# Reversible? Heating and Cooling 2

In this worksheet, students will be helped to place chemical changes into the context of an everyday occurrence: cooking. They are able to explore the relationship between baking a cake and whether chemical changes that happen are reversible or not.

Key stage:  KS 2

Curriculum topic:   Properties and Changes of Materials

Curriculum subtopic:   Reversible Changes

Difficulty level:

### QUESTION 1 of 10

Amy and Sam are doing a science experiment called 'Baking a Cake'! Sam loves chocolate cake so Amy has decided to use her method of baking to help Sam to understand some of the science behind cooking.

"What do we need to make the cake?" Sam asks Amy.

"Oh, lots of things: bowls, spoons..."

"No, No!" interrupts Sam, "I mean what ingredients do we need?"

"Well, there's flour, sugar, eggs and milk for a start." Amy replied.

"What about chocolate?" Sam wanted to know.

"Yes, and that of course!" laughed Amy. "Anyway, let's get started."

Which ONE of Amy's ingredients will dissolve completely in water?

flour

sugar

eggs

milk

chocolate

Amy placed all of the ingredients into a bowl.

"What do we do now?" enquired Sam.

"We need to combine them all together." replied Amy.

"How are we going to do that?" asked Sam.

What do you think they should do?

shake the mixture thoroughly

stir it with a wooden spoon

use an electric whisk or mixer

Once the mixture was well combined, Amy poured it into two cake tins and put it into the oven for 30 minutes.

When they took the cakes out Sam observed, "They don't look much like cakes - they're too flat."

Amy agreed that something was wrong. "Oh no, I forgot the baking powder and the bicarbonate of soda," she moaned, "they make the mixture rise."

"Never mind," said Sam, "we can just undo the mixing and start again."

Do you think Sam's right? Can they undo the cake and start again?

yes, it's easy to separate the ingredients - it's reversible

no, once the cake's been baked the ingredients are mixed - it's irreversible

no, once the cake's been baked the ingredients are combined as sponge - it's irreversible

"What do the baking powder and bicarb do anyway?" asked Sam.

"Oh, they're important - they make the cake mixture rise," explained Amy.

"How?" Sam wondered.

"Well, watch this." replied Amy. She sprinkled a little of the mixture into some water and it fizzed.

Why do you think the baking powder and bicarbonate of soda make the cake mixture rise?

the fizzing moves the mixture around quickly

the baking powder reacts with the cake mixture

the fizzing produces bubbles of gas which get trapped in the mixture

Amy and Sam made a new cake mixture using the baking powder and bicarbonate of soda as well and Amy put it into the oven for 30 minutes.

While they're waiting for it to bake answer this question: which TWO of their ingredients will melt if heated?

flour

butter

chocolate

baking powder

When a raw egg is broken into a pan and heated, in what way does it change?

it goes hard and then back into runny egg when it cools (reversible)

it goes hard and then stays like that (irreversible)

it starts to go hard and then melts into a runny mixture (reversible)

Once the 30 minutes were up Sam was keen to see what the cake looked like. Amy took the cakes out of the oven saying, as she did so, "There, that's more like it!"

"Can we eat it now?" asked Sam.

"Gosh, no - it's too hot!" chuckled Amy. "Anyway we've got to make the icing."

They mixed some chocolate and cream together and heated it gently in a saucepan until the chocolate had melted.

"If we put that chocolate back into the fridge now, would it go solid again?" asked Sam.

"Yes it would." his sister replied.

This is a reversible change - what word describes molten chocolate going solid?

freezing

melting

condensing

Match the following ingredients to explain, when they're heated, if they change permanently (IRREVERSIBLE) or whether when they cool they return to their original state (REVERSIBLE).

## Column B

milk
irreversible
butter
reversible
sugar
reversible
chocolate
irreversible
egg
irreversible

If milk is heated until it boils a gas is given off. What do you think is the main constituent of this gas?

milk vapour

steam

carbon dioxide

Here is what Amy and Sam's cake looked like when they had finished icing it:

Which ONE of the following do you think was the key process in changing the original ingredients into an edible cake?

mixing

baking

icing

• Question 1

Which ONE of Amy's ingredients will dissolve completely in water?

sugar
EDDIE SAYS
Only sugar will dissolve completely, breaking up into tiny particles of solid dispersed throughout the liquid. Milk is mainly water anyway.
• Question 2

Amy placed all of the ingredients into a bowl.

