10.3 Properties of Liquid

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Liquid properties affected by IMFs are mentioned below

  • boiling point (previously discussed) and melting point
  • viscosity of liquid
  • surface tension of liquid
  • capillary action of liquid

In this section, we are going to discuss them in detail.

What is a liquid?

Liquids occupy a rather peculiar place in the trinity of solid, liquid and gas. A liquid is the preferred state of a substance at temperatures intermediate between the realms of the solid and the gas. But if you look at the melting and boiling points of a variety of substances, you will notice that the temperature range within which many liquids can exist tends to be rather small. In this, and in a number of other ways, the liquid state appears to be somewhat tenuous and insecure, as if it had no clear right to exist at all, and only does so as an oversight of Nature.

How do we know it’s a liquid?

Anyone can usually tell if a substance is a liquid simply by looking at it. What special physical properties do liquids possess that make them so easy to recognize? One obvious property is their mobility, which refers to their ability to move around, to change their shape to conform to that of a container, to flow in response to a pressure gradient, and to be displaced by other objects. But these properties are shared by gases, the other member of the two fluid states of matter. The real giveaway is that a liquid occupies a fixed volume, with the consequence that a liquid possesses a definite surface. Gases, of course, do not; the volume and shape of a gas are simply those of the container in which it is confined. The higher density of a liquid also plays a role here; it is only because of the large density difference between a liquid and the space above it that we can see the surface at all. (What we are really seeing are the effects of reflection and refraction that occur when light passes across the boundary between two phases differing in density, or more precisely, in their refractive indexes.)

  1. Flow properties of liquids: the viscosity

The term viscosity is a measure of resistance to flow. It can be measured by observing the time required for a given volume of liquid to flow through the narrow part of a viscometer tube.

The viscosity of a substance is related to the strength of the forces acting between its molecular units. In the case of water, these forces are primarily due to hydrogen bonding. Liquids such as syrups and honey are much more viscous because the sugars they contain are studded with hydroxyl groups (–OH) which can form multiple hydrogen bonds with water and with each other, producing a sticky viscometerdisordered network.

Fig. 10.38

Source: https://www.chem1.com/acad/webtext/virtualtextbook.html

Specific viscosity of some liquids at 20°C.
SubstanceViscosity
water HOH1.00
diethyl ether
(CH3-CH2)2O
0.23
benzene C6H60.65
glycerin C3H2(OH)3280
Mercury1.5
motor oil, SAE30200
Honey~10,000
Molasses~5000
pancake syrup~3000

Even in the absence of hydrogen bonding, dispersion forces are universally present. In smaller molecules such as ether and benzene, and also in metallic mercury, these forces are so weak that these liquids have very small viscosities. But because these forces are additive, they can be very significant in long carbon-chain molecules such as those found in oils used in cooking and for lubrication. These long molecules also tend to become entangled with one another, further increasing their resistance to flow.

It’s worth taking a moment to examine this table, and to see how well you can relate the viscosities to the molecular structures.

Why viscosity changes with temperature

The temperature dependence of the viscosity of liquids is well known to anyone who has tried to pour cold syrup on a pancake. Because the forces that give rise to viscosity are weak, they are easily overcome by thermal motions, so it is no surprise that viscosity decreases as the temperature rises.

T/°C01020406080100
viscosity/cP1.81.31.00.650.470.360.28
Viscosity of Water as a Function of Temperature

Automotive lubricating oils can be too viscous at low temperatures (making it harder for your car to operate on a cold day), while losing so much viscosity at engine operating temperatures that their lubricating properties become impaired. These engine oils are sold in a wide range of viscosities; the higher-viscosity oils are used in warmer weather and the lower-viscosity oils in colder weather. The idea is to achieve a fairly constant viscosity that is ideal for the particular application. By blending in certain ingredients, lubricant manufacturers are able to formulate “multigrade” oils whose viscosities are less sensitive to temperatures, thus making a single product useful over a much wider temperature range.

How viscosity impedes flow

The next time you pour a viscous liquid over a surface, notice how different parts of the liquid move at different rates and sometimes in different directions. In order to flow freely, the particles making up a fluid must be able to move independently. Intermolecular attractive forces work against this, making it difficult for one molecule to pull away from its neighbors and force its way in between new neighbors.

Fig. 10.39

Source: commons.wikimedia.org/

The pressure drop that is observed when a liquid flows through a pipe is a direct consequence of viscosity. Those molecules that happen to find themselves near the inner walls of a tube tend to spend much of their time attached to the walls by intermolecular forces, and thus move forward very slowly.

Fig. 10.40

Source: https://www.chem1.com/acad/webtext/virtualtextbook.html

  • Surface Tension

Water acts as if it has a “skin” on it due to extra inward forces on its surface. Those forces are called the surface tension.

