Chemistry - A Molecular Science

Chapter 7 States of Matter and Changes in State

7.5

LIQUIDS

The energy of attraction between molecules in

the liquid state is comparable to their

thermal energy, so the molecules remain rela

tively close to one another while undergoing

random, but restricted, motion.

Liquids

adopt the shape of the bottom of the container but

maintain their own volume, which depends only slightly on the temperature. Most of the properties of liquids depend upon the strengths

of their intermolecular forces. We consider

two examples here: viscosity and surface tension.

a) Molecules on the surface experience attractive forces from the bulk but not from above, so there is a net inward force, which causes the liquid to minimize its surface area. b) Molecules in the bulk of a liquid experience forces in all directions, so there is no net force on the molecule. (a)

(b)

Figure 7.12 Meniscus shapes a) Molecules that have strong adhesive forces increase interactions with the walls by forming a concave meniscus. b) Molecules with weak adhesive forces maximize their cohesive forces by forming a convex meniscus.

(a)

(b)

Figure 7.11 Interactions in the liquid

Viscosity

is the resistance of liquids to flow. In order for a liquid to flow, molecules

must slide past one another, so intermolecula

r interactions must be broken and reformed.

If thermal energy is relatively low compared to

the energy of interaction, the molecules

cannot readily break the interactions, so the fl

ow is retarded and the liquid is said to be

viscous

. Thus, viscosity decreases as the liquid is heated. For example, oil is quite viscous

when cold, but flows easily

when hot. Viscosity also depends upon the shape of the

molecule: viscosity increases as the ease w

ith which molecules become entangled

increases.

Molecules on the surface of a liquid are a hi

gher potential energy than those in the

interior because surface mo

lecules are not involved in as many energy-lowering

intermolecular interactions (Figure 7.11). Thus

, water ‘beads’ into distorted spheres when

it is placed on a surface with which it does not interact because doing so minimizes its surface area and the number of higher-energy surface molecules. Alternatively, the contraction into spheres can be viewed as a

result of the net inward force exerted on the

surface molecules (Figure 7.11) that is ab

sent in the bulk because the competing forces

cancel in the bulk. The energy required to in

crease the surface area of a liquid by a fixed

amount is called the

surface tension

of the liquid. The units of surface tension are J/m

2. 
   

Surface tension increases with

increasing intermolecular forces

.

Forces between like molecules are called

cohesive

, while those between unlike

molecules are said to be

adhesive

. Viscosity and surface tension both result from cohesive

forces. However, when a liquid is placed in a c

ontainer, there may also be adhesive forces

between the liquid and the walls of the contai

ner, and the balance between the cohesive

forces that tend to ‘bead’ the liquid and the cohesive forces that tend to ‘wet’ the walls of the container dictates the curved shape of th

e top of the liquid, which is known as the

meniscus

. Glass contains many Si-O bonds, so wa

ter can hydrogen bond to the oxygens

on the surface of the wall. The strong adh

esive forces between water and glass tend to

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