Combustion: tracking O₂ and CO₂
Observe gas exchange during candle combustion in a sealed chamber by tracking O₂ and CO₂ concentrations with FizziQ Connect.
Activity Summary
The student tracks O₂ and CO₂ concentrations in a sealed chamber during candle combustion, observing O₂ decrease and CO₂ increase until the candle extinguishes.
Introduction
Everyone has observed that a candle eventually goes out under an inverted glass, but do you really know why? Combustion of candle wax (paraffin) is a chemical reaction that consumes oxygen and produces carbon dioxide and water. By monitoring O₂ and CO₂ concentrations in a sealed chamber with precise sensors, you can watch this chemical transformation unfold in real time and understand exactly why the flame dies.
Learning Objectives
- Measure O₂ decrease and CO₂ increase during combustion
- Understand why the candle goes out in a sealed chamber
- Distinguish complete from incomplete combustion
- Connect sensor data to the chemical equation of combustion
Instruments and sensors
Scientific instruments
- CO₂ meter
- O₂ meter
Sensors
- SCD40 sensor
- O₂ sensor
FizziQ Features
- Data export (Share) — Export the recorded O₂ and CO₂ concentration curves to a spreadsheet to analyze the combustion phases.
Required Materials
- - Smartphone or tablet with FizziQ Connect - SCD40 and O₂ sensors - M5 Stack module and multiport hub - Sealed chamber with lid - Tea light candles - Note: the protocol remains adaptable to any comparable gas-sensing data acquisition tool.
Experimental Protocol
Connect the SCD40 and O₂ sensors to the M5 Stack via the multiport hub (red I2C port).
If needed, reset the sensors by clicking RESET and confirming. Verify that 4 quantities are displayed: CO₂, temperature, humidity, and O₂.
Open FizziQ Connect on your smartphone and connect to the M5 Stack via Bluetooth. Select 'External sensors'.
Mount the two sensors inside the sealed chamber, positioning them away from the candle flame (heat and soot can damage sensors).
Fix a tea light candle to the bottom of the chamber. Verify that the lid seals properly.
Before lighting, note the initial concentrations in ambient air: O₂ (about 20.9%) and CO₂ (about 400-800 ppm). These are your reference values.
In FizziQ Connect, set the measurement interval to 250 ms from the M5 Stack menu. The fast rate captures the rapid changes during combustion.
Light the candle, then quickly close the chamber lid.
Watch the concentrations evolve in real time on the M5 Stack screen or in FizziQ Connect. The candle will eventually go out by itself when O₂ drops too low.
Wait 1-2 minutes after extinction for concentrations to stabilize, then stop recording.
Export the data in FizziQ Connect: select 'UART Sensor' for O₂, then 'Concentration' for CO₂. Use the 'Share' function to export to a spreadsheet if needed.
Analyze the graphs: identify the phases (before lighting, active combustion, extinction, stabilization) and note the initial and final values for O₂ and CO₂.
Expected Results
O₂ concentration decreases from 20.9% to about 15-16% at candle extinction. CO₂ increases from 400-800 ppm to about 25,000 ppm. The candle burns for 1-5 minutes depending on the chamber size. Temperature and humidity also increase. The O₂ decrease and CO₂ increase are clearly correlated.
Scientific Questions
- What are the products of complete paraffin combustion?
- Why does the flame have a blue base and a yellow top?
- What would happen if you used a larger chamber?
- How does the sealed chamber experiment prove that O₂ is consumed during combustion?
- What is the difference between a physical change and a chemical change?
Scientific Background
Combustion is a **chemical transformation**: it converts reactants (paraffin and oxygen) into products (carbon dioxide and water). The chemical nature of the substances changes.
Paraffin is the **fuel**: it is the substance that burns. Atmospheric oxygen is the **oxidizer**: it is the substance that sustains the burning. Both are necessary; remove either one and the flame goes out.
The reaction is **exothermic**: it releases energy as heat and light. This energy heats the wax, melts it, vaporizes it, and sustains the flame.
In the sealed chamber, the amount of air is limited. As combustion consumes O₂, its concentration drops. The flame goes out at about 15-16% O₂, not at 0% — the remaining O₂ is insufficient to sustain combustion.
The CO₂ produced accumulates in the chamber because it cannot escape. Its concentration rises from a few hundred ppm (ambient level) to 25,000+ ppm.
Candle wax is **paraffin**, a mixture of long-chain alkanes. It is modeled by the formula **C₂₅H₅₂**. The balanced equation is: **C₂₅H₅₂ + 38 O₂ → 25 CO₂ + 26 H₂O**.
If combustion is **incomplete** (insufficient O₂), it also produces carbon monoxide (CO, a toxic gas) and soot particles (unburned carbon). The yellow flame color is due to incandescent soot.
The bottom of the flame, blue in color, is the zone of complete combustion where O₂ supply is adequate. The yellow and luminous upper part contains soot particles that glow from the heat.
Extensions
- Vary the chamber size and measure how it affects burn time
- Investigate the effect of initial O₂ concentration by flushing with O₂-enriched air
- Use the data to calculate the mass of paraffin consumed
- Compare a short fat candle with a tall thin candle in the same chamber
Frequently Asked Questions
The candle goes out immediately after sealing.
The chamber may be too small. Use a larger container. Also ensure the seal is not so tight that it creates a vacuum when you close it.
The CO₂ sensor reads very high values right from the start.
Residual CO₂ from a previous experiment may remain. Ventilate the chamber thoroughly before starting.
Why does O₂ not drop to zero?
The flame cannot sustain combustion below about 15-16% O₂. The remaining O₂ stays in the chamber.
Is it safe to do this experiment in class?
Yes, with normal candle safety precautions. The sealed chamber contains the flame and extinguishes it automatically. Always have a fire extinguisher accessible.
Detailed Description
The student places a tea light candle in a sealed chamber equipped with FizziQ Connect O₂ and SCD40 sensors. They record the initial concentrations, light the candle, seal the chamber, and track the evolution of O₂ (decreasing) and CO₂ (increasing) until the candle goes out. FizziQ Connect displays the O₂ and CO₂ curves in real time via the M5 Stack module and lets students export the data to a spreadsheet for graphical analysis of the combustion phases.
Ready to start?
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