How surface tension works:

A molecule within the bulk of a liquid experiences attractions to neighboring molecules in all directions, but since these average out to zero, there is no net force on the molecule because it is, on the average, as energetically comfortable in one location within the liquid as in another.

Fig. 10.41

Source: https://www.chem1.com/acad/webtext/virtualtextbook.html

Liquids ordinarily do have surfaces, however, and a molecule that finds itself in such a location is attracted to its neighbors below and to either side, but there is no attraction operating in the 180° solid angle above the surface. Consequently, a molecule at the surface will tend to be drawn into the bulk of the liquid. Conversely, work must be done in order to move a molecule within a liquid to its surface.

Why liquids form drops??

Clearly, there must always be some molecules at the surface, but the smaller the surface area, the lower the potential energy. Thus, intermolecular attractive forces act to minimize the surface area of a liquid.

The geometric shape that has the smallest ratio of surface area to volume is the sphere, so very small quantities of liquids tend to form spherical drops. As the drops get bigger, their weight deforms them into the typical tear shape…. and bubbles

Fig. 10.42

Source: https://www.chem1.com/acad/webtext/virtualtextbook.html

Surface tensions of common liquids

Table 10.3

substancesurface tension
water H(OH)72.7 dyne/cm
diethyl ether (CH3-CH2)2O17.0
40.0
glycerin C3H2(OH)363
mercury (15°C)487
n-octane21.8
sodium chloride solution (6M in water)82.5
sucrose solution
(85% in water)
76.4
sodium oleate (soap) solution in water25

Surface tension is defined as the amount of work that must be done in order to create unit area of surface. The SI units are J m–2 (or N m–1) but values are more commonly expressed in mN m–1 or in cgs units of dyn cm–1 or erg cm–2.

The table shows the surface tensions of several liquids at room temperature. Note especially that

hydrocarbons and non-polar liquids such as ether have rather low values

one of the main functions of soaps and other surfactants is to reduce the surface tension of water. Mercury has the highest surface tension of any liquid at room temperature. It is so high that mercury does not flow in the ordinary way, but breaks into small droplets that roll independently.

Surface tension and viscosity are not directly related, as you can verify by noting the disparate values of these two quantities for mercury. Viscosity depends on intermolecular forces within the liquid, whereas surface tension arises from the difference in the magnitudes of these forces within the liquid and at the surface.

Surface tension changes with temperature

Table 10.4

°Cdynes/cm
075.9
2072.7
5067.9
10058.9

Surface Tension of Water

Surface tension always decreases with temperature as thermal motions reduce the effect of intermolecular attractions. This is one reason why washing with warm water is more effective; the lower surface tension allows water to more readily penetrate a fabric.

Wetting of surfaces

Take a plastic mixing bowl from your kitchen, and splash some water around in it. You will probably observe that the water does not cover the inside surface uniformly, but remains dispersed into drops. Do the same thing with a perfectly clean glass container, and you will see that the water forms a smooth film on the glass surface; we say that the glass is wetted by the water.

Anyone who has driven a car knows that water poured onto a freshly-cleaned windshield will form a uniform film, but after driving in heavy traffic, the glass surface gets coated with greasy material; running the wipers under these conditions simply breaks hundreds of drops into thousands.

When a molecule of a liquid is in contact with another phase, its behavior depends on the relative attractive strengths of its neighbors on the two sides of the phase boundary. If the molecule is more strongly attracted to its own kind, then interfacial tension will act to minimize the area of contact by increasing the curvature of the surface. This is what happens at the interface between water and a hydrophobic surface such as a plastic mixing bowl or a windshield coated with oily material.

Fig. 10.45

Source: https://www.chem1.com/acad/webtext/virtualtextbook.html

A liquid will wet a surface if the angle at which it makes contact with the surface is less than 90°. The value of this contact angle can be predicted from the properties of the liquid and solid separately. 

A clean glass surface, by contrast, has –OH groups (from hydrated SiO2 structures) sticking out of it which readily attach to water molecules through hydrogen bonding; the lowest potential energy now occurs when the contact area between the glass and water is maximized. This causes the water to spread out evenly over https://www.chem1.com/acad/webtext/states/state-images/wetleaf.jpgthe surface, or to wet it.

Fig. 10.46

You have undoubtedly noticed water droplets sitting on the hydrophobic surface of most plant leaves.  It is created by a thin layer of cells that secrete a waxy substance that prevents excessive water loss; few plants would survive dry weather without this coating.  Our own skin is fairly easily wetted when very clean, but is soon covered with an oily-waxy substance secreted by the sebaceous glands; its function is to lubricate and waterproof the skin. Its effect can be readily seen when perspiration turns into beads of sweat.

