2.2 UNITUNITmrjeffsecondaryscience1.weebly.com › uploads › 1 › 4 › 3 › 7 › ...SSciDim2CB_Ch02.indd 36ciDim2CB_Ch02.indd 36 111/1/06 3:59:34 PM1/1/06 3:59:34 PM. 37

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2.22.2Changes are happening to the substances around us all of the time: an aluminium can is squashed, a glass window is smashed, a sausage is fried, fireworks explode and the steel door of a car rusts. When something happens to a substance, its change can be classified as either physical or chemical.

Fig 2.2.1 Exploding fireworks involve both physical and chemical changes.

Physical changeA physical change occurs when a substance changes, but no new substance is formed. Physical changes are happening when a substance:• changes shape; for example, an aluminium can is

crushed or an elastic band is stretched• is crushed or broken into smaller pieces; for

example, a glass window is shattered or an aspirin is crushed to make it easier to swallow

• is dissolved; for example, sugar is dissolved in hot coffee or turpentine removes a paint stain

• is simply mixed with other substances; for example, concentrated cordial is diluted by mixing it with water

• changes its state by melting, evaporating, freezing or condensing; for example, an ice cube melts.

Fig 2.2.2Dry ice (frozen carbon dioxide) changes quickly into gaseous carbon dioxide when put in water.

This is a physical change.

Chemical changeA chemical change or chemical reaction has occurred whenever a new substance has formed. Some chemical changes are obvious: the substance formed looks or acts very differently to what was there before. Sometimes, however, the change may be difficult to detect. The only indication may be a change in colour, a change in temperature or the production of light or bubbles.

Some chemical changes that you meet almost every day include:• cooking; such as cooking a cake or grilling a

sausage• burning; such as lighting a match or burning

your toast• explosions; such as petrol exploding in the

cylinders of a car

SciDim2CB_Ch02.indd 36SciDim2CB_Ch02.indd 36 11/1/06 3:59:34 PM11/1/06 3:59:34 PM

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• ripening; such as a banana ripening from green to yellow to black

• rotting; such as a dead mouse behind the fridge beginning to smell.

Fig 2.2.3 Rusting is a chemical change: a new substance has formed.

Types of chemical reactionsIn a chemical reaction, the substances you start with are called the reactants. The substances you end up with are called its products. All chemical reactions can be represented by a chemical equation. The symbol → indicates that a chemical reaction has taken place. All chemical reactions can be shown as:

reactants → products

There are four basic types of chemical reactions: • combination reactions• breaking-down reactions• precipitation reactions • combustion reactions.

Prac 3 p. 43

Prac 2 p. 42

Prac 1 p. 41

Combination reactionsIn combination reactions, reactants combine to form a single new substance. Rusting of iron is an example of a combination reaction where iron (Fe) reacts with the oxygen gas (O2) found in air or water to form rust (the compound iron oxide Fe2O3).

This could be written as a word equation:

iron + oxygen gas → iron oxide

or it could be written as a formula equation using element symbols and chemical formulas:

Fe + O2 → Fe2O3

Sometimes subscript symbols (s), (l) and (g) are added to give extra information about whether the chemical is a solid, liquid or gas. The symbol (aq) indicates that the substance is dissolved in water as an aqueous solution.

Iron and rust are both solid, while oxygen is a gas. So the reaction can also be written as:

Fe(s) + O2 (g) → Fe2O3 (s)

This reaction is unbalanced because the numbers of atoms on each side of the equation do not match. You will learn how to balance equations in Science Dimensions 3, Chapter 2.

Combination reactions are also sometimes called synthesis reactions.

Breaking-down reactionsIn breaking-down reactions one substance breaks up into a number of smaller ones. For example, some photographic film is coated with silver chloride (AgCl), a substance that breaks down when light falls on it, leaving a dark image, or negative, on the film. Two products are formed from this reaction, silver (Ag) and chlorine gas (Cl2). The reaction could be written:• as a word equation:

silver chloride → silver + chlorine

• as an unbalanced formula equation:

AgCl → Ag + Cl2

• showing the states of matter:

AgCl(s) → Ag(s) + Cl2 (g)

Breaking-down reactions are also known as decomposition reactions.

