To balance the unbalanced chemical equation:
\[ 2 \text{NaHCO}_3 → \text{Na}_2\text{CO}_3 + \text{CO}_2 \]
we start with assessing the elements present on both sides of the equation. The reactants (2 NaHCO₃) yield products of Na₂CO₃ and CO₂.
Counting atoms:
- On the left: 2 Na, 2 H, 2 C, 6 O
- On the right: 2 Na, 1 C (from Na₂CO₃) + 1 C (from CO₂) = 2 C, 3 O (from Na₂CO₃) + 2 O (from CO₂) = 5 O
To balance hydrogen and oxygen, the simplest way to adjust would be to add water (H₂O) to the products.
If you add water (H₂O), you can adjust the numbers of hydrogen and oxygen to balance the equation overall. So, the correct answer to add to balance the equation is:
H₂O
Therefore, the answer is H₂O.