Investigating Science 11–12 · Year 11

The candle in a closed container: separating observation from inference

Module 1: Cause and Effect – Observing

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

What is seen (the flame goes out, the water rises) is an observation; “the oxygen was used up” is an inference, and a designed test shows the rise is caused mainly by heating and cooling of the trapped gas.

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

naked flame and hot wax
hot glass
glass breakage
loose hair and sleeves near the flame

Controls

safety glasses
tie back hair and roll up sleeves
heatproof mat and fire blanket at hand
let jars and wax cool before handling

Note

Heat or hot water is used: follow the NSW Department of Education Chemical Safety in Schools (CSIS) package, Section 1, and record a RiskAssess risk assessment before the lesson.

What you need

three identical straight-sided glass jars of about 500 mL, each marked in 25 mL steps by pouring in measured water
tealight candles, fixed to a small dish or floated on a cork disc
a wide shallow dish holding 2 cm of water coloured with food colouring
matches or a long-nosed lighter, heatproof mat, ruler, stopwatch, phone for video

How to do it

Calibrate a jar by adding water from a measuring cylinder in 25 mL steps and marking each level; record the total volume.
Stand one lit candle in the dish, lower the jar over it in one smooth motion and start filming.
Record qualitative observations (flame shrinking, soot, fog on the glass, bubbles escaping at the rim) and quantitative ones (time until the flame goes out, water level at that moment from the video, final level after 2 minutes).
Repeat three times and average the final rise as a fraction of the jar volume.
Repeat with three candles under the same jar, and with one candle while lowering the jar slowly, three times each.
In a two-column table separate every statement into observation or inference.
Compare the results with the largest volume change the combustion chemistry could cause, and decide which explanation the data support.

What you should see

The flame goes out within seconds to tens of seconds; most of the water rise happens after the flame goes out, as the hot gas cools, and bubbles escape at the rim while the jar is lowered. Paraffin burns as C₂₅H₅₂ + 38 O₂ → 25 CO₂ + 26 H₂O, so even if all 20.95 % oxygen were consumed and the water vapour condensed, the gas volume would fall by only 20.95 % × (1 − 25/38) = 7.2 %, and less because the flame goes out first. Vera, Rivera and Núñez (2011) found in sealed-volume runs that the final gas volume is nearly equal to the initial volume, and trace the observed rise to thermal expansion, trapped hot air and bubbling. The heat explanation predicts a larger rise with three candles and with slow lowering; the oxygen explanation predicts the same rise, so the comparison is the discriminating test the learner reports.

What changes

What you change: number of candles (1 or 3) and speed of lowering the jar
What you measure: final water rise as a fraction of the jar volume
What you keep the same: same jar and marked scale
same water depth in the dish
same candle type
level read 2 minutes after the flame goes out
three trials per condition

Common misconceptions

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

The water rises because the oxygen is used up.
The water is sucked up by a vacuum, when it is pushed up by the outside air pressure.
The flame goes out only when all of the oxygen is gone.

Curriculum references

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

Investigating Science Stage 6 Syllabus (2017), NESA; currentINS11-8 INS11/12-1 INS11/12-3 INS11/12-4
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

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