SUPPLEMENTARY TOPIC 1 STATES OF MATTER.

Transcription

1 SUPPLEMENTARY TOPIC 1 STATES OF MATTER. States f matter. Matter is generally regarded as existing in three pssible phases - slid, liquid r gas. Fr example, water can be in the slid frm (ice) r the familiar liquid frm r the gaseus frm (water vapur). Each physical state has its wn assciated prperties. The SOLID STATE is characterised by having a fixed shape with rigid bundaries. The LIQUID STATE still has a definite vlume and des maintain a bundary but that bundary is nt fixed, instead adapting t the cntainer. The GASEOUS STATE has n fixed vlume - the cmpnent mlecules mve thrughut the cntainer t unifrmly fill it. If the gas is nt trapped in a cntainer, it spreads thrughut the atmsphere. As an example, a smelly chemical demnstratin dne at the frnt ultimately makes its presence smelt at the rear f the lecture theatre. These prperties which are assciated with each state can be explained by the KINETIC MOLECULAR THEORY OF MATTER. Nte that when substances underg changes f state such as thse illustrated by water abve, the H2O mlecules themselves are nt brken dwn t cmpnent atms. It is nly the weak frces that perate between the mlecules which are vercme as the mlecules gain mre energy, thereby allwing them t separate frm each ther and escape int the gaseus phase. Thus water is still present as H2O mlecules in the slid, liquid and gas phases, but in the gaseus phase the mlecules are far apart and mve abut independently f each ther instead f having reduced freedm t mve as in the liquid state r being anchred in a fixed psitin as in the slid. The kinetic mlecular thery. (a) Gases. In this thery, the mlecules f a substance in the gas phase are regarded as being in cnstant mtin, mving at very high speeds and clliding with each ther r with the walls f their cntainer many times each secnd. The speed f the mlecules and thus their energy increases with increasing temperature. The mlecules are regarded as having n vlume f their wn and it is assumed there are n attractive frces between them. The mlecules are travelling rapidly and have enugh energy fr them t rebund frm cllisins withut any lss f energy. The cllisins with the walls f the cntainer are the cause f gas pressure - the mre cllisins per secnd that ccur, the greater is the resultant pressure. Fr example, a H 2 mlecule at 0 C is mving at 5 times the speed f sund and 10 clliding with ther mlecules at the rate f 5 10 cllisins per secnd. The abve is nly a mdel. Hwever, this mdel des apprximate well t the behaviur f real gases under nrmal temperature and pressure cnditins. The mlecules f all real gases f curse d have a finite vlume, althugh it is indeed nly a tiny fractin f the cntainer vlume at nrmal pressures but is significant at high pressures. Mst f the vlume f a cntainer f a gas is actually empty space with the real vlume f the gaseus mlecules at nrmal pressures being less than 1%. Thus gases are relatively easy t cmpress - e.g. using a ST1-1

2 ST1-2 bicycle pump t blw up a tyre. Real gases als experience attractive frces between their mlecules which causes the cllisins t be "sticky" t sme extent and this becmes significant at lw temperatures. Hwever, the kinetic mlecular mdel is adequate t explain and predict the behaviur f gases prvided the temperatures are nt very lw r the pressures nt very high. (b) Liquids. In the liquid phase, the cmpnent mlecules are much clser tgether than in the gaseus state. This is seen in the fact that 1 ml f liquid water cnverted t steam at the biling pint prvides 1670 ml f the gas. The amunt f empty space in a liquid is abut 3% f the ttal vlume and cnsequently liquids are almst incmpressible. In the liquid phase there is still TRANSLATIONAL MOVEMENT f the mlecules, but this is very much slwer and mre restricted than fr gases. Because the mlecules in the liquid phase are much clser tgether, the attractins between them are much strnger, giving rise t the phenmenn f surface tensin and thus t the defined bundary f the liquid. Surface tensin is a cnsequence f unbalanced attractive frces experienced by the utermst layer f mlecules in the liquid. This is well illustrated by the bservatin that drps f liquid such as water adpt a spherical shape in which the surface area is smallest fr any given vlume. (c) Slids. The cmpnent atms, mlecules r ins in the slid are anchred by even strnger frces than thse in the liquid phase. They are held at fixed psitins in a crystal lattice and are unable t mve within the slid. Instead they can nly underg mtins such as vibratin which d nt require any translatinal mvement. Hence the slid has a rigid bundary and a fixed vlume with negligible empty space and cannt be cmpressed t any significant extent.. Cnversin between states - latent heat. Cnsider water in the liquid state being heated. The temperature rises until the water starts t bil at a temperature f 100 C at sea level. Measurements wuld shw that 4.18 J f energy wuld be required fr each gram f water heated by 1 C. Therefre t heat 1 mle (18 g) f water frm 25 C t 100 C wuld require J f heat = 5.6 kj. This heat is all used t increase the mtin f the water mlecules which mve faster at higher temperatures.

