# Burning Materials

In this worksheet, students will be offered a chance to explore what happens when substances burn: what sort of products are formed, what can be observed and whether the changes are reversible.

Key stage:  KS 2

Curriculum topic:   Properties and Changes of Materials

Curriculum subtopic:   Non-reversible Changes

Difficulty level:

### QUESTION 1 of 10

Charlie and Suzi are spending the afternoon with their Uncle Mike, finding out what happens when different substances burn.

To start off with they place a small piece of paper on to a metal plate and their uncle strikes a match and places it on to the paper which catches fire and burns with a yellow flame, giving off some smoke.  A grey crumbly ash is left behind.

Let's explore what's going on.

How many new substances can you definitely tell have been made when the paper burned?

one

two

three

Which of the following pieces of evidence show that something different has been made when the paper burned (select all that apply)?

a grey ash is left behind

the paper burned with a yellow flame

it smelled different

some smoke was produced

the paper curled up as it burned

the ash weighs less than the paper did

Charlie reckoned that it would be possible to reverse the process by combining the gases released from the burning paper with the ash it left behind. Suzi said that that was impossible.

What do you think?

Charlie's right - paper can be made again

Suzi's right - it's impossible.

they're both right - if you could collect the gas you could make paper again but, as the gases have gone into the air, it's impossible

Next Uncle Mike put a small nightlight candle on a plate in front of them and lit it.

Charlie and Suzi both watched the candle over the next few minutes. Their uncle asked them what they thought was happening. Charlie said that he thought the candle was melting but Suzi disagreed - she said it was burning.

What do you think?

Charlie's right - the wax is melting

Suzi's right - the wax is burning

they're both right - the wax is melting and burning

Charlie said that he could prove it was melting: he blew the candle out and they watched the wax turning to a solid.

What is the word that describes the change as liquid wax cools to turn to solid wax?

condensing

freezing

melting

"There you are!" exclaimed Charlie. "That proves it - it's a reversible change!"

Suzi still disagreed and asked her uncle to light the candle again.

"Look," she said, "you can see it's burning."

"But that's just the wick." replied Charlie.

Suzi asked her uncle for some kitchen scales. They blew the candle out and placed it on the scales which read 22g. Uncle Mike then lit it again and they allowed it to burn for 10 minutes after which they weighed it again. The new reading was 20g.

Which of the following do you think is the most likely explanation for the change?

2g of wax has melted away

the candle burned to become 2g lighter

the burning candle released 2g of gas into the air

To prove her point Suzi asked their Uncle Mike to strike another match. They watched the match burn and observed the changes: a flame burned giving off smoke, heat and light.

A black crumbly stick was left behind.

It was too light to weigh so Suzi asked Charlie whether he thought it was heavier, lighter or hadn't changed its weight.

What do you think?

the match was heavier after burning

the match was lighter after burning

the match was the same weight after burning

Where do you think that most of the chemicals that were in the match are now?

in the black stick

in the flame

in the air

If you found a box of matches lying around, what do you think would be the best thing to do?

throw them in the bin

leave them alone

From their experiments Charlie and Suzi came to the conclusion that when things burn the changes cannot be reversed.

What do you think is the best explanation for this?

when substances burn new chemicals are formed and cannot be returned to the original substances

when substances burn gases are always lost into the air and cannot be retrieved

when substances burn they always become lighter and so something is always lost

• Question 1

How many new substances can you definitely tell have been made when the paper burned?

two
EDDIE SAYS
Once the flame went out a "grey crumbly ash" was left behind and during the burning "some smoke" was given off, so that's definitely two. Those two may be made of several substances themselves but that would need some fairly advanced science to find out so..... two!
• Question 2

Which of the following pieces of evidence show that something different has been made when the paper burned (select all that apply)?

a grey ash is left behind
some smoke was produced
the ash weighs less than the paper did
EDDIE SAYS
You're looking for evidence that something new has been made by the burning: grey ash and smoke are new, while the yellow flame is simply the process that creates them. The smell is part of the smoke but the fact that it weighed less is evidence that part of the paper has gone into the air and so been changed.
• Question 3

Charlie reckoned that it would be possible to reverse the process by combining the gases released from the burning paper with the ash it left behind. Suzi said that that was impossible.

What do you think?

Suzi's right - it's impossible.
EDDIE SAYS
When things burn the changes are irreversible: often a gas is released and the solid changes to new chemicals, so changing them back again is a non-starter.
• Question 4

Next Uncle Mike put a small nightlight candle on a plate in front of them and lit it.

Charlie and Suzi both watched the candle over the next few minutes. Their uncle asked them what they thought was happening. Charlie said that he thought the candle was melting but Suzi disagreed - she said it was burning.

What do you think?

they're both right - the wax is melting and burning
EDDIE SAYS
If you watch a candle burn you can see the heat melting the wax and some of it runs down the side, but that molten wax is drawn up the wick where it burns, so they've both made correct observations.
• Question 5

Charlie said that he could prove it was melting: he blew the candle out and they watched the wax turning to a solid.

What is the word that describes the change as liquid wax cools to turn to solid wax?

freezing
EDDIE SAYS
When a liquid cools and turns to a solid that's called freezing - easy to understand for water turning to ice (as it's cold) but much harder to accept for molten iron turning to solid iron as it freezes at 1500°C - yes, it's true!
• Question 6

"There you are!" exclaimed Charlie. "That proves it - it's a reversible change!"

Suzi still disagreed and asked her uncle to light the candle again.

"Look," she said, "you can see it's burning."

"But that's just the wick." replied Charlie.

Suzi asked her uncle for some kitchen scales. They blew the candle out and placed it on the scales which read 22g. Uncle Mike then lit it again and they allowed it to burn for 10 minutes after which they weighed it again. The new reading was 20g.

Which of the following do you think is the most likely explanation for the change?

the burning candle released 2g of gas into the air
EDDIE SAYS
If wax melts, none is lost so it'll weigh the same whether it's solid or molten. It did burn to become 2g lighter but that's an OBSERVATION rather than an EXPLANATION; the most likely explanation is that those missing 2g have gone into the air as gas.
• Question 7

To prove her point Suzi asked their Uncle Mike to strike another match. They watched the match burn and observed the changes: a flame burned giving off smoke, heat and light.

A black crumbly stick was left behind.

It was too light to weigh so Suzi asked Charlie whether he thought it was heavier, lighter or hadn't changed its weight.

What do you think?

the match was lighter after burning
EDDIE SAYS
The fact that smoke was given off from the burning match means that something was lost - it's going to be lighter.
• Question 8

Where do you think that most of the chemicals that were in the match are now?

in the air
EDDIE SAYS
The match head contains a bunch of chemicals including phosphorus and sulphur and then the match-stick itself is made of wood. When these burn most of the products of the burning are gases, like carbon dioxide, which disappear off into the air.
• Question 9

If you found a box of matches lying around, what do you think would be the best thing to do?

EDDIE SAYS
Best not throw them in the bin and leaving them there may leave another, younger, person in potential danger. If you inform an adult they can be placed somewhere safe.
• Question 10

From their experiments Charlie and Suzi came to the conclusion that when things burn the changes cannot be reversed.

What do you think is the best explanation for this?

when substances burn new chemicals are formed and cannot be returned to the original substances
EDDIE SAYS
This is quite tricky: burning is a process that forms new chemicals - quite often that includes gases but not always and, in fact, you will learn that the new chemicals formed from the burning weigh MORE than the original substance ... look out for why that should be when you study it! However it cannot be reversed into the original substances that existed before the burning took place.
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