Chemical change involves substances reacting to form new substances (ACSSU225): identify the differences between chemical and physical changes. identify evidence that a chemical change has taken place. investigate simple reactions such as combining elements to make a compound. recognise that the chemical properties of a substance, for example its flammability and ability to corrode, will affect its use.
A Task
Rust is the chemical reaction of iron with oxygen in the presence of moisture. Plan and conduct an investigation to answer the question: What effect does salt have on the rusting of a nail?
Chemistry of Fire Otto von Guericke, 1650, showed that a candle would not burn in a container from which the air had been removed. Robert Hooke, 1665, proposed that air had a substance that combined with flammable substances to produce a flame. Antoine-Laurent Lavoisier, 1772, found that the burning of sulfur produced a heavier substance. He thought that the extra weight was the sulfur combining with a gas in the air that he called 'oxygen'. Lavoisier was known as the father of modern chemistry. For example, by carefully weighing the reactants and the products he showed that if a piece of wood was burned to ashes, the total mass remained unchanged. Antoine-Laurent Lavoisier (1743 - 1794)
Chapter 7 Chemical Reactions 145 Chemical Change
Physical Change Chemical change happens when a new substance is formed. Physical change happens when a substance Chemical change happens when a substance changes from one form to another form combines with another substance to form one but keeps exactly the same chemical or more new substances. composition. Chemical change happens when a substance A physical change can affect the colour, size, breaks down into two or more substances. or shape but no new products are formed. Chemical change is not reversible except With a physical change the substances keep with further chemical change. their property (Salt dissolved in water still tastes salty). Sometimes a physical change The rusting of iron is a chemical reaction. Iron combines with can be reversed. The choc bar is changing from one form, solid, to another oxygen and water to produce a new compound. 4Fe + 3O + 6H O 4Fe(OH) form, liquid, but is keeping the same chemical composition. 2 2 3 It still tastes the same which suggests a physical change.
Examples of physical change: Examples of chemical change: • A sheet of paper is put through a shredder. The substance has changed from • Toasting bread. Burning is always a • The particles are always moving. one form to another but kept the • Milk starting to smell bad (bacteria in the Dissolving is always same chemical composition. chemical change. • A yellow paint is mixed with a blue paint to a physical change. milk forming new substances). produce green. • Burning a match. • Salt is mixed with water. • Frying an egg. This change is irreversible Chemical Physical (Try adding carbon dioxide Change • Cutting bread into slices. Change • Vinegar + baking soda carbon dioxide. to water and produce vinegar and baking soda). • Boiling water. • A battery producing a current. • Making lemonade by squeezing lemon juice and adding sugar and water. Exercise
1 What is a chemical change? Exercise 2 Indicate why each of the following are 1 What is a physical change? chemical changes: Fireworks a) The rusting of iron. 2 Indicate why each of the following are Frying and egg physical changes: b) Toasting bread. Precipitation a) The freezing of water (changing from 3 Which of the pictures on the left show a liquid water to solid ice). chemical change? Explain. Mixture of b) Dissolving sugar in water. 4 Can you think of 10 more examples of sugar, lemon juice, and water 3 Which of the pictures on the left show a chemical change? with melting ice Sliced bread physical change? Explain. Melting ice 4 Can you think of 10 more examples of Candle physical change? Fire
146 7.1 Chemical Change Chapter 7 Chemical Reactions 147 How do you know if a Colour Change Chemical Change chemical change has taken place? A colour change can suggest that a chemical The following evidence can suggest that a reaction has taken place. chemical, chemical reaction, has taken place: Chemical The rusting of iron is evidenced by the • Change of colour. Change typical reddish-brown rust colour. • Change in temperature. The colour change due to the ripening of • Change of smell. fruit is caused by enzyme reactions. These • Change of form. reactions also change the taste and smell of The colour change of ripening fruit. the fruit. Are ripe fruits sweeter than unripe fruits. • Production of gases. • Production of a precipitate. • Decomposition of organic material. • Light or sound is produced. I Chemical change Activity Although the above changes may suggest Watch online videos on 'chemical change' Milk and colour and evidence/indications of chemical change. that a chemical change has taken place, the Materials: Milk, plate, food colouring, only conclusive evidence of a chemical detergent, cotton buds. change is a chemical analysis. Method: a) Cover the plate with milk. b) Add a drop of each food colouring close together in the centre of the plate. Temperature c) Put a drop of detergent on the end of a cotton The colours follow the reactions of the detergent with the milk. The detergent breaks apart the water, bud and place it in the middle of the milk proteins, and fats in the milk. The detergent chases Many reactions either produce energy or and hold for about 20 seconds. the fat molecules. absorb energy. A reaction that produces energy will become warmer. A reaction that uses energy will become cooler. What is the chemical formula for "banana"? Exercise Reactions that produce heat are called The burning of wood, an exothermic reaction, is essentially BaNa exothermic reactions. The burning of the reaction of cellulose (a sugar), and oxygen to produce 2 1 Give 5 kinds of evidence that might suggest carbon dioxide and water. C H O + 6O 6CO + 6H O of fossil fuels such as coal, oil, and gas 6 12 6 2 2 2 that a chemical change has taken place? produces heat. These are exothermic reactions. Challenge 2 What is an exothermic reaction? Reactions that absorb heat are called 3 What is an endothermic reaction? Activity Endothermic reactions endothermic reactions. 