"What do we do now?" enquired Sam.

"We need to combine them all together." replied Amy.

"How are we going to do that?" asked Sam.

What do you think they should do?

use an electric whisk or mixer
EDDIE SAYS
James Bond would probably shake them and stirring with a wooden spoon is a reasonable method; however, when you want something to dissolve or to react quickly and well, the better you mix it the better the result - an electric mixer will do the job best.
• Question 3

Once the mixture was well combined, Amy poured it into two cake tins and put it into the oven for 30 minutes.

When they took the cakes out Sam observed, "They don't look much like cakes - they're too flat."

Amy agreed that something was wrong. "Oh no, I forgot the baking powder and the bicarbonate of soda," she moaned, "they make the mixture rise."

"Never mind," said Sam, "we can just undo the mixing and start again."

Do you think Sam's right? Can they undo the cake and start again?

no, once the cake's been baked the ingredients are combined as sponge - it's irreversible
EDDIE SAYS
Like dough, once a sponge mixture has been baked a chemical reaction has occurred which produces new substances - all the flour, eggs, etc. no longer exist as such - they are combined into new substances which taste even better!
• Question 4

"What do the baking powder and bicarb do anyway?" asked Sam.

"Oh, they're important - they make the cake mixture rise," explained Amy.

"How?" Sam wondered.

"Well, watch this." replied Amy. She sprinkled a little of the mixture into some water and it fizzed.

Why do you think the baking powder and bicarbonate of soda make the cake mixture rise?

the fizzing produces bubbles of gas which get trapped in the mixture
EDDIE SAYS
Both bread and sponge are especially good to eat because they have bubbles of gas trapped within them that make them light and squashy. The baking powder/bicarb mixture releases a gas and the bubbles form within the mixture causing it to rise.
• Question 5

Amy and Sam made a new cake mixture using the baking powder and bicarbonate of soda as well and Amy put it into the oven for 30 minutes.

While they're waiting for it to bake answer this question: which TWO of their ingredients will melt if heated?

butter
chocolate
EDDIE SAYS
Both butter and chocolate are at their best if kept cool - above 20C they become fairly runny as they start to melt.
• Question 6

When a raw egg is broken into a pan and heated, in what way does it change?

it goes hard and then stays like that (irreversible)
EDDIE SAYS
Eggs are great chemicals to play with in the kitchen! When the egg is heated the colourless part starts to go hard and white as it is irreversibly changed into a new substance. The yellow part of the egg also follows this pattern, but a little more slowly.
• Question 7

Once the 30 minutes were up Sam was keen to see what the cake looked like. Amy took the cakes out of the oven saying, as she did so, "There, that's more like it!"

"Can we eat it now?" asked Sam.

"Gosh, no - it's too hot!" chuckled Amy. "Anyway we've got to make the icing."

They mixed some chocolate and cream together and heated it gently in a saucepan until the chocolate had melted.

"If we put that chocolate back into the fridge now, would it go solid again?" asked Sam.

"Yes it would." his sister replied.

This is a reversible change - what word describes molten chocolate going solid?

freezing
EDDIE SAYS
"Freezing" describes what happens when a molten substance is cooled down into a solid. That's fairly easy to understand for butter or chocolate as they are put in the fridge, but try this: molten iron FREEZES to solid iron at the very high temperature of 1500C! It's true!
• Question 8

Match the following ingredients to explain, when they're heated, if they change permanently (IRREVERSIBLE) or whether when they cool they return to their original state (REVERSIBLE).

## Column B

milk
irreversible
butter
reversible
sugar
irreversible
chocolate
reversible
egg
irreversible
EDDIE SAYS
Milk, when heated, steams (so some is lost) and its flavour and structure changes permanently. Butter and chocolate, as long they're heated to melting point (and not beyond) can be frozen again. Sugar turns to a brown treacly substance while egg turns hard, both irreversibly.
• Question 9

If milk is heated until it boils a gas is given off. What do you think is the main constituent of this gas?

steam
EDDIE SAYS
Milk is mainly water so when it boils that water turns to steam.
• Question 10

Here is what Amy and Sam's cake looked like when they had finished icing it:

Which ONE of the following do you think was the key process in changing the original ingredients into an edible cake?

baking
EDDIE SAYS
They're all important, of course, but the chemical reactions that change the runny mixture into cake sponge is caused by the heat of the oven - without that there wouldn't be a cake to ice. What a lot of science there is in the kitchen!
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