Homework book 2.5 Combination reactions

2.22.2


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>>>Precipitation reactionsSometimes a solid forms when two solutions are mixed together. The solid formed is called a precipitate. It starts as a fine powder that then settles out into a layer of sediment at the bottom of a clear solution. Precipitates are insoluble, meaning they do not dissolve in water. Some precipitates are very colourful and are often used as paint pigments.

Silver nitrate is a clear, colourless and poisonous solution that looks just like water (this is why you must never taste anything in a laboratory). When added to salt water (another clear and colourless solution) it immediately forms a cloudy, white, solid powder, which then settles over time.

This reaction could be written:• as a word equation:

silver sodium silver sodium nitrate + chloride → chloride + nitrate

solution solution precipitate solution

• an unbalanced formula equation:

AgNO3 + NaCl → AgCl + NaNO3

• showing the states of matter:

AgNO3 (aq) + NaCl(aq) → AgCl(s) + NaNO3 (aq)

Fig 2.2.4 A precipitation reaction

Combustion reactionsCombustion reactions happen whenever you burn or explode something. A substance reacts with oxygen, usually from the air around it. New (often gaseous) substances form and heat and light may be released, sometimes as a flame or explosive flash.

A Bunsen burner works because methane gas from the gas supply reacts with oxygen drawn through its air hole. Carbon dioxide, water vapour and a flame (heat and light energy) are produced:

methane gas + oxygen

gas →carbon dioxide

gas+ water

vapour

CH4 (g) + O2(g) → CO2 (g) + H2O(g)

When magnesium ribbon burns, it combines with oxygen in the air to produce a white powder, magnesium oxide:

magnesium metal + oxygen

gas →magnesium

oxide powder

Mg(s) + O2 (g) → MgO(s)

Prac 4 p. 43

Prac 5 p. 44

Why do you eat?To be able to live you need lots of energy. You need energy to keep warm, to move and to keep your organs working properly. The food you eat is converted into glucose, a type of sugar. When you need energy, you burn some of this glucose in a combustion reaction:

glucose solution + oxygen

gas →carbon dioxide

gas+ water

vapour + energy

C6H12O6 (aq) + O2 (g) → CO2 (g) + H2O(g)This reaction draws oxygen from your blood and that is why you need to breathe. The carbon dioxide produced is then carried back to your lungs to be breathed out. This special combustion reaction is something that all animals

do and is called respiration.

Speeding up reactionsWhen fireworks explode, the rate of reaction is very fast: the firework chemicals are used all at once. In contrast, iron rusts very slowly. You can judge the rate of a reaction by observing how quickly new substances are produced or how quickly reactants disappear.

Physical and chemical changePhysical and chemical change


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2.22.2Reason: The rhodium attracts the harmful gases and oxygen so more of each gas comes together and reacts. The rhodium does not actually react with either gas; it just speeds up the reaction.

• enzymes (found in the body)

Example: Digestive enzymes help break down large molecules such as starch into smaller molecules like glucose. Glucose is more easily digested than starch and ultimately gets used by your body cells.

Reason: Enzymes act like a pair of scissors. They cut up the starch molecules but don’t combine with them. Enzymes are a type of catalyst. They simply speed up chemical reactions.

Rate of reaction can be affected by:

• the amount or concentration of reactants

Example: A stain may be removed more quickly by adding more stain remover, or a stronger stain remover.

Reason: More molecules are available to take part in the reaction, so products are produced more quickly.

• temperature

Examples: Fruit ripens more quickly in warmer weather; food keeps longer when kept cool in a refrigerator.

Reason: Reactants move quicker at higher temperatures and have more energy available to break bonds to allow the formation of new substances.

