Particles: The Big Idea in Chemistry PDF

Pa rticles: the big idea in chemistry All chemistry is based on one big idea: that everything is made of very tiny pieces called particles. Most single particles are far too small to see. When you look at a jar of a chemical in the lab, you are looking at a collection of countless trillions of tiny...

Pa rticles: the big idea in chemistry All chemistry is based on one big idea: that everything is made of very tiny pieces called particles. Most single particles are far too small to see. When you look at a jar of a chemical in the lab, you are looking at a collection of countless trillions of tiny particles. Think about that! And when you observe a chemical reaction - for example, an acid dissolving a piece of metal - you are seeing the result of trillions of tiny particles reacting together. This is one of the things that makes chemistry so exciting. It took a while for the big idea to develop... For centuries, people had guessed that water and air were made of tiny A All made of particles ! particles. But they could not see the particles, so could not prove it. Then in 1 827, a botanist called Robert Brown was studying pollen from a flower, in water, under a microscope. He noticed that the pollen granules jiggled around. They were not alive - so why were they moving? 78 years later, Albert Einstein came up with the answer. The granules were moving because they were being struck by tiny invisible particles of water. In fact their movement proved that water is made of tiny particles. Today, everyone accepts that things are made of particles. The evidence around you Most particles are so small that we cannot see them directly, even with the most powerful microscope. But there is evidence for them all around you. Think about these four examples. Outside the la b 1 In sunlit rooms, you sometimes see dust 2 Cooking smells spread. The 'smells' are dancing in the air. It dances because the dust due to particles which spread because they specks are being bombarded by tiny particles are bombarded by the particles in air. This is in the air, too small to see. Just like those an example of diffusion. (See pages 3 and 1 1.) pollen granules in water. Some end up in your nose! 2  S TAT E S O F M AT T E R I n the la b air particle bromine particles and air particles water now fu l ly m ixed particle '....... WJ#'. 7 particles from the crysta l mix I among the brom ine water particles particle 3 Place a crystal of purple potassium manganate(VII) 4 Place an open gas jar of air upside down on an open in a beaker of water. The colour spreads through the gas jar containing a few drops of red-brown bromine. water, because particles leave the solid crystal - it The colour spreads upwards because the particles of dissolves - and spread among the water particles. bromine vapour mix among the gas particles in the air. Diffusion Look again at the two examples above. The particles mix and spread by colliding with other particles, and bouncing off in all directions. This mixing process is called diffusion. It takes place in liquids and gases. The result is that particles spread from where they are more concentrated, until all the particles are evenly mixed. Look at the drawings on the right. So what are these tiny particles? The movi ng particles col l ide, then The smallest particles, that we cannot break down further in bounce apart i n all directions. chemical reactions, are called atoms. In some substances, the particles are just single atoms. For example, air contains single atoms of argon. In many substances, the particles consist of two or more atoms 0 joined together. These particles are called molecules. Water and bromine exist as molecules. Air is mostly nitrogen and oxygen molecules. 0 In the end they a re all mixed up. In other substances the particles are atoms or groups of atoms that ( But they keep on movi ng.) carry a charge. These particles are called ions. In example 3 above, the particles in potassium manganate(VII) are ions..&. The process of diffusio n. 0 You'll learn more about all these particles in Chapters 2 and 3. 'Seei ng' atoms We ca n 't see atoms through any m icroscopes that use l i g ht. But i mages of them can be ta ken using electron m icroscopes. These use beams of electrons, not l i g ht. (We meet electrons later ! ) 1 Dust specks appear to dance i n a i r. Why d o they 4 a Describe diffusion. move? b Use the idea of d iffusion to exp l a i n how the 2 Example 3 above proves that potass i u m smell of perfu me travels across a roo m. manganate(VII) is m a d e o f particles. Explain why. 5 a List t h e three types o f particle you wi l l meet i n 3 B rom ine vapour is heavier than a i r. Even so, it chemistry. spreads u pwa rds, in example 4 above. Why? b What is the difference between an atom and a molecule? 