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The Chemistry of What We Breathe

Combustion reactions, sensor physics and the molecular basis of air quality

Grades 9–10 45 min experiment live IESH data

Students write and balance combustion equations, learn how MQ sensors detect gases through electrical resistance changes, and connect chemistry theory to live sensor readings from the device.

Learning objectives

  • Write and balance chemical equations for complete and incomplete combustion of carbon fuels.
  • Explain how metal oxide gas sensors detect gases through changes in electrical resistance.
  • Interpret the voltage divider circuit used to protect the MCP3208 ADC from MQ sensor output.
  • Relate chemical combustion theory to real sensor readings and Nepal's fuel transition.

Background

Combustion is a rapid oxidation reaction between a fuel and oxygen that releases energy as heat and light. For a simple carbon fuel in complete combustion: C + O₂ → CO₂ + energy. For incomplete combustion (insufficient oxygen): 2C + O₂ → 2CO + energy. In a wood fire, both reactions occur — the ratio depends on oxygen availability, fire temperature and fuel moisture content. A well-stoked fire with good airflow produces mostly CO₂; a smouldering fire with wet wood produces CO and soot.

MQ gas sensors work through a semiconductor principle. The sensing element is a ceramic tube coated with a metal oxide — typically tin dioxide (SnO₂). In clean air, oxygen molecules adsorb onto the surface of the SnO₂, creating a potential barrier that limits electron flow — the resistance is high. When a reducing gas like CO is present, it reacts with the adsorbed oxygen: CO + O⁻(ads) → CO₂ + e⁻. This releases an electron and lowers the resistance. The sensor circuit converts this resistance change to a voltage change, which the ADC reads as a higher numerical value.

Our device uses a voltage divider before the ADC to protect it: two 4.7kΩ resistors halve the MQ output voltage from 0–5V to 0–2.5V, which is within the MCP3208's 3.3V reference range. This means the raw ADC values must be multiplied by 2 to get the true sensor output voltage — which is why the dashboard shows a "sensor voltage" field alongside the raw ADC reading.

Procedure

  1. Read current MQ-7 and MQ-135 raw ADC values and their displayed sensor voltages.
  2. Write the balanced equation for complete combustion of methane (CH₄ + O₂ → ?).
  3. Write the balanced equation for incomplete combustion of methane (CH₄ + insufficient O₂ → CO + H₂O). Balance it.
  4. Now write the complete and incomplete combustion equations for ethanol (C₂H₅OH), which is used in some cleaner cookstoves.
  5. The MQ-7 reads higher when CO concentration increases. Draw a circuit diagram showing: MQ sensor resistance (RS) + load resistor (RL, 1kΩ) + 5V supply + ADC input. Label where the ADC reads its voltage.
  6. Calculate: today's MQ-7 raw ADC = ___. Sensor voltage (ADC voltage × 2) = ___V. Sensor resistance RS = 1000 × (5V ÷ sensor_voltage − 1) = ___ Ω.

Discussion

  1. Why does a smouldering, oxygen-starved fire produce more CO than a hot, well-aerated flame? Relate this to the chemistry of combustion.
  2. Nepal is transitioning from wood cooking fires to LPG (liquefied petroleum gas, primarily propane C₃H₈). Write the balanced complete combustion equation for propane. Does this produce CO? Under what conditions might it?
  3. If a sensor reads higher when resistance is lower, what would happen to the MQ-7 reading inside a room with a running car engine? Why is this dangerous?

Worksheet

1. Balance the following equation for incomplete combustion of butane (used in camping stoves): C₄H₁₀ + O₂ → CO + H₂O [4 marks]

Answer guide (for teachers)

2C₄H₁₀ + 9O₂ → 8CO + 10H₂O. Check balancing: C: 8=8; H: 20=20; O: 18=18. Award marks for correct balancing even if student uses different integer multiples.

2. Explain the chain of events from a rise in CO concentration to a higher number on the MQ-7 ADC reading. Use the terms: reducing gas, metal oxide, resistance, voltage. [5 marks]

Answer guide (for teachers)

CO (a reducing gas) reacts with adsorbed oxygen on the tin oxide surface of the sensor, releasing electrons and reducing the electrical resistance of the sensor. Lower resistance means a larger share of the supply voltage falls across the fixed load resistor. The ADC reads this higher voltage as a higher digital number.

3. Today's MQ-7 ADC reading is 350. Calculate (a) the ADC voltage (350/4095 × 3.3V), (b) the sensor voltage (× 2 for divider), (c) the sensor resistance RS = 1000 × (5/Vsensor − 1). [6 marks]

Answer guide (for teachers)

(a) 350/4095 × 3.3 = 0.282V. (b) 0.282 × 2 = 0.564V. (c) RS = 1000 × (5/0.564 − 1) = 1000 × 7.87 = 7,870 Ω. Accept ±5% for rounding.

Vocabulary

Incomplete Combustion
Combustion with insufficient oxygen, producing carbon monoxide (CO) and/or soot rather than only CO₂ and water.
Metal Oxide Semiconductor Sensor
A gas sensor using a metal oxide (typically SnO₂) whose electrical resistance changes when target gases are present.
Adsorption
The adhesion of molecules from a gas or liquid onto a solid surface (distinct from absorption, which is bulk uptake).
Voltage Divider
A circuit of two resistors in series that produces an output voltage proportional to but smaller than the input voltage.

Extension

Research the electrochemical CO sensor used in professional CO alarms. Compare its operating principle to the MQ-7 metal oxide sensor. List advantages and disadvantages of each type. Which would be more appropriate for a medical-grade air quality monitor, and why?

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