Junior Science | Combustion, Oxidation and Conservation of Mass: Tutor Prep Guide (Fire Triangle, Magnesium Gain, Four Errors)

2026-11-13

Tutor lesson-prep guides · Junior Science

Every student has burnt a candle, yet few can explain why a beaker over the flame puts it out, or how a candle can lose mass while iron gains mass as it rusts. This lesson does three things: split burning into three conditions, turn oxygen into grams, and separate closed from open systems.

Objectives — Students can: (1) name the three sides of the fire triangle and use one side to explain three ways of extinguishing a fire; (2) write the complete combustion equation for methane, CH4 + 2O2 giving CO2 + 2H2O, and calculate the oxygen mass needed; (3) explain that incomplete combustion produces carbon monoxide and why it is more dangerous than carbon dioxide; (4) distinguish closed from open systems and use 2.4 g of magnesium becoming 4.0 g of oxide to show the extra mass comes from air; (5) state that rusting needs both oxygen and water.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Elements and compounds	Is magnesium oxide an element or a compound?	A compound of two elements
Formula mass	Mg is 24 and O is 16, so what is the formula mass of MgO?	40
Air composition	What fraction of air is oxygen?	About 21 percent by volume

02 Lesson flow (45 minutes)

Time	Stage	Tutor does	Student does
0-3 min	Prior knowledge check	Ask three quick questions on oxides, formula mass and air; note gaps	Answer orally, write misses on the first line
3-12 min	Fire triangle demos	Burn paper, then cap a beaker over it, then use a wet cloth, removing one side each time	Name the missing side and sketch the triangle
12-22 min	Complete and incomplete burning	Write both equations; calculate 64 g of oxygen for 16 g of methane and 9.5 times the air volume	Copy one worked example with masses on both sides
22-32 min	Conservation of mass	Burn 2.4 g of magnesium in a closed flask (4.0 g), then burn a candle in the open and explain the loss	Draw one arrow per system showing where mass went
32-40 min	Independent practice	Hand out the four questions; circulate and ask, do not answer	Compare answers and name the wrong step
40-45 min	Exit check	Five oral questions; anyone who misses stays for catch-up	Answer orally; released only when all five are correct

03 Teaching script (read it out)

Scene 1: The fire triangle: remove one side and it stops

Run three demonstrations, each removing one side only. Burn paper: fuel, oxygen and heat are all present, so it keeps burning. Cap a beaker over the flame: oxygen runs out, the flame shrinks and dies, and droplets appear on the glass because hydrogen burns to water. Cover it with a wet cloth: water absorbs heat, the temperature falls below the ignition point, and it stops.

Draw a triangle labelled fuel, oxygen, ignition temperature. Every method maps to one side: sand or a fire blanket cuts oxygen, water cools, closing the gas valve removes fuel. Ask students to give three methods and name the side each one cuts.

Close with the honest line: putting out a fire is not magic, it is removing one side of the triangle, which is why an electrical fire must never be hit with water.

Scene 2: Complete and incomplete burning: the oxygen supply decides

Write two equations. Complete: CH4 + 2O2 giving CO2 + 2H2O. Check the masses: 16 g of methane with 64 g of oxygen gives 44 g of carbon dioxide plus 36 g of water, and both sides total 80 g.

When oxygen is short: 2CH4 + 3O2 giving 2CO + 4H2O. Carbon monoxide binds to haemoglobin far more strongly than oxygen does and occupies the carrying sites, so dizziness and collapse can come quickly. Carbon dioxide does not do that; its danger is displacing air. Make that contrast explicit, or students treat the two gases as one.

Then the volume: oxygen is only 21 percent of air, so 1 L of methane needs 2 L of oxygen, which is about 9.5 L of air. That is exactly why burning charcoal in a sealed room produces carbon monoxide: the oxygen runs out.

Scene 3: Conservation of mass: ask about the system boundary first

Put 2.4 g of magnesium ribbon in a closed flask, heat it, let it cool, and weigh: 4.0 g of white oxide. From 2Mg + O2 giving 2MgO, 48 g of magnesium takes 32 g of oxygen to give 80 g of oxide; 2.4 g of magnesium uses 1.6 g of oxygen, and 2.4 plus 1.6 is 4.0.

Then burn a candle in an open cup and the mass drops. Students will object that mass should be conserved. The answer is that carbon dioxide and water vapour leave into the air; weigh the whole thing under a bell jar and the total does not change.

Finish with rust: 4Fe + 3O2 giving 2Fe2O3, where 5.0 g of steel wool gives about 7.1 g of rust, a gain of 2.1 g that is entirely oxygen. Ask students to repeat one sentence back: ask whether the system is closed before asking whether the mass changed.