3 What's the difference? It is easy to tell the difference between a solid, a liquid, and a gas: A solid has a fixed shape and a A liquid flows easily. It has a A gas does not have a fixed fixed volume. It does not flow. fixed volume, but its shape volume or shape. It spreads out It cannot be compressed into a changes. It takes the shape of to fill its container. It is much smaller space. Are you sitting on the container you pour it into. lighter than the same volume of a solid? solid or liquid. Water: solid, liquid a nd gas Water can be a solid (ice), a liquid (water), and a gas (steam). Its state can be changed by heating or cooling: thermometer thermometer shows 1 00 ° c shows O c steam (i nvisible) 0 water vapour water boi lin g water ice cu bes melting.... heat.... heat 1 Ice changes to water when it 2 When the water is heated, 3 Now the water is boiling. is warmed. This change is called its temperature rises. As it It evaporates fast. The gas it melting. The thermometer warms up, some of it changes forms is called steam. The shows 0 °C until all the ice has from liquid to gas, which goes thermometer shows 1 00 °C melted. So 0 °C is called its into the air. This is called while the water boils off. 1 00 °C melting point. evaporation. is its boiling point. And when steam is cooled, the opposite changes take place: cool to cool to below 1 00 ° c below 0 ° ( condenses to fo rm water freezes or solidifies stea m to form ice You can see that: condensing is the opposite of evaporating freezing is the opposite of melting the freezing point of water is the same as the melting point of ice, 0 °C. 4  S TAT E S O F M AT T E R Other thi n gs ca n change state too It's not just water. All substances can exist as solid, liquid, and gas. Even iron and diamond can melt and boil! Compare the melting and boiling points in this table: I Substance Melting point / D C I Boiling point / D C I oxygen -2 1 9 I_ -1 83 7 mercury - 39 357 ethanol -1 5 78 water 0 1 00..&. Molten i ron bei ng pou red out sod i u m 98 890 at an i ron works. Hot - over ta ble salt (sod i u m chloride) 801 1 465 1 540 ° ( ! i ron 1 540 2900 d i a mond 3 5 50 4832 Why are they all different? It's because their particles are_ different. You will find out more about all these substances during your course. The state at room temperatu re What usually matters most to us is the state of a substance at room temperature. (Room temperature is taken to be 20 °C.) Look again at the figures in the table. At 20 °C, oxygen is a gas. That's lucky, since we cannot do without it, and we breathe it in. Iron has a very high melting point, and is hard and strong at room..&. Water - liquid at room temperature. So we use it for building structures. Mercury is a metal too tem peratu re. How does that - but it is liquid at room temperature, so no good for building anything. benefit us?..&. E njoying water i n its solid state, i n Anta rctica...&. Once u pon a time, we depended on steam for rai l transport. 1 G ive two properties of: 4 Look at the su bstances in the table above. a a solid b a liquid c a gas a Which of them has the lowest melti ng point? 2 What do we cal l the substance that forms when water: b Which has the highest boi l i n g point? a freezes? b evaporates at its boi l i n g point? c Which wil l boil off fi rst on heating : iron or table salt? 3 G ive a word which means the opposite of: d Which a re l iq uids at room temperature? a evaporate b freeze e Why a re the melting and boi ling poi nts so different? 5 How the particles a re a rranged, and move Water can change from solid to liquid to gas. Its particles do not change. They are the same in each state. But their arrangement, and movements, change. The same is true for all substances. Look: State Solid __________...... , How the particles are arranged, and move Diagra m of particles The particles in a solid are arranged in a regular pattern or lattice. Strong forces hold them together. So they cannot leave their positions. The only movements they make are tiny vibrations to and fro. Liquid The particles in a liquid can move about and slide past each other. They are still close together, but not in a lattice. The forces that hold them together are weaker than in a solid. Gas The particles in a gas are far apart,..· O and they move about very quickly. / JI There are almost no forces holding them together. They collide with each k/ other and bounce off in all directions. Changing state So why do substances change state when you heat them? It is because the particles take in heat energy - and this changes how they move. Melting When a solid is heated, its particles get more energy and vibrate more. This makes the solid expand. At the melting point, the particles vibrate so much that they break away from their positions. The solid turns liquid. ooo o 0 o oo o°o oo heat heat energy at 0 0 0 0 0 00 0 0 energy melti ng point 00 0 0 0 0 0 0o o 0 0 0 o sol id a liquid is formed 6  S TAT E S O F M AT T E R Boiling When a liquid is heated, its particles get more energy and move faster. They collide more often, and bounce further apart. This makes the liquid expand. At the boiling point, the particles get enough energy to overcome the forces between them. They break away to form a gas. Look: t),/'Q., 1 rr()/'(:l n,n ran't:5 > heat -□"o",to'··o'· ,o:·-"' ,.,-.:. >$ \ ,\ I )I heat energy at energy v.O.O;D,_ K,H :; - fr. boiling point ,, ,,: "o·'J--f', -. D'" ✓ 1 \ ,, O'.. ,-.:. 