4 The rusting of iron can be summarised by the Vinegar and baking soda An everyday example of an endothermic symbolic equation: Materials: Vinegar, baking soda, thermometer, reaction are cold packs. Bend the pack and the pack becomes cold. 4Fe + 3O + 6H O 4Fe(OH) clear small container. 2 2 3 Method: What chemical substances are in the pack? a) What is the symbol for iron? a) Place about 10 ml of vinegar in a clear b) What is the evidence that rusting is a Exothermic and Endothermic I container. Add a thermometer and record the Challenge chemical change? Watch online videos about exothermic and temperature of the vinegar. 5 What is the evidence that the ripening of endothermic reactions. c) Add about half a teaspoon of baking soda. Colour change fruit is a chemical change? Record the temperature during the reaction. Is it possible to have a colour change d) Is the reaction of vinegar and baking soda an without a chemical change? exothermic or endothermic reaction? Can you provide five examples? 148 7.1 Chemical Change Chapter 7 Chemical Reactions 149 Bubbles Light
Bubbles can suggest that a chemical reaction During a chenical reaction energy can be is producing a gas. released in the form of light. Bubbles can also indicate a physical change. Fireflies, some fungi, and some deep sea The bubbles in softdrink is carbon dioxide animals use chemical reactions to produce being released after being dissolved in the light. drink. Most combustion reactions produce energy Bubbles are also produced when water is in the form of light. Combustion reactions boiled. Boiling water is a physical change. always involve oxygen. A fire is a combustion reaction. The reaction of vinegar and baking soda producing bubbles The combustion of the sparkler with oxygen is producing of carbon dioxide gas. The bubbles slow in activity as energy in the form of heat and light. Challenge either the vinegar or baking soda are used up in the reaction. Sparkler + O2 burnt sparkler + energy Bubbles Is it possible to have bubbles If you are not part of without a chemical change? the solution then you Activity Precipitate are in the precipitate. Can you provide five examples? Coke-Cola and baking soda Materials: Coke-cola, baking soda, When two or more reactants are mixed together and a precipitate falls to the bottom Challenge thermometer, clear small container. Exercise Method: of the container then this provides evidence Coke-Cola that a chemical reaction is occurring. 1 Smell and taste can be used as evidence of a) Place about 10 ml of coke-cola in a clear a chemical change. Why should the use of The reaction between vinegar and baking container. Add a thermometer and record the A precipitate is the formation of a solid in a smell and taste be avoided? soda is similar to the reaction between temperature of the coke-cola. solution during a chemical reaction. coke-cola and baking soda. 2 What is combustion? b) Add about half a teaspoon of baking soda. Vinegar is an acid (acetic acid). 3 What is a precipitate? Record the temperature during the reaction. Is coke-cola an acid? 4 Which of the following is a chemical change c) Is the reaction of coke-cola and baking soda What is a test for an acid? Activity and which is a physical change? an exothermic or endothermic reaction? a) Building a sand castle on the beach. d) Are bubbles (gas) produced? Precipitate Smell Materials: Baking soda, calcium chloride, clear b) Painting a desk green. containers (plastic cups). c) Lighting a candle. When two or more substances react, a smell, Method: d) Cooking biscuits. scent, or odour can be produced. a) Make a solution of about 2 g (half teaspoon) e) Bending/snapping a light stick to of calcium chloride and 20 mL of water. The smell of decomposing organic material produce a glow. suggests that chemical reactions are involved b) Make a solution of 2 g (half teaspoon) of 5 The chemical reaction of baking soda and as organic material decomposes/rots. baking soda and 20 mL of water. calcium chloride produces a precipitate. The symbolic equation is: As many chemical reactions produce c) Carefully pour the clear baking soda solution 2NaHCO + CaCl CaCO +2NaCl + CO + H O poisonous products the use of smell and taste into the calcium chloride solution. 3 2 3 2 2 should be avoided. d) Did you notice a precipitate? Can you deduce the symbolic formula of the precipitate? e) What is the evidence that a chemical reaction occurred?
150 7.1 Chemical Change Chapter 7 Chemical Reactions 151 Combustion
Combustion reactions, or burning, occur when a compound combines with oxygen from the air. The compound usually contains Hydrogen + Oxygen Water carbon. Combustion reactions are normally exothermic - They produce heat. O H Example: H H O Burning coal is an example of a combustion H reaction. Coal is essentially carbon. carbon + oxygen carbon dioxide + heat A coal power station in Victoria. The combustion of carbon + O produces considerable heat to produce steam to turn turbines
C + O2 CO2 + heat to produce electricity. C + O2 CO2 + heat
H H H O
Roughly 1.9 tonnes of CO2 is produced for every tonne of coal burnt.