• surface area

Example: Iron in the form of iron filings reacts more quickly with acid than the same amount of iron does in the form of a single block.

Reason: Iron filings have a greater surface area than a single block, allowing more atoms of iron to be attacked at once by the acid.

• helper chemicals (catalysts)

Example: In a car’s catalytic converter, the element rhodium helps harmful exhaust fumes react with oxygen to produce less harmful products.

Prac 6 p. 44

Prac 7 p. 45

starch molecule

enzyme cuts bondsin starch molecule

Fig 2.2.5 Enzymes breaking up a starch molecule

Prac 8 p. 45

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[ Questions ]Revision questions

Physical change 1 Define a physical change.

2 List an example of each of the five forms of physical change (other than those given in the text).

Chemical change 3 What is the difference between a chemical and a

physical change?

4 List an example of each of the five forms of chemical change (other than those given in the text).

5 List four observations that suggest a chemical change has taken place.

Types of chemical reactions 6 What are four types of chemical reaction?

7 What are the ingredients of a chemical reaction called?

8 What is a precipitate?

9 Explain what →, (s), (l), (g) and (aq) mean in a chemical equation.

10 Copy and complete these word equations:a methane gas + ______ gas → ______ gas + water

vapourb ______ metal + oxygen gas → magnesium ______

powder

Hardening fillingsDentists use a special

paste to fill holes in teeth. The paste is then hardened quickly by using ultraviolet

(UV) light as a catalyst.

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c glucose ______ + oxygen gas → carbon dioxide gas + ______ + energy

d silver ______ → silver + ______ gase ______ + oxygen gas → iron ______

11 Copy and complete these unbalanced chemical equations:a Fe + ______ → Fe2O3

b ______ → Ag + Cl2 12 Copy and complete these unbalanced chemical

equations, showing the state of each substance:a Mg(s) + ______ (g) → MgO(s)

b AgNO3 (__) + NaCl(__) → AgCl(__) + NaNO3 (__)

c CH4(g) + ______ (g) → ______ (g) + H2O(g)

d C6H12O6 (__) + O2 (__) → CO2 (__) + ______ (g)

Speeding up reactions 13 What would be the effect of each of the following on a

wood-fire heater?a A log is chopped into several small pieces before

being added to the fire.b A vent is closed so less air gets to the fire.

14 Give an example of a fast and a slow reaction.

15 Give four ways a chemical reaction may be sped up.

16 What is a catalyst?

17 How does an enzyme help speed digestion?

Thinking questions

18 Decide if these changes are physical or chemical.a Grass is mown.b Wood burns.c Tree branches are mulched.d An egg is fried.e Finger nails are filed down.f Milo is dissolved in milk.g Cornflakes go soggy.h A green tomato ripens and turns red.i Rain turns the sports oval muddy.j Vegetable scraps in the compost bin produce a

rich soil.k Dynamite explodes.l Concrete hardens.m Margarine melts in a frypan.n Leaves turn red in autumn.o Wet clothes on the line dry.p A match burns.q Unrefrigerated milk goes ‘off’.r Butter burns in a frypan.s Bread goes mouldy.

t Water freezes to make ice cubes.u Red wines tastes like vinegar after many years.v A soluble aspirin fizzes away in a glass of water.

19 Write word equations for each of the following:a Hydrogen gas and oxygen gas explode together to

form water.b Sodium dissolves in water to produce sodium

hydroxide solution and hydrogen gas.c Solid mercury oxide is broken down into liquid

mercury and oxygen gas.

20 Use the chemical formulas on page 31 to write chemical equations for each of the following.

a carbon + oxygen gas → carbon dioxideb hydrogen gas + oxygen gas → water vapourc hydrogen gas + chlorine gas → hydrochloric acid

vapour 21 Write the following as a word equation.