Surfactants: The surface tension of water can be reduced to about one-third of its normal value by adding some soap or synthetic detergent. These substances, known collectively as surfactants, are generally hydrocarbon molecules having an ionic group on one end. The ionic group, being highly polar, is strongly attracted to water molecules; we say it is hydrophilic. The hydrocarbon (hydrophobic) portion is just the opposite; inserting it into water would break up the local hydrogen-bonding forces and is therefore energetically unfavorable. What happens, then, is that the surfactant molecules migrate to the surface with their hydrophobic ends sticking out, effectively creating a new surface. Because hydrocarbons interact only through very weak dispersion forces, this new surface has a greatly reduced surface tension.

Fig. 10.47

Fig. 10.48

Detergent Washing: Chemistry of washing

Fig. 10.49

Source: https://www.chem1.com/acad/webtext/virtualtextbook.html

All of us at one time or another have enjoyed the fascination of creating soap bubbles and admiring their intense and varied colors as they drift around in the air, seemingly aloof from the constraints that govern the behavior of ordinary objects — but only for a while! Their life eventually comes to an abrupt end as they fall to the ground or pop in mid-flight

Fig.10.50

https://www.chem1.com/acad/webtext/virtualtextbook.html

The walls of these bubbles consist of a thin layer of water molecules sandwiched between two layers of surfactant molecules. Their spherical shape is of course the result of water’s surface tension. Although the surfactant (soap) initially reduces the surface tension, expansion of the bubble spreads the water into a thiner layer and spreads the surfactant molecules over a wider area, deceasing their concentration. This, in turn, allows the water molecules to interact more strongly, increasing its surface tension and stabilizing the bubble as it expands.

The bright colors we see in bubbles arise from interference between light waves that are reflected back from the inner and outer surfaces, indicating that the thickness of the water layer is comparable the range of visible light (around 400-600 nm).

Once the bubble is released, it can endure until it strikes a solid surface or collapses owing to loss of the water layer by evaporation. The latter process can be slowed by adding a bit of glycerine to the liquid. A variety of recipes and commercial “bubble-making solutions” are available; some of the latter employ special liquid polymers which slow evaporation and greatly extend the bubble lifetimes. Bubbles blown at very low temperatures can be frozen, but these eventually collapse as the gas diffuses out.

  • Capillary Action: Cohesion & Adhesion

Capillary action (or capillarity) describes the ability of a liquid to flow against gravity in a narrow space such as a thin tube.

This spontaneous rising of a liquid is the outcome of two opposing forces:

Cohesion – the attractive forces between similar molecules or atoms, in our case the molecules or atoms of the liquid. Water, for example, is characterized by high cohesion since each water molecule can form four hydrogen bonds with neighboring molecules.

Fig.10.51

Adhesion – the attractive forces between dissimilar molecules or atoms, in our case the contact area between the particles of the liquid and the particles forming the tube.

The capillarity of the liquid is said to be high when adhesion is greater than cohesion, and vice versa. Hence, knowledge of the liquid is not sufficient to determine when capillary action will occur, since we must also know the chemical composition of the tube. These two, together with the contact area (the tube’s diameter), comprise the key variables. For example, water in a thin glass tube has strong adhesive forces due to the hydrogen bonds that form between the water molecules and the oxygen atoms in the tube wall (glass = silica = SiO2). In contrast, mercury is characterized by stronger cohesion, and hence its capillarity is much lower.

Fig.10.52

Source: https://www.chem1.com/acad/webtext/virtualtextbook.html

The height (h) of a liquid inside a tube is given by the formula

So what’s going on here?

In case the forces of adhesion are greater than those of cohesion and gravity (when it exists), the molecules of the liquid cling to the wall of the tube. We will observe that the upper surface of the liquid becomes concave (the height of the liquid at the contact area is higher than its height at the center of the tube). The cohesive forces between the molecules of the liquid are “attempting” to reduce the surface tension (i.e. to flatten the upper surface of the liquid and thus prevent the increased surface area in the concave state). In doing so, the molecules keep climbing up until a steady state between cohesion and adhesion is achieved (with or without the gravity component).

This also explains why this phenomenon occurs exclusively in thin tubes (also in the absence of gravity). In wider vessels, only a small fraction of the liquid comes into contact with the vessel walls, and so adhesive forces are negligible and there is hardly any rising of the liquid.

Many everyday phenomena are a result of capillary action, including:

(1) A kerosene lamp or a candle “sucking up” oil or liquid wax, respectively.

(2) Water climbing up the microscopic fibers of paper towels.

(3) Located at the inner ends of each eye, the lacrimal ducts drain our tears using 

      capillary action.

(4) In chromatography, a method for separating solutes, different solutes climb up the

      surface of a stationary phase at different rates, resulting in separation (see picture of

      thin layer chromatography below).

Fig.10.50

Source: Commons.wikimedia.org/

Fig.10.52

Source: Commons.wikimedia.org/

Video: Capillary action in plants