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C2.7 · Design an inquiry comparing complete and incomplete combustion

Learn to design an inquiry comparing complete and incomplete combustion through clear examples and targeted practice.

Ontario Grade 11 Chemistry

Chemical Reactions

Plan a fair, safe comparison and decide what evidence can support your conclusion

A burner flame can look different when its air opening is adjusted. A flame with less air may produce visible soot, while one with more air may burn with little visible soot. These observations can help guide an inquiry, but appearance alone does not identify every product. In this lesson, you will plan a safe, fair comparison of complete and incomplete combustion. The plan is proposed; it is not a report of measurements already collected.

What you will learn

Describe the difference between complete and incomplete combustion using the products formed.
Plan an inquiry that changes oxygen supply while keeping other conditions as similar as possible.
Choose observations and measurements that can help compare the two types of combustion.
Identify safety steps and limits in the evidence before drawing a conclusion.

1. From visible changes to a particle model

Combustion is a chemical reaction in which a substance reacts with oxygen and releases energy, often as heat and light. In a classroom inquiry, the useful starting point is what can be observed: a flame, water vapour, or black solid soot. Soot is made mainly of tiny carbon particles.

A fuel contains atoms that can form new substances during combustion. For a simple course-level model, propane contains carbon and hydrogen atoms. Oxygen from the air can react with these atoms. The atoms are rearranged; they are not created or destroyed.

With enough oxygen available for the fuel to react, complete combustion of propane forms carbon dioxide and water. With a limited oxygen supply, incomplete combustion can form carbon monoxide and water, carbon particles and water, or a mixture of products. Carbon monoxide is a poisonous gas that cannot be detected by sight or smell. Never use flame appearance to decide that air is safe.

\text{fuel} + \text{oxygen} \rightarrow \text{combustion products}

Complete combustion forms carbon dioxide and water in the model used here.
Incomplete combustion can form carbon monoxide and/or carbon particles, as well as water.
Visible soot is evidence that carbon particles formed; a clean-looking flame does not prove that no carbon monoxide formed.

2. Use equations to make predictions

A chemical equation represents a reaction with formulas. The small numbers in a formula are subscripts. They show how many atoms of each element are in a particle. A coefficient is a number placed before a formula. It counts particles or amounts of that substance. When balancing an equation, change coefficients, not subscripts, so the formulas remain correct.

These balanced equations show possible outcomes for propane. State symbols give the physical state: gas is shown as (g), and solid is shown as (s). Water is written as gas here because it is represented as water vapour in the hot combustion products. The incomplete-combustion equations are possible models, not a claim that only one product forms in every flame.

Balancing provides a prediction about the atoms in the represented reaction. It does not by itself show which products an experiment actually made. That requires suitable evidence.

\mathrm{C_3H_8(g) + 5O_2(g) \rightarrow 3CO_2(g) + 4H_2O(g)}

The same fuel can have different products when the oxygen supply differs.
A balanced equation has the same number of each type of atom on both sides.
Equations predict products to test for; they do not replace observations.

3. Plan a fair and safe comparison

An inquiry is a planned investigation that uses evidence to answer a question. A useful question is: How does changing the air supply to a burner affect evidence of complete or incomplete combustion? The independent variable is the factor deliberately changed. Here, it is the burner air opening, used as a classroom control for air supply. The dependent variable is the evidence recorded, such as visible soot on a cool surface or a carbon dioxide reading from suitable equipment.

Keep other conditions as similar as possible. Use the same burner and fuel source, the same observation time, the same distance and position for any approved sampling equipment, and the same surface for checking soot. These are controlled conditions. Record the air-opening setting for each trial. If the setting is not measured directly as an oxygen amount, describe it honestly as an air-supply setting rather than a measured oxygen concentration.

Safety is part of the design, not an optional extra. Combustion must take place only with teacher approval and direct supervision, using an approved burner setup in suitable ventilation, such as a fume hood when required by the school procedure. Keep faces and hands away from the flame. Never seal the burner or trap its exhaust. Carbon monoxide may form without visible warning, so use the school’s required carbon-monoxide precautions and never deliberately breathe or collect exhaust. Stop if the flame becomes unstable or the teacher’s safety conditions are not met.

Before collecting data, decide what counts as evidence and how it will be recorded. A soot observation can support a claim that carbon particles formed. A carbon dioxide sensor or other teacher-approved method can provide evidence about carbon dioxide. Soot alone cannot identify carbon monoxide. If the available equipment cannot safely identify a product, state that limit rather than claiming it was detected.

Change one main factor: the air-supply setting.
Keep fuel source, timing, equipment position, and observation method consistent.
Plan safe procedures and product evidence before lighting a burner.
Record observations as observations; do not invent readings or treat predictions as results.

4. Record evidence and make a limited conclusion

A clear data table can include the air-opening setting, the observation time, flame appearance, soot evidence, and any approved sensor reading with its unit. Record what happened without changing the wording to fit a prediction. For example, “black deposit visible on the test surface” is an observation. “Incomplete combustion occurred” is an interpretation that needs evidence and careful reasoning.