HCl + Na → NaCl + H2

22 Identify the type of changes happening in the following.a To cook toast, the bread needs to first dry out

then burn.b To make toffee, sugar first needs to be melted and

then burnt.c A candle burns, the wax dripping down its side.

23 Enzymes speed up digestion. Why is this a good thing?

24 Reaction rates slow as time goes on. Suggest why.

25 Which type of headache tablet might work faster—a capsule containing fine grains or a solid tablet?

Analysis questions

26 Identify the reactants and products in the reaction Mg + HCl → MgCl2 + H2.

27 Label each of these reactions as combination, decomposition, precipitation or combustion reactions.

a zinc + sulphur → zinc sulphideb Pb(NO3)2 (aq) + NaCl(aq) → PbCl2 (s) + NaNO3 (aq)

c NaCl → Na + Cl2d ethane + oxygen gas → carbon dioxide + water

vapour 28 A catalyst is used in a reaction. Which of the following

best describes the amount of catalyst left at the end of the reaction compared to the amount present at the start?A noneB lessC the sameD more



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[ Practical activities ]2.2

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IT Ripening fruitA chemical reaction happens whenever fruit ripens or goes brown. Chemicals called antioxidants slow the ripening process.

You will needTray or plate (preferably white), permanent marker, ruler, apple, banana, potato, lemon, knife to cut fruit, vitamin C tablets

What to do 1 Rule up a grid of nine squares on the plate or tray.

2 Crush the vitamin C into fine powder and squeeze the lemon, collecting its juice.

3 Carefully cut the apple, banana and potato so that you end up with three slices of each. Place them in the squares as shown in Figure 2.2.6.

4 Cover the fruit in row 1 with the crushed vitamin C.

Prac 1 Unit 2.2

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apple slices banana slices potato slices

tray

Fig 2.2.6

2.22.2U

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at workSurfing

Companion WebsiteConnect to the Science Dimensions 2 Companion Website at www.pearsoned.com.au/schools, select Chapter 2 and click on the Web Destinations button to find answers to these questions about physical and chemical changes.

1 What are the new substances formed when food decomposes?

2 How can you ensure that vegetable scraps will produce good compost suitable for the garden?

3 How is sheet metal made from raw materials? What are the physical and chemical changes involved?

4 Describe the process of galvanising that is used to prevent rust.

5 What are exothermic and endothermic reactions and how can they be used?

6 What enzymes are produced in the body, where are they made and what do they do?

7 What happens when baking soda and vinegar react with each other? Carry out an experiment and test the gas

produced by this reaction by holding a lit match in it. Suggest an equation for this reaction.

8 What are the enzymes sometimes used in washing powders? How do they help?

Present your work as a set of answers to the questions.

Companion WebsiteFind out about balancing chemical equations by connecting to the Science Dimensions 2 Companion Website at www.pearsoned.com.au/schools, selecting Chapter 2 and clicking on the Web Destinations button.

Investigating

Rusty nailsObtain several identical jars and place a nail in each. Compare the rate at which iron nails rust in the following situations: nail partly under water, nail fully under water, nail in salty water, nail in water with a crushed vitamin C tablet (vitamin C is an antioxidant).

Present your work as an experimental report. Include all the normal features, such as aim, materials, method, results and conclusion.

2.2 SCIENCE


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Extinguishing fireFire is a common chemical reaction that always has oxygen as a reactant. The gas carbon dioxide (CO2) is commonly used as a fire extinguisher that works by blocking oxygen from the flame.

You will need100 mL measuring cylinder, 250 mL beaker, spatula, baking soda (bicarbonate of soda), vinegar (acetic acid), long matches, birthday candle, Blu-Tack or Plasticine

What to doPart A 1 Put two to three heaped spatulas of baking soda into a

250 mL beaker.

2 Use some Blu-Tack or Plasticine to stand a birthday candle in the centre of the beaker, so that it is surrounded by the baking soda.

3 Light the candle and let it burn for 30 seconds or so.

4 Carefully pour vinegar into the beaker until the baking soda is just covered. The gas carbon dioxide (CO2) is immediately produced.