3 ST1-3 Once the biling pint is attained, if heat cntinues t be supplied, there is n increase in temperature but instead the liquid cnverts t the vapur (gaseus) state at 100 C. The energy being supplied at this pint is used by water mlecules t escape the attractive frces f ther water mlecules in the liquid phase and underg the transitin t the gas phase. If heating is carried ut fr a lng enugh time, all the liquid sample will be cnverted t gas and nly after this has ccurred will the temperature f the steam start t rise again assuming it is trapped. If this experiment were carried ut under cnditins which allwed the energy supplied t be measured, it wuld be fund that each mle f water required 41 kj fr the cnversin frm liquid t gas at 100 C. This very large amunt f heat energy was used by the water mlecules t vercme the attractive frces which exist between them in the liquid state, especially the frces knwn as hydrgen bnding (see Tpic 13). Because n temperature rise is bserved, this energy used fr the transitin frm liquid t vapur state is called the "LATENT HEAT OF VAPORISATION". All liquids require the latent heat f vaprisatin t be supplied in rder t make the transitin frm the liquid t the vapur, althugh mst have a smaller value than water. The latent heat f vaprisatin f water is exceptinally large because f the extensive hydrgen bnding frces which must be brken befre individual mlecules can escape t the vapur state. Similarly, cnsider water in the slid phase (ice). Ice melts at 0 C t frm liquid water at the same temperature. Again, heat must be supplied merely t achieve the cnversin frm slid t liquid phase withut any increase in temperature. In this case the amunt f heat required is nly 6 kj per mle f ice melted, and this is called the "LATENT HEAT OF FUSION". The energy required fr ice t melt is much less than that required fr water t bil because hydrgen bnds between H2O mlecules are retained in the melting prcess but must be brken fr the mlecules t enter the gaseus state. In bth these examples, heat was required fr the cnversins f liquid t gas r slid t liquid. Fr the reverse cnversins, heat wuld be released. Thus steam cndensing t liquid water at 100 C releases the same amunt f heat as was absrbed in the vaprisatin prcess, viz 41 kj per mle. This is the basis fr steam sterilisatin where the large amunt f latent heat released is used t kill micrrganisms. In rder t cnvert liquid water t ice, heat must be remved, again equal t that amunt absrbed when ice melts, viz 6 kj per mle. These prcesses can be summarised as fllws: heat absrbed melting biling SOLID heat f fusin LIQUID heat f vaprisatin GAS freezing cndensing heat released In sme situatins, slids may cnvert directly t the gaseus state in a prcess called "SUBLIMATION" in which case the heat invlved wuld be the sum f the heats f vaprisatin and fusin. Liquefactin f gases. All gases can be cnverted t the liquid state. This is achieved by lwering their energy sufficiently fr the attractive frces between the mlecules t cause them t stick tgether in large aggregates, frming drplets which ultimately calesce. The energy f the gaseus mlecules is lwered by cling the gas. High pressure als