H I Burning Magnesium I Hydrogen lit splint test Watch online videos demonstrating the burning of magnesium. Exercise Watch online videos that show the 'lit splint test for hydrogen'. 1 What is a combination reaction? Be careful. The reaction of Magnesium, Mg, is one of the most common 2 In the chemical reaction: X + Y XY, hydrogen with oxygen (lit elements on Earth. Pure magnesium is a soft which are the reactants and which are the Combination splint) is explosive even with silver white metal. It burns in air with a strong small amounts of hydrogen. products? white light. Magnesium is used in flares and 3 What is the test for hydrogen? Combination reactions occur when two fireworks. or more substances combine to form a new magnesium + oxygen magnesium oxide 4 Hydrogen will react explosively with oxygen to produce water. Write the symbolic substance. Activity 2Mg + O2 2MgO The reactants are the starting substances. equation. Make and Test for Hydrogen The products are the result of the chemical 5 What is a combustion reaction? Materials: Dilute hydrochloric acid, zinc, test reaction. tube, test tube rack, splint, bunsen burner. I Burning Magnesium in Dry Ice 6 Write the symbolic equation for the burning X + Y XY of carbon. Method: Watch online videos demonstrating the 7 Write the symbolic equation for the burning In the above chemical equation, the reactants a) Place a small amount of zinc in a test tube. burning of magnesium in dry ice. of magnesium. X and Y combine to form the product XY. Add about a quarter of a test tube of dilute hydrochloric acid. 8 When heated, a mixture of iron (Fe) and Example: Burning magnesium in dry ice shows that sulphur (S) will combine chemically to 2HCl + Zn ZnCl2 + H2 Using a lit splint to test for hydrogen (In combustion can happen without oxygen. produce iron sulphide (FeS). Attempt to the test, the reactants hydrogen and oxygen b) Light a splint and place it in the mouth of magnesium + carbon dioxide magnesium oxide + carbon write the symbolic equation for this chemical combine to produce the product water). the test tube. If it 'pops' then hydrogen is 2Mg + CO 2MgO + C reaction. present. 2 hydrogen + oxygen water 9 Attempt to write the symbolic equation for c) If there is no 'pop' then you probably don't the burning of sulphur to produce sulphur 2H2 + O2 2H2O have enough hydrogen. Hydrogen is lighter Dry ice is solid carbon dioxide (SO ). CO 2 that air, so catch it with another test tube dioxide. A temperature of 2 about -80°C is needed to inverted over the first test tube? solidify carbon dioxide gas.
152 7.2 Simple Reactions Chapter 7 Chemical Reactions 153 http://themalloryfamily.net/chemistry/chem51l/exp10_combination-and-decomposition-reactions.pdf good decomposition stuff
Decomposition Some Decomposition Reactions Add water here I Electrolysis of Water Decomposition reactions occur when a Heating sugar (sucrose): Oxygen substances breaks down to form two or more Watch online videos about sugar carbon + water 'electrolysis of water'. new substances. Hydrogen C H O C + H O The reactant is the starting substance. 12 22 11 2 The products are the result of the chemical Challenge decomposition. Electrolysis of water: Electrolysis of Water water hydrogen + oxygen XY X + Y + Design your own electrolysis of water H O H + O Anode - Cathode In the above chemical equation, the reactant 2 2 2 equipment using only household materials. + - XY decomposes to form the products X and Test for oxygen. Heating limestone: Y. Battery Switch Test for hydrogen. calcium carbonate calcium oxide + carbon dioxide Example: CaCO CaO + CO Passing an electric current through water separates The reactant calcium carbonate (limestone), 3 2 it into the elements hydrogen and oxygen. This is when heated, decomposes into the products called 'electrolysis of water'. What is the chemical formula for coffee? Are there any jokes calcium oxide and carbon dioxide. Heating Malachite: CoFe about sodium? malachite copper oxide + carbon dioxide + water 2 calcium carbonate calcium oxide + carbon dioxide Na CuCO .Cu(OH) CuO + CO + H O 3 2 2 2 CaCO3 CaO + CO2 Activity Decomposition of Calcium Carbonate Exercise Materials: Limestone (calcium carbonate), bunsen burner, tripod, heat mat, gauze mat, 1 What is a decomposition reaction?
