The anomalous expansion of water

Heating does not always lead to expansion. Figure 11.19 shows an

arrangement that can be used to demonstrate how the volume of a fixed mass of water varies with temperature. The

distance, h, can be used as a measure of change in volume of water since h is proportional to the volume of water in a uniform tube.

The graph in figure 11.20 shows that the minimum volume of the water occurs at 4°C. Thus, the maximum density of water occurs at 4°C (since density = _volumemass and the volume is smallest at this temperature). The graph shows that water actually expands while being cooled from 4°C to 0°C. Further,

liquid-in-glass thermometer ❯ bimetallic strip ❯ thermostat ❯ CHAPTER 15 IPTL[HSSPJZ[YPW JVU[HJ[Z LSLJ[YPJILSS

Figure 11.18 Bimetallic strip fire alarm

circuit. O UHYYV^NSHZZ[\IPUN TL[YL Y\SL [OLYTVTL[LY PJLZHS[ TP_[\YL =VS\TL ;LTWLYH[\YL‡* PJL PJLHUK^H[LY ^H[LY TH_PT\TKLUZP[` ¶    

is because at temperatures below 4°C, the water molecules fit together in a different, more open pattern.

The expansion produced during cooling is described as ‘anomalous’

expansion, since substances usually contract, rather than expand, when

cooled and so this behaviour is seen as strange. In cold countries, the

anomalous expansion of water may cause pipes carrying water to burst when the water in them freezes during winter. However, the burst is not discovered until the ice thaws!

The fact that ice has a larger volume than the water from which it is formed also means that ice is less dense than water. Thus, ice floats in water (figure 11.21).

Figure 11.22 shows that the top of a pond may be frozen even though there is water underneath. As the pond cools from about 10°C to 5°C, the density of the water increases and the cold water sinks to the bottom of the pond. As the temperature drops lower than 4°C, the water becomes less dense and the colder water now rises, leaving water at 4°C at the bottom of the pond. As cooling continues, the temperature at the top drops to 0°C and ice begins to form.

Since the density of ice is less than

that of water, the ice remains on top of the water in the pond. Ice, like water, is a poor conductor of heat. Thus, the water beneath the pond does not get cooled as quickly as before by the cold air above the pond. Hence, it is possible that, for an entire winter season, water can remain in a pond under a sheet of ice formed on the surface. Fish are therefore able to remain alive in the pond throughout the winter by dwelling in this body of water, which is at 4°C approximately.

anomalous expansion ❯

ITQ10

Why do cans of soda (soft drinks) burst when left in a freezer too long?

ITQ11

According to the graph in figure 11.20, does water show ‘anomalous’ behaviour above 4°C?

Figure 11.21 An iceberg floats because

ice is less dense than water. Since ice is only slightly less dense than water, we can see the tip of the iceberg when we are above the water! PJL ^H[LY ‡* ‡* ‡* ‡*

Figure 11.22 Life in a pond during very

cold weather.

Chapter summary

• The kinetic model of matter assumes that matter is made up of tiny particles, called molecules. These molecules are always in motion, have strong forces between them and, when they collide, they do so elastically.

• Evidence for the kinetic model comes from the regular shapes and cleavage planes of crystals, Brownian motion, cohesion, adhesion and the elastic behaviour of substances. • The kinetic model regards solids as being made up of molecules that are very closely

packed. At such close spacing, molecules exert strong forces of attraction on each other.

• In liquids, the forces between molecules are not as great, since the average separation distance between molecules in liquids is a little larger than in solids. • In gases and vapours, the molecules are so far apart that they experience practically

no force of attraction and therefore move about fairly independently of one another. • Energy supplied to a solid (or liquid) can do work against the molecular force of

attraction, causing the mean separation distance of molecules to increase. As the average separation distance between molecules increases, the substance may show an overall thermal expansion.

• At certain temperatures and pressures, a substance may show a change of state, such as melting and boiling as it goes from solid to liquid and liquid to vapour, respectively. In the reverse process, the substance may undergo condensation and freezing.

In this chapter, the kinetic model is discussed in relation to molecular substances.

• During a change of state of a pure crystalline substance, the temperature of the substance remains constant. During the process, heat is either released or absorbed depending on the change that is taking place.

• Temperature is explained by the kinetic model as being related to the average speed of the molecules in a substance. An increase in temperature means an increase in average speed.

• Gas pressure and vapour pressure result from bombardment of the internal walls of the container by the molecules of the gas or vapour present within the container.

