Science: Air Composition & Properties PDF

s e i t r e p o r p s t Airand i 7.1 4‘0‘@ Air is all around us. But do you know what air is? Share what you know about The ventilation is poor. What is wrong with the air? air with your classmates. 'Fresh’ air makes me feel great! What is He is breathing in gases other than air. Is air the...

s e i t r e p o r p s t Airand i 7.1 4‘0‘@ Air is all around us. But do you know what air is? Share what you know about The ventilation is poor. What is wrong with the air? air with your classmates. 'Fresh’ air makes me feel great! What is He is breathing in gases other than air. Is air the only ‘gas’ that allows you to survive? inside air? In the above discussion, we should realize that air is not a single gas. It actually consists of various gases. Now, let’s see what air is made up of, and the properties of its components. What is air made up of? Air forms the atmosphere that surrounds the Earth. It is important to the survival of living things. Scientists have found that air is a mixture of gases as shown below. Percentage (by volume in air) 78% Nitrogen 4 See 'Snapshot' on p. 14 for more information about argon. atmosphere X&/E nitrogen @ oxygen & carbon dioxide =& Lk argon Now, we should understand that 7.1 Making a pie chart § A pie chart is a circle divided into sectors to show the composition of something. Let's make a pie chart to show the composition of air. Step 1: Calculate the angles of the sectors. 4 Round up the numbers to the first decimal point. Step 3: Label each sector with the name of the gas. T P S Step 2: Draw the sectors according to the angles above. Q e S e L e e Ty & Steps 2 and 3 .!’ Step 1 aE— Oxygen 21% Angle of sector in pie chart 360° X Others " ) PN s L T e ik o bt i b e el Pt o T R et 10l e W Fe[ g Gas Percentage in air atmosphere Fig. 7.1 d e d n u o r r u s s i h t r The Ea . e r e h p s o m t a e h t by How can we test for various gases in air? Gases in air are colourless and odourless. How can we ¥ tannuage 1p distinguish between them? In the following experiments, we are _tess suffix without, e.q. going ple and oing to learn about simple t tests for oxygen, en, carbon dioxide 10Xi colourless- without colour, i water vapour. .Q_ (7} 7.1A 0\&' Testing for gases in air g n i n i a t n o c s e b u t g n i l i o b stoppered 4 for each gas e oxygen e carbon dioxide e nitrogen ) e air lime water hydrogencarbonate indicator 4 wooden splints gas lighter '. 8 A Wear safety goggles. & Be careful when handling a naked flame. Steps Prepare 4 boiling tubes containing oxygen. Label them from o \ A\ /) A to D. Follow steps 2 to 5 and observe what happens. {i 9 Light a wooden splint with a lighter. Remove the stopper from boiling tube A and quickly put the burning splint into the tube. What happens to the burning splint? Y QY burning splint o gas being tested 7 Living Things and Air om fr r pe op st e th ve mo Re n. ow Prepare a glowing splint as sh e th to in nt li sp g in ow gl e th t pu y boiling tube B and quickl 9 ? nt li sp g n i w o l g e th to s n e p p tube. What ha ‘k ( ) BENPNY, \\ blow '\ ¢ \\ burning | Y splint | bl'/ 2 gas being tested \ @ glowing splint / i glowing splint / Remove the stopper from boiling tube C. Add 2 cm’ of lime water to the tube. Stopper the tube and shake it gently. What happens to the lime water? gas being tested lime water 7 shake gently Remove the stopper from boiling tube D. Add 2 cm’ of hydrogencarbonate indicator to the tube. Stopper the tube and shake it gently. What gas being tested happens to the indicator? shake gently 6 Repeat steps 1 to 5 with carbon dioxide, nitrogen and air in turn. ' = " :' ' Results < N 3dses L |= 0 E 2 i E | Sanat ) ? Oxyge — g T e ?*t oy SRR fi L T O s LR 3 -~ - s ] 4 ] b Nitrogen Carbon dioxide ' - "-.F;.d. R o ik =T B A E w:. i N 3 Air "" r 'E‘ | E 1 % { SR > i el . = Q. x ey ;"" - :‘_":I = N ii,.. LY Burning splint Glowing splint | | e --- Hydrogencarbonate indicator Analysis Based on the above results, suggest suitable test(s) for the following gases. Write ‘nil’ if no suitable test has been found for the gas. Carbon dioxide | - ~ Suggestion 1 What you expect Suggestion 2 o, What you expect e L e L e U More to discuss 1. Air contains oxygen, carbon dioxide and nitrogen. In the above tests, why does air not give exactly the same results as these gases? 