Science 7–10 · Year 8

What a burning candle makes: testing for water and carbon dioxide

Change

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

Burning is a chemical reaction with oxygen that produces new substances, and the products of a hydrocarbon flame can be caught and identified.

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 tubing and hair
hot funnel
cobalt chloride paper is harmful and cobalt(II) chloride is classed as a carcinogen by inhalation; anhydrous copper(II) sulfate is harmful and an eye irritant
limewater is an irritant

Controls

adult lights the candle and runs the pump
anhydrous copper(II) sulfate preferred; if cobalt chloride paper is used it is handled with tweezers and binned as chemical waste
eye protection
long hair tied back

Note

NSW Department of Education, Chemical Safety in Schools (CSIS) 2021 Technical Update: Section 1.7 (risk assessment before use) and Volume 2 Appendix D (generic assessment advice, user codes and DoE categories). Complete a RiskAssess or CSIS site-specific risk assessment before the lesson.

What you need

tea-light candle on a heat-resistant mat, matches (adult)
glass funnel clamped upside down over the flame, connected by tubing to a U-tube in iced water and then to a boiling tube of limewater (10 mL)
filter pump or hand suction pump to draw the gases through, anhydrous copper(II) sulfate (a spatula tip on a watch glass) or blue cobalt chloride paper, dry beaker, thermometer

How to do it

Teacher assembles the train: inverted funnel, tubing, cooled U-tube, limewater tube, pump.
Light the candle and start the pump so the hot gases are drawn through the apparatus for 3 to 5 minutes.
Learners record what collects in the cooled U-tube and what happens to the limewater.
Test the liquid in the U-tube with a few grains of white anhydrous copper(II) sulfate (or blue cobalt chloride paper); hold a dry cold beaker briefly above a second candle and note the mist.
Control: run the pump for the same time with the candle unlit and compare the U-tube and the limewater.
Write the word equation for the complete combustion of the wax (a hydrocarbon).

What you should see

Colourless liquid collects in the cooled tube and turns white anhydrous copper(II) sulfate blue (or blue cobalt chloride paper pink), so it is water; the limewater turns cloudy, so carbon dioxide is present. The cold beaker held over a flame mists with condensation within seconds and a dark soot mark shows incomplete combustion where oxygen is short. The control run collects some liquid of its own: room air carries water vapour, and an ice-cooled tube condenses part of it out of every litre the pump draws through, so the comparison is how much and how fast rather than liquid against none. The learner knows it worked when both tests are positive with the candle lit and the control collects far less liquid in the U-tube and, as the RSC notes, takes much longer to show any change in the limewater.

What changes

This activity lists no variables to change, measure and keep the same.

Common misconceptions

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

The candle wax just disappears into heat and light.
Water from the flame comes from the wick being wet.
Carbon dioxide from a candle is the same as the air it started with.

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)SC4-CHG-01 SC4-WS-01
Australian Curriculum v9AC9S8U07AC9S8I03

Sources

The pages the author read to write this activity.

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