4 ST1-4 can be used t frce the mlecules clser tgether and by these means allw the attractive frces t suppress the mtin f the gaseus mlecules. Examples f gases cnverted t the liquid phase and stred under pressure at rm temperature are liquid butane and prpane present in LPG cylinders and used as fuel fr prtable stves and mtr vehicles. Anther example f gas liquefactin is seen in the frmatin f cluds frm water in the gaseus frm. Cluds cntain fine drplets f misture that have cndensed frm gaseus water in the atmsphere and which under the apprpriate cnditins calesce t frm rain drps r even cndense t the slid frm as hail r snw. The large amunt f heat released in the cndensatin f water vapur t liquid phase has significant effects n the weather. Evapratin cmpared with biling. Liquids can cnvert t the gas phase withut being biled by heating directly. The level f water left in a glass in the pen gradually falls and ultimately the water disappears cmpletely. This prcess is called EVAPORATION and nly differs in that the mlecules leaving the liquid phase d s frm the surface whereas, in biling, the mlecules clse t the heat surce gain the energy t cnvert t gas and frm bubbles which then rise thrugh the liquid and escape t the surrundings. Bth prcesses require exactly the same amunt f heat t ccur, but evapratin is much slwer because the heat is gained gradually frm the surrundings rather than quickly frm an applied heat surce. Evapratin is accmpanied by cling f the liquid, and this is the basis fr evaprative clers and als fr the bdy's wn cling mechanism whereby sweat frm pres n the surface f the skin evaprates, absrbing 41 kj f heat energy per mle as it des s. Air mtin abve the surface f a liquid speeds up the prcess f evapratin by remving gas phase mlecules and preventing their return t the liquid. Check yur understanding f this sectin. On a ht day a persn may cnsume an additinal litre f water which is subsequently cnverted t sweat. Calculate the heat remved frm the bdy by the evapratin f this sweat, taking the mass f 1.0 L f water = 1.0 kg. Effect f pressure n biling pint. Because it is ever present, we tend t be blivius t the high pressure resulting frm the atmsphere abve us - equivalent t 1.0 kg n each square centimetre f ur bdy. Pressure can be expressed in many units, ften as atmspheres but in SI units, pressure is measured in pascals (Pa). The reductin in pressure with increasing height abve sea level requires pressurisatin in high flying aircraft and presents difficulties fr muntain climbers n tall peaks. Athletes cmpeting at venues such as Mexic City require cnsiderable time there t acclimatise t the lwer pressure f xygen available. As the external pressure decreases, mlecules can escape frm liquid t gas phase mre readily, requiring less energy and thus biling at a lwer temperature. At sea level where average atmspheric pressure = kpa, pure water bils at 100 C but at the tp f Mt. Ksciusk (2230 m) it bils at 92.6 C due t the pressure being reduced t 77.3 kpa. Atp Mt.Everest (8840 m) the biling pint wuld be a mere 69.6 C at a pressure = 30.7 kpa. The same principle als applies in reverse - if external pressure is increased then

5 ST1-5 the biling pint will increase as the escaping mlecules require mre energy (ie higher temperature) t leave the surface f the liquid. Use f this is made in pressure ckers and autclaves where water may be cnverting t steam at temperatures as high as 121 C. The cmbinatin f the latent heat f vaprisatin released when the steam cndenses and the high temperature causes cking t be faster and, in the case f autclaves, is sufficient t destry all micrrganisms - even heat resistant bacterial spres. Sublimatin. SUBLIMATION is the name given t the prcess whereby a slid is cnverted directly t the gas phase withut first frming the liquid phase. The reverse prcess, frm gas directly t slid, is termed DEPOSITION. Sublimatin ccurs when the frces that hld the mlecules t each ther in the slid state are very weak - thus inic cmpunds will nt sublime at nrmal pressures, as the electrstatic frces that bnd ins in an inic crystal lattice are strng. Hwever, many cvalently bnded cmpunds ften have nly weak frces perating t hld the mlecules clse t each ther in the slid state. Sme f these cmpunds have such weak frces between the mlecules that, when heated sufficiently t break dwn the slid, the mlecules can escape immediately int the gaseus phase. Carbn dixide is a cmmnly knwn cmpund that underges sublimatin, this prperty being used in "dry ice" because it cnverts frm the slid state t the gaseus state withut passing thrugh the liquid phase. Again, nte that when sublimatin ccurs, the mlecules themselves d nt break dwn t their cmpnent atms. Only the weak frces perating between the mlecules break, thereby allwing them t separate frm each ther and escape int the gaseus phase. Unlike water, carbn dixide des nt have hydrgen bnds perating between the CO 2 mlecules and s it can underg the change frm slid t gas directly at rm cnditins withut passing thrugh the liquid state. Latent heat and the weather. The heat energy absrbed when water evaprates and released when water vapur cndenses is a very significant factr cntrlling the planet's weather. When water evaprates frm the ceans and bdies f fresh water, the latent heat f vaprisatin (41 kj fr every 18 ml f water evaprating) is absrbed by the evaprating water mlecules. This heat energy is btained mainly as infra-red radiatin frm the sun. The water vapur s prduced rises in the atmsphere and ultimately cndenses t frm water drplets (rain) r the slid phase (hail, snw). When the water cndenses in the upper atmsphere, it necessarily releases heat energy equal t the latent heat f vaprisatin which was absrbed when the evapratin ccurred. Thus the prcess f evapratin and cndensatin f the water transfers vast amunts f energy up int the atmsphere and this energy in turn is the driving frce fr destructive strms and flds. Gases such as carbn dixide in the atmsphere assists in increasing the amunt f the sun's energy that is available fr the evapratin prcess. Cncern abut the rise in Earth's temperature underlies attempts t reduce the amunt f carbn dixide released. Mlar vlumes f liquids and slids. The vlume f ne mle f any element r cmpund is called its MOLAR VOLUME. In Tpic 9 the intercnversin f mass and mles was emplyed t calculate the masses f reactants and prducts invlved in chemical reactins. The