A bit of trivia Decomposition: the separation tongs. 2 In the chemical reaction: XY X + Y, of a substance into simpler Explosives, containing which are the reactants and which are the • substances as a result of heat, Method: Activity nitroglycerin, when light, chemical activity, or products? detonated decompose Place a small amount of calcium carbonate biological activity. Decomposition of Sugar into carbon dioxide and on the edge of the gauze mat. Heat the 3 The electrolysis of water decomposes water water. calcium carbonate for 5-10 minutes. into hydrogen and oxygen. Materials: Household sugar, test tube, test tube An explosion is a rapid • holder, test tube rack, bunsen burner. expansion of gases. calcium carbonate calcium oxide + carbon dioxide a) Write the symbolic equation for this Cement is made by • chemical reaction. Method: heating limestone CaCO3 CaO + CO2 to around 1500°C, b) What is a test for hydrogen? a) Place a small amount of sugar (sucrose) and grinding the What is the evidence that a chemical c) What is a test for oxygen? decomposed calcium in a test tube. Heat it gently over a bunsen reaction has taken place? burner until a black sticky solid remains in carbonate into a powder mixed with clay. 4 When calcium carbonate is heated it the test tube (Don't try to clean the test tube). decomposes into calcium oxide and carbon dioxide (CaCO3 CaO + CO2). C12H22O11 C + H2O What would you expect to happen if
1 What is the evidence that a chemical magnesium carbonate (MgCO3) was heated? reaction has taken place? When heated gently, the sugar decomposes into 5 When sucrose (C12H22O11) is heated it 2 What is the sticky black substance at the carbon and water. decomposes into carbon and water bottom of the test tube? (C12H22O11 C + H2O). 3 Did you notice water moisture forming at the What would you expect to happen if glucose cooler top of the test tube? (C6H12O6) was heated?
154 7.2 Simple Reactions Chapter 7 Chemical Reactions 155 Chemical properties of Iron (Fe): The abundance, low cost, and high strength of iron has resulted in iron being the most used metal. Steel is iron mixed with small amounts of other elements such as carbon to make the iron harder and stronger. An alloy is a mixture of elements. Steel is an Chemical Properties alloy of iron and mainly carbon.
The chemical property of a substance Reactivity: Reactivity: describes the chemical changes that may Iron rusts in damp air but not in dry air. Iron, with moisture, will react with the occur with the substance. oxygen in the air to form rust. The presence of salt and moisture increases the rusting. The chemical and physical properties of a Flammability describes iron + oxygen iron oxide substance determines its use. how easily something will 4Fe + 3O 2Fe O burn or catch fire. The prevention of rusting is helped by coating the steel 2 2 3 Example: with zinc. This is called galvanisation.
Hydrogen is the most flammable of Stainless steel is an alloy of iron with chromium added. all known substances. Hydrogen, in The stainless steel is protected by an outer layer of combination with oxygen, is used to power chromium oxide. The outer chromium reacts with rockets. oxygen in the air to form the protective layer. An Atlas V rocket using kerosene and liquid oxygen to 2H2 + O2 2H2O power its first stage. The rocket uses liquid hydrogen and chromium + oxygen chromium oxide liquid oxygen to power the upper stage. 4Cr + 3O2 2Cr2O3
Chemical properties of Hydrogen (H ): Uses of Hydrogen (H ): 2 2 Word Bank Exercise Flammability: Flammability: Add the following words to your word bank 1 What is meant by the chemical property of a Hydrogen reacts slowly with oxygen to form water. Hydrogen is used as a rocket fuel. by writing the word, a definition of the word, substance? ► and a sentence or phrase using the word: However, when a spark or catalyst such as platinum is 2 A chemical property of hydrogen is that it H2 + O2 H2O added the chemical reaction of hydrogen and oxygen Word Definition A sentence will react explosively with oxygen. becomes explosively violent. Fuel cells change the chemical energy Physical change a) Attempt to write the symbolic equation of the reaction between hydrogen and Chemical change for the reaction of hydrogen with O 2 oxygen into electricity. Fuel cells can be Exothermic reaction oxygen. H2 used to power engines. Endothermic reaction b) Suggest a possible use for hydrogen Reactant X and Y are The reactants given that it will react explosively with Electrolysis of reactants in the are the starting oxygen. water H2 fuel cell substances in following reaction a chemical 3 Briefly describe a hydrogen fuel cell. X + Y XY reaction Reactivity: Reactivity: 4 What is the difference between iron and At high temperatures hydrogen becomes very reactive Hydrogen is used to make ammonia. Combination reaction steel? and is one of the few substances that will react with ► Ammonia is an important source of Decomposition nitrogen. fertilisers. reaction 5 The rusting of steel is essentially a reaction between iron and oxygen in the air. H + 3N 2NH Chemical property 2 2 3 a) Attempt to write the symbolic equation Hydrogen, in the presence of a catalyst, will react with ► Hydrogen is thus used to produce solid Learning Power for the reaction of iron (Fe) with oxygen. unsaturated fats and oils to produce solid fats. fats such as margarine from unsaturated Can you spend about 30 seconds looking at a b) Suggest three ways of preventing rust vegetable oils. word, and then write down the word and its (basically preventing the oxygen from definition without looking? making contact with the iron). Can you write down all words, and their definition, without looking? 156 7.3 Chemical Properties Chapter 7 The Water Cycle 157 Hydrogen Fuel Cell
