132 the Nature of Liquids Section Review Answers

Learning Objectives

Past the finish of this section, y'all will be able to:

• Distinguish between adhesive and cohesive forces
• Define viscosity, surface tension, and capillary rise
• Describe the roles of intermolecular bonny forces in each of these properties/phenomena

When you pour a glass of water, or fill a car with gasoline, yous notice that water and gasoline menses freely. Simply when you cascade syrup on pancakes or add oil to a auto engine, you annotation that syrup and motor oil do not flow equally readily. The viscosity of a liquid is a measure of its resistance to flow. Water, gasoline, and other liquids that catamenia freely have a depression viscosity. Honey, syrup, motor oil, and other liquids that do non flow freely, similar those shown in Figure ten.15, have college viscosities. We can mensurate viscosity by measuring the rate at which a metal brawl falls through a liquid (the ball falls more than slowly through a more viscous liquid) or past measuring the rate at which a liquid flows through a narrow tube (more viscous liquids flow more slowly).

Figure 10.xv (a) Love and (b) motor oil are examples of liquids with loftier viscosities; they flow slowly. (credit a: modification of piece of work past Scott Bauer; credit b: modification of work past David Nagy)

The IMFs betwixt the molecules of a liquid, the size and shape of the molecules, and the temperature determine how easily a liquid flows. Every bit Table ten.2 shows, the more structurally complex are the molecules in a liquid and the stronger the IMFs between them, the more difficult information technology is for them to movement past each other and the greater is the viscosity of the liquid. Equally the temperature increases, the molecules move more speedily and their kinetic energies are better able to overcome the forces that concord them together; thus, the viscosity of the liquid decreases.

Viscosities of Mutual Substances at 25 °C

Substance	Formula	Viscosity (mPa·s)
water	HiiO	0.890
mercury	Hg	1.526
ethanol	C2H5OH	1.074
octane	C8H18	0.508
ethylene glycol	CH2(OH)CHtwo(OH)	16.ane
honey	variable	~2,000–10,000
motor oil	variable	~50–500

Table 10.2

The diverse IMFs betwixt identical molecules of a substance are examples of cohesive forces. The molecules within a liquid are surrounded by other molecules and are attracted equally in all directions by the cohesive forces within the liquid. Yet, the molecules on the surface of a liquid are attracted just by about half every bit many molecules. Considering of the unbalanced molecular attractions on the surface molecules, liquids contract to course a shape that minimizes the number of molecules on the surface—that is, the shape with the minimum expanse. A pocket-sized drop of liquid tends to presume a spherical shape, as shown in Figure 10.sixteen, because in a sphere, the ratio of surface area to volume is at a minimum. Larger drops are more than greatly affected by gravity, air resistance, surface interactions, and then on, and as a result, are less spherical.

Effigy 10.16 Attractive forces result in a spherical water drop that minimizes surface area; cohesive forces concur the sphere together; agglutinative forces keep the drib attached to the web. (credit photo: modification of work past "OliBac"/Flickr)

Surface tension is defined as the free energy required to increase the surface area of a liquid, or the strength required to increase the length of a liquid surface by a given amount. This belongings results from the cohesive forces between molecules at the surface of a liquid, and information technology causes the surface of a liquid to acquit like a stretched safety membrane. Surface tensions of several liquids are presented in Table 10.3. Among common liquids, water exhibits a distinctly high surface tension due to strong hydrogen bonding between its molecules. As a result of this high surface tension, the surface of water represents a relatively "tough skin" that can withstand considerable forcefulness without breaking. A steel needle carefully placed on water will float. Some insects, like the one shown in Figure 10.17, even though they are denser than water, move on its surface because they are supported by the surface tension.

Surface Tensions of Common Substances at 25 °C

Substance	Formula	Surface Tension (mN/m)
water	HtwoO	71.99
mercury	Hg	458.48
ethanol	C2HvOH	21.97
octane	C8H18	21.14
ethylene glycol	CH2(OH)CH2(OH)	47.99

Table 10.3

Figure 10.17 Surface tension (correct) prevents this insect, a "water strider," from sinking into the water.