'P the particles get enough slow-moving particles the particles energy to escape in liquid move faster Evaporating Even well below the boiling point, some particles in a liquid have enough energy to escape and form a gas. Evaporation takes place at the surface of the liquid, and over a range The kinetic particle theory 0 of temperatures. It is a slower process than boiling. (Boiling takes Look at the key ideas you have met place throughout the liquid, and at a specific temperature.) A su bsta nce ca n be a sol id, a l i q u i d , or a gas, and change How m uch heat is needed? from one state to another. The amount of heat needed to melt or boil a substance is different It has different cha racteristics for every substance. That's because the particles in each substance in each state. (For exa m ple, are different, with different forces of attraction between them. sol ids do not flow) The stronger the forces, the more heat energy is needed to overcome The differences a re due to the them. So the higher the melting and boiling points will be. way its particles a re a rranged, and move, in each state. Reversing the changes Together, these ideas make u p You can reverse those changes again by cooling. As a gas cools, its t h e ki netic particle theory. particles lose energy and move less quickly. When they collide, they (Kinetic means about motion.) do not have enough energy to bounce away. So they stay close, and form a liquid. On further cooling, the liquid turns to a solid. Look: on heating, the particles gain energy --+ ___ wat e r el s ( I i q u i d ) __a_s_it_w...,_ m_s_up,_s,,..o_ a r_ m_e_e_ ; --.i steam (gas) ra_te_s_ va_.p_o_ ice ( sol i d ) m__ 1----- the rest boils at 1 00 °(.,. at 0 c freezes (solidifies) as you cool it below 1 00 ° C, the water ice water stea m at 0 ° C vapour begins to condense or liquify +-- on cooling, the particles lose energy and move more slowly; as they get closer together the forces of attraction take over 1 D raw diagrams to show the a rrangement of particles 4 Why d o pudd les o f rai n d ry i n t h e sunshine 1 in a sol id, a liquid, and a gas. Add notes to each 5 State three differences between evaporation and diagra m. boi l i n g. 2 U s i n g t h e idea o f particles, exp l a i n why: 6 Oxygen is t h e g a s we breathe i n. It ca n be separated a you can not pour solids b you can pour l i q u ids from the a i r. It freezes at -2 1 9 ° C and boils at -1 83 ° C. 3 D raw a d iagram to show what happens to the a I n which state i s oxygen, at: i 0 ° C ? i i -200 ° C ? particles, when a l i q u i d cools to a sol i d. b How would you turn oxygen gas i nto solid oxygen? 7 The heating cu rve for water Heating curve for water Suppose you heat up some ice at a steady rate. G)1 50 You might expect the temperature to keep 1 25 rising as the ice warms up and turns to water, 1 00 and then from water to steam. But this does 75 not happen. te mperature consta nt at 1 00 °C, -+---+---1+-- the boi l i n g point of wate r Instead, the temperature remains constant for 50 a time in two places, even though you keep on 25 heating. Look at the two horizontal lines on 0 --,--+,,--'-- tem perature consta nt at 0 °C, ___,_____, graph (D. s lid t h e melting poi nt o f ice 7 -2 5 This graph is called a heating curve. 0 2 3 4 5 6 8 Ti me si nce sta rt of h eati ng / m i n utes A heating curve shows how the temperature of a substance changes as you heat it up. Note the axes labels. The time is a measure of how much heat energy has been added, on heating at a steady rate. As you can see from the graph: Temperatures remain constant while water changes state. These temperatures are its melting and boiling points. Explai ni ng the heati n g cu rve The shape of the heating curve can be explained by the way the particles behave. Read the notes on this larger copy of the graph. Start at the bottom. 