5 Record your observations.

6 Do not move the beaker or candle. If the candle goes out, try to relight it using a long match.

Part B 7 Put two or three spatulas of baking soda into the

measuring cylinder.

8 Pour enough vinegar in to cover the baking soda.

9 As it bubbles, lower a lit long match into the top of the measuring cylinder.

10 Record what happens to the match.

Questions

1 How do you know that a gas was produced?

2 Is the reaction of baking soda and vinegar a physical or chemical change?

3 What does carbon dioxide (CO2) do to a flame?

Prac 2 Unit 2.2

5 Dribble the lemon juice over the fruit in row 2.

6 Don’t do anything to the fruit in row 3.

7 Leave the fruit unrefrigerated and uncovered for at least one hour.

8 Copy the diagram into your workbook and record which fruit went brown.

4 How do you know CO2 was present?

5 Predict whether CO2 is heavier or lighter than air. Explain your answer.


Questions

1 How do you know that a physical change happened when the fruit went brown?

2 When it went brown, which gas, contained in air, do you think the fruit reacted with?

3 Did vitamin C and lemon juice slow down the reaction in all the fruit?

4 Vitamin C contains ascorbic acid and lemon juice contains citric acid. Suggest which chemicals make good antioxidants.

5 Suggest why antioxidants are called antioxidants.

6 Suggest other ways of slowing the ripening reaction.

measuringcylinder

baking soda and vinegar

long match

beaker

candle

Blu-Tack

Do not pour in too much vinegar

Fig 2.2.7 C02 as an extinguisher

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2.22.2

Coating nailsYou will needSmall iron nails, copper sulfate solution, copper nitrate solution, three test tubes, test-tube rack

What to do 1 One-third fill one test tube with water, one with

copper sulfate solution and one with copper nitrate solution, and place the tubes in the rack.

2 Place one iron nail in each test tube.

3 Leave the test tubes to stand for 5 minutes or more; preferably overnight.

Questions

1 Which solution produced the thickest coating on a nail?

2 What was the purpose of the test tube containing only water with a nail?

3 Suggest what the coatings may be.

Prac 3 Unit 2.2

test tuberack

water

copper nitratesolution

copper sulfatesolution

Fig 2.2.8

A precipitation reaction 5 Dip the burnt end of the match into the filtered

precipitate so some of the precipitate stays on the match.

6 Using tongs, place the match end with the precipitate on it into the blue flame of a Bunsen burner for about 10 seconds.

7 Observe the end of the match. Can you see any traces of another substance?

Questions

1 Describe the precipitate. Suggest what it may contain, given that one part of each reactant’s name is involved.

2 What chemicals may remain in solution?

3 Suggest what the substance obtained in Part B is, giving reasons.

Prac 4Unit 2.2

You will needPotassium iodide solution, lead nitrate solution, filter paper, funnel, conical flask, matches, tongs, test tube, test-tube rack

What to doPart A 1 Place 2 cm of potassium iodide solution in a test tube.

2 Add a similar amount of lead nitrate solution.

3 Leave the test tube to stand in a rack for several minutes and observe the contents.

Part B 1 Use filter paper, a conical flask and a funnel to filter out

the precipitate.

2 If possible, leave the solution filtering overnight.

3 Light a match and blow it out after about 1 cm of the lit end has turned to charcoal.

4 Remove the ‘lump’ (the burnt, previously red end) from the end of the match.


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Burning magnesium

CAUTION: Do not look directly at the magnesium as it will burn the eye.

