So do I!

In fact, one of my favourite books is “Fahrenheit 451” by Ray Bradbury. The story takes place in the future where owning books is illegal, and “firemen” of the country burn any books that are found to try and control the thoughts of its citizens.

The reason why the book is titled as such is because 451 degree Fahrenheit (°F), which is around 233 degrees Celsius (°C), is the temperature at which paper burns and catches fire!

So how do objects, like paper, burn and start a fire?

Let me introduce the fire triangle below, which identifies the 3 components required for us to ignite and start a fire.

Speaking of burning paper, this reminds me of a question from the 2020 Raffles Girls’ Primary School (RGPS) P6 SA2 Examination Paper where an experiment on the topic of Heat Energy is conducted.

This experiment involved wrapping a piece of paper around 2 different materials before the paper was heated. Miraculously, the piece of paper wrapped around each material took a different amount of time to start burning!

Let’s take a closer look at the question first before we discuss the reason behind the miraculous observation above.

## Question Analysis

Source: Raffles Girls’ Primary School – 2020 P6 SA2 Examination Paper [Q26]

For this question, we can see that there are 2 parts to this experiment.

As such, we will split our analysis of the question into 2 parts as well.

## Part 1: Heating Materials A And B With Wax And Thumbtacks Attached To Them

Let us first identify and highlight the keywords/key phrases in Part 1 of the question.

Source: Raffles Girls’ Primary School – 2020 P6 SA2 Examination Paper [Q26]

🟨 ASame amount of wax

🟦 B – Hold the identical thumbtacks

🟥 C – Materials are of identical length

🟩 D – The thumbtacks were placed at equal distance away from the heat source.

### Thought Process

For experiment-centric questions, I always advise my students to do the following:

“Find out what the changed variable and measured variable are from the experiment.”

Identifying the changed variable and measured variable from the experiment allows students to identify the aim of the experiment.

“How do we know what is the changed variable?”

What is the difference between the experiment set-ups? Could it be the amount of wax, the type of thumbtack, the length of the material or the distance from the heat source to the thumbtack?

From key phrases A to D as identified earlier:

🟨 ASame amount of wax

🟦 B – Hold the identical thumbtacks

🟥 C – Materials are of identical length

🟩 D – The thumbtacks were placed at equal distance away from the heat source.

We realise that all the 4 components above are kept the same and none of them can be the changed variable (difference).

Look at the question again. Do you realise that the type of material (materials A and B) is different? Thus, the changed variable is the type of material.

Source: Raffles Girls’ Primary School – 2020 P6 SA2 Examination Paper [Q26]

Here comes the next important question:

“How do we know what is the measured variable?”

So what did the experiment observe/record?

Since the question stated that Alison observed that the thumbtack on material B dropped off first, that means the measured variable is the time taken for the thumbtack on each material to drop off.

Source: Raffles Girls’ Primary School – 2020 P6 SA2 Examination Paper [Q26]

Since we are measuring the time taken for the thumbtacks on materials A and B to drop off, what is the aim of this experiment?

The aim should be to find out which material is a better/poorer conductor of heat.

💡 A summary for the experiment set-up so far:

Changed variable: Type of material

Measured variable: Time taken for the thumbtack on each material to drop off

Aim: To find out which material is a better/poorer conductor of heat

Since the thumbtack on material B dropped off first, here are some questions that should cross your mind:

1. How did the thumbtack on each material drop off?
2. Why did the thumbtack on material B drop off before the thumbtack on material A? Is material B a better/poorer conductor of heat as a result?

Now, let’s discuss these questions.

(a) How did the thumbtack on each material drop off?

From the first paragraph of the question, the thumbtack is attached to the material with the wax.

What do you think will happen to the wax when the material is placed over a heat source?

🔥 Key Concept: The material will conduct heat from the heat source to the wax, causing the wax to melt.

After the wax has melted, what will happen to the thumbtack? The thumbtack will drop off from the material.

(b) Why did the thumbtack on material B drop off before the thumbtack on material A? Is material B a better/poorer conductor of heat as a result?

Since we have understood from (a) that the thumbtack drops off the material due to the wax melting, now ask yourself this:

“If the thumbtack on material B dropped off first, did the wax on material B melt faster or slower?”

Faster.

“Since the wax on material B melted faster, did material B conduct heat from the heat source to the wax faster or slower?”

Faster.

“Since material B was able to conduct heat faster than material A, what can we conclude about the property of materials A and B?”

Material B is a better conductor of heat than material A.

## Part 2: Heating A Piece Of Paper That Is Wrapped Around Materials A And B

Source: Raffles Girls’ Primary School – 2020 P6 SA2 Examination Paper [Q26]

### Thought Process

After reading the second part of the question, here are some questions that you should have thought of:

1. How does the piece of paper wrapped around the materials burn?
2. Which material would cause the paper to burn first? Why?

Let us discuss these questions and the concepts behind them.

(a) How does the piece of paper wrapped around the materials burn?

Recall the fire triangle that we mentioned at the start of this blog post. For the paper to burn and start a fire, do you still remember what are the 3 components required?

We require heat, fuel, and oxygen. The paper is the fuel and oxygen is readily available in the surroundings. What about heat?

For the paper to catch fire and burn, the paper will also need to gain enough heat from the heat source to reach a certain temperature before it can start burning.

(b) Which material would cause the paper to burn first? Why?

At the end of our previous discussion in Part 1,we have established that material B is a better conductor of heat than material A.

Can we then say that since material B is a BETTER conductor of heat, heat from the flame of the heat source will be transferred to material B FASTER, which is then transferred from material B to the paper FASTER, causing the paper wrapped around material B to burn first?

Let’s check if the sequence of the heat transfer above makes sense.

Referring to the diagram in the question, notice that the heat source is placed below the papers that are wrapped around each material. As such, is the flame of the heat source in direct contact with the paper or materials A and B?

The paper.

As a result, is it accurate to say that heat from the flame of the heat source will be transferred to the material first, then from the material to the paper, as mentioned above?

No!

What should be the correct sequence of heat transfer then?

Heat from the flame of the heat source should be transferred to the paper first, then from the paper to material A and B.

Now that the sequence of heat transfer is verified, let us discuss the amount of heat transfer at the different stages.

From the flame of the heat source to the paper

Since the same flame is in direct contact with the piece of paper wrapped around each material, will the piece of paper wrapped around each material gain heat from the flame at the same rate or different rate?

Same rate.

For example, 5 units (5u) of heat is transferred from the flame to the paper wrapped around each material.

From the paper to the material

Recalling that material B is a better conductor of heat than material A, where:

🔥 Key Concept: A better conductor of heat (material B) will conduct heat faster.

A poorer conductor of heat (material A) will conduct heat slower.

Should heat be transferred away from the paper to material B at a faster rate or slower rate, as compared to material A?

Faster rate.

Thus, in total, which paper will gain more heat? Is it the paper wrapped around material A or material B?

The paper wrapped around material A.

For example, the paper wrapped around material A gained more heat in total (3u) while the paper wrapped around material B gained less heat in total (1u).

As such, which paper will first reach the minimum temperature required before it can burn?

The paper wrapped around material A.

Finally, which paper will burn first?

The paper wrapped around material A.

Since we need to choose the paper that burns first, it would be the paper wrapped with material A and we are left with options (1) and (2).

Why is the answer material A? As discussed previously, material A, being a poorer conductor of heat, conducted heat AWAY FROM the paper to itself slower, causing the paper to gain more heat in total and burn first.

Thus, the correct answer is option (2).