2. Between the two tests for oxygen, which one will you use later on? e B A e T i e s e e A . e = B FERERR- S Nitrogen s e e e e e el — - /7 Lliving Things and Air {; P e l p m a s r ai g n i n i a t n o c s e b u t g n i l i o b stoppered . Q e l p m a s r ai g n i n i a t n o c s e b u t g n i l i o b stoppered lime water r o t a c i d n i e t a n o b r hydrogenca wooden splint gas lighter :21 | | : l_ Steps Your teacher will give you four boiling tubes. Two of them contain gas P and two of them contain gas Q, They may be oxygen or carbon dioxide. il o According to part A, do simple tests to distinguish between them and record the results. Results Analysis Based on the above results, gas P is (a) and gas Q.is (&) 6 Wear safety goggles. & Be careful when handling a naked flame. ‘& 7.1B Testing water vapour i Q+? A. Testing water : i iccator®) desicc a (in dry cobalt chloride paper dry forceps r e t a w le tt li a g n i n i a t n o c dropper 1 piece et ) o Cobalt chloride paper is usually stored in a desiccator. A desiccator contains a drying agent™ and is airtight. Steps Observe dry cobalt chloride paper Add a drop of water to a piece of dry m and record its colour. cobalt chloride paper. Observe what | o : a A e Ee o ot el oi Additional info LT happens to the paper and record the result. il o i ’ - T TP -3 desiccator : r ’ sl S e o g ¥ ¥ Cip g} & g kil dry forceps ’ dddsr dry paper Chlorlde cobalt paper vy i P R o Ll A chloride S i ke e dry cobalt Results | Condition Colour of cobalt chloride paper )), | Dry stoppered boiling tube containing gas R stoppered boiling tube containing gas 5 dry cobalt chloride paper (in a desiccator) dry forceps 1 1 | 2 pieces 1 pair . el e W SRR o e h I s e el s e o e e e B. Distinguishing dry air and wet air desiccator ¥2/% 45 drying agent 7 /¢ 7 Living Things and Air Steps s, le mp sa air ng ni ai nt co s be tu g in Your teacher will give you two boil a do A, rt pa g to in rd co Ac . air t we or air y dr R and S. They may be simple test to distinguish between them and record the results. Results PO e e N g W R - -u-:s-r--—-n'*t-v'-'-‘_\": T ¥ Air sample 5 Gi T LA e E..' S ;o L P R S S - e A ¥ TNy Y A T gl R S R s = e e R p Analysis Based on the above results, we find that air sample R is and air sample S is ’ - ' - - , i, gy ™v\AJINN JVVI] v d _L-..Jr =) 1 s C (‘*IL S0 =i - ~ . o k- NG ) 1S UIme = wm W a wdter Y fl ol v i e & :fi.-.. | Cr o~ -_‘—V,_f.: I i % al gLow SPUTIL o :I — wvd .L""-.a’..""l .f“'p"-l-*lfr‘ o DI 1:-._(:]);.-{1 ter. Parss 17oIm colourtess ‘_.f. P N T yarogencarbonate \vAdroadencarh ANnate b = P i |- Ly to m _!,!{L_&g".\fz indicatnr fe rom yod A vval | Ava, indicator trom red to /vellow Nitrogen is chemically inactive and there is no simple test for it. § VoA iy - J . e - XN T e e What properties do the gases in air have? Now let us learn more about some of the gases in air and how we make use of them in our daily life. 1. Oxygen Oxygen makes up about one-fifth of air. It supports life and most living things cannot live without oxygen (Fig. 7.2). Oxygen has a low boiling point (-183 °C). It is slightly denser than air. it , gen oxy h wit ns bur l fue a en Wh g. nin bur ts Oxygen suppor 5 Vo gives € Oxygen is essentialfor burning. 6 h wit l fue gas a ns bur * der wel a t tha is on ati lic app off energy. An ). 7.3 g. (Fi als met g tin cut for me fla oxygen to produce a very hot It PP Revise section 1.3 of Book TA ;pout the fire triangle, if e T = & i Fig. 7.2 Which living things shown above need oxygen? ) e o7 Fig. 7.3 Oxygen reacts with other materials and gives off a large amount of energy. welder 1514 T Ih m Oxygen toxicity Patients who have difficulty in breathing are given air with a higher oxygen content. However, this does not mean the more oxygen the better for our body. If we breathe in pure oxygen, this can damage our brain, lungs and eyes. Such a phenomenon is called oxygen toxicity™. 2. Carbon dioxide There is only a small amount of carbon dioxide in air. However, you will find in this chapter how it affects all living things on our Earth. 4 Unlike oxygen and most Carbon dioxide has a higher boiling point than oxygen (-78.5 5 : . substances, carbon dioxide . . shaes fom salid o gasat is °C). We may use dry ice (solid carbon dioxide) to keep boiling point. food cold (Fig. 7.4). Also, carbon dioxide is denser than air. Unlike oxygen, carbon dioxide does not support burning. It can be used to put out fires as it cuts off the oxygen supply to the burning objects (Fig. 7.5). 