6 ST1-6 mass and vlume f any substance is cnnected by its DENSITY. The density f any bject is defined as the rati f its mass and vlume. Using the symbls m fr mass, V fr vlume and d fr density, this relatinship can be written as d = m/v. The density f any pure substance is a characteristic f that substance and tabulatins f densities are available fr mst elements and cmpunds. The vlume f mst materials increases as the temperature rises because the mtin f the cmpnent atms, mlecules r ins increases due t the extra energy supplied as heat. Cnsequently the density f liquids and slids depends t a small extent n the temperature and this must be specified alng with its numerical value. Fr example. the density f liquid water has the fllwing values at varius temperatures: Temperature ( C) 3 Density (g cm ) The units used t express density must be mass per unit f vlume. Generally in chemistry the units used are grams fr mass and cubic centimetres fr vlume, but millilitres are ften used instead f cubic centimetres because they are identical. Mlar vlumes f gases. While the mlar vlume f any slid r liquid at a given temperature depends n the specific element r cmpund, fr gases the vlume f a mle f any gas at a given temperature and pressure is always the same. This is because the attractive frces between gas phase mlecules are s weak that gases expand t cmpletely and unifrmly fill their cntainer regardless f which gas is present, s the vlume f a mle f gas is determined nly by the temperature and pressure. In the next Tpic, this prperty is further examined. Example 1. Calculate the vlume f 1.00 mle f water at 30 C given the density f water at 3 that temperature is g cm. Density = mass vlume Mlar mass f H2O = 18.0 g = 18.0 / mlar vlume mlar vlume = 18.0 / = 19.0 cm ml = 1 cm, mlar vlume = 19.0 ml

7 ST1-7 Objectives f this Tpic. When yu have cmpleted this Tpic and its assciated tutrial questins, yu shuld have achieved the fllwing gals: 1. Knw the three cmmn states f matter and their distinguishing features. 2. Knw the principles f the kinetic mlecular thery f slids, liquids and gases. 3. Understand why (latent) heat changes accmpany phase changes. 4. Understand the prcesses by which gases liquefy and liquids bil r evaprate and hw biling differs frm evapratin. 5. Understand hw pressure affects the biling pint f a liquid. 6. Understand why sme slids sublime when heated at nrmal pressures. 7. Recgnise the rle f latent heat assciated with phase changes f water as a factr affecting the weather. 8. Be familiar with the cncept f mlar vlume and understand why the mlar vlume f a gas is independent f the particular gas examined. SUMMARY. Matter may exist in the slid, liquid r gaseus phases. Each state has its wn prperties which are a cnsequence f hw far apart the cmpnent particles (atms, mlecules r ins) are and hw fast they are mving. When mlecular substances underg a phase change, their mlecules remain intact and d nt decmpse t cnstituent atms. The kinetic mlecular thery fr an ideal gas envisages gases as cnsisting f rapidly mving particles separated by relatively large distances and clliding withut lss f energy, due t the absence f attractive frces between them. This mdel applies t idealised gases where there are n attractive frces perating between the cnstituent particles and thse particles have n vlume. Cnstituent particles f real gases d ccupy a vlume and d experience attractive frces between themselves. Hwever, while this is nly a mdel, it adequately explains the prperties f real gases at nrmal cnditins f temperature and pressure. In the liquid state, the cmpnent particles are much clser tgether with strnger attractins between them. Translatinal mtin is present but restricted in the liquid state. Particles in slids have even less freedm t mve with their mtin being restricted t vibratins and rtatins with n translatinal mtin. Cnsequently slids maintain a fixed shape. The cnversin between different physical states is accmpanied by energy changes - energy absrbed fr slid t liquid r liquid t gas phase changes and an identical amunt f energy released fr the reverse phase changes. These energy changes are called "latent heats" because they are nt accmpanied by any temperature change, being related slely t the changed distances between the particles as the phase change ccurs.