Surface tension is affected by a variety of variables, including the introduction of additional substances on the surface. In the late 1800s, Agnes Pockels, who was initially blocked from pursuing a scientific career merely studied on her ain, began investigating the bear upon and characteristics of soapy and greasy films in h2o. Using homemade materials, she developed an instrument known as a trough for measuring surface contaminants and their effects. With the support of renowned scientist Lord Rayleigh, her 1891 newspaper showed that surface contagion significantly reduces surface tension, and also that changing the characteristics of the surface (compressing or expanding information technology) besides affects surface tension. Decades later, Irving Langmuir and Katharine Blodgett built on Pockels' work in their own trough and important advances in surface chemical science. Langmuir pioneered methods for producing unmarried-molecule layers of film; Blodgett applied these to the development of not-reflective drinking glass (critical for picture show-making and other applications), and besides studied methods related to cleaning surfaces, which are important in semiconductor fabrication.

The IMFs of attraction betwixt two dissimilar molecules are chosen adhesive forces. Consider what happens when h2o comes into contact with some surface. If the agglutinative forces between water molecules and the molecules of the surface are weak compared to the cohesive forces between the water molecules, the water does non "moisture" the surface. For example, water does not wet waxed surfaces or many plastics such as polyethylene. Water forms drops on these surfaces considering the cohesive forces within the drops are greater than the adhesive forces between the water and the plastic. H2o spreads out on drinking glass because the adhesive strength between water and drinking glass is greater than the cohesive forces within the water. When water is confined in a glass tube, its meniscus (surface) has a concave shape considering the water wets the glass and creeps up the side of the tube. On the other manus, the cohesive forces between mercury atoms are much greater than the adhesive forces between mercury and drinking glass. Mercury therefore does not moisture glass, and information technology forms a convex meniscus when confined in a tube because the cohesive forces within the mercury tend to draw information technology into a drop (Figure ten.18).

Figure 10.18 Differences in the relative strengths of cohesive and adhesive forces result in unlike meniscus shapes for mercury (left) and h2o (correct) in drinking glass tubes. (credit: Mark Ott)

If you place one stop of a paper towel in spilled wine, as shown in Figure 10.19, the liquid wicks up the newspaper towel. A similar process occurs in a cloth towel when you lot utilize it to dry off after a shower. These are examples of capillary action—when a liquid flows within a porous material due to the allure of the liquid molecules to the surface of the material and to other liquid molecules. The adhesive forces between the liquid and the porous material, combined with the cohesive forces within the liquid, may exist potent enough to move the liquid up confronting gravity.

Effigy 10.nineteen Wine wicks up a paper towel (left) because of the strong attractions of water (and ethanol) molecules to the −OH groups on the towel's cellulose fibers and the stiff attractions of h2o molecules to other water (and ethanol) molecules (right). (credit photograph: modification of work by Mark Blaser)

Towels soak upward liquids like water because the fibers of a towel are made of molecules that are attracted to h2o molecules. Most cloth towels are made of cotton wool, and paper towels are generally made from paper pulp. Both consist of long molecules of cellulose that incorporate many −OH groups. H2o molecules are attracted to these −OH groups and form hydrogen bonds with them, which draws the H₂O molecules upwardly the cellulose molecules. The water molecules are also attracted to each other, and so large amounts of water are drawn up the cellulose fibers.

Capillary action tin can also occur when i stop of a minor diameter tube is immersed in a liquid, as illustrated in Figure 10.xx. If the liquid molecules are strongly attracted to the tube molecules, the liquid creeps up the within of the tube until the weight of the liquid and the adhesive forces are in balance. The smaller the bore of the tube is, the higher the liquid climbs. It is partly by capillary action occurring in plant cells called xylem that water and dissolved nutrients are brought from the soil upward through the roots and into a plant. Capillary activity is the basis for thin layer chromatography, a laboratory techniques commonly used to split up minor quantities of mixtures. You depend on a constant supply of tears to keep your eyes lubricated and on capillary action to pump tear fluid away.