Fuel cells change the chemical energy of the reaction Ripening and Rotting Fruit between hydrogen and oxygen into electricity. Fuel cells have been used to power engines in cars, buses, boats, submarines, etc. When a fruit is picked from a plant it immediately Micobes, small organisms and fungi, feed loses its supply of oxygen, water, and nutrients. off the sugars in the fruit and use the energy The cells of the fruit then begin to die because of produced by the breakdown of the sugar to The major source of hydrogen is from the steam keep alive. treatment of natural gas. High temperature this lack of oxygen, fruit, and nutrients. steam reacts with methane to produce carbon fructose + oxygen carbon dioxide + water dioxide and hydrogen. C6H12O6 + O2 CO2 + H2O steam + methane hydrogen + carbon dioxide Rotting fruit I H O + CH H + CO 2 4 2 2 Watch online videos of 'rotting fruit'. How to prevent the rotting of fruit: Note the evidence of chemical change. )) Keeping fruit in fridges and freezers Fuel cells essentially consist of an anode (positive), slows down the chemical decay a cathode (negative), an electrolyte, and fuel such as reactions. hydrogen and oxygen. The stages of a piece of fruit: )) Add preservatives to prevent the The inputs of the fuel cell are hydrogen over the 1. Unripe fruit contains a lot of starch and is involvement of oxygen and microbes The fuel/electrolyte pushes electrons from the anode anode and oxygen from the air over the cathode. sour. to the cathode and through connections to power an The outputs are electrons and water. in the chemical decay reactions. The net equation is: H + O H O application such as an electric motor. 2 2 2 2. As the fruit ripens, the starches change )) Cook and bottle to keep the oxygen to sugars (glucose and fructose) and and microbes away from the fruit. Debate the following: becomes sweeter. The advantages of a hydrogen fuel cell: Governments should make it a priority to starch + water fructose ) ) 1 kg of hydrogen may do as much develop hydrogen fuel cells. Exercise work as 2 L of petrol mainly because C6H10O5 + H2O C6H12O6 1 What is the evidence that rotting fruit is a hydrogen fuel cell is more efficient 3. When the fruit is picked most of the a chemical change? than an internal combustion engine. starches have changed to sugars and Exercise microbes begin to feed off the sugars. 2 Essentially the ripening of fruit is a )) A fuel cell has no greenhouse gas chemical change from starch to fructose emissions (1 L of petrol can produce 1 Describe a hydrogen fuel cell. (a sugar) summarised by: 2 kg of carbon dioxide). 2 The net chemical reaction in a C6H10O5 + H2O C6H12O6 Activity hydrogen fuel cell is: a) Is this a combination or Test for starches and sugars in fruit. decomposition reaction? H2 + O2 H2O Disadvantages of a hydrogen fuel cell: Materials: Fruit and Iodine b) What is the difference between a) Is this a combination reaction or )) An expensive source of power Method: starch and fructose (a sugar)? a decomposition reaction? compared to the combustion of fossil b) What are the products of a Apply a drop of iodine to fruit flesh: 3 What are the two factors mostly fuels. hydrogen fuel cell? • Dark-blue indicates starches. responsible for the rotting of fruit? • Taking 2-3 seconds to become dark-blue )) The extraction of hydrogen from 3 What are three advantages and 4 Is the breakdown of fructose an indicates that some starch is present. natural gas also produces carbon three disadvantages of hydrogen exothermic or endothermic reaction? • Yellow indicates mostly sugar. dioxide. fuel cells? Explain.
158 7.4 Science Knowledge Chapter 7 The Water Cycle 159 Science Investigations Activity
Science investigations are activities in which Conduct investigations to answer each ideas, predictions or hypotheses are tested of the following questions. and conclusions are drawn in response to a Planning & Conducting question or problem.
Science Inquiry Think about and organise an experiment to Investigations can involve a range of answer the question. activities, including experimental testing, Science inquiry skills are important in Questioning & Predicting field work, locating and using information science, and in any situation that requires Planning an experiment is to describe in detail, sources, conducting surveys, and using critical thinking. The process of thinking in the step-by-step procedures to follow. How much sugar is needed to get optimum modelling and simulations. logical steps allows us to answer questions production from a teaspoon of yeast? about the world around us. Collect your data and write it in a prepared data table. Prediction: Well? What is your prediction? Science inquiry skills include: Yeast and sugar • questioning and predicting. Put a teaspoon of yeast in a bottle. Processing & Analysing • planning and conducting. Add a teaspoon of sugar to the bottle. • processing and analysing. Summarise the data in the form of a graph or Add half a cup of warm water to the bottle and Questioning & Predicting chart to help in understanding the data and to • evaluating. mix the contents. identify relationships. Is carbon dioxide produced when yeast feeds on Put a balloon over the mouth of the bottle. • communicating. Charts, graphs, and tables are also a great way of sugar? presenting investigation data to others. How quickly will the balloon fill? Prediction: Well? What is your prediction? The analysis of the data in a graph involves looking for trends, patterns and relationships in How large will the balloon become? Variables the graph. Are you able to draw a conclusion from your experimental data? The independent variable is the variable Questioning & Predicting Yeast and sugar that is changed. In graphs, the independent I variable is plotted on the x-axis. Watch a couple of videos on Does temperature affect the rate at which yeast feeds on sugar? The dependent variable is the variable Evaluating 'balloon and yeast'. that is measured. In graphs, the dependent Did your experiment provide an answer to the variable is plotted on the y-axis. Prediction: Well? What is your prediction? question? Yeast and sugar The controlled variables are all of the How good was your data? Baker's yeast is used to produce lighter and other variables that are to be kept constant. Would you do anything different if you tastier bread. Yeast is a single celled organism. repeated your experiment? The feeding of yeast on sugar can be summarised as: Questioning & Predicting sucrose + water ethanol + carbon dioxide Is the mass of the reactants equal to the mass of 2C12H22O11 + H2O 2C2H5OH + 2CO2 I Investigation variables Communicating the products after yeast feeds on sugar? Write a report using scientific language. Watch a couple of 'investigation variables' Prediction: Well? What is your prediction? or 'fair testing' videos. Baker's yeast. Present your report to your target audience using The packet has digital technology. just been opened. Examples of reports are shown in Chapter 1.
