Balance the photosynthesis chemical equation
Count atoms while keeping conditions separate from coefficients.
Section 6.1 · Photosynthesis
Exam scope and assessment notes
Extended only (Supplement) · Biology 0970, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked; human subject-teacher review, supervised practical competence and cumulative assessment remain pending. Cambridge states that IGCSE 0970 has the same subject content as IGCSE 0610, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Recall chloroplasts, enzymes and the difference between photosynthesis and respiration.
- Use the stated observations, quantities and controls; diagrams are schematic.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
How many oxygen atoms are in 6CO₂?
What you will learn
- Count atoms while keeping conditions separate from coefficients.
- Use the stated evidence to explain the method and its limits.
Use the required overall equation
The Extended balanced chemical equation is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, with light and chlorophyll as conditions. A coefficient multiplies a whole formula; the small subscript belongs to the formula and should not be changed merely to balance the equation.
Carbon counts are 6 on each side and hydrogen counts are 12 on each side. Oxygen on the left totals 12 from carbon dioxide plus 6 from water; the right totals 6 in glucose plus 12 in oxygen gas. Both sides have 18 oxygen atoms in the stated molecule-count model.
Use ratios without inventing yield
The coefficients give the simplified overall ratio 6 carbon dioxide : 6 water : 1 glucose : 6 oxygen. This is not a rate measurement, and a whole plant’s net gas release need not follow that ratio because it also respires and the measurement can miss gas.
For a specified idealised conversion, two glucose molecules correspond to 12 carbon dioxide and 12 water molecules in the overall equation. That count exercise does not require a plant to produce only whole glucose molecules at a particular moment or establish a real experimental yield.
Pause and explain
How many carbon dioxide molecules correspond to two glucose molecules in the stated equation ratio?
Worked example
Check whether 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ conserves the three atom types.
Show the worked solution
- Carbon: 6 left and 6 right.
- Hydrogen: 12 left and 12 right.
- Oxygen: 12 + 6 = 18 left; 6 + 12 = 18 right.
Answer The equation is balanced: C = 6, H = 12 and O = 18 on each side.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Write the balanced overall photosynthesis equation required for Extended.
Give me a hint
Use the word-equation substances with their fixed formulas.
Compare my reasoning
- Place 6 before CO₂ and H₂O.
- Use C₆H₁₂O₆ and 6O₂ as products.
- Include light and chlorophyll as conditions.
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, with light and chlorophyll.
Look for these in your work
- Correct formulas and coefficients.
- Retain conditions separately.
Count total oxygen atoms on each side of the balanced equation.
Give me a hint
Include oxygen in both products and both reactants.
Compare my reasoning
- Left: 6 × 2 + 6 × 1 = 18.
- Right: 6 in glucose + 6 × 2 = 18.
18 oxygen atoms on each side.
Look for these in your work
- Include glucose oxygen.
- Compare both totals.
A plant’s collected oxygen release does not match the ideal equation ratio during an experiment. Why is that not automatically evidence that atoms are unbalanced?
Give me a hint
The apparatus observes net collected release, not every reaction molecule.
Compare my reasoning
- Respiration uses oxygen.
- Some gas can dissolve or escape collection.
- The equation is an overall chemical ratio, not a guaranteed collection yield.
Net exchange and collection losses can differ from the overall reaction ratio without violating atom conservation.
Look for these in your work
- Distinguish equation and measurement.
- Give at least one relevant experimental effect.
Common mistakes
- Do not change H₂O into another substance to balance atoms.
- Coefficients describe a ratio, not a measured gas rate.
What can you explain now?
How many carbon dioxide molecules correspond to two glucose molecules in the stated equation ratio?
Compare with the explanation
12.
Each glucose corresponds to six carbon dioxide molecules.
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Make it stick
Reconstruct the equation and independently count each atom type.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.