04 Question types and worked steps

Type	Example	Steps	Answer
Triangle mapping	A CO2 extinguisher puts out a fire. Which side does it cut?	Reason from what the device does: it excludes air	Oxygen (and it cools as well)
Equation and mass calculation	What mass of magnesium oxide forms from 2.4 g of magnesium?	Balance first, then use the 24 to 40 ratio	2.4 times 40 divided by 24, giving 4.0 g
Volume conversion	What mass of oxygen does 16 g of methane need?	Find moles first, then multiply by 32	2 mol of oxygen, which is 64 g
System reasoning	A candle loses mass in the open. Does that break conservation?	Define the boundary, then track each product	No: CO2 and water vapour escape into the air

Sanity check (teach this): Sanity check to teach every time: after reaching 4.0 g of oxide, ask where the extra 1.6 g came from, and note that the gain can only come from a gas, so 2.4 g can never become 24 g. A student who writes 24 g has treated 40 as the mass of one magnesium atom; a student who writes 1.6 g has reported the oxygen instead of the product. Since oxygen is only 21 percent of air, doubling the mass by burning is never realistic in a school lab.

05 Independent practice (3 questions)

Question	Answer	What to watch
1. A wet cloth stops a flame. Which side of the triangle is cut?	Ignition temperature: the water absorbs heat	Can they say cooling rather than just wetting?
2. What mass of oxide forms when 2.4 g of magnesium burns fully?	4.0 g, using 1.6 g of oxygen	Did they use the 24 to 40 ratio?
3. What mass of oxygen does 16 g of methane need for complete combustion?	64 g, that is 2 mol	Did they treat methane as 1 mol and double it?
4. What mass does 5.0 g of steel wool reach when fully rusted?	About 7.1 g, gaining 2.1 g of oxygen	Can they name oxygen as the source of the gain?

06 Common mistakes and what to say

Mistake	Student writes	Say this
Assuming every fire has a flame	Burning iron has sparks, so iron must have a flame	A flame is burning gas. Iron glows red hot and throws off sparks, but that is not a flame, and you lose marks for writing it in observations.
Applying conservation blindly to open systems	Mass after burning must equal the mass before	Ask whether the system is closed first. In an open system gas escapes, so the reading falls; to prove conservation you must cover it and weigh again.
Treating rust as iron changing by itself	Iron just goes bad if you leave it	You need oxygen and water together. That is why dry sealed iron does not rust, and why paint or grease works by keeping both out.
Saying carbon dioxide is poisonous	CO2 is toxic, so extinguishers are dangerous	CO2 is not a poison, it suffocates by displacing air. The gas that binds haemoglobin and poisons you is CO. Keep the two apart.

07 Homework (paper, section and question numbers)

Science chapter index — Open the states of matter chapter and compare gas volume conversions
Burning observation record — With an adult, burn a candle and record flame colour, droplets on a cold glass and how long it takes to go out under a cup
Calculation set — Do questions 1 to 4 in the exercise set, writing the ratio used in each

08 Exit check (last 5 minutes)

Question	Answer	Decision
What are the three sides of the fire triangle?	Fuel, oxygen and ignition temperature	Fuel, air and fire needs correcting to ignition temperature
What mass does 2.4 g of magnesium give?	4.0 g of oxide	2.4 g means the oxygen was left out
Why is CO more dangerous than CO2?	It binds haemoglobin and blocks oxygen transport	A bare poisonous needs the mechanism
How does the mass change when steel wool burns in a closed flask?	It increases, because oxygen joins the metal	Unchanged prompts the question of where the oxygen went
Roughly what fraction of air is oxygen?	About 21 percent, one fifth	78 percent is nitrogen, not oxygen

09 Tutor FAQ

We have no closed combustion flask. How can we still show conservation of mass?

Use a heatproof bottle with a rubber stopper, or a sealed bag with enough air to burn a short ribbon. An easier route is steel wool with water and a splash of vinegar in a sealed bottle: it rusts over two days and clearly gains mass.

Students mix up carbon monoxide and carbon dioxide. Any memory hook?

One letter, one danger: CO grabs haemoglobin; CO2 crowds out air. Then give the sentence they can reuse: CO is toxic, CO2 is suffocating.

Must the calculation use moles?

Not in junior secondary. Formula-mass ratios such as 24 g of magnesium to 40 g of oxide are enough. What matters is that they always write mass divided by formula mass, then multiply by the target formula mass.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.