Chemical reactions occur when the chemical structure of a substance changes. During photosynthesis, the bonds within the carbon dioxide and water are broken and reformed to make oxygen and glucose molecules. All the atoms in carbon dioxide and water molecules become a part of glucose or oxygen. We know this by counting the atoms on each side of the equation. The number 6 in front of H2O

H
2
O
in the photosynthesis reaction is a coefficient; it indicates that there are 6 water molecules. The small 2 after the hydrogen is a subscript; it indicates that there are two hydrogen atoms per water molecule. If there are no subscripts or coefficients, assume the number equals one.

1 answer

You are correct in your description of how chemical reactions, such as photosynthesis, involve the breaking and forming of chemical bonds, leading to the transformation of reactants into products. In the case of photosynthesis, the overall reaction can be summarized by the balanced chemical equation:

\[ 6 \text{CO}_2 + 6 \text{H}2\text{O} \rightarrow C_6H{12}O_6 + 6 \text{O}_2 \]

This equation shows that six molecules of carbon dioxide (\( \text{CO}_2 \)) and six molecules of water (\( \text{H}2\text{O} \)) react to produce one molecule of glucose (\( C_6H{12}O_6 \)) and six molecules of oxygen (\( \text{O}_2 \)).

As you noted:

  • The coefficient (the number in front of the molecule) indicates how many molecules are involved in the reaction. For example, the coefficient "6" in front of \( \text{H}_2\text{O} \) indicates there are 6 water molecules.

  • The subscript (the number that appears after an element in a chemical formula) indicates the number of atoms of that element in a molecule. For example, in \( \text{H}_2\text{O} \), the "2" indicates that there are two hydrogen atoms in each water molecule.

Understanding the distinction between coefficients and subscripts is crucial for manipulating and balancing chemical equations, as it affects the overall stoichiometry and conservation of mass in any chemical reaction.

In summary, the total number of each type of atom on the left side of the equation (reactants) must equal the total number of each type of atom on the right side (products), demonstrating that matter is conserved during the reaction.