0 1 50 Heating curve for water gas/- as you keep on heating, the gas p a rticles move faste r; the 1 25 temperatu re keeps r i s i n g I 1 00 L 1 ---- but here, at 1 00 °C, the heat energy tha t is taken i n is used to 75 overcome the forces between the p a rtic les; the tem perature l i q u id wi l l n ot rise again u nti l all the wate r ha s turned to g a s / I-- 50 I I I I I the particles i n the l i q u i d take i n heat energy a n d 25 s l i d e past each other faster; the temperature rises I 0 so l i d/- 0 1 2 4 5 -2 5 3 6 7 8 Ti me si nce sta rt of heat i n g / m i n utes but h e re, at 0 0 C, the heat energy that is ta ken i n is used to the pa rticles in the ice l attice overcome the forces h o l d i n g the p a rticl es; together i n the l attice; take in heat energy and v i b rate the tem pe rature wi l l n ot rise aga i n u n t i l a l l the ice has me lted faster; the tem perat u re rises 8  S TAT E S O F M AT T E R The cooling cu rve for water The graph below is a cooling curve for water. A cooling curve shows how the temperature of a substance changes as you cool it down. As you can see, this curve is the mirror image of the heating curve. The temperature remains constant at 1 00 °C and 0 °C, as the water changes state. This time, read the notes on the graph from the top down. Cooling curve for water 0 , 50 as the gas cools, the particles move less q uickly, \ with less energy; they don't bou nce so far apart 1 25 gas\ when they col lide t 1 00 by 1 00 °C, the particles in the gas do n ot have enough energy to \ bou ncea part when they col lide; the forces of attraction between L liquid them take over; the gas condenses to a liquid; this releases heat, so even though you keepon cooling, the tem peratu re does not fa l l :3 7 5 50 again u ntil a l l the g a s has liquified \ by O °C the particl es in the liq uid move so slowly that stronger 25 \ r--- forces of attraction take over and the lattice sta rts to form - the so id\ water freezes(solidifies) to ice; this process a l so releases heat, so 0 -2 5 the temperatu re doesnot fa l l again u ntil a l l the water has frozen Ti me si nce sta rt of coo l i n g / m i n utes 0 2 3 4 5 6 7 8 Not j u st water... You can draw heating and cooling curves for any substance. They are different for each substance. That's because they depend on the particles Heating curve for iron 0 3 500 F and the forces between them. The heating curve on the right is for iron. 3000 D u What if a su bsta nce is not pu re? --- 2 500 E 0 The graphs here are for pure water and iron. A pure substance has only 2000 one type of particle. And it has sharp melting and boiling points, which 1 500 you can find from the heating curve. 1 000 500 But if other types of particle are mixed in, they affect the forces between 0- A particles. Changes of state will now occur over a range of temperatures, Time since sta rt not sharply. So lines like BC and DE on graph @ will be tilted, not flat. of heating / m in utes This means that melting and boiling points can be used to check whether a substance is pure. You can find out more on page 250. 1 What is a heating curve? 5 Look at cool ing cu rve @, for water. 2 Explain how you can find the melting and boiling point a About how long does the temperatu re stay at of a substance from a heating curve like graph G). 1 00 ° ( ? 3 Look at g raph (1). The tem peratu re stays steady at b Explain why the temperature stays steady for this 0 ° C for over half a m i n ute, even though heati ng time, even though you keep on cooling the container. contin ues. Explain why, using the term particles in 6 Look at graph @ , for i ro n. you r answer. a What is happen ing between poi nts D and E ? 4 Which takes more energy: melti ng ice, or boi l i n g b Between which poi nts is i ron turn ing i nto a l iq uid? water? G ive evidence t o support you r answer. c Give approximate melting and boiling points for iron. 9 What is gas pressure? When you blow up a balloon, you fill it with air particles. As they move about, they collide with the sides of the balloon, and exert pressure on it. This pressure keeps the balloon inflated. In the same way, all gases exert pressure on the walls of their containers. When you change the tem perature of a gas Look at the container of gas below. The piston can move freely up or down, until the pressure is the same inside and outside the container..& The more a i r i n , the h igher piston moves up, so the the pressu re. --- piston can pressu re does move up 0 a n d down freely not change piston moves down, so the pressu re does not change are now heat cool The pressure is the same inside and... more often, and with more force. But if you cool it, the particles outside the container. But if you So the piston moves up, as shown slow down, and strike the walls heat the gas, its particles will move here. Now the gas takes up more less often, with less force. The faster, and collide with the walls... space: its volume has increased. piston moves down. The gas The same is true for all gases, at constant pressure: volume decreases. When you heat a gas, its volume increases. When you cool a gas, its volume decreases. When you change the pressure of a gas piston pushed down 0 piston moves up aga i n 0 0 0 a-_ U ,_o\lu 'b gas compressed 0 0 i nto a smaller 0 0 0 