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Meat cooking on a barbecue, sugar dissolving in a cup of tea, ice-blocks melting in a glass of soft drink and grass decomposing in your backyard rubbish pile are all examples of change. Change is an essential part of life. Some changes, such Co

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as those that occur in the cooking of food, are useful and we often use heat to bring about the change. Other changes, such as the wearing out of your bike tyres, are unwanted and expensive. Either way, change involves particles interacting with each other or their environment. By understanding change you may help the environment, reduce expensive waste or learn how to use different materials in new and exciting ways.

Matter changesMatter changes around us all the time. Changes in matter, such as the burning of petrol inside the engine of a car, occur very quickly, while other changes, such as the rusting of a car, occur very slowly. Some

changes to matter result in new substances being produced while other changes do not. Changes in matter can be divided into two main groups, as shown in Figure 5.5.1.

MATTER CHANGES

YES NOIs a new substance produced?

CHEMICAL CHANGE PHYSICAL CHANGE

for example for example

cooking a pieceof meat

petrol burning ina car engine

food producingenergy in your body

sand papering a piece of wood

ice blocks melting

coffee and sugardissolving

Fig 5.5.1 Changes in matter can be divided up into two main types: physical change and chemical change.

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Physical changeWhen a physical change occurs there are no new materials produced, even though the matter changes in some way. Changes of state (or phase) are examples of this. For example, when water boils it goes from a liquid state to a gaseous state but no new materials are produced.

Water boiling is a physical change, as no new materials are produced.

watermoleculesin gas

watermoleculesin liquid

evaporation/boiling

Fig 5.5.2

Physical changes occur all around us and, because they often affect us in some way, it can be helpful if you think of physical change as useful or non-useful.

Useful physical changesThink of the many useful physical changes you see or feel on a daily basis—clothes drying on the clothes line, sugar dissolving in your cup of tea, or the floorboards in your house being sanded.

Physical changes also occur at a global level and are responsible for our weather patterns. Rainfall and snowfall, for example, involve physical changes.

Non-useful physical changesMost examples of non-useful physical changes involve wearing and breakages. Wearing occurs whenever materials have to move against each other.

Fig 5.5.3 Physical changes occur at a global level. The formation of clouds, for example, is a physical change involving water evaporating and then condensing to form clouds.

Fig 5.5.4Tyre wear is a physical process that can result in dangerous tyres.

Metal fatigue results in the sudden failure of metal, such as this bolt breaking. Fig 5.5.5

Metal fatigue is a physical process that occurs when metals experience strong forces. This can occur in building structures as well as in aeroplane wings and car bodies. It can result in the sudden breakage of the metal through the development of cracks.

Physical and chemical changePhysical and chemical change

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On a global level, the wearing of soil through air and water movement can cause change to our landscape and environment.

Chemical changeIn the last focus you discovered that in chemical reactions the atoms can rearrange to form new materials or compounds. This is a key feature of chemical change. In chemical change new substances are formed. You can tell when a chemical reaction occurs because one or more of the following occurs:1 A new substance is formed which is clearly

different from the materials you started with.2 One of the original substances no longer exists or is

greatly reduced in size.3 There is a colour change.4 A solid or gas is formed.5 An odour (indicating a new material) is produced.6 There is a temperature change where the reaction

took place–it can become hotter or colder.

These observations don’t tell us for sure that it is a chemical reaction, because many of them can also occur during a physical change.

Fig 5.5.6When two colourless liquids are mixed and there is a colour change (like here to green), we have

evidence of a chemical reaction.

Chemical equations describe chemical changeScientists use chemical equations to describe and summarise what has happened during a chemical change. Your teacher may demonstrate the chemical reaction that occurs between hydrogen gas and oxygen gas.

To summarise what has happened during this reaction, you can write a word equation. The substances to the left of the arrow are called the reactants and the substances to the right of the arrow are called the products. The arrow itself indicates that a change has occurred.

hydrogen gas + oxygen gas ! water reactants change product

As with physical changes, it is helpful to divide chemical changes up into useful and non-useful chemical changes.

Useful chemical changesThere are many examples of useful chemical change which result in products that benefit our environment or lifestyle. Some of these are summarised in Figure 5.5.8.