8 ST1-8 Gases are liquefied by lwering the energy f the particles by cling (t slw the particles) and using high pressure (t frce the particles clser tgether). Ultimately the attractive frces between the atms r mlecules cause them t clump tgether as a liquid. The reverse prcess is biling r evapratin, bth f which absrb energy, but evapratin ccurs at temperatures belw the biling pint and results in cling f the remaining liquid because the highest energy particles escape frm the surface t the gas phase. Biling ccurs at the site where the heat surce is applied and the temperature f the biling liquid remains cnstant because mre heat is supplied t the liquid frm the surce t replace the energy remved by the escaping gas phase cmpnent. The temperature at which biling ccurs is influenced by the external pressure n the surface f the liquid. Higher external pressure raises the biling pint f a liquid and lwer pressure decreases it. Sme slids can be cnverted directly t the gas phase at nrmal pressures in the prcess called sublimatin. This ccurs when the frces perating between the mlecules are particularly weak. The latent heat assciated with the evapratin and cndensatin f water is a very significant factr assciated with the earth's weather. The vlume f a given mass f any substance can be calculated frm its density. The density f all pure substances is a characteristic and fr mst elements and cmpunds, is tabulated at varius temperatures. This is necessary because the vlume f mst materials increases with temperature due t faster mtin f the cmpnent particles. If the amunt f an element r cmpund is ne mle, then the vlume is its mlar vlume. Unlike slids r liquids hwever, the mlar vlume f all gases is the same at a specified temperature and pressure because the particles f gas experience such small attractive frces that any gas expands t cmpletely fill its cntainer. Recmmended fllw up chemcal mdules: Sectin: General Chemistry Mdule: States f Matter Tpic: Vapur pressure f liquids, phase diagrams and their interpretatin. TUTORIAL QUESTIONS - SUPPLEMENTARY TOPIC Name the three cmmn states f matter and give the characteristics f each state. 2. What are the assumptins made as the basis fr the kinetic mlecular thery f gases? 3. In what ways d real gases deviate frm the ideal gases described by the kinetic mlecular mdel? 4. What is the cause f pressure in a gas trapped in a cntainer? Hw des the temperature f the gas influence the pressure?

9 ST What is the rigin f surface tensin? Why are drps f liquid spherical? 6. Why are liquids almst incmpressible but gases can be cmpressed easily? 7. Why d slids maintain a fixed shape whereas liquids and gases d nt? 8. Water reaches its biling pint when heated t 100 C at atmspheric pressure. Further heating causes the water t cntinue biling withut any temperature increase. What happens t the heat being supplied? 9. Why des water have an exceptinally large heat f vaprisatin? 10. T melt 1 mle f ice at 0 C requires supplying 6 kj f heat. Hw much heat wuld have t be remved frm 1 mle f water at 0 C t freeze it? 11. Why are steam burns much mre severe than scalding by biling water? 12. Dry ice (slid carbn dixide) is ften used t keep sme types f fd frzen. Give a reasn why it may be preferred t nrmal ice? 13. What is the advantage assciated with liquefying natural gas? Hw is this dne? 14. Why is water vapur able t cndense t drplets after it rises int the upper atmsphere? 15. Cmpare the prcesses f evapratin and biling. Indicate in what ways they are similar and hw they differ. 16. Hw d evaprative clers wrk? 17. A can cntaining a little water is biled t generate steam. If the cap is then screwed n, the can cllapses as it cls. Why des this ccur? 18. Hw culd ne make a really ht cup f tea n tp f Mt.Everest? 19. Hw culd yu demnstrate the existence f atmspheric pressure? 20. What prperty f a cmpund causes it t sublime when heated? Give an example f a cmpund that sublimes at atmspheric pressure. 21. Calculate the mlar vlumes at 25 C f: 3 (a) idine (density at 25 C = 4.9 g cm ) 3 (b) sdium chlride (density at 25 C = 2.2 g cm )