Observe your teacher:

1 Wind a 3 cm piece of magnesium into a spiral and place in a crucible.

2 Weigh it accurately on an electronic balance.

3 Place a lid on the crucible, leaving it slightly ajar so that air can enter but no-one can look into it.

4 Heat the crucible strongly over the blue flame of a Bunsen burner.

5 Weigh the crucible after it is cool.

Questions

1 Describe the magnesium and what is produced by the reaction.

2 How do you know this was a chemical and not a physical change?

3 The new substance is magnesium oxide. Where did the oxide part of the product come from?

4 The crucible probably weighed more after heating than it did before. Why?

5 The reactants of this reaction were Mg and O2 and the product was MgO. Write this as a formula equation showing the states of matter.

Prac 5 Unit 2.2

Teacher demonstration

Reaction rate—effect of temperature and concentration

Questions

1 How does temperature affect reaction rate for this experiment?

2 How does concentration of the hydrochloric acid affect reaction rate?

3 Can you be sure your answers to the above questions apply to all reactions?

Prac 6Unit 2.2

timer

10 mL hydrochloric acid

observe cross

50 mL sodium thiosulfatesolution

Fig 2.2.9

You will needSodium thiosulfate (‘hypo’) solution (0.1M), hydrochloric acid (1M), hydrochloric acid (2M), cold water, hot water, conical flask, measuring cylinder (10 mL), large beaker, safety glasses

What to do 1 Place 50 mL of sodium thiosulfate solution into a

conical flask.

2 Sit the conical flask in a beaker of cold water (put ice blocks in the water if they are available) for 5 minutes.

3 Remove the conical flask from the beaker and dry its base.

4 Draw a cross on a piece of white paper and place the conical flask on top of the cross.

5 Add 10 mL of hydrochloric acid (1M strength) to the conical flask, and time how long it takes before you can no longer see the cross under the base of the flask.

Alternatively, use a light sensor on one side of the flask and a light source on the other to measure the amount of light transmitted through the contents of the flask as the reaction progresses. Note the time taken for the cloudiness or ‘turbidity’ of the solution to stabilise.

6 Repeat steps 1 to 5, but use hot water at step 2 instead of cold.

7 Repeat steps 1 to 5, but use 2M strength hydrochloric acid instead of 1M strength.



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2.22.2

Reaction rate and surface area

2 Grind one of the Alka Seltzer tablets into a fine powder.

3 At the same time place a whole tablet in one beaker and the crushed tablet in the other beaker, stirring for a few seconds.

4 Observe the time taken for each to finish reacting with the water.

Questions

1 Which tablet (crushed or whole) had the greater surface area?

2 What is the effect of greater surface area on the rate of this reaction?

3 What factors should be kept the same for both beakers? Why?

Prac 7 Unit 2.2

stirring rod

whole tablet crushed tablet

Fig 2.2.10

Catalysts and enzymesYou will needFour test tubes, test-tube rack, hydrogen peroxide solution, manganese (IV) dioxide, fresh liver, small piece of apple or potato, knife, cutting board, wax taper, safety glasses

What to do 1 Dice the piece of liver and place some of it in a

test tube.

2 Place some manganese dioxide in another test tube.

3 Place a small piece of apple or potato in another test tube.

4 One-quarter fill another test tube with hydrogen peroxide only. Hydrogen peroxide slowly decomposes into water and bubbles of oxygen. Can you see any bubbles of oxygen forming?

5 Now add the same amount of hydrogen peroxide to the other three test tubes and observe carefully, comparing rates of bubble formation in all three test tubes.

Questions

1 In which test tube was oxygen produced most rapidly?

2 Were liver and manganese dioxide left in the test tubes, or were they consumed by the reaction? What does this tell you about these substances?

3 Compare the effect of the apple or potato with that of the manganese dioxide.

4 How could you test that the gas produced was in fact oxygen?

5 Do you think cooked liver would work as well? If time permits, test it also.

Prac 8Unit 2.2

hydrogen peroxide

liverhydrogenperoxide only

piece ofapple orpotato

manganesedioxide

Fig 2.2.11

You will needTwo Alka Seltzer tablets, two beakers (250 mL), two stirring rods, mortar and pestle or other grinding tools

What to do 1 Place 100 mL of water in each beaker.


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