0 0 Q 0 q 0.:::. ,, 0 o volume 0 00 Suppose you push the piston... strike the walls more often. As But i f you now release the down. The gas pressure will rise, you can see, increasing the gas piston, it will move up again. because the particles are now in a pressure in this way means that So the pressure of the gas falls smaller space, so they will... the volume of the gas decreases. again - and its volume increases. The same is true for all gases, at constant temperature: An increase in pressure means a decrease in volume, for a gas. A decrease in pressure means an increase in volume, for a gas. In fact, if you increase the pressure on a gas enough, you can push its particles so close together that a gas turns into a liquid. 10  S TAT E S O F M AT T E R The rate of diffusion of gases On page 3 you saw that gases diffuse because the particles collide with other particles, and bounce off in all directions. But two gases at the same temperature will not diffuse at the same rate. The rate depends on the mass of their particles. An experi ment to com pa re rates of diffusion A The typical random path of a The particles in hydrogen chloride gas are twice as heavy as those in gas particle d u ri n g diffusio n. ammonia gas. So which gas do you think will diffuse faster? Let's see: After each col l ision, it bounces Cotton wool soaked in ammonia solution is put into one end of a off in a d ifferent d i rection. But long tube (at A below). It gives off ammonia gas. overa l l , it travels to where the gas is less concentrated. At the same time, cotton wool soaked in hydrochloric acid is put into the other end of the tube (at B). It gives off valid hydrogen chloride gas. Gases diffuse along the tube. White smoke forms where they meet: A B cotton wool soa ked g lass wh ite smoke cotton wool soaked in a m m o n i a solution tube forms here i n hyd rochloric acid A Imagine the marble on the right is the heavier one. Which The white smoke forms closer to B. So the ammonia particles have marble will bounce further and travelled further than the hydrogen chloride particles - which means faster, when they col l ide? they have travelled faster. The lower the mass of its particles, the faster a gas will diffuse. This makes sense when you think about it. When particles collide and bounce away, the lighter particles will bounce further and faster. The particles in the two gases above are molecules. The mass of a molecule is called its relative molecular mass. So we can also say: The lower its relative molecular mass, the faster a gas will diffuse. Note that everything in the experiment above needs to be at the same temperature (room temperature) for a valid test. That is because, as you A The particles that carry scent spread by diffusion. No need for know, the temperature also affects how fast gas particles move. help from you ! 1 What causes the pressure i n a gas? 5 Look at the diagrams labelled a). You can force the gas 2 What happens to the vol u me of a gas: i nto a smaller volume by push ing the piston down. a as you heat the gas? b as you cool the gas? Using the idea of particles, explain why its pressure rises. 3 How does the vol u m e of the gas change if the 6 In the diffusion experiment above, there is a i r i n pressu re is: t h e tube. a i ncreased ? b reduced ? a Explain how the ammonia particles move from A to B. 4 In the diagrams labelled G), the piston can move up and b Explain why the hydrogen ch loride particles don't down freely. Using the idea of particles, explain why: move as fa r as the a m monia particles. a the vol ume of the gas i ncreases as the gas is heated 7 Of all gases, hydrogen d iffuses fastest at any g iven b the volume of the gas decreases as the gas is cooled temperatu re. What can you deduce from this? 11 Checkup on Chapter 1 Revision checklist Questions Core syllabus content Core syllabus content Make sure you can... 1 Give the correct name for each change of state: D state that everything is made of particles, and a solid liquid b liquid solid name three types of particles c gas liquid d liquid gas D explain why this shows that particles exis t: 2 A large crystal of potassium manganate(VII) LJ - the movement of dust specks in air was placed in the bottom of a beaker of cold - the purple colour spreading when a crystal of water, and left for several hours. potassium manganate(VII) is placed in water D describe diffusion , and use the idea of particles to explain why it occurs cold wa ter □ name the three states of matter, and give the properties of each state crystal of potassium mangana te(VII) D explain that heating a substance will bring a Descri be what would be seen: about a change of state i after five minutes ii after several hours D describe the change of state that occurs during: b Explain yo ur answers using the idea of melting boiling evaporating condensing