4. $h_{2}(g) + o_{2}(g) \\rightarrow h_{2}o_{2}(l)$

Explanation:

Step1: Identify the number of atoms for each element

Reactants: $H = 2$, $O = 2$. Products: $H = 2$, $O = 2$.

Step2: Compare the atom counts

The number of Hydrogen and Oxygen atoms is equal on both sides.

Step3: Determine the coefficients

Since the equation is already balanced, the coefficients are all 1.

Answer:

$\text{H}{2}(g) + \text{O}{2}(g) \rightarrow \text{H}{2}\text{O}{2}(l)$