10 ST1-10 ANSWERS TO TUTORIAL TOPIC ST1 1. Slid, liquid and gas. In the slid state, the cmpnent particles (atms, mlecules r ins) are restricted in their mtins t nly vibratins due t strng frces between them. Cnsequently slids retain a fixed shape and are almst incmpressible. In the liquid state, the cnstituent particles have sme degree f translatinal mtin because the attractive frces are nt as strng as in the slid state. Even s, liquids are als almst incmpressible and the frces are strng enugh fr liquids t maintain defined bundaries althugh unlike slids, n fixed shape. In the gaseus state the attractive frces between the cnstituent particles are much weaker that thse perating fr the same substance in either the slid r liquid states. The cnstituent particles in the gaseus state are in cnstant and rapid mtin. Cnsequently gases have n fixed vlume r defined bundaries and will expand t cmpletely and unifrmly fill their cntainer. 2. The kinetic mlecular thery f gases assumes that the cmpnent particles are in rapid mtin in straight lines and frequently experience perfectly elastic cllisins with each ther and with the walls f their cntainer with n lss f energy. Pressure frm gases is envisaged as a result f thse particles clliding with the walls f the cntainer. Anther assumptin f the mdel is that the cnstituent particles f ideal gases have zer vlume. 3. The atms r mlecules f real gases d exhibit sme degree f interactin when cllisins between them ccur, smetimes called sticky cllisins. They als d have a finite vlume. Hwever, the deviatin frm ideal gas behaviur caused by these factrs is nt significant at nrmal temperature and pressure cnditins. 4. Gas atms r mlecules are in rapid mtin and cllide with the walls f their cntainer, giving rise t the bserved pressure attributed t the gas. At higher temperatures the particles have mre energy, mve faster and impart a greater pressure n the cntainer s walls. 5. In the liquid state, attractive frces perate between adjacent mlecules even thugh the mlecules have sme degree f translatinal mtin. Fr mlecules that are nt near t the surface f the liquid, these frces cancel ut in all directins verall s there is n nett frce experienced by them. Hwever, thse mlecules in the uter layer f the liquid experience unbalanced frces due t there being n mlecules f the liquid utside that layer. The result is that the uter layer f mlecules acts like a skin n the surface f the liquid and this is the rigin f surface tensin. Liquids typically frm spherical drps because this shape cntains the largest vlume f liquid fr the smallest surface area, thereby minimising the verall frces that the uter mlecules experience 6. The mlecules in a liquid are very clse tgether, typically ccupying abut 97 % f the ttal vlume. Cnsequently there is little scpe t frce them t be clser tgether by cmpressing the liquid. In a gas, the cmpnent particles are far apart and at rm temperature and pressure their actual

11 ST1-11 vlume is nly abut 1% f their cntainer. Cnsequently there is cnsiderable scpe t cmpress a gas. The mre pressure applied t gases, the clser their cmpnent particles are frced tgether and the mre difficult it is t further cmpress the gas int a smaller vlume. 7. In a slid, there is almst n vid space between the cmpnent particles and the frces f interactin between them is s strng that they have n translatinal mtin, s their bundaries d nt change. The cmpnent particles in liquids and gases are further apart and d have translatinal mtin, much mre s in the gas phase than the liquid. In bth cases, that translatinal mtin means that they have n fixed shape. 8. After liquid water has been heated t its biling pint, further heating cntinues t supply energy t the water mlecules and they use that energy t escape frm the liquid phase t the vapur phase. The energy is used t vercme the attractive frces that exist between the mlecules in the liquid, especially the frces called hydrgen bnding. Cnsequently the additinal heat energy supplied des nt lead t a rise in temperature because it is being used in the vaprisatin prcess. This heat energy is called the latent heat f vaprisatin and fr water and is equal t 41 kj per mle f water. 9. Water has a particularly large heat f vaprisatin because it has hydrgen bnds attracting adjacent H2O mlecules t each ther as well as the weaker types f intermlecular frces which all mlecules in the liquid phase experience. 10. The energy difference between slid water and liquid water at its melting/freezing pint is 6 kj. This amunt f energy is required t be supplied t the ice t melt it r must be remved frm liquid water t freeze it. The actual amunt f heat t be transferred is the same, regardless f the directin f the phase change. 11. Burns frm steam are mre severe than burns frm water at its biling pint because steam cndensing n the skin releases the latent heat f vaprisatin f water, 41 kj per mle. Liquid water at that temperature has nne f this latent heat stred s less heat is released t the skin and the burn is less severe (but still painful!). 12. The advantage f using dry ice rather than nrmal water ice is that the dry ice sublimes at atmspheric pressure and n liquid is frmed in the prcess. This avids the material being cled becming wet as it wuld when water ice melts t frm liquid water. 13. Gases ccupy a very large vlume fr a given quantity f the substance cmpared with the vlume required fr the same amunt f that substance in the liquid phase. Natural gas, methane, is much mre easily transprted as a liquid using cled tankers. It is liquefied by using high pressure t cmpress the gas, frcing the mlecules clser tgether and als the gas is cled s that the mlecules mve mre slwly. This cmbinatin allws the attractive frces between the CH 4 mlecules t cause clumping and finally liquefactin.