160 7.5 Science Inquiry Chapter 7 The Water Cycle 161 A colour change can suggest that a chemical The following evidence can suggest that a reaction has taken place. chemical reaction has taken place: The rusting of iron is evidenced by the typical • Change of colour. reddish-brown rust colour. The colour change • Change in temperature. due to the ripening of fruit is caused by enzyme reactions. These reactions also change the taste • Change of smell. and smell of the fruit. • Change of form. Physical change happens when a substance Chemical change happens when a new Bubbles can suggest that a chemical reaction is • Production of gases. changes from one form to another form but keeps substance is formed. producing a gas. exactly the same chemical composition. • Production of a precipitate. Chemical change happens when a substance Bubbles can also indicate a physical change. A physical change can affect the colour, size, or combines with another substance to form one or • Decomposition of organic material. The bubbles in softdrink is carbon dioxide being shape but no new products are formed. more new substances. • Light or sound is produced. released after being dissolved in the drink. With a physical change the substances keep Chemical change happens when a substance Although the above changes may suggest that Bubbles are also produced when water is boiled. their property (Salt dissolved in water still tastes breaks down into two or more substances. a chemical change has taken place, the only Boiling water is a physical change. salty). Sometimes a physical change can be Chemical change is not reversible except with conclusive evidence of a chemical change is a During a chenical reaction energy can be released reversed. further chemical change. chemical analysis. in the form of light. Fireflies, some fungi, and some deep sea animals use chemical reactions to produce light. Many reactions either produce energy or absorb Examples of physical change: Examples of chemical change: Most combustion reactions produce energy in energy. the form of light. Combustion reactions always • A sheet of paper is put through a shredder. • Toasting bread. Reactions that produce heat are called involve oxygen. A fire is a combustion reaction. • The particles are always moving. • Milk starting to smell bad (bacteria in the exothermic reactions. The burning of of fossil When two or reactants are mixed together and a milk forming new substances). • A yellow paint is mixed with a blue paint to fuels such as coal, oil, and gas produces heat. precipitate falls to the bottom of the container produce green. • Burning a match. These are exothermic reactions. then this provides evidence that a chemical reaction is occurring. • Salt is mixed with water. • Frying an egg. Reactions that absorb heat are called endothermic reactions. A precipitate is the formation of a solid in a • Cutting bread into slices. • Vinegar + baking soda carbon dioxide. solution during a chemical reaction. • Boiling water. • A battery producing a current. When ready, cover the information above and answer the questions below.
When ready, cover the information above and answer the questions below. Exercise Exercise 1 Give 5 kinds of evidence that might suggest 8 Which of the following is a chemical change Exercise Exercise that a chemical change has taken place? and which is a physical change? 2 What is an exothermic reaction? a) Building a sand castle on the beach. 1 What is a physical change? 4 What is a chemical change? 3 What is an endothermic reaction? b) Painting a desk green. 2 Indicate why each of the following are 5 Indicate why each of the following are 4 The rusting of iron can be summarised by the c) Lighting a candle. physical changes: chemical changes: symbolic equation: d) Cooking biscuits. 4Fe + 3O + 6H O Fe(OH) a) The freezing of water (changing from a) The rusting of iron. 2 2 3 e) Bending/snapping a light stick liquid water to solid ice). a) What is the symbol for iron? to produce a glow. b) Toasting bread. b) Dissolving sugar in water. b) What is the evidence that rusting 9 The chemical reaction of baking soda and c) A rotting piece of fruit. is a chemical change? calcium chloride produces a precipitate. c) Making lemonade by squeezing lemon d) Tarnishing, browning, of silver. 5 What is the evidence that the ripening of The symbolic equation is: juice and adding sugar and water. 6 Can you think of 5 more examples of fruit is a chemical change? 2NaHCO + CaCl CaCO + 2NaCl + CO + H O 3 Can you think of 5 more examples of 3 2 3 2 2 chemical change? 6 Smell and taste can be used as evidence of Can you deduce the symbolic formula of the physical change? a chemical change. Why should the use of precipitate? smell and taste be avoided? 7 What is a precipitate?