4 Not all fire extinguishers use carbon dioxide. Fig. 7.4 Using dry ice to keep ice cream frozen Sinking ‘mist’ The ‘mist’ from boiling water rises in air, but that from dry ice sinks. Why? & o, ¥ - - & by J | “ . & ot "5 — * - . i - * g™ &, : Fig. 7.5 Putting out a fire with carbon dioxide mM—_—J Lfi&_—_—h““ 3. Nitrogen Nitrogen makes up the largest percentage of air. It has a low boiling point (-196 °C), and its density is similar to air. Liquid nitrogen can be used to cool things down rapidly. For example, it is used to store living cells at a low temperature in a laboratory (Fig. 7.6). " . '& 'N'- A e, P B L Five = b 4 Fig. 7.6 Putting living cells into a tank containing liquid nitrogen Nitrogen is chemically inactive. Also, it does not support lives. Therefore, it can be used to fill food packages. This helps keep the food fresh for longer (Fig. 7.7). ~ ni—" Fig. 7.7 We can fill the package of potato chips with nitrogen. EN RS RN REE SRR NN N NEEE EEEE NN NN R NE NN NN N R EEER EEXEEEEE Argon Argon is the third most abundant gas in air. It is ch.emica.l[y N filled with | argon e T— inactive (argon means ‘lazy’ in Greek). If nitrogen is not inactive enough for a certain use, argon can be used instead. Also, argon does not conduct heat well. Therefore, a window can be filled with a layer of argon for better heat insulation. Cutaway of a special kind of window 7 Living Things and Air Let’'s do a simple comparison between the above gases. Nitrogen Carbon dioxide Oxygen 21% (high) 0.04% (low) 78% (highest) yes no no Boiling point -183 °C (very low) -78.5 °C (low) -196 °C (very low) Density 1.4 kg m™ 1.8 kgm™ 1.3kgm™ (air = 1.3 kg m™) (similar to air) (denser than air) (similar to air) Examples of use e supporting life e welding * keeping food cold storing living cells (using liquid nitrogen) Percentage in air s Supports burning? — — (using dry ice) (burning gas fuels) * putting out a fire e packaging gas Table 7.1 Comparison of oxygen, carbon dioxide and nitrogen From the above discussion, we have learnt that: nNg points ana densities aTall Oorts i m L bu o ':- % g . | ;—-I"'l F 1 atlall in el s \ ™ but Cdr bon I Y 1 I:t ey P jt-;\ T "l‘ "‘.l Y Vil Srpiey di oxiae r-l [ -~ .-_I_.._": y Jl — .;;__;l_n__wg_i nicrogen e~ pm rj o ':. J..t g e g e AR Separation of gases in air Different gases have different boiling points and densities. We can @ Air flows in B AN RN AR EE R RN RN E NN NN RN NRE N 2432000800800 (5] _In this separation tower, air ~ @ Nitrogen floats to the top of is further cooled to below the separation tower and is collected there the boiling point of oxygen @ Impurities are removed make use of these properties to separate the gases in air. The cartoon on the right shows a simplified process of how nitrogen, oxygen and argon are separated. ® The air is CompEE s @ Argon is collected at = = J| @ The air is cooled so as to remove water and carbon dioxide [ the middle with special techniques @® Liquid oxygen sinks to the bottom and is collected there 8ddoam ,’ 7.1 Making your own oxyge n absorber You may find a small packet of oxygen absorber in some food ' Language tip packages. Do you know what its purpose is? Oxygen absorber in Chinese is called BE & while in Japanese is called BiEE & | . The small packet contains chemicals that can remove oxygen from inside the food packages. This helps keep the food fresh for longer. You can also make your own oxygen absorbers. Think about the following and search for related information on the web: 1. What chemicals can be used to absorb oxygen? 2. How and where can | get all the materials needed? 3. How should | pack the chemicals so that gases but not moisture can pass through? 4. What steps should | take to ensure safety and cleanliness? 5. How should | test its oxygen absorbing power? In fact, oxygen and carbon dioxide can react with other materials easily. Why does their content remain fairly stable in the atmosphere? To answer this question, we need to know two important processes carried out by living things. They will be discussed in the next two sections. Living Things and Air Checkpoint 1. e id ox di on rb ca , air ng ni ai nt co s jar s ga to Glowing splints are put in s ep ke nt li sp e th me ti e th to g in rd co ac s se and oxygen. Rank the ga burning, from the shortest to the longest. 2. £ (c) < (b) (a) ' Three gases are pumped through certain indicators for a long time. == gas out gas in m=) indicator Write down what happens to the indicators below: Gas carbon dioxide Indicator hydrqgerjcarbonate indicator lime water | air ; lime water ‘ 1 | a What happens 3. | | The chart below shows the composition of air. | others (variable) argon (0.93%) i ; C (0.04%) (@) Name the gases A, B and C. (b) (i) (ii) Name a gas whose percentage in air varies. How can you test for this gas? | | | |