcome, and so the more viscous the liquid.

    PROBLEM \(\PageIndex{5}\)

    It is often recommended that you let your car engine run idle to warm up before driving, especially on cold winter days. While the benefit of prolonged idling is dubious, it is certainly true that a warm engine is more fuel efficient than a cold one. Explain the reason for this.

    Answer

    The fluids in the engine are warmer, decreasing their viscosity, which aids in lubricating the moving parts of the engine causing it to run smoother.

    PROBLEM \(\PageIndex{6}\)

    The surface tension and viscosity of water at several different temperatures are given in this table.

    Water Surface Tension (mN/m) Viscosity (mPa s)
    0 °C 75.6 1.79
    20 °C 72.8 1.00
    60 °C 66.2 0.47
    100 °C 58.9 0.28
    1. As temperature increases, what happens to the surface tension of water? Explain why this occurs, in terms of molecular interactions and the effect of changing temperature.
    2. As temperature increases, what happens to the viscosity of water? Explain why this occurs, in terms of molecular interactions and the effect of changing temperature.
    Answer a

    As the water reaches higher temperatures, the increased kinetic energies of its molecules are more effective in overcoming hydrogen bonding, and so its surface tension decreases. Surface tension and intermolecular forces are directly related.

    Answer b

    The same trend in viscosity is seen as in surface tension, and for the same reason.

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