Effigy 10.20 Depending upon the relative strengths of adhesive and cohesive forces, a liquid may rise (such as h2o) or fall (such as mercury) in a drinking glass capillary tube. The extent of the rise (or autumn) is directly proportional to the surface tension of the liquid and inversely proportional to the density of the liquid and the radius of the tube.

The height to which a liquid volition ascent in a capillary tube is determined past several factors as shown in the post-obit equation:

$$h=\frac{2T\text{cos}\theta }{r\rho k}$$

In this equation, h is the peak of the liquid inside the capillary tube relative to the surface of the liquid outside the tube, T is the surface tension of the liquid, θ is the contact angle betwixt the liquid and the tube, r is the radius of the tube, ρ is the density of the liquid, and k is the acceleration due to gravity, nine.8 chiliad/s². When the tube is made of a fabric to which the liquid molecules are strongly attracted, they will spread out completely on the surface, which corresponds to a contact angle of 0°. This is the state of affairs for water rise in a glass tube.

Example 10.4

Capillary Rise

At 25 °C, how high will h2o ascent in a glass capillary tube with an inner bore of 0.25 mm?

For water, T = 71.99 mN/thousand and ρ = one.0 g/cm³.

Solution

The liquid volition rise to a meridian h given by: $h=\frac{2T\text{cos}\theta }{r\rho \mathrm{thou}}$

The Newton is defined as a kg g/south², and then the provided surface tension is equivalent to 0.07199 kg/s². The provided density must be converted into units that will cancel appropriately: ρ = 1000 kg/thousand^three. The diameter of the tube in meters is 0.00025 one thousand, and so the radius is 0.000125 m. For a glass tube immersed in water, the contact bending is θ = 0°, so cos θ = one. Finally, acceleration due to gravity on the earth is g = 9.8 m/sⁱⁱ. Substituting these values into the equation, and cancelling units, we have:

$$h=\frac{\mathrm{ii}\left(0.07199{\text{kg/s}}^2\right)}{\left(0.000125\text{thousand}\right)\left(1000{\text{kg/thou}}^3\right)\left(9.8{\text{m/south}}^{\mathrm{ii}}\right)}=0.12\text{m}=\text{12 cm}$$

Bank check Your Learning

H2o rises in a glass capillary tube to a peak of 8.4 cm. What is the diameter of the capillary tube?

Answer:

diameter = 0.36 mm

Chemistry in Everyday Life

Biomedical Applications of Capillary Action

Many medical tests require drawing a minor amount of blood, for example to determine the amount of glucose in someone with diabetes or the hematocrit level in an athlete. This procedure can be easily done because of capillary action, the ability of a liquid to flow upward a small tube against gravity, as shown in Figure 10.21. When your finger is pricked, a drop of blood forms and holds together due to surface tension—the unbalanced intermolecular attractions at the surface of the drib. Then, when the open end of a narrow-diameter glass tube touches the drop of blood, the adhesive forces betwixt the molecules in the claret and those at the glass surface draw the blood up the tube. How far the claret goes upwardly the tube depends on the diameter of the tube (and the type of fluid). A minor tube has a relatively big surface area for a given book of blood, which results in larger (relative) attractive forces, assuasive the blood to be fatigued farther upward the tube. The liquid itself is held together by its own cohesive forces. When the weight of the liquid in the tube generates a down force equal to the upward forcefulness associated with capillary activeness, the liquid stops rise.

Figure 10.21 Blood is collected for medical assay by capillary action, which draws claret into a small diameter glass tube. (credit: modification of piece of work past Centers for Disease Control and Prevention)

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Source: https://openstax.org/books/chemistry-2e/pages/10-2-properties-of-liquids

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