俗话说：响水不开,开水不响.烧水的时候,水加热到一定的程度,就会在茶壶里“吹口哨”,而水烧开后,声音就消失了.下面这篇文章就来为您解开这个谜团. 　　Ifyouhaveeverpeekedinsideakettleasitboilsyoumighthavenoticedtinybubblesappearingontheelementandrisingtothesurface.Thesebubblesarecausedbecausetheheatingelementiscausingthefilmofwatersurroundingittoreachtheboilingpointandturnintosteam.Becausesteamisagasandislessdensethanwateritwillrisetothetopofthekettle.Timeforsomequickrevisionincaseanyoneiswondering:Densityisameasurementoftheamountofspaceacertainamountofmatteroccupies.Soifyouweretomeasurethespacetakenupbyacertainamountofliquidwaterandthenturnitallintosteamandseehowmuchspacethegastakesup,youwouldseethatthesteamtakesupmoreroomandisthereforelessdense.OK,enoughofthat.OK,backtothekettle.Soourbubblesofgaseouswater(steam)arecurrentlyrisingtowardsthesurfaceofthewaterbecauseoftheirlowerdensity.Butbecausethewaterisfurtherawayfromtheheatingelementisnowherenearboilingtemperature(becausetheyarefurtherawayfromtheelement),thebubblesofsteamgetcooleddownandreturntoliquidform.Theprocessofthelower-densitybubblesofgassuddenlybecominghigher-densitywateragainiswhatmakesthenoise.Ifyouthinkaboutit,theliquidwatertakesuplessroomthanitdidwhenitwasabubbleofgasasitisdenser.Soasitchangesitsstate,watermoleculessurroundingthebubblewillrushintofillupthespacethatitusedtooccupywhenitwaslessdenseandtookupalargerspace.Thisisonlyonaverysmallscalebutitisstillverynoisy.Asthewatergetsheatedmoreandmore,thebubblesofgasgetbiggerandastheyburstbeforetheyreachthesurface,itmakesmoreandmorenoise.Thensuddenly,allwillgocalmandtherewillbenonoise.Thiscalmbeforethesteam(haha)isbecauseallthewaterhasfinallyreachedboilingtemperatureandthebubblesofgascanreachthesurfaceoftheliquidandescapeintotheair.Thisdoesn'tmakeasmuchnoiseaswhenallthebubbleswerebursting.Ofcourse,whenallofthewaterstartsboilingitgetsnoisyagainasitisallbeingchurnedaboutasithappilybubblesaway.Now,ifyoureboilwaterthathasalreadybeenboiledorhadbeenleftstandingitshouldintheorymakelessnoise.Forabubbleofgastoformittakesabitofeffort.Thinkofallthatwaterthathastobepushedoutoftheway.Onceabubblemanagedtoform,itgetseasyforotherwatermoleculestogetinontheactandturnintogastoo.Waterfromthetapalreadyhasdissolvedgasinitthatcanbeusedtoseedbubbles.Butifitsalreadybeenboiled,alotofthedissolvedgashasgone.Lessbubbles=lessnoise.Thisbringsmeontowhatcanhappeninamicrowave.Youheatupsomewateranditdoesn'tlooklikeit'sboiling,thenthesecondyoutouchit,thewholethingexplodesoutofthemugandburnsyou.Thisisduetosuperheating.Thisistheprocessofheatingsomethingaboveitsboilingtemperaturewithoutitactuallyboiling.Waterboilsat100°Cifthereisalreadyabubbleofsteam(orair)present.Butintheabsenceofbubbles,watercanbeheatedabove100°C.Amughasverysmoothsides(unlikeakettleorsaucepan)sothereisnowhereforlittlebubblesofairtogettrappedandactasbubbleseeds.Plus,themugiscoolerthanthewaterinside,unlikewhenyouheatwaterinasaucepanorkettlewherethemetalandheatingelementsgetshotterthanthewater.Thismeansthatyou'llgetsmallareasofsuperheatinginakettleofsaucepanbutthewholethin