Compare trials only after checking that the planned conditions were kept similar. If more soot appears with a smaller air opening, that pattern is consistent with more carbon particles forming under the lower-air setting. It supports, but does not prove by itself, every detail of the reaction. A carbon dioxide reading may add evidence, but it does not show that carbon monoxide is absent.

A strong conclusion answers the inquiry question, refers to the recorded evidence, and names a limitation. It should distinguish between what was directly observed and what is inferred from the model. If a result is unclear or trials differ, report that honestly and suggest a practical improvement, such as making the observation time more consistent.

Separate direct observations from explanations.
Use more than flame colour to compare combustion.
State what the evidence supports and what it cannot establish.

Worked example

Build a comparison plan

A class wants to compare combustion with a burner air opening set to two different positions. Design a safe investigation that could provide evidence about complete and incomplete combustion. Do not invent results.

Set the question and changed factor
Ask how the burner’s air-supply setting affects evidence of combustion products. Change only the air-opening position between the two conditions. Treat this setting as a control for air supply, not as a direct measurement of oxygen concentration.
Choose what to keep consistent
Use the same burner and fuel source, observation time, test-surface material, and equipment positions. Record the settings and repeat the same procedure for each condition. These controls make the comparison fairer.
Choose evidence and safety controls
With teacher approval and direct supervision, record flame appearance and whether soot is visible on an approved cool test surface. Use only teacher-approved equipment for any gas reading. Work under the school’s required ventilation and carbon-monoxide precautions. Do not collect or breathe exhaust, and stop if safety conditions are not met.
Plan how to interpret the evidence
Soot would be evidence of carbon particles. A suitable carbon dioxide reading could provide evidence about carbon dioxide. Neither a clean-looking flame nor a carbon dioxide reading alone proves that carbon monoxide is absent. The conclusion must be limited to what the chosen evidence can show.

Answer: The plan changes air-opening position, keeps the main comparison conditions consistent, records approved observations, and includes safeguards for carbon monoxide and flame use. It does not claim any outcome before the inquiry is carried out.

Check: The design distinguishes the changed factor, controlled conditions, evidence, and safety limits. No experimental measurements are assumed.

Common mistakes and how to avoid them

Treating a blue or yellow flame as proof of a particular product.

Correction: Flame appearance can be recorded, but it does not identify all products. Use suitable evidence and state its limits.

Claiming that no carbon monoxide formed because no soot was seen.

Correction: Soot is carbon, not carbon monoxide. Carbon monoxide is invisible and odourless, so soot observations cannot establish its absence.

Changing fuel, timing, and air setting at the same time.

Correction: Change the air-supply setting as the main factor and keep other conditions as similar as practical.

Writing a predicted equation or expected observation as if it were a measured result.

Correction: Label predictions before the investigation. Report only evidence actually recorded after an approved investigation.

Lesson summary

Complete combustion of propane is represented as forming carbon dioxide and water; incomplete combustion can form carbon monoxide and/or carbon particles, along with water.
A fair comparison changes the air-supply setting while keeping other conditions as similar as possible.
Plan suitable evidence and safety controls before conducting any combustion inquiry.
A conclusion must connect to recorded evidence and explain what that evidence cannot show.

Check your understanding

Question 1

A test surface has a black deposit after a supervised burner trial. What does this observation directly support?

Carbon particles formed.
Carbon monoxide was the only product.
No carbon dioxide formed.
The exhaust contained no harmful gas.

Show answer and explanation

Carbon particles formed.

A black soot deposit supports the presence of carbon particles. It does not identify all gases in the exhaust or establish that the exhaust is safe.

Question 2

Which plan best compares two air-supply settings?

Use different fuels and change the air setting each time.
Use the same burner and fuel, change the air setting, and keep observation time consistent.
Change the air setting and test-surface position together.
Use flame colour alone to identify every product.

Show answer and explanation

Use the same burner and fuel, change the air setting, and keep observation time consistent.

Changing one main factor and keeping other conditions consistent makes the comparison fairer. Flame colour alone cannot identify every product.

Key terms

Combustion: A reaction in which a substance reacts with oxygen and releases energy, often as heat and light.
Complete combustion: Combustion represented here as producing carbon dioxide and water when a fuel containing carbon and hydrogen has enough oxygen available.
Incomplete combustion: Combustion that can produce carbon monoxide and/or carbon particles, as well as water, when oxygen supply is limited.
Independent variable: The factor deliberately changed in an investigation.
Dependent variable: The evidence or result recorded to see how it changes when the independent variable changes.
Controlled condition: A factor kept as similar as possible between comparison trials.

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About this lesson and its review

Published by DoAssignment. This reviewed lesson follows Ontario Grade 11 Chemistry (SCH3U), expectation C2.7. It is a study resource, not an official curriculum publication.

Before publication, content is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability. Errors can still occur, so corrections are welcomed.

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