12 ST The temperature f the atmsphere is lwer at higher altitudes. The lwer temperature allws water mlecules in the gaseus phase t lse energy and mve mre slwly, ultimately cndensing as liquid water drps. 15. Evapratin and biling f a liquid bth result in liquid phase mlecules escaping t the vapur phase by vercming the attractive frces that hld them tgether in the liquid. When a liquid is biled, heat is supplied frm a heat surce such as a gas burner and the mlecules clse t the surce f the heat receive the extra energy which allws them t mve t the surface and escape as a gas. Evapratin ccurs when mlecules f the liquid near t its surface are able t absrb sufficient heat frm the surrundings t make the transfer t the gas phase. Because the mst energetic mlecules escape in this way, the temperature f the remaining liquid decreases and the result is evaprative cling f the liquid, a prcess used by humans t cl their bdies by sweating. 16. Evaprative clers make use f the large heat f vaprizatin (latent heat) f water t cl a stream f air blwn ver water. The latent heat absrbed by the water mlecules as they transfer t the vapur state is enhanced by the greater rate f evapratin resulting frm the air travelling acrss the water s surface. Because the air mving acrss the water surce cntains mre mlecules f water vapur than the surrunding air, the humidity is increased. 17. When the water is biled, gas phase water mlecules displace the mlecules in the air within the can. If the can is then sealed and cled, the water mlecules cndense back t the liquid phase where they ccupy far less vlume. This results in the pressure f the atmsphere utside the can n lnger being balanced by the pressure inside it, leading t the can being crushed by the weight f air utside. 18. The biling pint f a liquid increases as the external pressure n its surface increases, because the mlecules need mre energy t escape frm the surface. Cnversely, if the external pressure is reduced, mlecules f a liquid requires less energy t escape t the vapur phase and s will bil at a lwer temperature. This is why the biling pint f water n a high muntain is less then the nrmal 100 C experienced at sea level. One can elevate the biling pint f water by heating it in a sealed cntainer called a pressure cker. This is used smetimes in dmestic cking t ck fd faster (and destry it in the prcess!) and culd be used t bil water fr the tea at a suitably high temperature. 19. Air pressure can be demnstrated by taking a glass tube that is sealed at ne end, filling it with water and inverting the tube int a beaker f water. The bulk f the water remains in the tube, supprted by the pressure f the atmsphere n the surface f the water in the beaker. This is the principle underlying the barmeter which is used t measure air pressure. Mercury is used in a barmeter because its large density reduces the length f the tube required. At nrmal atmspheric pressure, the length f the clumn f mercury required is abut 76 cm high. Atmspheric pressure wuld supprt a clumn f water abut 16 times this length.

13 ST Sublimatin is the prcess whereby a slid cnverts t the gas phase withut passing thrugh an intermediate liquid state. Fr this t happen, the frces f attractin between cnstituent particles in the slid must be particularly weak r the external pressure very lw. Slid carbn dixide is a cmmn example f a substance that sublimes at atmspheric cnditins. 21. (a) density = mass vlume vlume = mass density mass f 1.0 mle f idine = g = 254 g 3 density f idine at 25 C = 4.9 g cm vlume f 1.0 mle f idine at this temperature = = 52 cm (b) mass f 1.0 mle f sdium chlride = = 58.5 g 3 density if sdium chlride at 25 C = 2.2 g cm vlume f 1.0 mle f sdium chlride = = 27 cm 3 3