Air, Oxygen, Carbon Dioxide and Water Air, oxygen and carbon dioxide are important chemicals in our everyday lives. Knowledge of their properties helps us to develop an understanding of the role they play. Acids and bases are present in many everyday materials, including common household substances, and salts are produced when acids and bases react. Air and Oxygen Air as a mixture of gases because the components are not chemically joined its composition can vary it components can be separated by physical means Component Abundance Nitrogen [N 2 ] 78% Oxygen [O 2 ] 21% Carbon Dioxide [CO 2 ] 0.03% Water Vapour [H 2 O] Variable Other gases [Methane, Noble Gases] The balance Show that approximately One Fifth of the Air is Oxygen Set up the apparatus as shown Heat the copper strongly Slowly pass the air over the hot copper using the syringes Repeat several times Allow the system to cool Note the volume of air left in the syringe The change in volume is the volume of oxygen (20 cm 3 ) % oxygen = change in volume 100 = 20 100 = 20% ( 1 / 5 ) Note A similar result can be achieved by placing a large measuring cylinder over a burning candle floating on water It is not as accurate because the candle goes out before all the oxygen is used. Show that there is CO 2 and Water Vapour in Air Set up the apparatus as shown Connect to the suction pump as indicated Turn on the suction pump Result 1. The cobalt chloride paper turns from pink to blue 2. The lime water turns milky Conclusion 1. Air contains water vapour 2. Air contains carbon dioxide Prepare a sample of Oxygen by decomposing H 2 O 2 using MnO 2 as a Catalyst Place some Manganese Dioxide (MnO 2 ) into a flask Slowly drop Hydrogen Peroxide (H 2 O 2 ) from a dropping funnel Allow some bubbles of impure gas to escape Collect the pure oxygen gas over water in a gas jar Place a gas jar full of water on the beehive shelf as shown When the gas jar is full lift it slightly and seal it with a lid Remove the gas jar and replace it with a fresh one 1
Investigate the ability of Oxygen to Support Combustion in a Wooden Splint and a Candle Light a wooden splint (or candle) Blow it out so that it splint remains glowing Place into a gas jar full of oxygen Result: the splint (or candle) bursts into flame Conclusion: oxygen supports combustion. This test can be used to identify oxygen Uses of oxygen Respiration in cells to produce energy Needed for combustion (burning) Properties of Oxygen Colourless, odourless, tasteless Slightly heavier than air Neutral to litmus Supports combustion Reacts with substances forming oxides Burning Carbon and Magnesium in Oxygen Collect some gas jars of Oxygen Using a combustion spoon burn some carbon (charcoal) in air Quickly plunge it into a gas jar of oxygen Then add some damp blue and red litmus paper into the jar Result: The charcoal burns more brightly forming carbon dioxide The red litmus is unchanged but the blue litmus turns red Conclusion: carbon dioxide is an acidic gas Repeat using magnesium instead of carbon Result: Magnesium burns with a white flame Magnesium burns more brightly forming magnesium oxide The red litmus turns blue but the blue litmus is unchanged Conclusion: magnesium oxide is a basic oxide Prepare Carbon Dioxide Set up the apparatus as shown Add dilute hydrochloric acid to the marble chips using the dropping funnel Collect the CO 2 in a gas jar over water The gas jar is full of CO 2 when a lighted splint is extinguished at the top Put a lid on the gas jar and collect several other jars To Show that CO 2 turns Limewater Milky Add lime water to a test tube of CO 2 and shake it The lime water turns milky so the gas is CO 2 2
