According to the law of conservation of matter, we know that the total number of atoms does not change in a chemical reaction and thus mass is conserved. The reactant in this model is hydrogen peroxide. Hydrogen peroxide decomposes to produce water and oxygen gas. How many water molecules must be added to complete this model?

To analyze the decomposition of hydrogen peroxide (H₂O₂) into water (H₂O) and oxygen gas (O₂), we can start with the balanced chemical equation for the reaction:

\[ 2 H₂O₂ \rightarrow 2 H₂O + O₂ \]

From this equation, we see that two molecules of hydrogen peroxide decompose to produce two molecules of water and one molecule of oxygen gas.

In the provided information, you have 2 red dots (representing the hydrogen peroxide molecules) on the left side of the equation. Since the reaction produces 2 molecules of water and 1 molecule of oxygen gas, to complete this model, you need to show the two molecules of water on the right side of the equation.

The question mark likely represents the need for you to add two additional water molecules to balance the equation visually.

Thus, **you would need to add 2 water molecules to complete the model** based on the decomposition of hydrogen peroxide.