Three candles

Study the convection of hot gases and CO₂ stratification under a bell jar by placing three SCD40 sensors at different heights.

When a candle burns under a glass bell jar, the hot combustion gases rise by convection. But where does the carbon dioxide go? Does it spread evenly or settle to the bottom because it is denser than air? By placing three CO₂ sensors at different heights under the bell jar, you can track the CO₂ concentration and temperature at each level in real time and discover that convection initially carries CO₂ upward — despite it being heavier than air — because the combustion gases are hot.

Visão geral da atividade:

The student tracks CO₂ and temperature at three heights under a bell jar with a burning candle, discovering that hot combustion gases rise by convection before the denser CO₂ eventually sinks.

Nível:

Middle and high school

Serge Paupy

Autor:

Duração (minutos):

30

O que os alunos farão:

- Observe the convective transport of hot combustion gases
- Measure CO₂ concentration at three heights simultaneously
- Understand why hot CO₂ rises despite being denser than air at the same temperature
- Identify the chronological sequence of CO₂ arrival at different heights
- Connect the observations to atmospheric convection and gas behavior

Conceitos científicos:

- Gas convection
- Buoyancy and density
- Hot gas versus cold gas behavior
- CO₂ properties (molar mass 44 g/mol vs air 29 g/mol)
- Combustion products
- Thermal stratification

Sensores:

- SCD40 sensor (CO₂ in ppm, temperature) × 3

Materiais necessários:

- Smartphone or tablet with FizziQ Connect
- Three M5 Stack modules with SCD40 sensors
- A large glass bell jar
- A candle
- A vertical support (lab stand)
- Optional: 3 smartphones for simultaneous recording

Procedimento experimental:

Prepare three M5 Stack modules, each equipped with an SCD40 sensor. Turn them on and verify each displays CO₂ concentration and temperature.
Mount the three modules on a vertical support (lab stand, camera tripod) at three distinct heights: bottom (a few cm from the base), middle (halfway up), and top (near the top of the bell jar).
Place the candle at the center of the base, next to the support. The flame should be below the middle sensor.
Connect each M5 Stack to FizziQ Connect (you can use three smartphones/tablets, or record sequentially from each module).
Note the initial CO₂ and temperature values for all three sensors in ambient air. They should be close to each other.
Light the candle, then carefully place the bell jar over the entire setup (candle + support + sensors).
Start recording simultaneously on all three modules (or use the REC mode on each M5 Stack).
Record for 60 to 80 seconds, until the candle goes out from lack of O₂.
Stop the recording and export the data from each module in FizziQ Connect.
Superimpose the three CO₂(t) curves and the three temperature(t) curves on the same graph to compare the chronological arrival of CO₂ and heat at each height.

Resultados esperados:

The upper sensor detects CO₂ first (up to 18,000 ppm) and the highest temperature (up to 26°C), followed by the middle sensor (up to 14,000 ppm, 25°C), then the bottom (up to 10,000 ppm, 24°C). After candle extinction, CO₂ gradually redistributes and concentrations begin to equalize. The chronological sequence clearly demonstrates upward convective transport.

Questões científicas:

- Why does the upper sensor detect CO₂ first even though CO₂ is denser than air?
- What happens to the CO₂ after it cools at the top of the bell jar?
- How would the results change if the candle were at the top instead of the bottom?
- Why does the candle go out after about a minute under the bell jar?
- What role does convection play in the Earth's atmosphere and weather?
- How is this experiment related to the principle of a hot air balloon?

Explicações científicas:

Convection is a heat transfer mode specific to fluids (liquids and gases). When a gas is heated, its density decreases and it rises through buoyancy. Cooler, denser gas sinks to replace it.

The hot gases produced by candle combustion (CO₂, H₂O, warm N₂) rise to the top of the bell jar. This is why the upper sensor detects CO₂ first and shows the highest concentration and temperature.

Pure CO₂ is denser than air (M_CO₂ = 44 g/mol versus M_air ≈ 29 g/mol). However, in this experiment, the CO₂ is produced at high temperature in the flame and is therefore much less dense than the surrounding cool air.

It is only after cooling on contact with the walls and top of the bell jar that the CO₂ slowly descends. This is why the lower sensors detect CO₂ later and at lower concentrations.

This phenomenon underlies many natural processes: atmospheric convection currents, wind circulation, cloud formation, and even ocean currents. Hot air rises and cold air sinks — the engine of weather and climate.

Atividades de extensão:

Perguntas frequentes:

Q: I only have one M5 Stack module. Can I still do the experiment?
R: You can repeat the experiment three times, placing the sensor at a different height each time. The results will be less precise because conditions vary between trials, but the trend should be visible.

Q: The bell jar is too small for three sensors.
R: Use a large glass vase, a plastic container, or even a large inverted plastic bottle with the bottom cut off. The key is that it must be transparent enough to see the candle.

Q: All three sensors show the same CO₂ at the end.
R: This is expected. After the candle goes out, convection stops and the gases gradually mix. Given enough time, all three sensors will read the same concentration.

Q: The candle goes out almost immediately.
R: The bell jar may be too small. Use a larger container to give more air (and O₂) for the candle to burn longer.