Answers to ITQs

ITQ1 Yes! During the manufacture of doped semiconductors, atoms of impurities are made to diffuse into solid chips (see pages 474–475).

ITQ2 Tiny molecules of air travelling at high speeds in random directions are probably colliding with the specks of soot, resulting in the jerky movement of the latter.

ITQ3 The drop spreads on the glass surface because the adhesion forces between the glass and water molecules are greater than the forces of cohesion between the water molecules. The drop is spherical on the leaf surface because the force of adhesion between the waxy leaf surface and water is less than the cohesion forces among the water molecules.

ITQ4 (a) The graph is horizontal, indicating that there is no change in temperature. This means that the heat being supplied is not being used to increase the kinetic energies of the molecules, but rather to move them apart during the change of state.

(b) This suggests that the ‘wax’ might not be of one pure material, but might consist of several ‘waxes’.

ITQ5 Since only faster particles (i.e. those with greater kinetic energy) are able to leave the surface of the liquid, the average kinetic energy of the molecules remaining in the liquid is reduced – the temperature of the liquid drops, since temperature is a measure of average kinetic energy of molecules.

ITQ6 Unlike water in an earthenware goblet the temperature of the water in the plastic pitcher (see figure 11.13) is not likely to drop by much, since there are no pores in the wall of the container from which water can evaporate and produce a cooling effect. However, if the container is open, some evaporation will take place from the surface and there could be a small temperature fall.

ITQ7 On dry days, the air contains little or no water vapour. The wet clothes lose many more water vapour molecules to the air than they gain from it. On humid days, the wet clothes receive as well as lose water vapour molecules, since both the air and the clothes contain water. Hence the clothes will dry faster on dry days (at the same temperature)

ITQ8 If the external pressure is reduced, a lower vapour pressure within bubbles (corresponding to a lower temperature) can equal the external pressure. Hence boiling can occur at temperatures less than 100°C.

ITQ9 Particles in a solid are very close to each other, and therefore the forces between them are very large. Hence, as the bar contracts it exerts a tremendous force on the nail.

ITQ10 The water in the soft drink expands as ice is being formed from the

water in the drink. The large volume of ice formed pushes against the closed can, causing the latter to burst.

ITQ11 No. Water expands as the temperature rises from 4°C. Substances

Examination-style questions

1 When smoke is brightly illuminated and viewed through a microscope, small, bright specks of light showing jerky movement are seen. These specks are:

A electrons in the air B molecules of air

C smoke particles D photons of light 2 Bridges are built with gaps between sections to allow for:

A contraction during hot weather B contraction during cold weather

C expansion during hot weather D expansion during cold weather 3 Which of the following substances does not contract when cooled from 3°C to 1°C?

A ethyl alcohol B water C copper D steel 4 The conversion of liquid to vapour at a specific temperature and pressure is called:

A boiling B condensation C evaporation D vaporisation 5 The diagram shows a bimetallic strip inside an electric iron. The strip is made of metals A

and B. knob electrical contact, C to power supply heating element of iron screw springy metal A B

(i) When the iron is plugged into the power supply and switched on, the heating element gets hot. The bimetallic strip next to the heating element curves and moves away from the contact C. What does this tell you about the relative expansions of the metals A and B on heating?

(ii) Explain what happens to flow of electricity in the heating element of the iron: (a) when the strip moves away from C.

(b) several minutes after the strip has moved away from C.

(iii) In what direction (up or down) must the screw be adjusted for a ‘low’ temperature setting on the iron? Explain your answer.

(iv) The bimetallic strip ensures that the iron does not get too hot nor too cold when switched on, that is, it regulates the temperature of the iron. What is the name given to a device, like the bimetallic strip, which helps to keep the temperature of an appliance fairly steady?

6 The table below summarises a comparison of the properties we associate with solids, liquids and gases. Copy and complete the table.

State Spacing of particles Movement of particles Volume and shape Special properties Solid Closely packed in a regular

formation, giving high density

Limited to vibrations about a fixed position

Can be cut, stretched, bent, twisted, and polished. Incompressible

Liquid Can be poured, forms drops, takes the

shape of its container. Incompressible Gas Far apart, giving low density Independent random motion









used as the basis for measuring temperature













T = θ + 273, where T is the temperature in kelvins and θ is the temperature in °C

temperature measurement of temperature practical thermometers liquid-in-glass bimetallic strip electrical electronic scales of temperature

and fixed points

physical properties and temperature measurement