particles. D define these terms : c Name the two processes that took place melting point boiling point freezing point during the experiment. D describe how the particles are arranged, and move, in each state of matter 3 Use the idea of particles to explain why: D explain why a gas exerts a pressure a solids have a definite shape □ describe how this affects the volume of a gas : b liquids fill the bottom of a container - changing the temperature c you can't store gases in open containers - changing the pressure d you can't squeeze a sealed plastic syringe that is completely full of water E xte n d ed sylla b u s co ntent e a balloon expands as you blow into it. Make sure you can also... □ exp lain the changes of state that occur when 4 Below is a heating curve for a pure substance. It a substance is heated, or cooled, in terms of shows how the temperature rises over time, when particles and their movement the substance is heated until it melts, then boils. D define these terms : heating curve cooling curve - D identify these on heating and cooling curve s: I'! 1 1 5 ······· ·············· ·----- 2 changes of state melting / freezing points liq u id boiling / condensing points E D interpret heating and cooling curves in terms Ti me of particles and their movement D explain, in terms of particles, the effect of a What is the melting point of the substance? temperature and pressure on the volume of b What happens to the temperature while the a gas substance changes state? D outline an experiment to show that a gas will c The graph shows that the substance takes diffuse faster than another gas with heavier longer to boil than to melt. Suggest a reason particles for this. □ explain the effect of relative molecular mass d How can you tell that the substance is not on the rate of diffusion of gases water? e Sketch a rough heating curve for pure water. 12  S TAT E S O F M AT T E R 5 A cooling curve is the opposite of a heating 9 You can measure the rate of diffusion of a gas curve. It shows how the temperature of a using this apparatus. The gas enters through the substance changes with time, as it is cooled from thin tube: a gas to a solid. Here is the cooling curve for one hydrogen ,___......-1._ plug of substance: gas i n ====; - 1 porous plaster OJ gas 78 water risi ng in tube 0 -15 water solid Time a What is the state of the substance at room The measuring tube is sealed at the top with temperature (20 °C)? a plug of porous plaster. Air and other gases b Use the list of melting and boiling points on can diffuse in and out through the tiny holes page 5 to identify the substance. in the plug. c Sketch a cooling curve for pure water. The water rises in the measuring tube if the 6 Using the idea of particles, explain each of these. chosen gas diffuses out through the plug faster a When two solids are placed on top of each than air diffuses in. Air is mainly nitrogen and other, they do not mix. oxygen. b Heating a gas in a closed container will increase its pressure. a When you use hydrogen gas, the water rises in c Poisonous gases from a factory chimney can the measuring tube. Why? affect a large area. b What does this tell you about the rate of d In a darkened cinema, dust particles appear to diffusion of hydrogen, compared with the gases dance in the beam of light. in air? c Explain your answer to b. Use the term mass ! 7 a Pick out an example of diffusion. d The molecules in carbon dioxide are heavier i a helium-filled balloon floating in the air than those in nitrogen and oxygen. ii an ice lollipop melting Predict what will happen to the water in the iii a creased shirt losing its creases when you measuring tube, when you use carbon dioxide. iron it with a hot iron Explain your answer. Gas Formula Relative atomic or iv a balloon bursting when you stick a pin molecular mass in it 10 methane CH 4 v a blue crystal forming a blue solution, 16 hel i u m He 4 when it is left sitting in a glass of water oxygen 32 vi brushing paint onto a wall from a paint tin 02 n itrogen 28 vii the yolk hardening, when you boil an egg b For your choice in a, draw a diagram showing N2 the particles before and after diffusion. chlorine Cl 2 71 c Explain why this diffusion occurs. Look at the table above. Extended syllabus content a Which two gases will mix fastest? Explain. 8 This is about the cooling curve in question 5. b Which gas will take least time to escape from a Using the kinetic particle theory, describe and a gas syringe? explain the changes in arrangement of c Would you expect chlorine to diffuse more particles that occurs at 78 °C. slowly than the gases in air? Explain. b Explain how the movement of particles is d An unknown gas diffuses faster than nitrogen, changing in the part of the curve labelled: but more slowly than methane. What you can i gas ii liquid deduce about its relative molecular mass? 13