Photosynthesis and Plant Nutrition: Question 10

Worked solution

Step 1: Count the atoms on the left-hand side of option A

For 6CO₂ + 6H₂O:

• Carbon: 6 × 1 = 6
• Oxygen: (6 × 2) + (6 × 1) = 12 + 6 = 18
• Hydrogen: 6 × 2 = 12

Step 2: Count the atoms on the right-hand side of option A

For C₆H₁₂O₆ + 6O₂:

• Carbon: 6
• Hydrogen: 12
• Oxygen: 6 + (6 × 2) = 6 + 12 = 18

Carbon (6 = 6), hydrogen (12 = 12) and oxygen (18 = 18) all match, so option A is correctly balanced.

Step 3: Show why the other options fail

• Option B: the right-hand side oxygen becomes 6 + (12 × 2) = 30, which does not equal the left-hand side’s 18 oxygen atoms. This equation is not balanced.
• Option C: halving every coefficient gives only 3 carbon atoms and 6 hydrogen atoms on the left, but C₆H₁₂O₆ on the right needs 6 carbon atoms and 12 hydrogen atoms, a single molecule of glucose cannot be built from half the raw materials.
• Option D: this writes water as a product instead of oxygen, so it produces no oxygen gas at all, which contradicts oxygen being a known product of photosynthesis. Counting atoms confirms it is also unbalanced: 12 hydrogen atoms and 18 oxygen atoms on the left do not match the 24 hydrogen atoms and 12 oxygen atoms this equation gives on the right.

Final answer

The correctly balanced symbol equation for photosynthesis is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, option A.