162 7.6 Chapter Review Chapter 7 The Water Cycle 163 Combination reactions occur when two or more Decomposition reactions occur when a substances combine to form a new substance. substances breaks down to form two or more new Jab/stab a potato with a straw The reactants are the starting substances. substances. I The products are the result of the chemical The reactant is the starting substance. A Sweet Trick Watch some online videaos showing reaction. how to do this trick. The products are the result of the chemical X + Y XY decomposition. In the above chemical equation, the reactants X XY X + Y and Y combine to form the product XY. In the above chemical equation, the reactant XY Example: decomposes to form the products X and Y. Using a lit splint to test for hydrogen (The Example: reactants are hydrogen and oxygen. The product Calcium carbonate (limestone), when heated, is water). decomposes into calcium oxide and carbon hydrogen + oxygen water dioxide. 2H + O 2H O 2 2 2 calcium carbonate calcium oxide + carbon dioxide Combustion reactions, or burning, occur when CaCO3 CaO + CO2 a compound combines with oxygen from the air. Other examples: The compound usually contains carbon. Heating sugar (sucrose): 1 Challenge your audience 2 Fold over the straw so that 3 Holding the straw tightly, Example: sugar carbon + water to stick a straw into a it fits in your fist and traps quickly jab the straw into potato. air in the straw. the potato. Burning coal is an example of a combustion C12H22O11 C + H2O reaction. Coal is essentially carbon. Electrolysis of water: carbon + oxygen carbon dioxide water hydrogen + oxygen C + O2 CO2 H2O H2 + O2 When ready, cover the information above and answer the questions below. A Couple of Puzzles Exercise Exercise 1 The net shown has 1 What is a combination reaction. 9 Attempt to write the symbolic equation for the burning of sulphur to produce sulphur been folded to make a 2 In the chemical reaction: X + Y XY , dioxide (SO ). box. Which box? which are the reactants and which are the 2 10 What is a decomposition reaction? products? 11 In the chemical reaction: XY X + Y, 3 What is the test for hydrogen? which are the reactants and which are the 4 Hydrogen will react explosively with oxygen products? to produce water. Write the symbolic 12 The electrolysis of water breaks decomposes 2 Place each of the numbers equation. water into hydrogen and oxygen. Write the 1, 2, 3, 4, 5, 6, 7, 8, 9 in 5 What is a combustion reaction? symbolic equation for this chemical reaction. each square so that the 6 Write the symbolic equation for the burning a) What is a test for hydrogen? sum of each row and 3 Cog A has 16 teeth and of oxygen. b) What is a test for oxygen? column is as shown.
cog F has 8 teeth. How
13 When calcium carbonate is heated it 7 Write the symbolic equation for the burning many turns must cog F A
of magnesium. decomposes into calcium oxide and carbon make so that both letters
dioxide (CaCO CaO + CO ). 18 3 2 are upright? 8 When heated, a mixture of iron (Fe) and F What would you expect to happen if sulphur (S) will combine chemically to 7 magnesium carbonate (MgCO ) was heated? produce iron sulphide (FeS). Attempt to 3 14 When sucrose (C H O ) is heated it write the symbolic equation for this chemical 12 22 11 20 reaction. decomposes into carbon and water (C H O C + H O). 12 22 11 2 14 13 18 What would you expect to happen if glucose
(C6H12O6) was heated? 164 7.6 Chapter Review Chapter 7 The Water Cycle 165 The chemical property of a substance describes Fruit ripening can be summarised as starches the chemical changes that may occur with the changing to sugars (glucose and fructose) and substance. The chemical and physical properties becoming sweeter. H of a substance determines its use. starch + water fructose Hydrogen is the most flammable of all known C6H10O5 + H2O C6H12O6 substances. Hydrogen, in combination with The rotting of fruit can be summarised as small C H oxygen, is used to power rockets. organisms and fungi breaking down the sugars Competition Questions H2 + O2 H2O for living energy Hydrogen will combine with nitrogen. fructose + oxygen carbon dioxide + water Hydrogen is used to make ammonia. Ammonia C H O + O CO + H O 6 12 6 2 2 2 A catalyst is a substance that speeds up a chemical reaction. HH is an important source of fertilisers. A catalyst is not used in the reaction and only needs to be present H2 + 3N2 2NH3 Fuel cells change the chemical energy of the in small quantities. reaction between hydrogen and oxygen into Iron, with moisture, will react with the oxygen in 1 In the example shown on the immediate right, which electricity. Fuel cells have been used to power The major source of hydrogen is from the steam the air to form rust. substance is the catalyst? engines in cars, buses, boats, submarines, etc. treatment of natural gas. High temperature iron + oxygen iron oxide a) Steam (H O). steam reacts with methane to produce carbon Fuel cells are more efficient than internal 2 4Fe + 3O 2Fe O b) Methane (CH ). dioxide and hydrogen. 2 2 3 combustion engines. 4 Stainless steel is an alloy of iron with chromium c) Nickel (Ni). Ni Fuel cells don't have green house emissions. d) Carbon dioxide (CO ). H2O + CH4 H2 + CO2 added. The outer chromium reacts with oxygen 2 Fuel cells are an expensive source of power. in the air to form a rust protective layer. The extraction of hydrogen, as fuel for fuel cells, chromium + oxygen chromium oxide from natural gas produces carbon dioxide. 2 At the top right of this page is an atomic model of one of 4Cr + 3O 2Cr O 2 2 3 the compounds involved in the steam treatment of methane. Which compound is represented by the atomic model?