Show that CO 2 is Acidic when it Dissolves in Water Place a piece each of moist red and blue litmus paper in a gas jar and shake Result: The red paper remains red but the blue litmus paper turns red Conclusion: This tells us that CO 2 is an acid when it dissolves in water Show that CO 2 is Heavier than Air and Does Not Support Combustion Place a burning candle in a beaker Pour CO 2 into the beaker Result: The candle goes out Conclusion: Carbon dioxide is heavier than air Carbon Dioxide does not support combustion Uses of Carbon Dioxide Fire Extinguishers - because it is heavier than air and does not support combustion Fizzy drinks it gives them the sharp taste and the fizz Hardness in Water Hard water is water that forms a scum with soap Soft water is water that forms a lather with soap Temporary Hardness: hardness that can be removed by boiling Permanent Hardness: hardness that cannot be removed by boiling. Hardness is caused by dissolved substances in the water e.g. calcium and magnesium Hardness can be removed by ion exchange (swopping calcium and magnesium ions for sodium ions) Test Water for Hardness (Soap Test) Dissolve some soap in water Place some hard water [bottled water] in a test tube Place some soft water [deionised water] in another test tube Add soap solution to both Stopper the test tubes and shake vigorously Result: Soft water forms suds [a lather] Hard water forms a scum [does not lather] Conclusion: Hard water forms a scum or does not lather with soap. Soft water forms a lather with soap Water Treatment: makes water clean and safe to drink Screening: wire mesh removes large pieces of debris from water e.g. nappies, bottles etc. Flocculation: chemicals [Aluminium sulphate] are added to the water to make small particles clump together Sedimentation: Large tanks where water moves very slowly allows these heavy particles to sink to the bottom of the tanks. Clean water passes on to the next stage of treatment. Filtration: water is passed through beds of sand which catches particles that are too small to sink Chlorination: small amounts of chlorine are added to the water to kill any bacteria and make the water safe to drink. Fluoridation: small quantities of fluorine are added to the water to prevent tooth decay Storage: water is stored under cover before distribution to prevent it being contaminated 3
Detect the Presence of Dissolved Solids in a Water Sample Set up the apparatus as shown Place a sample of tap water on the clock glass Heat the water on the clock glass so that it evaporates Result: when the water on the clock glass has evaporated it leaves a white residue on the clock glass Conclusion: water contains dissolved solids Distillation of Sea Water to obtain Pure Water Place some sea water in the flask Connect the Liebig condenser as shown with the water supply to the lower connection and the upper connection to the sink Heat the sea water with a bunsen burner Result: water evaporates and is condensed and pure (distilled) water is collected in the beaker Conclusion: Distillation separates pure water from sea water Electrolysis of Water Set up the apparatus as shown Fill the voltameter with water plus a little sulfuric acid Connect the electrodes to the battery Result A chemical reaction occurs as the current passes through the liquid. This is electrolysis. Hydrogen collects at the cathode. Identified by burning with a slight pop Oxygen collects at the anode. Identified by relighting a glowing splint There is twice as much hydrogen as oxygen Conclusion: Water is made of two parts hydrogen to one part oxygen. H 2 O 4
Acids and Bases Common Strong Acids and Bases Strong Acids HCl [Hydrochloric acid] H 2 SO 4 [Sulfuric acid] HNO 3 [Nitric acid] Strong Bases NaOH [Sodium hydroxide] KOH [Potassium Hydroxide] Ca(OH) 2 [Calcium hydroxide] Alkalis are bases that are soluble in water Reactions of Acids and Bases Neutralisation of an Acid with a Base using an Indicator When an acid reacts with a base, a salt and water are formed This is called neutralisation Learn all the equations is this chapter carefully - first as word equations and then as chemical equations Titrate HCl against NaOH, and prepare a sample of the salt NaCl Titration Put exactly 20 cm 3 of sodium hydroxide solution into a conical flask Add a few drops of Universal Indicator solution Slowly add hydrochloric acid from the burette and mix by swirling When the solution goes green stop adding the acid Record the amount of acid added Extracting the salt Take a fresh conical flask and add exactly 20 cm 3 of sodium hydroxide solution Add exactly the same volume of acid to it and mix Place the solution in an evaporating basin Evaporate the water using a Bunsen burner Result: sodium chloride salt crystals form in the basin Conclusion: Neutralising NaOH with HCl forms the salt NaCl and water 5