Figure 5.5.8 illustrates that chemical change occurs all around us and is essential in the production of pharmaceuticals, structural materials and energy. Chemical change is also essential in food and drink production, for the recycling of materials from plastics and in the decomposition of materials in the soil.

The chemical reaction that occurs when hydrogen gas and oxygen gas react to

produce water Fig 5.5.7

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X-rays for aeroplanes

X-rays are not only used in looking for broken bones; they can also be used for

looking for broken metals! Metal fatigue in metals can result in disastrous

breakages in the wings and bodies of aeroplanes.

By using X-rays, technicians can identify metal fatigue in major

structural parts before it becomes visible to the eye.

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Non-useful chemical changesNot all chemical change is useful. Meat goes bad and milk goes sour, particularly if left out of the refrigerator. In these cases, chemical change is occurring, producing new chemicals that do not taste or smell very nice and can even be dangerous.

Metals such as iron corrode (rust), particularly if the steel is exposed to the air and moisture. The change that occurs with iron when it corrodes is shown in the chemical reaction below:

steel + oxygen + water ! iron oxide (rust) grey colourless colourless browny-red metal gas liquid flaky solid

It is easy to identify that a chemical change has occurred because a new substance (rust) is produced that has different properties from any of the starting materials. Metals other than iron can also corrode by reacting with the oxygen and water in the air.

Chemical changes also occur in our environment, with some nasty side-effects. For example, the burning of fuels produces a number of substances, such as carbon monoxide, carbon dioxide and sulfur dioxide. These materials not only pollute the air, making it difficult for people to breathe, they can also react with the moisture in the air to produce ‘acid rain’, which can damage buildings and vegetation.

Some examples of useful chemical changes

recycling ofmaterials

is used in the

is used to make

is used to make

helps in

is used to produceuseful

chemicalchange

energy including burning offuels–coal,oil and gas

cooking

producesresults in reducing reducing

cooked meatand

vegetables

differentflavours

decompositionof rubbish

pollution landfill

pharmaceuticals

tofight

disease

for

cancertreatments

structuralmaterials

such as

metals plastics ceramics

and developing including

less polluting energy such as hydrogen fuel cells

portable electricity such as batteries

is usedin

such as such as

Fig 5.5.8

How fast can chemical change occur?Some chemical changes occur very quickly. The burning of petrol in a car engine or an explosion at a mine site are examples of this. In these chemical changes, the new products are produced very quickly, along with large amounts of heat and light.

Fig 5.5.9This chemical change occurs very rapidly.

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Other chemical changes, such as the rusting of a car or the breakdown of plastic in the environment, occur very slowly.

The speed (or rate) at which chemical changes occur can sometimes be controlled. Temperature has an effect on all chemical change, which is one reason you store food in a refrigerator. The lower temperature reduces the speed at which the food decomposes.

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[ Questions ]Use your book

Matter changes 1 What is the difference between a physical

change and a chemical change?

Physical change 2 Explain why the freezing of water is a physical change. 3 Describe two useful and two non-useful physical

changes that occur in your home. In your answer, explain why each is useful or non-useful.

4 Describe two useful and two non-useful physical changes that occur in our environment. In your answer, explain why each is useful or non-useful.

Chemical change 5 Describe how you would recognise if a chemical change

had occurred. 6 Use the concept map shown in Figure 5.5.8 to describe

six examples of useful chemical changes that occur. You may like to set your answers up in a table which describes the change, why it is useful and the materials that are produced.

7 Describe two non-useful chemical changes that occur in your environment.

8 What does the term ‘reactants’ mean when describing a chemical reaction? What are the products?

How fast can chemical change occur? 9 Give an example of a chemical reaction that we like to

occur very slowly. 10 Give an example of a chemical reaction that occurs

very quickly.

Use your head 11 Copy the table below into your notebook. Complete

the table in your notes by classifying each change as ‘physical’ or ‘chemical’. You also need to explain your choice.