When ready, cover the information above and answer the questions below. a) Steam (H2O).
b) Methane (CH4). c) Nickel (Ni). Exercise Methane and Exercise d) Carbon dioxide (CO ). 2 Organic material carbon dioxide 1 What is is meant by the chemical property of 5 What is the evidence that rotting fruit is a a substance? chemical change? Methane 3 A biogas generator is essentially the chemical change 2 A chemical property of hydrogen is that it 6 Essentially the ripening of fruit is a chemical (breakdown) of organic material into methane and carbon will react explosively with oxygen. change from starch to fructose (a sugar) dioxide. What is probably the purpose of the caustic soda a) Attempt to write the symbolic equation summarised by: (NaOH) solution in the diagram of a biogas generator? for the reaction of hydrogen with C H O + H O C H O 6 10 5 2 6 12 6 a) Take the moisture out of the biogas. NaOH oxygen. Is this a combination or decomposition b) Take the methane out of the biogas. reaction? b) Suggest a possible use for hydrogen c) Take the carbon dioxide out of the biogas. Rough sketch of a biogas generator given that it will react explosively with b) What is the difference between d) Take any microorganisms out of the biogas. hydrogen. starch and fructose (a sugar)? 3 What is the difference between iron and 7 Is the breakdown of fructose an exothermic 4 What could you probably conclude about the physical steel? or endothermic reaction? Explain. properties of methane and carbon dioxide? 4 The rusting of steel is essentially a reaction 8 Describe a hydrogen fuel cell. a) Both methane and carbon dioxide are soluble in a between iron and oxygen in the air. caustic soda (NaOH) solution. 9 The net chemical reaction in a hydrogen fuel b) Methane is soluble in a caustic soda (NaOH) solution. a) Attempt to write the symbolic equation cell is: for the reaction of iron (Fe) with oxygen. c) Carbon dioxide is soluble in a caustic soda (NaOH) H + O H O b) Suggest three ways of preventing rust 2 2 2 solution. Is this a combination reaction or a (basically preventing the oxygen from d) Neither methane nor carbon dioxide is soluble in decomposition reaction? making contact with the iron). a caustic soda (NaOH) solution. 10 What are three advantages and three disadvantages of hydrogen fuel cells? 166 7.6 Chapter Review Chapter 7 The Water Cycle 167 Harder Test Questions
1 When sugar and yeast are mixed together, the yeast, a fungus, feeds on the sugar. This is described as fermentation because alcohol and bubbles of carbon dioxide are produced. Sugar ethanol + carbon dioxide + energy C6H12O6 2CH3CH2OH + 2CO2 + energy a) Give two pieces of evidence that 3 The above battery essentially consists of suggest that this is a chemical reaction. a carbon rod, negative terminal, a zinc casing, b) Is the feeding of yeast on sugar a positive terminal, and a manganese oxide combination reaction or a decomposition paste in between the carbon and the zinc. reaction? a) Is the anode positive or negative? c) What is the evidence that fermentation b) When used, why does the battery is an exothermic reaction? eventually become flat?
2 A student investigated the question 'What is 4 Natural gas mostly consists of mixtures the effect of temperature on the feeding of of methane and ethane with small amounts yeast on sugar?'. Using the same quantities of propane, butane, and pentane. of yeast, sugar, and water at different water Each of these gases are compounds of temperatures in a thin measuring tube, the carbon and hydrogen. following data was obtained: a) Copy and complete the following table: Temperature Bubble height Gas Formula 5°C 60 mm Methane CH 25°C 100 mm 4 Ethane C H 40°C 160 mm 2 6 Propane C H 80°C 70 mm 3 8 Butane Pentane a) Draw a graph of temperature vs height. b) Write an answer to the question 'What b) Combustion is the burning of a fuel and is the effect of temperature on the an oxidant, such as oxygen, producing feeding of yeast on sugar'? heat. Symbolic chemical equations are given for the burning of methane and c) Would you be able to write an answer ethane. Attempt to write the chemical to the question 'Which temperature equations for the burning of propane. does yeast prefer'? CH4 + O2 CO2 + H2O C2H6 + O2 CO2 + H2O
168 7.6 Chapter Review