Science 7–10 · Year 9

Weighing a flame: how much carbon dioxide a burning candle adds to the air

Science understanding: Earth and space sciences (ACARA Year 9 carbon cycle and conservation of mass; NSW Stage 5 focus areas Environmental sustainability and Reactions, the latter through its content group Law of conservation of mass)

PracticalMedium risk

School laboratory, not for home

In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

The idea

Combustion moves carbon that was stored for millions of years in petroleum into the air as carbon dioxide, and because oxygen from the air joins it, the carbon dioxide produced weighs about three times as much as the wax burned.

Safety card

Medium riskA teacher supervises

Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

Hazards

open flame near hair, sleeves and paper
hot liquid wax and a hot metal cup
limewater irritates the eyes

Controls

an adult lights and extinguishes the candle; long hair tied back and sleeves clear
the tea-light stays on the heat-resistant mat and is not moved while lit
eye protection while handling limewater
a bucket of water or fire blanket within reach

Note

Write the risk assessment before the lesson under CSIS 1.7 (Risk Assessment, a pre-requisite for risk control) of the NSW Department of Education Chemical Safety in Schools package (Section 1, 2021 Technical Update), or through RiskAssess; keep the Safety Data Sheet for every chemical with the class register.

What you need

3 tea-light candles in their metal cups
an electronic balance reading to 0.01 g, a heat-resistant mat and a draught shield (three sides of a cardboard box lined with foil)
a dry, cool 250 mL glass beaker for the water test; for the carbon dioxide test, as in the RSC practical, a glass funnel clamped over the flame and joined by tubing to a boiling tube holding about 20 mL of fresh limewater (saturated calcium hydroxide solution), with a filter pump drawing a gentle stream of gas through it
matches or a long lighter (adult), a stop clock and eye protection

How to do it

Weigh a tea-light in its cup to 0.01 g and record the mass.
An adult lights it on the heat-resistant mat inside the draught shield; start the clock.
After 1 minute hold the dry beaker upside down over the flame for 5 seconds and look for mist inside (water).
After 10 minutes of burning, blow the candle out, wait 1 minute and weigh it again.
Calculate the mass of wax burned, then the carbon dioxide and water produced using 3.12 g of carbon dioxide and 1.33 g of water per gram of wax.
Repeat with the other two tea-lights and find the mean burning rate in grams per minute and the carbon dioxide produced per minute.
Carbon dioxide test, run by the adult as in the RSC practical: relight one tea-light under the clamped funnel, turn on the filter pump so a gentle stream of gas is drawn through the limewater, and leave it for a few minutes until the limewater turns milky; then run the pump for the same time with no tea-light, as the control, and note how much longer any change takes.

What you should see

Mist forms inside the cold beaker (water), and the limewater that the pump draws the combustion gases through turns milky within a few minutes (carbon dioxide) while in the control run without a flame it changes far more slowly; these are the two products the RSC practical identifies. A beaker held briefly over the flame collects too little carbon dioxide for a reliable limewater test, which is why the gases are pumped through it. Taking paraffin wax as alkanes from C20H42 to C40H82 (an assumption stated here), 1.00 g of wax is 85.0 to 85.3 percent carbon by mass and burns to 3.12 g of carbon dioxide (3.115 to 3.126 g across that range) and 1.33 g of water, using 3.45 g of oxygen from the air: 1.00 + 3.45 = 3.12 + 1.33 g, so mass is conserved. So if a tea-light loses 0.50 g in 10 minutes, it has added 1.56 g of carbon dioxide, holding 0.43 g of carbon, to the air. The learner knows it worked when the three burning rates agree within 20 percent and both product tests are positive.

What changes

What you change: burning time
What you measure: mass of wax burned (and so carbon dioxide produced)
What you keep the same: same brand of tea-light
draught shield
weighing after the same 1 minute cooling

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Burning destroys matter; the wax's atoms leave as invisible gases that weigh more than the wax.
A candle only gives out light and heat; it also releases carbon dioxide and water vapour.
The carbon dioxide from fuel was already in the air; its carbon had been locked underground in petroleum.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Science 7-10 Syllabus (2023), NSW Education Standards Authority (implemented from 2026; codes read at curriculum.nsw.edu.au on 22 and 23 September 2026)SC5-ENV-01 SC5-RXN-01 SC5-WS-01 SC5-WS-04
Australian Curriculum v9AC9S9U03AC9S9U07AC9S9I03AC9S9I04

Sources

The pages the author read to write this activity.

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