Change Physical or chemical Explanation

A piece of paper being burnt

A snow flake forming from water drops in a cloud

Clothes drying out on a clothes line

A cake baking in your oven

Grass decomposing in your compost heap

Sugar dissolving in your cup of coffee

12 Name three everyday examples of:a a chemical change being encouraged to occurb at attempt being made to stop chemical change

occurring. 13 Look back at the equation showing the rusting of steel

(page 240). Use this chemical equation to explain the following observations.a Cars in humid climates tend to corrode faster than

cars in dry, cool climates.b Steel objects are often painted to prevent them from

rusting. 14 A student carried out the following investigations.

She recorded the observations as she went and decided that, based on the observations, they were all examples of chemical changes:a Vinegar (dilute ethanoic acid) is added to baking

soda (sodium hydrogen carbonate). A fizzing sound is heard, the test tube gets warm and lots of bubbles are produced, which pop when they get to the surface.

b Silver nitrate solution is added to salt water and forms a milky, white solid.

c When sodium is added to water it fizzes violently and whizzes around the surface of the water. Bubbles of gas can be seen, along with blue flames.

For each of the reactions above, explain why the student might have considered them to be chemical changes.

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Homework book 5.6 Physical and chemical changes at home

Exploding wheatChemical reactions can occur very rapidly if the materials in the reaction

are ground very finely. For example, although you

would not normally regard wheat as explosive, wheat dust can explode in wheat silos if it comes in contact

with a flame and plenty of oxygen gas. You can

demonstrate the potential for this by sifting flour onto

a Bunsen flame.

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15 Kim adds a piece of magnesium to hydrochloric acid. There is some fizzing and a gas is produced. The magnesium disappears. Kim infers that magnesium chloride and hydrogen gas are produced in this chemical reaction.a What evidence is there that a chemical reaction has

occurred?b What are the reactants for this chemical reaction?

What are the products for this chemical reaction?c Write a word equation for this chemical reaction.

Investigating questions 16 Ask your teacher if you can try the

investigations described in Question 14. Describe all the changes you observe and explain how these support the idea that these are chemical changes.

17 Josh and Kate set up an investigation where they measured the time that it took for a piece of magnesium

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ribbon to totally react with hydrochloric acid at both 20°C and 50°C. Their results are summarised in the table above.a What might have Josh’s and Kate’s hypothesis for

this investigation have been?b What do the results tell us about the effect of

temperature on reaction rate?c Make a list of the variables that Josh and Kate would

have controlled for this to be a ‘fair investigation’.d Why were there three trials for each temperature?

18 Carry out the investigation as described in Question 17. Make sure that you take into account all the controlled variables you suggested.

Temperature (°C)

Trial 1 (seconds)

Trial 2(seconds)

Trial 3(seconds)

Average (seconds)

20 120 110 132 121

50 72 84 80 79

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Observing chemical reactionsPurposeTo observe chemical reactions and identify the key signs that a chemical change has occurred.

RequirementsSafety note: Make sure you wear safety glasses at all times when you are handling or heating chemicals.Safety glasses, six test tubes, test tube rack, potassium iodide solution (0.1 M), lead nitrate solution (0.1 M), hydrochloric acid (2M), copper sulfate solution (0.1 M), two pieces of magnesium ribbon, copper (II) carbonate, test tube holder, Bunsen burner, marble chips, granulated zinc.

ProcedureIn each of the six reactions, take care to note how the test tubes feel so that you can identify if any of the reactions produce heat or become cooler. Carry out the reactions below in separate test tubes as directed by your teacher. Draw up a three-column table to record any observations.

Prac 1 Focus 5.5

Label the columns ‘Reactants’, ‘Observations’ and ‘Evidence of chemical change’. 1 Add 10 drops of potassium iodide to a test tube. Then

add 10 drops of lead nitrate solution. 2 Add 10 drops of hydrochloric acid to one piece of

magnesium. 3 Add 10 drops of copper sulfate solution to a piece of

cleaned magnesium. 4 Add about 1 cm of copper carbonate to a test tube.

Carefully heat the copper carbonate in a blue Bunsen flame using a test tube holder.

5 Place a few marble chips into a test tube. Cover these chips with hydrochloric acid.

6 Add a couple of pieces of granulated zinc to a copper sulfate solution. Let this sit for a few minutes.

Questions 1 Complete your table. 2 Write a summary of the changes that can occur when a

chemical change takes place.

Physical and chemical changePhysical and chemical change